Originally Posted by Cloudstrewn
The map was about 2 metres/6 feet away and the trees about 50 metres/150 feet. I used autofocus on all the tests.
What I did was to fix a random shutter speed then fix the iris wide open (with 0db gain), then repeat the process with a new shutter speed which gave a better exposure, and keep doing this until I got the shutter speed which gave the correct exposure for a wide open iris and then took the test shot. Then for successive f values you can use the fact that a one stop closing of the iris (eg from f/2 to f/2.8 or from f/5.6 to f/8) requires doubling the exposure time (eg 1/250 to 1/125) - so if you want to do a test shot with f/2.8 after f/2 you can first select double the exposure time then select the new iris setting.
I found that the simple doubling of exposure time didn't always work precisely for whatever reason, so you may have to play around with shutter speeds a bit more to get the right exposure for a chosen iris setting. Hope this makes some sense - it's difficult to put into words.
Ah, ok. I was simply using shutter priority
My method was to set up the camera on the subject, then before shooting I would find and note down that range of shutter speeds that would give me apertures from 2.8 to to 6.8, then start shooting and work my way through the shutter speeds while the camera was recording.
At each shutter speed, I would momentarily have a look at the aperture menu to make sure it had selected the stop as per my notes.
When I did this, I noted that there was a slight to noticable decrease in luminosity.
It seems that when you are viewing and adjusting the shutter speed, the aperture is free to move to any fractional [infinitely variable] position between stops to give exact exposure.
But when you switch to the aperture menu, the iris is forced to set itself to the nearest higher full stop position, which may be less exposure.