Exposure Query on my D90

[chelt_strobist]

I have been testing out my D90 to check the exposure accuracy.

I have taken a series of test shots all at 1 stop increments using a 35mm f1.8 lens and at ISO 400. (note, I have also done the same with my 50mm and my 18-55mm, and seen the same type of affect happening).

2.8 @ 1/20
4.0 @ 1/10
5.6 @ 1/5
8.0 @ 1/2.5

The aim for me was to prove that the exposure (i.e. the histogram) would remain the same for each of these exposures as I understand the theory being that if I take an exposure at a set aperture and shutter speed, and then increase my aperture by 1 stop and decrease my shutter speed by 1 stop, I should end up with the same exposure.

Well, I have done this under what I believe is a controlled light situation, i.e. in the living room, using house lights at only, with no natural light coming in from outside. I have the camera mounted on a tripod and the subject is a still life, in this case a mug to take out any possible movement.

But, what is annoying me as I don't understand it, each one of my exposures is slightly, but noticeably different.

For example, in relation to the histogram rather than the image on the LCD: the 4.0 is lighter than the 2.8, the 5.6 is slighty darker than both of them, then the 8.0 is lighter than the 5.6 and is somewhere in between the 2.8 and 4.0.

Has anyone else tried this with their D90 and found the same issue of having different exposure results when they should be the same.

I'm a little baffled.

Cheers for any light you can shed on this.
Matt

 

[HoppyUK]

The histogram is extremely sensitive and shows changes almost imperceptible to the eye.

But the image actually is changing, even though the exposure is the same (or should be). Depth of field is changing and that blurs/sharpens things so that the tonal distribution is altered. For example, if you take a picture of of piece of card that is one half white and the other half black, with a sharp line between the two, that will deliver a very different histogram to the same piece of card shot completely out of focus so that the sharp line is very blurred.

The other thing is that lens apertures are not always dead accurate, due to mechanical tolerances. And at very high f/numbers, like f/22, they are often not absolutely consistent either. If you shoot a series at f/22, chances are there will be a very slight difference between some of the shots, and they may not be exactly one stop different to f/16. Shutter speeds are usually pretty accurate and consistent these days, but you'll probably see slight differences there at the highest speeds like 1/8000sec.

I have found ISO changes to be dead accurate pro-rata.