Thank you for that link. This document is very interesting: it gives the right words to the concepts and explains them well! I’ll use it in the end of my post.
I’m gonna give some precision about the pictures I do. I use a little drone to make aerial photography. I use it in the same incident light conditions (direct sunlight in the middle of the day) with the purpose to make a map. So I’m looking for pictures that have the same exposure.
Here is how I proceed (all in the CHDK menu):
- If the weather is sunny, I set the ISO value on 50 ISO (=market value; the real ISO is 30; the setting is 3 x 30). I want the best quality.
- I set the ND filter state on Out
- I take a picture of a printed checked pattern and set the shutter speed according to the histogram (I don’t have any exposure meter…), so I avoid over- or under-exposure. Usually, it is around 1/2000 s. I need a high shutter speed to avoid motion blur. From 1/800, it seems to be enough to avoid it.
- I take some pictures around to check the settings and I mount the camera on the drone. Then the drone can make 2 things: turning the camera on/off and taking pictures. Until now, all the pictures have the same exposures and are excellent, except when the surface is a little bit brighter (like drier grass or a clear road): in these cases, the neutral density filter swings in and the exposure is far to be correct… (with my setting, it would have been bright, but not over-exposed at all).
So if I check my exposure with the equation in your document, waterwingz, we have:
Av + Tv = Iv + Sv
and using the tables on p. 3 to 5 to transform my usual settings:
3 (=F/2.8 ) + 11 (=1/2000) = 10 (=direct sunlight, according to Wikipedia) + 3 (=30 ISO)
which means the exposure is almost correct, a little bit too dark.
If we see only the incriminated pictures, we probably have a higher average scene luminance, which means a higher exposure value (Ev), which explains the ND filter “reflex” to avoid a picture too bright, if we see this equation: Ev = Av + Tv
The shutter speed is indeed supposed to be at its maximum, so the only thing the camera could try to change is the aperture, by using this filter.
So I tried other settings, with the ND filter in:
- Shutter speed: 1/800 (Tv=9.5)
- ND filter state: In (equ. F/8, Av=6)
- Override ISO Value: 15; Value Factor: 10 (real ISO 150, Sv=5.5)
- Illumination: direct sunlight (Iv=10)
According to the first equation, it should be the “proper exposure”. And it is the case on many pictures. But it happens the same thing in the other way: When the subject is less bright, the ND filter comes off and the picture is overexposed.
I don’t know how to use/write scripts, may be it could allow a solution… For the moment, the only way out I see is to buy an other camera that allows to really override the aperture/ND filter.
