ISO and exposure compensation – what’s the relationship?

If I shoot a digital SLR – let’s say a Nikon D200 or a Nikon D40 – at 800 ISO and set +1.0 exposure compensation, is that the same as shooting at 400 ISO?

I wanted a theoretical answer to that question because it seemed to me that increasing the sensitivity of the sensor by increasing the ISO is not the same as setting the sensitivity at a certain ISO and then pumping more light onto the sensor by increasing exposure.

But perhaps that is not so; and I say that because I do not know how the hardware or the software inside cameras work. I only know what works.

First of all, why is exposure compensation necessary?
The answer to that is that the metering system in most SLR cameras (film as well as digital) measures light reflected off the subject, and the light passing through the lens is measured inside the camera.

According to Nikon, their Matrix Metering System can set optimal exposure for virtually every situation.

However, they caution that the internal metering system is designed to provide optimal exposure for a subject with standard reflectivity. The ‘standard’ is an 18% gray, which is halfway between black and white in reflectivity terms.

But when the reflectivity of the subect is very different, the metering system may not be up to dealing with it.

Nikon, and all manufacturers and experts, recommend increasing exposure compensation for scenes that are likely to be underexposed using the metering system alone.

These are scenes where the subject is white or yellow or where sky fills a lot of the scene. Conversely, decreasing exposure compensation is suggested for scenes with a lot of deep green, or for dark subjects, or where a good part of the subject is in shadow.

The bride and groom present a real problem for wedding photographers – she’s wearing a white dress and he is wearing a dark suit.

And the fact that cameras can deal with these shots, shows that film and digital are able to handle the problem with proper technique.

Back to the problem
Well I haven’t got an answer to that theoretical question of the relationship between exposure compensation and ISO in digital cameras, but I have done some tests using the Nikon D40 and the D200. The really interesting thing for me is that the two cameras did not behave the same way.

How I did the test
I did not shoot in Aperture Priority or Exposure Priority or any of the programs where the camera decides the appropriate speed or aperture side of the exposure equation.

Rather, I shot in Manual mode and adjusted exposure using the bar that appears in the LCD. Once the indicator on the bar showed that exposure was correct, I shot the frame.

So, in detail, I shot at 400 ISO, then changed the ISO to 800, adjusted the exposure in the bar to zero and shot again.

Finally, I left the ISO at 800 and changed the exposure compensation from zero to +1.0 but did not adjust the exposure in the bar.

I did this for each camera in turn. The day was sunny and cloudless, so the overall light levels remained more or less the same throughout the test.

Exposure Bar
exposure bar

What I expected is that with +1.0 exposure compensation the shot would be overexposed. After all, it had received twice as much light as the meter suggested it should receive to expose the shot correctly. And so it was with the D40.

I opened the three frames together in Camera Raw 4.2 and looked at the exposure. The shot taken at 800 ISO with +1.0 exposure compensation was lighter than the other two. One of the neat things one can do with Adobe Camera Raw 4.2 is to hold down the ALT button on the keyboard (I use a Mac) and then press the Exposure slider in the program.

The frame blacks out and only shows those highlights (if any) that are blown outside the maximum exposure range.

If the highlights are not blown, the scene will be black. (Of course, a shot could be underexposed, but there is another slider to deal with that.)

With the D40, all the shots had some areas of blown highlights but the shot with +1.0 overexposure showed substantially more blown highlights than the other two shots. I needed to pull exposure back with that shot to get the blown highlights to disappear.

What I actually did was to pull the exposure back precisely one stop, which seemed about right.

With the D200 it was a different story. When I looked at all three shots they all showed very nearly the same areas of blown highlights, which were in the white window frame that occupied one side of the frame. No shot showed a greater area than the other two.

So in terms of comparing one shot to the others, they were all exposed correctly.

I do not understand how the camera could expose correctly vis a vis the other settings when I had put in the exposure compensation.

I shot in Manual mode so there is no possibility that the camera adjusted the exposure, yet the shot with +1.0 exposure compensation looked the same as the others.

I spent some time looking at the histograms for all three shots, and there were only very small differences. It remains a mystery, but what the sets of shots did show can be seen here.

I don’t like to thrust my opinion or interpretation of visual results onto other people. People can judge for themselves. They can ask, is there as much detail? Does one image look degraded compared to the others?, and so on.

But if you ask me, they all look the same. And if there are small differences, you will only see them in 100% crops.

Which means you can use 800ISO without worrying about the image being worse than at 400IS0. You can add exposure compensation and it will not degrade the image.

You might have to modify the exposure in post-processing (easier to do with RAW files than jpegs) but if you have time to look at the histograms in the camera (not so easy with the D40 admittedly as you have to dig into the menus) while you are shooting, you can control that anyway.

And +1.0 is a lot of exposure compensation. I tend to use minus amounts, if anything, and -0.7 is about as much as I normally go, even in the brightest conditions. So here are the shots, and for me the bottom line is – be confident the camera can handle it – and go out and shoot pictures.

D200 panel of three shotsclick on the thumbnail for a much larger view (1000 pixels wide)
D200 three shot panel

D40 panel of three shotsclick on the thumbnail for a much larger view (1000 pixels wide)

D40 three panel