AC90: Auto exposure: how do it do that??

A

AndersonSoundRecording

Member
Greetings all.

During the process of fooling around with my camera, I noticed that when switched to auto, the AC90 seems to be able to increase the gain while still holding the iris at f2.8. Not sure if it might be playing games with the shutter speed, but otherwise I can't figure out how to do this manually.

Anyone know how to accomplish this?
 
Depends on the zoom setting, likely. The iris can open up more when the lens is zoomed out more, and it can only open up to 2.8 when zoomed fully in, so perhaps the iris is already as open as possible and therefore gain is allowed?
 
Here's the thing though - when shooting in auto, the iris display says f2.8 and next to it is "std"

When I flip over to manual, "std" disappears, the iris display says "open", and I am also able to see how much gain the camera applied.
 
STD means that it's in standard auto-iris mode (as opposed to BLC, which would mean backlight compensation was applied, and SPT, which means it's in spotlight compensation). So that alone means nothing, that's really just telling you it's in autoiris mode, which of course disappears when you're in manual.

As for the gain, it's exactly the same. You're seeing a difference in display (again, only a difference in display, not in operation) that changes from "AGC" (meaning, automatic gain is applied) to when you go to manual, where it tells you the exact number. But look at the brightness of the image -- there's no difference. There is no more gain in auto, than you could have gotten in manual.

The camera isn't gaining any brightness/gain capability when it's in auto. It's just reporting it to you differently.
 
Thanks Barry - all of that I understand (except I didn't know what the STD stood for).

My confusion arises where the camera is able to hold the iris closed somewhat while still applying gain (or at least it seems so, since it indicates that it is holding at f2.8) and when I switch to manual, it shows me how much gain was applied, but I can't apply gain manually until the iris is completely open.

This could a useful thing to be able to duplicate manually in some situations, if it is possible.
 
It can't do that. There is no additional capability. You're just being confused by the labels they're applying. F/2.8 is the same as "open" when you're at maximum zoom. To reiterate: it is impossible to have gain on an AC90, unless the iris is opened to its absolute maximum first. And that absolute maximum is different depending on where in the zoom range you are. At full wide angle, "open" is f/1.5. At full telephoto, "open" is f/2.8.
 
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