New Understanding Auto Exposure

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Understanding Auto Exposure

This is the first in my series of “How Photography Works” topics.

If you ever wished you understood more about how photography works then I hope you will find the answers to your questions here.

I will break this Understanding Auto Exposure topic into sections because the number of sample photos required exceeds the number allowed in a single post. It will take a few posts to complete this topic so please allow me to get all the pieces in an unbroken line before commenting. Afterwards I will invite discussion if people still have questions.

I’m going to jump right into the heart of the matter first and if you are curious you can also read the extra details at the end.

Except for a few camera apps with manual control all the rest are using automatic exposure.

Let’s start with a photographic definition.

Here are some examples of an “average scene”.

“Average” is not a description of the ordinary-ness of the subject matter but a photographic definition of the range of tones in the image. Blue sky, green grass, green trees, maybe some water such as in a river or lake, maybe a road or parking lot, no great extremes or large amount of bright or dark tones. Overall it reflects quite close to 18% of the light falling on it from the sun. That 18% is consistent enough that the camera light meter is calibrated to that 18% reflectance.

Another way to identify an “average” scene - a picture that comes out properly exposed, requiring no adjustments when photographed with no adjustments to camera settings.

In looking for examples of “average scenes” I found I had to look quite hard. The vast majority of photos in my collection don’t fit into this category, containing subject matter either lighter or darker than “average”.

Here is a picture of a standard 18% grey card.

The 18% grey card is a standard photographic reference tone and probably the only one you might ever need.

This is how an “average scene” looks to your camera. If you spin your “average scene” photo at high speed it will blur quite closely into this tone.

The 18% grey card is useful for a variety of exposure problems and calibration. You can make exposure readings off of a grey card in situations where the camera’s built in reflected meter reading cannot provide reliable results.

Of course, a photograph isn’t just 18% grey but contains a full range of tones from white to black.

The important thing is that, in an average scene, this full range of tones averages out to 18% reflectance.

Here are the 3 cards, white, 18% grey, and black, at full size. These 3 cards represent the full range of tones that the photographic process is capable of recording in a photo.

Now I’m going to move in to make a close-up photo of the grey card, with nothing else in the picture except the card. For these grey card photos I’m using the standard native camera app with no edits.

The resulting photo looks just like the grey card. No surprise because this is exactly what the light meter is programmed to do.

So far so good.

Let’s do the same thing with the white card.

Observe what happens as I gradually move from the grey card at the top left to the white card.

Do you see that as the camera moved away from the grey card to the white card how the white card gradually got darker, and when the white card was the only thing in the picture it looks just like the grey card?

The lower right frame in the picture is the white card and it looks just like the 18% grey card in the upper left frame. What’s going on? How does white turn into grey?

Now let’s try that with the black card.

You can see the same progression taking place. As the black card becomes the only thing in the picture it gradually becomes grey instead of black. Once again, the final result is grey. In fact, the same grey as the grey card.
(The amount of automatic exposure boosting required to turn black into grey brought out the slight texture in my black card but it was the best black surface I could find. I made the picture out of focus to offset the distraction of the texture.)

So here’s the final result of the test: Each of the 3 cards photographed with automatic exposure and no adjustments.

The left one is the grey card, the middle is the white card, the right frame is the black card.
Below is how they looked at the start in a single exposure:

WHAT??!! These are straight unedited shots from the native Apple camera. No matter which card you photograph it comes out looking like the grey card. If you don’t believe me try it yourself. In fact, I recommend you do this test yourself since it will really bring home the reality of the situation. You might find it isn’t easy to get completely even illumination.

Remember - that’s exactly what your camera’s light meter is programmed to do. It will adjust your exposure to give you the 18% tone, every time. As soon as you grasp exactly what the light meter is doing you will understand why exposure readings can be quite far off in some situations. Just remember, nothing in the picture except the card, no background edges. That includes no cropping to exclude a background after shooting. Nothing in the picture except the card.

Let me assure you that this isn’t just some quirk of the native Apple camera. Every camera light meter does the same thing in this test.

In automatic exposure the light meter is a single state device. It can only say “this is the right exposure for this amount of light”. The light meter has no idea what your subject matter is. It only knows how much light is coming into the camera. It cannot make any decisions whether a tone should be lighter or darker. That’s your job.

What does this mean so far?

As long as you photograph subjects of “average” reflectance everything is just fine. Fortunately, a great many subjects fall into this category of “average reflectance” so the light meter works quite well a lot of the time.

It is when you start doing more adventurous things that you start to run into trouble.

The next part is about recognizing situations where exposure error is guaranteed, and what to do about it.
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Awesome... thanks Brian! Now I just have to start practicing... :grimacing:


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Part 2. Here are the top 2 situations where people have exposure problems.

1 – Scenes containing a higher than average amount of dark tones.
  • Such as pictures in the dark woods.
  • Pictures of dark coloured objects.
  • Light coloured flowers against a darker background.
  • Large areas in shadow.

A forest area like that typically found around waterfalls has a greater than average amount of dark tones, as seen in the lower half, above. What you end up with is the wish-washy photo in the top half because the camera decides the image needs more exposure to meet the 18% tone it is calibrated for.


These are fairly extreme examples to make the situation obvious. In the top half the white flowers are set against a fully shaded background. You can imagine how that would turn out when rendered as an 18% grey tone. Totally overexposed white flowers with no detail. The dreaded burned-out highlights.
The lower half, above, shows a cliff as seen from the shady side but looking through to full sunlight on the other side. When you average out the large area of black and the lesser bright area you would end up with a badly overexposed central area while the dark shadows would be brightened quite a bit.

• When you point your camera at a darker than average subject you will get an overexposed picture - every time, unless you take steps to compensate for the exposure error. Dark toned areas will lose their solid richness. Light tones become washed out. If the picture contains important highlights they will be burned out. Overexposed colours become weak or washed out. Not at all what you saw.

2 - Scenes containing unusually light tones.
  • such as snow scenes, white sand beaches,
  • close-ups of light coloured flowers, etc.
  • Bright fog & mist.

A lighter than “average” scene will result in an underexposed photo. Every time. The photo looks dull overall. Whites look grey. Snow scenes typically pick up a blue cast due to reflected skylight.
When you point your camera at a bright scene, like the bottom one above, it will end up looking like the grey card tone, top. That’s what you meter is programmed to do.


A very bright fog or mist, like in the lower half above, will always cause your photo to be underexposed as the meter tries to achieve the 18% tone, top.

Exposure Compensation

Now that I have highlighted the 2 most common exposure error situations I need to provide some solutions.
Remember that we are still using automatic exposure and not getting bogged down in full manual controls.

The standard method of correcting automatic exposure error in regular cameras is to use exposure compensation.


You simply press the exposure compensation button, the exposure compensation scale pops up, and you use a control wheel to bias the exposure lighter or darker by your chosen amount.

Even though this refers to regular cameras and high-end iPhone camera apps I want to start here because it makes it easier to explain.
  • To keep the dark in a picture you need to underexpose a bit - negative values on the exposure compensation scale. Typically -2/3 to -1 or even more.
  • To keep the lightness in a photo you need to overexposed a bit. Typically +1/3 to +1.
Now you know how exposure compensation is achieved so lets look at how this is implemented on a smartphone.

Generally, only the high-end camera apps have exposure compensation scales. PureShot and 645 Pro, the Moment - Pro Camera app, Halide, ProCam, ProCamera, NightCap Pro, and a few others. They don’t all use the same units. Some use the familiar fractional exposure stops while others use decimal EV numbers, but it amounts to the same thing.

Seeing Photographically

You may be familiar with the acronym WYSIWYG (what you see is what you get). Wouldn’t it be wonderful if that were true in photography. In this case we are more concerned about when you don’t get what you see, and why.

Seeing Photographically refers to being able to pre-visualize how the scene you see with your eyes will be rendered by the photographic process, in other words - the difference between what you see and what the final result actually turns out to be. One aspect of that is how exposure will effect the final result.

Once you become familiar the appearance of an “average scene” you will become more aware when your scene is a bit lighter or darker than “average” and take steps to override the camera’s light meter to get a correct exposure.

You can tell how well you are doing at recognizing and correcting exposure when photos deviate from “average” by how often and by how much you have to adjust your image exposures in editing. Over time your images will require less editing to look well exposed.

Most smartphone apps use a visual on-screen slider to adjust the exposure.

Pictured above is the Apple native camera app.

You simply tap on the screen to select the area of the image you want to meter and when you slide your finger up or down the screen the adjustment scale appears beside it. Slide up the screen for brighter or down the screen for darker. But how do you know how much?

This exposure slider works well enough but has some problems. First, it is a visual process and when you can’t see the screen very well you are just guessing. Next, you can’t tell how much of an adjustment you are making - there’s no scale.

Another problem is you that have to adjust each image because the slider resets itself after every photo. If you are making a series of photos in the same conditions you have to adjust each one individually and since there is no scale it is impossible to adjust every one the same. That makes consistent results impossible.
The other thing missing from the native camera app is any indication of what settings will be used to make the photo. What shutter speed and ISO?

With an exposure compensation control you can set a value and make as many photos as you want with that setting, as long as the conditions remain the same.

If you are nervous about venturing into other camera apps, fear not. The adjustment is just as easy as the native camera except you now have an indication of how much override you have selected.


Top left - PureShot: A long-time favourite of mine. Right now the camera is in ISO priority mode as shown by the mode dial at the bottom right. Exposure compensation is always available in the right-hand side red circled area. You simply tap the - or + symbol to shift the brightness in that direction. Just beside it on the left, still inside the circled area, it shows -2/3 for the current setting. The other circled area shows the overall exposure scale. Another useful feature is that the current exposure settings are always shown at the bottom left.
Middle left - Camera+ 2: When you tap on the screen the reticule appears defining the focus and exposure area. Sliding your finger up or down causes the exposure compensation scale to appear and a numerical indication appears.
Bottom left - Halide: Tap on the screen and slide up or down to adjust exposure compensation. A reminder stays on-screen in the circled area.
Top right - Moment: Tap the EV item on the sliding menu bar and the current selection turns blue and the scale pops up. While sliding up or down the scale a temporary indication appears at the bottom centre. Once the adjustment is made you tap the blue button again to dispense with the scale. A reminder of the setting remains.
Middle right - Prime: Focus is adjusted by sliding up and down and exposure compensation is adjusted side-to-side. While sliding left-right on the screen the scale on the left appears as well as the exposure amount at the top. When adjusting is finished those two items disappear and only the reminder in the white box remains.
Bottom right - ProCamera: Tapping the Exposure Compensation icon changes it to the indicator circled on the left. Sliding your finger up & down the scale sets the adjustment amount.

How much is -1?
The simple answer is that a setting of -1 reduces the exposure by half. The effect of -1 on exposure using the shutter speed as the controlled value would be like going from 1/30 to 1/60 sec. +1 doubles the amount of exposure. Like going from 1/30 to 1/15 sec.

What is 1 Stop?
To answer this we need to look at the standard camera controls going back to the manual camera days. The standard shutter speed scale went from 1 second, 1/2 second, 1/4 sec., 1/8 sec., 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000. This is a simple numerical scale and you can see that 1/2 second is half as long as 1 second. 1/250 is twice as long as 1/500. *note below.

Each step is referred to as 1 stop. Each step to a faster shutter speed on the scale reduces the exposure by half and each step going slower doubles the exposure.

In those days there was no such thing as 1/9 sec, or 1/45 sec., as there can be on digital cameras with electronic shutters. Only the standard increments. Electronic controls allows the use of 1/3 stop increments, or even decimal values such as .7.

iPhone cameras don’t have adjustable lens apertures. Instead they have a fixed aperture of ƒ1.8 for the wide angle lens so we can skip talking about apertures and ƒ-stops since they are not a variable in our exposure calculations.

*(Eagle-eyed readers might notice some anomalies in the numeric scale. For instance 1/30 should really be 1/32 and 1/60 should really be 1/64. It is just some rounding to simplify the scale but the real values are there behind the scenes).

How much to correct?
Now there’s a good question. To some degree you simply get a feel for it after working with it for a while. It also depends on the amount of overall contrast in the scene. Except for those really extreme situations you will not likely venture beyond +/- 1 & 1/3, or 1.3. Much more likely will be corrections between 2/3 and 1, either + or -.

The best way to learn is to recognize situations likely to cause exposure errors, make a photo with no correction as a baseline, and try to estimate how much correction you think it needs. Go ahead and try. Then look at your results. What setting, in that situation, gave you the “correct” exposure, such that no exposure correction was necessary. You should be able to consistently nail your exposures to within 1/3 stop.

Metering Systems
There are a few different ways that light meters have been designed to provide exposure readings. Each was devised to overcome problems encountered in the others.
Averaging - The typical type of light metering averages the light from the entire image area. That type of metering is the most prone to the type of exposure errors mentioned.
Centre-weighted averaging - Averages just the central 2/3 of the image, and typically biased towards the bottom of the image, making it more focused on the subject area. Often a practical compromise.
Spot metering - Measures only a small area at the centre of the image, perhaps 2-5% of the overall image area. This type of metering is ideal when the subject area is small and surrounded by a very different background tone. Aiming the camera around a scene the exposure may vary wildly and you may wonder what is going on until you realize the metering area is very small and responds to every different tone in the image. You really need to know what you are metering to use this type successfully. Favoured in Zone System Photography.
Matrix metering - A more sophisticated metering system that analyzes individual tones in the image and tries to map them in the dark-to-light dynamic range of a photo, picking an exposure setting based on a median point in the distribution of tones. Typically takes the brightest tone and assumes it should be white. This is another step up but when the brightest tone isn’t supposed to be white it still results in exposure errors.

Did you know that the fastest shutter speed on an iPhone 8 is 1/91,000! Typically a DSLR would have a maximum shutter speed of 1/2000 or 1/4000 so 1/91,000 is an astounding number. You might mention that to some stuffy DSLR user you come across.

Don’t worry, be happy.
At this point I can anticipate that some people will be thinking, “Why should I bother with all this if I can correct it later in editing.” Well, in actual fact, you can correct minor errors with no problems. However, larger errors will result in important tonal values being bumped off-scale and not recorded in the image file. For instance, overexposure can drive highlight tones off-scale which means you are left with empty featureless white where highlight detail should be. The dreaded burned-out highlights - one of the most common errors that can trash an otherwise nice photo. Likewise, shadow detail can disappear if you underexpose an image. All the middle tones will look OK with editing but the tonal information you lost due to exposure error is gone for good and cannot be retrieved by editing. So it is important to at least get close to the correct exposure if you can. More on this later when I get to the histogram display.

By now you should be able to anticipate what the likely exposure errors would be in the next two photos.


Coming next: Two more common exposure problems.
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