Category: Technology

Ideas about the mechanics, techniques, and technology behind image making.

Posted on

The Magic of the Lens

Hotel deVille, Paris

Do you ever stop to think about your lenses, besides wanting a shinny new one? There is a magic of the lens that we seldom consider and perhaps do not even understand.

Many constraints

My perception is colored by my background as an engineer. I see a modern lens as serving so many constraints that it is a wonder they do the job as well as they do. We expect high resolving power and “good” bokah. It needs to have a good zoom range but be small. It must be weather sealed and rugged, but inexpensive. And, of course, issues like low chromatic aberration and great edge to edge sharpness and low distortion and minimal light falloff (vignetting) and minimum flare are all givens. Oh and blazing fast auto focus, too.

The poor lens designers are in a tight place. Luckily for them computer design tools have advanced greatly. Also, new materials are available to help overcome some of the design problems of the past.

Still though, we ask a lot of a professional grade lens. Probably more than we realize.

Simple lens

We have an idea in mind of how a lens works. You probably did an experiment in High School Physics with a simple lens. Then you took it out and fried some ants.

What we normally picture is a biconvex lens. Don’t let a fancy word scare you. That just means both sides are thicker in the middle than on the edge. Like this:

By DrBob at the English-language Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2065907

The rays (red lines) illustrate how the lens focuses on a point. That focusing is what images the outside world sharply onto our sensor.

This is true. It works. But nothing in life is simple anymore.

Reality

The reality is that, because of our high expectations and the piles of constraints to satisfy, real lens design has to be much more complex.

I am going to use the Nikon Z 24-120 f/4 zoom as an example. For two reasons: it is a representative high quality modern design, and I like it – a lot. It is my go-to lens for everyday use.

Lens design has gone far beyond the “simple” lens pictured above. Here is a cutaway of the Nikon lens:

Photography Life: https://photographylife.com/reviews/nikon-z-24-120mm-f-4-s

We can see that it has many lens elements (a word for a piece of glass in a lens) – 16 of them to be exact. Few of them are simple biconvex elements. Some of them are exotic glass. Things like high refractive index (they bend light more sharply than regular glass) or other properties. Some are aspherical. This means they are quite complex designs to achieve specific results. These are hard to design and manufacture. Usually they are necessary to correct for effects of other things and make the resulting image better.

Zoom

Let’s look at a few specific features. This lens has a 5x zoom range, from 24mm to 120mm. Now you would think that, for the lens to zoom 5x, it would have to get 5 times longer. This would be true for a straightforward design.

However, us users of the lens would not like that. It would have to be very big and bulky to do that. And it would be awkward when zoomed all the way out. It would be long and off balance the camera.

But complex design magic and some of those special lens elements allow them to shortcut physics. it zooms over the 5x range while only extending to less than twice it’s collapsed length. Amazing and very welcome.

Reflections

The real world is not a well behaved bundle of parallel rays coming into the lens, like in the simple lens picture above. Light is coming from everywhere. Most of it is what we want to end up on the sensor. But a lot isn’t. Light coming in from a sharp angle tends to “bounce around” inside the lens and cause a lowering of contrast. Kind of a fog look.

Modern lenses have special coatings on the glass and use some of the special types of glass i mentioned to fight this. These go a long way to canceling the reflections.

It used to be that shooting in the direction of a very bright source, like the sun would always cause unwanted internal reflections that degrade the image. Now it is amazing how little that happens. I really only worry about that if the sun is directly or nearly directly in view.

Abstract study in texture and shape©Ed Schlotzhauer

Chromatic aberration

Chromatic aberration is something we seldom consider, except when we are getting down to the last details of a final print. One of the nasty realities of physics is that each “color” of light is a different frequency. The amount of “bend” the lens gives to light is dependent on the color (frequency) of the light. This means not all the colors focus at the same point. That’s bad.

Have you every looked very closely at magnified blowup of a sharp edge in one of your photos? Especially if it is in a high contrast lighting situation. You may see a slight fringe of green or purple around the edge. This is called chromatic aberration. Not all the colors focusing together.

One of the purposes of the exotic glass and all the elements in modern lenses is to minimize this. They do a pretty good job.

But they are not perfect. Luckily it is a simple check box in Lightroom Classic to have the software automatically remove chromatic aberration.

Other considerations

If you ever carry a camera around all day you learn to appreciate light weight. Lens designers would like to design their lenses with a very sturdy metal shell and structure. But we would not like to carry that. Modern plastics and design techniques have allowed the designers to create our lenses at a more user friendly weight while still being sturdy enough to hold up to hard use. Thank you.

Did you know that some lenses make the light come into the sensor is a certain direction to make the sensor receive the photons better? Did you know that most of our zoom lenses, especially, have quite a bit of distortion and vignetting and resolution falloff at the edges? Those are some of the things that are part of the tradeoffs. But one reason they are traded off is there’s a bit of perceptual and software slight of hand.

First, we don’t notice it much. Really. We are not as sensitive to it as you would think. Unless you spend your time photographing test charts. Second, many of us set Lightroom Classic to look at the model of the lens and automatically apply a “correction” to the image we see. Adobe has a database of lenses with mathematical models to correct their distortions. This is a good thing.

As a matter of fact, Nikon has a special deal with Adobe such that the great Z 24-70 f/2.8 lens is automatically corrected in Lightroom Classic, whether or not the user selects that. It is impossible to defeat it. Hardware and software are joining in a symbiotic relationship. Making an image is a blend of both and it will only increase with time.

Almost everything done to solve one problem creates another. This is why designs are so complex and expensive. Everything is a tradeoff. It is all a question of how good can we make this property while not letting that other one get worse than a certain level.

Example black & white image©Ed Schlotzhauer

Magic

I am in awe of these brilliant designers. They achieve beautiful balance. Like I said, I regularly use this example lens I have talked about and I am generally very happy with it. But let me emphasize that pure, unexcelled technical perfection is not usually my goal. A lens like this is “good enough” for 99.9% of my needs.

For me, as a user, I take the camera out and start using it. What I see and feel is more important than technology. Sometimes, though, my engineer nature kicks in and makes me marvel at the complexity. But really, I shoot and expect my great gear to capture what I want. And it usually does. Marvelous.

The magic of the. lens. Like most good magic, how it works is invisible to us. But occasionally stop to consider how lucky we are and what an incredible piece of technology we have attached to our camera body.

Feature image

The image at the top was shot with this Nikon Z 24-120mm f/4 zoom lens I have been using as an example. This is the Hotel de Ville in Paris – their Town Hall. You can’t really tell in this small jpg, but I am completely happy with the capabilities of this lens. If the opportunity arose I am sure I could make a very good 60″ print of this. Here is a section of it zoomed to 100%.

100% section, ©Ed Schlotzhauer

Let me assure you that I am not affiliated with or sponsored by Nikon. I am just using this nice lens that I use frequently as a representative example of what a modern zoom lens is and is capable of doing.

Posted on

Out of Focus

Interpretation of starry sky at night.

A few months ago I wrote about being in focus, both technically and mentally. I want to go a little deeper into how technical focus happens in modern cameras and an an experience I had recently where what I did was out of focus.

What is focus

Technically, focus is simple when the lens is adjusted so that the part of the subject you are most interested in is sharply defined. Your lens has a focus ring to use to manually focus. Most of us probably use the camera’s built in auto focus capability. This is much more precise than my old eyes. And a lot faster than most of us can do manually.

Focusing physically moves one or more of the lens elements inside the lens barrel. This is required to adjust the focus point.

I will let you argue whether focus is an absolute, precise point or just an acceptable range. I will just say that I am swinging away from being adamant about absolute technical perfection and leaning more toward artistic judgement and intent. Set your own values you will live by.

Whether we manual focus or use auto focus, we observe in the viewfinder the image moving from a fuzzy blob a crisp, detailed representation of the scene before us. Unless we have a very old piece of technology in our camera with something called a split image viewfinder. I had this in my first SLR. It was magic and awesome for most of the subjects I shot.

The split image viewfinder showed the image sharp regardless of focus. The image was divided into 2 pieces in the central circle. The pieces were offset from each other when out of focus. Use the focus ring to bring the 2 halves into alignment and the image was sharply focused. Magic. Enough trivia, though.

Little did I know this was a type of and precursor to what we now call phase detection auto focus. Let’s get a little deeper into the technology.

How does it work?

Auto focus in a DSLR or mirrorless camera is complex and requires many precise components. But it works so well now that we tend to take it for granted.

There are 2 basic technologies in modern cameras. The older one is called contrast detection and the newer and better one is called phase detection.

I have written on histograms, a subject I consider vitally important to photography. Histograms and their interpretation are the basis of contrast detection auto focus. It is brilliantly simple in concept and in process as what we do when we are manually focusing.

If an image in the viewfinder is out of focus, the pixels are blurred together. Kind of like looking through a fog. A result is that in the histogram, the values are clustered in the center. This is an indication of low contrast. But when an image is sharp, there is a wider range of brighter and darker pixels. This illustrates it:

From https://digital-photography.com/camera/autofocus-how-it-works.php

Focus process

So conceptually, the system moves the focus a little and measures again to see if the histogram got more narrow (more out of focus) or wider (sharper) . If it got more in focus, continue moving that direction and measuring until the peak contrast if found, But if it got more out of focus, move the focus the other direction and continue the process. It is a hunting process to find the optimum focus point. Just like we do to manually focus.

Unfortunately, this process is slow. It can take seconds to arrive at the focus. This is why phase detection auto focus came to prominence.

In phase detection auto focus, some of the light coming through the lens is split off to a separate sensor. Like the split image viewfinder I mentioned above, it is further split into two paths. Through some brilliant engineering, they can determine in one measurement how far off focus is and in what direction. The focus moves there quickly. Note that in mirrorless cameras all the light goes directly to the sensor, so these auto focus sensors are built directly into the sensor.

I said that phase detection is “better” than contrast detection. That is true as far as being very fast. Actually, contrast detection can achieve more precise focus. There is a kind of system called hybrid the combines the strengths of both. I will not discuss that or go into the bewildering variety of focus areas or focus modes.

Out of focus

This is all great as far as technology goes. It works quite well in the cases it is designed for. We are lucky to have it.

But all of these systems rely on the sensor having enough light to see some contrast. It doesn’t work in the dark. Yes, there is another variation on auto focus that is called active auto focus. It shoots a red beam from the camera to illuminate the focus area. This has a very short range and does not help the scenario I’m about to describe.

Recently I was in Rocky Mountain National Park, over on the west slope where there is little light. It was full dark on a moonless night. The mountains all around provided lovely silhouettes. The stars were astonishing. Beautiful. I had to stop and get some star images.

A trailhead parking lot provided a great and convenient place to set up – wondering if those occasional sounds I heard in the dark were bears. I guess not. It was perfect. Except. There was not enough contrast to focus, even at 6400 ISO. And the viewfinder image was too noisy to be useful for manual focus. I did not have a powerful enough flashlight to cast enough light on the nearest object, over 100 yards away, to allow the focus system to work.

Adding to the problem, the lens I brought on this outing did not have a focus scale (a curse of modern zoom lens design). Normally, in low light, I switch to manual focus and set the lens to infinity for a scene like this. I guessed, but missed badly for a big section of the images. They were uselessly out of focus. I am ashamed to show an example, but like this:

A blurry night shot©Ed Schlotzhauer

Experience is a great teacher

I write frequently advocating that we study our technology to become expert with it. And to practice, practice, practice to know how to use our gear, even in the dark. I failed. I encountered too much dark and a lens I had never tried to use in low light. The combination tripped me up. I am ashamed to admit I did not follow my own advice well enough.

But every failure is a learning opportunity, right? It can be a great motivator and reinforcer. I did some research and discovered a “hidden feature” I never knew my camera had. It should save me the next time I do this.

My Nikon camera has a setting I had never paid any attention to called “Save focus position”. When On (the default) it remembers the focus position of the current lens when the camera is turned off and restores it on wake up. But when Off – this is the brilliant part – it sets the lens to infinity on wake up. Now I will have a known infinity focus setting, even in total darkness! This setting is now in my menu shortcuts so I can access it quickly.

I would never have learned about this feature if I had not failed so spectacularly. Experience really is a great teacher.

So dig into those obscure settings you never bother with. There sometimes is gold there.

Keep learning and failing!

The featured image

That night’s shooting was not all bad. I nailed the focus on this star shot. It was purely of the stars and had no foreground. This foreground has been substituted from another blurry image that night (actually, redrawn by hand).

This is artistic expression rather than literal reality. I do that a lot. As photography progresses and matures, I believe that is more and more the norm.

Posted on

Diffraction

High DOF at f/22. Hang the diffraction.

Today I would like to try to help us understand a little about what diffraction is. Not getting too deep in the theory. Just enough to demystify it a bit.

Scary

Diffraction is probably a scary word to most of us. Even if we don’t know what it really means, we have heard of it and have been taught that it is a “bad thing”.

Have you been taught to avoid using apertures smaller than f/11? Note that when I say a “small” aperture I am referring to the physical size. Remember that as the aperture numbers get bigger the actual opening in the lens gets smaller. This simple graphic illustrates that:

Progression of physical f-stop sizes

The lore is that very small apertures (large f-numbers, like f/22) make an image too blurry to be useful. Don’t believe everything you hear without testing it.

Light theory

I’m going very light on theory (yes, pun intended). We’re just going to graze the surface without taking a deep dive in. (Here is a source to start at if you want to go deeper. Abandon all hope ye who enter…)

Light behaves as waves (most of the time). Actually, a number of things are waves: light, water waves, sound waves, gravity waves. Quantum mechanics theorizes that even matter is waves. Too deep for me.

We tend to visualize light going through our lens as rays. That is, straight lines. Yes and no. That is one useful model of looking at it. But light also behaves as waves. An interesting and important property of waves is that every point on a wave is a wave. So if the wave is blocked by a small opening, the wave spreads on the other side of the opening.

This picture by Verbcatcher does a marvelous job of illustrating that for waves in water:

Diffraction in water waves

See how the waves spread after going through the small opening to the sea? The smaller the opening (aperture) the more pronounced the effect. That is, a small aperture opening causes waves to spread out more.

What does it really mean

This is the basis of the recommendation to use physically large apertures (small f-numbers). Apertures that are large relative to the wavelengths of light do not cause much “bend” of the waves. Small apertures (large f-numbers) “bend” the light more.

What we can actually see in practice is that using small apertures causes our images to have a mildly “fuzzy” look. Because the waves spread more after going through a small aperture, the individual waves cover a larger pixel area. This slight spreading of the light causes the image to appear less sharp.

The best discussion of diffraction for photographers I have found is from this article by Spencer Cox. But even this gets too deep into theory.

I borrowed this image from it to illustrate the practical effects of diffraction as we change aperture:

Effects of diffraction with aperture

See how the larger apertures (small f-numbers) are sharper than the smaller ones?

This illustration below, also from Spencer Cox) gives a great conceptual representation of what is happening. Take that the grid represents pixels in your sensor. At f/4, the point of light only strikes one pixel. It will be seen as very sharp. But at small apertures, the waves spread some onto adjacent pixels and create a kind of fog.

Should you fear it?

Should you fear it and always shun small apertures? No, it is just a reality of physics. It is no more to be feared than gravity. As one of my sons would say, it is what it is. Be aware of what is going to happen and consciously decide how far you need to go.

All of the exposure determinations we make daily are tradeoffs. How much to stop motion? How much depth of field do we need? Is there enough light for a good exposure? What ISO setting should I use? All of these things and more have to be balanced in the moment of shooting, besides composition and esthetic issues.

Each setting costs something. As experienced photographers we must understand the tradeoffs and be able to judge what is right for us at the moment.

Diffraction is one of those tradeoffs. Know what it is going to do and how to use it or avoid it.

Sometimes you need more

But why would we ever intentionally make our image less sharp? We seldom actually choose to make it less sharp, but sometimes we need other things. I can give 2 easy examples.

The first and most common one is to increase depth of field (DOF). It is counter intuitive, but making the aperture smaller increases the perceived depth of field. So on the one hand we are making the image less sharp, but on the other hand we are making it appear sharper throughout. When we need to make a certain range of the field of view acceptably sharp we stop down the aperture until we achieve our goal. A tradeoff.

Depth of field with small aperture© Ed Schlotzhauer

The second case that comes to mind is to reduce the shutter speed. I often intentionally shoot motion blur. But I usually forget to bring a neutral density filter for the lens I am using at the time. I can generally achieve the effect I want by using my polarizer, reducing the ISO to the lowest setting, and cranking the aperture down to the smallest possible one. This will probably give me a shutter speed in the range I want to use. Yes, the small aperture increases diffraction and makes the image less sharp. But it is handheld at a long shutter speed. It is already intentionally blurred.

Intentional blurring based on small aperture.© Ed Schlotzhauer

But maybe more importantly, in a great video on Lumminous Landscape, Charles Cramer said “sharpness is something we have to get over.” He explained that if we take a picture just because it is sharp, it probably won’t be very interesting. We have to forget about how sharp is it and instead react to the scene before us on an emotional level.

Shoot the picture

Diffraction is a side effect of physics and our photographic technology. Don’t be afraid of it. Don’t blindly follow some rule you learned in the past about what you can or can’t do. Understand enough about it to recognize it and know how to use it to your advantage.

Look at the image above of the woman’s face. Even at f/32 – an extreme case – it is acceptable. Extra sharpening can be applied in your editing tool to compensate for it.

So diffraction is just there. Allow it to happen if that is the tradeoff you need to make. Just like using a high ISO adds noise, that is acceptable most of the time and better than missing the shot.

I know many of us don’t want to deal with what we perceive as increased complexity or too much technical detail. We just want to go take great pictures. My hope is that topics like this will actually make your photography life simpler by providing some grounding for information you may have heard in the past. Rather than trying to remember rules for how to use your equipment, you now have a model for what diffraction is doing and how strong its effect is. I hope you will be able to stop fearing it and accept it is just part of the tradeoffs of the technology.

Today’s image

This is a great old WWII era truck I found in my town. It is a Coleman. This was actually a Colorado company. It was designed and manufactured in the Denver area.

I needed enough depth of field to span from the great rust and paint patterns on the near outside through most of the cab. So it is shot at f/22. Diffraction? Works for me.

What do you think?

Posted on

They Told You Wrong About ISO

Candles, Catholic Church, Regensburg Germany.

Many of us have a wrong idea about ISO settings. I will just say they told you wrong about ISO. It was a misunderstanding. Whoever “they” are.

Statement of faith

It is stated as a “strong suggestion“, especially when we are learning landscape or portrait work. Never shoot with ISO over 100. Maybe it is stated as only shoot at the native ISO setting for your camera. Either way, these are given as rules.

I hate rules, especially for my art. Rule of thirds. Rules of composition. Never put the subject in the center. Never shoot at midday. Always use a tripod. The list goes on.

Like with religion, most of the so-called rules are based on good ideas, but over time they are repeated as commands and the underlying reasons are lost. Just do it. (I don’t think that is what Nike meant.) The rules become a statement of blind faith that cannot be challenged.

What is noise?

All digital cameras have noise. Noise is randomly generated in the sensor and in the electronics of the signal path until the pixels have been digitized by the analog to digital converter (ADC). The noise is a fundamental property of physics.

The question is how much noise is there relative to the desired data. This is called signal to noise ratio in engineering. When we amplify a signal by increasing the ISO setting, all the signal including the noise is increased. This is why images shot at high ISO settings tend to look noisy. The image is usually not less sharp, but there is more noise obscuring things.

It is true for a low cost point and shoot camera or a high end medium format camera. What changes are the relative amounts of noise and the limits the image can be pushed to.

What is ISO?

You’re familiar with the exposure triad: the combination of aperture, shutter speed, and ISO that determine exposure. That’s it. Many other things affect the composition and quality of an image, but only those 3 control the exposure.

Aperture is the size of the diaphragm opening in the lens. It controls, among other things, the amount of light coming in. Shutter speed is the length of time the shutter is open to let light come in. And the ISO setting is kind of like a volume control. It sets the gain or amount of amplification of the sensor data.

Going way back to early film days, there were no agreed on standards for the measure of how sensitive film was. So a couple of the largest standards organizations (the ASA and DIN scales) came together and created a standards group under the International Organization of Standards. They adopted the acronym of the standards organization (in English) as the name. By the way, officially “ISO” is not an acronym, it is a word, pronounced eye-so.

Long way around, but now there are defined standards for exposure. For a given combination of aperture and shutter speed, the ISO settings on all cameras give the same exposure.

Why use higher ISO settings

OK then, in concept, the ISO setting is a volume control for exposure. Turning it up (increasing the ISO value) amplifies the exposure data. But as I mentioned, it is not free. Amplifying the exposure also amplifies the noise in it.

It is true that low ISO settings produce less noise in the captured image. Modern sensors are much better than early ones. This is one of the wonders of engineering improvements that happen as a technology matures.

Then, we should not use high ISO settings, right? Well, everything is a tradeoff. We need to use a minimum shutter speed to avoid camera shake when hand holding or to stop subject movement. We need to use a certain aperture to give the depth of field we want. These decisions must be balanced in the exposure triad, often by increasing the ISO.

Can’t I just underexpose?

When you accept that we must use the lowest ISO setting, the logical conclusion is that you could massively underexpose the image and “correct” it in post processing. Unfortunately this doesn’t work well. You are still boosting the noise unacceptably.

The camera manufacturer knows more about it’s sensors than your image processing software does. The camera’s built-in ISO amplification can take into account it’s characteristics and do a better job. And modern sensors and electronics do a very good job.

Are you wrong about ISO?

If you are following a rule dictating you must or can’t do something, yes you are wrong. There are no rules in art. No ISO-like standards body specifies what your image must look like. There are always groups wanting to do this (are you listening camera clubs?), but they have no authority.

If you are hand holding a shot, it is better to boost the ISO to steady the movement than follow a rule about using low ISO. The noise will be secondary to the reduced shake. Or I sometimes use the lowest ISO setting in my camera to create blur. I enjoy intentional camera movement (ICM) shots and will occasionally force an artificially slow shutter speed.

If it is night and you want to shoot stars or street scenes, are you not going to do it because you would have to violate a rule by the ISO police?

Use the ISO setting that lets you express what you want to do. It is your art. There are no rules. Besides, luminance noise looks like film grain. It can be an interesting artistic technique in itself. Do what feels right to you.

Apology

I used fairly strong language about this. The reality is that most photography writers have softened their recommendations on ISO. Most of them freely recommend using high ISO. This is healthy.

But I know many of us were “imprinted” by early mentors who left us feeling there was something dirty about going above 100 ISO. I want to free you if you still have those self-imposed limits. Using even a very high ISO and getting the shot is always better than missing it because you wouldn’t want to chance increased noise.

Today’s image

Since I’m advocating it, here is an extreme case that I’m happy with. This was shot hand held with an old Nikon D5500 camera – at ISO 22800. I have corrected out some of the luminance and chromance noise and I am perfectly OK with what remains. Getting the shot made me happy, even if the noise is high.

Posted on

The Histogram is Just Data

High contrast but well controlled histogram.

I don’t mean to be insulting, but I cannot understand when people look at histograms as some magical, mysterious, and intimidating technical artifact. It is not. It is just data about what our sensor is seeing. The histogram is just data, and it is useful. Use it. Do not be afraid of it.

Trigger

A newsletter I received today triggered this semi-rant. But looking back, I see it has been over 3 years since I wrote about histograms, so it is probably time to revisit the subject. This actually is a subject I feel some passion for and believe it needs to be better understood by photographers.

The newsletter author declared that our histograms lie. I realize that click-bait is commonly used to try to get people to read articles, but I still feel it is being somewhat underhanded. Now, in fairness, the newsletter author made some valid points – except for the part about histograms lying.

What is a histogram?

We see this graph of some data and maybe it does look complex and mysterious if you are not used to working with data and don’t know where the data comes from. Let’s get over that by understanding how simple but effective it is.

By convention we play like our cameras measure light in a range of 0 to 255. There are no units: 0 represents black and 255 is pure white. The convention came from the history of early digital cameras. It is obsolete today, but still used. That is a topic for another day.

So there are 256 possible values of brightness (0-255). If we go through and count the number of pixels of each value – the number of pixels in the image that have value 0, the number of pixels in the image that have value 1, etc. – and put them on a graph, we have a histogram.

Here is a simple example:

Again, black is on the left going to white on the right. Even without me showing the actual image, we can see that there is a “bump” of dark values on the left and a larger hill of bright values on the right. In between is a relatively low and even count.

What can we learn from this? It is a black & white image, because there is no separate data for red, green and blue. There is high contrast because of the hills at the dark and bright ends. It is bright but not overexposed. There are deep blacks, but not enough to have lost important information. So, even without seeing the image, we can tell a lot about it. Is the image exposed “correctly”? Ah, that is the question my rant is based on.

This is why histograms are useful. They are useful data about our image. It gives simple information to help us understand our exposure better.

Benefit

Today’s mirrorless cameras bring us the amazing benefit of real-time histograms. We can select to see the histogram live in our viewfinder or on the display on the back.

What is the benefit? We see an immediate graphical view of the exposure the camera is determining. In the example above, we can see that the light tones are very bright, but not overexposed.

I routinely use it to watch for “clipping” of brights or darks. If there is a large hump of data jammed up against the left or right edge, that is probably a problem. I will often choose to override the camera’s exposure determination to avoid these peaks.

Again using the example above and knowing that my camera was in aperture priority mode, we see that it chose 1/750 second as the shutter speed. That works OK in this case, but if I did not agree, I would have easily used the exposure compensation dial to adjust the exposure. I do this a lot.

So the histogram is a quick and easy to get a feeling for the “shape” of the exposure.

They don’t lie

Now coming to the basis of my rant: histograms do not lie (actually, they do; I will say how later and why it doesn’t matter).

The newsletter author gave the example of a picture of some fruit on a dark table with a black background. She said the histogram lied because the camera did not give the exposure she wanted. It tried to make the whole image evenly exposed.

No, the histogram is just a straightforward measurement of the data. If you take your temperature but don’t like the reading you get, it is silly to say the thermometer lied.

What the author was describing was that she wanted to expose to have the same look as the scene she saw. This was a case of disagreeing with the camera’s matrix metering calculation. It was doing it’s job of trying to capture all the data that was there and preventing blown out blacks. But she decided to use exposure compensation to force the camera to expose the scene the way she wanted.

The histogram did not lie. As a matter of fact, she relied on it to do her exposure compensation values. She used the histogram to determine how to override the camera exposure calculation.

Actually, I would have used the camera’s original exposure determination. I like to have all the data available to work with. This is called exposing to the right. Bringing the brightness down in post processing to the level she wanted is simple, non-destructive, and does not add noise. Capturing the compensated image the way she wanted irreversibly crunches the blacks.

They lie

I said they don’t lie, but they do a little. For speed and efficiency the histogram is derived from the jpg preview of the image. Same as the preview shown in the viewfinder or camera back. If you study jpg processing you will see that it alters and discards a lot of information to give a good perceptual result.

So the histogram is not actually looking directly at the literal RAW data from the sensor. But there is little observable discrepancy. On my camera, I find that it exaggerates the highlight values very slightly. Still, I typically back the exposure off to avoid highlight clipping, so it adds a little conservatism into the process.

Trust the data you see. It is good enough.

They’re not the photographer

The histogram gives you data. It does not determine exposure. People talk about “good” or “bad” histograms. This is a misunderstanding. There are no absolute good or bad ones. What counts is did you get the exposure you wanted.

There are valid artistic reasons for shooting what some people would consider bad histograms. If it is what the artist wants, it is correct.

Histograms give us a reading of the exposure. They do not determine what is right. It gives some insight on what the automatic exposure calculation in the camera is trying to do.

Use it

The histogram is a brilliantly simple and wonderfully useful tool. We are lucky to have real-time histograms available to us now. It is a game changer. But it is just data. Do not be afraid of it.

The histogram does not lie. But it does not automatically ensure that the exposure is exactly what you want. You have to sometimes take change and override the camera settings. When you do, the histogram is there showing you the result of your decisions.

It is not magical or mysterious. It is a great tool. Use it. A craftsman know how to use his tools.