Exposure & the Histogram, 2

Before going any further, you should understand your camera’s

  • histogram
  • exposure compensation (EV) settings and
  • the concept of stops as it applies to exposure

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In this post we will look at how to get the exposure you want by changing EV based on information provided by the histogram. The process is very simple –

  1. Shoot (using whatever shooting mode and metering mode is appropriate; note this approach doesn’t apply when using manual shooting mode)
  2. Examine the histogram
  3. Underexposed? (darker than you want) increase EV from the current setting
  4. Overexposed? (lighter than desired) decrease EV from the current setting
  5. Do not change any settings except EV and repeat 1-4 as often as needed (shouldn’t take more than two repeats – at most)
  6. Although a regular histogram check is good practice, it shouldn’t need changing often after it’s been set via 1-5 for a given lighting situation (except if you’re using spot metering and don’t watch where the spot is placed).

Your camera probably offers a feature called exposure bracketing. You should at least try it to see the difference between it and 1-5 above. Bracketing is a film carry over and, for me, is less useful in the age of the histogram (but does have a place).

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Let’s start with an easy example – pure black, middle gray, pure white (or RGB values respectively of 0/0/0, 128/128/128, 255/255/255) in equal sizes. You can create a 4×6 card like this in a program like Photoshop, print it and use it to test your camera. Also, when we cover Color in a later lesson, you’ll see how to use it to help set color balance.

Camera default settings (EV=0), Aperture priority, Matrix Metering

As above with EV = +1/3

As you can see from the top histogram, the image is slightly underexposed – the histogram is shifted to the left. This is a sign that in situations like this one my camera/lens combination exhibits a tendency to slightly underexpose. If we want the exposure more “balanced“, with underexposure we have to increase EV. The effect of changing EV from 0 to +1/3 is shown in the lower image. It didn’t take much.

As a test of both your camera and your understanding thus far, you should shoot the same scene with the same setting varying only EV in fixed steps as I did in a previous post’s example. The main point of doing this is to show yourself how far the histogram shifts with different sizes of EV changes. If you’re going to change exposure using steps 1-5 above, it’s important to know roughly how large of a change is needed in step 3/4. If you’re shooting in aperture or shutter priority the camera is going to get you very close on the first try. Tricky lighting (mostly dark – black cat in a coal mine or mostly light – a field of snow) may throw the initial exposure off but typical scenes won’t take much, if any, EV change as seen above. By contrast, a very bright snow scene may take an change of +1 EV or more (if you don’t understand why the change for a bright scene is plus (+), you need to stop right here and go back to the section on metering!).

We’ll discuss details in a later post, but for now accept the following. For the highest quality images Expose to the Right. This means use EV to get the histogram as near to the right edge as possible without touching it. If the result is brighter than desired it can be changed in post-processing (PP) with no harm to the image quality. If you do the opposite – shift the histogram unnecessarily to the left – you can fix that in PP also, but the image quality will be degraded (lots of noise in the dark areas). EXPOSE TO THE RIGHT unless you have reason not to (there are some).

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A final example –

Both are aperture priority, matrix meter, top EV = 0, bottom EV = +1

Follow the Expose to the Right guideline, an EV change of +1 did the trick. The big spike on the histogram’s right is the featureless sky.

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Now let’s look at the same scene with “operator error”. The capture was made with metering set to spot “by accident”. Further the spot was in the center of the sky – the brightest part of the scene. The trusty light meter said to itself “That’s way brighter than middle gray; less light in order to make the sky darker”. Per step #2, we look at the histogram, not knowing why the scene is underexposed – but never fear, we know exactly how to fix it. Per step #3, put back more light; increase EV!

I took a guess at +2 EV. That was enough to get me the same exposure that I got in the 1st example using matrix metering and 0 EV. Another +1 EV, just as in the 1st example, would finish the job in two re-shoots. If you know your gear you should be able to do this entire 3-shot process in about 10 seconds or less – taking your eye from the viewfinder only long enough to glance at the histogram and then making the EV change “by feel” while putting your eye back to the viewfinder. This isn’t any longer than it would take to do auto-bracketing and is guaranteed to get the exposure you want.

Don’t read any more new material before getting out with your camera and trying this yourself.