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Shutter overrides speeds / Setting ISO

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Offline mikeage

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Shutter overrides speeds / Setting ISO
« on: 09 / October / 2008, 14:56:53 »
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Hi,

I've been playing around with the shutter override on my SD870 (IXUS860), and I've noticed that unless I manually set the ISO, all of my shots seem to come out very washed out. I'm not sure I understand why this is happening; surely the ISO being chosen should match default shutter speed, which is significantly slower (aka brighter) than the speed I choose.

If setting the ISO manually is correct, how should I decide what to use? Given that I have no actual aperture (just a ND filter), how can I make sure that the camera will be exposing pictures properly?

Thanks
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Offline fudgey

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Re: Shutter overrides speeds / Setting ISO
« Reply #1 on: 09 / October / 2008, 15:47:29 »
Override really really overrides. CHDK obeys you totally and only overrides what you tell it to override. Canon's firmware doesn't know CHDK is going to override until it's already shooting, so ISO will remain the same regardless of the override.

Tv override is not a Tv mode for cameras that don't have one (are there any, is yours one?). To some extent it probably could be made to behave like that, though, you can search through the feature requests section. Usually Tv override is used either to exceed artificial Tv limits (to set faster or slower shutter speeds than Canon firmware allows) or simultaneously with Av override and manual ISO (or ISO override) to achieve M mode like behavior on the many cameras that don't have an M mode.

I'm not sure what you're excepting and what you know, but you could study the exposure value system on e.g. Wikipedia... your camera probably has Ev compensation to come to your rescue unless you're overriding too far (i.e. more than your camera's ISO range can adjust for). Also note that Ev compensation can be scripted to +-4 Ev even though your camera's menus only show +-2 Ev.
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Offline mikeage

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Re: Shutter overrides speeds / Setting ISO
« Reply #2 on: 09 / October / 2008, 22:39:07 »
My camera doesn't have a Tv mode; my understanding is that most (all?) of the SD series doesn't. The only manual shutter controls it provides are a long exposure mode; from 1s to 15s.

I've had good results using it with ISO 80 and the flash at 1/100K (which I'm positive is not really 1/100K, but is something fast enough that it makes nice water drop photos). My concern was more that I wanted to be sure what I was doing was _right_, not just that I happen to have gotten lucky. How do I calculate the impact of the flash, in this case?
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Offline fudgey

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Re: Shutter overrides speeds / Setting ISO
« Reply #3 on: 10 / October / 2008, 03:11:24 »
Quote from: mikeage on 09 / October / 2008, 22:39:07
I've had good results using it with ISO 80 and the flash at 1/100K (which I'm positive is not really 1/100K, but is something fast enough that it makes nice water drop photos). My concern was more that I wanted to be sure what I was doing was _right_, not just that I happen to have gotten lucky. How do I calculate the impact of the flash, in this case?

Flash exposure is guesswork. You'll just have to take test shots until exposure is good. Histogram may help a lot in judging this. You don't need to break a bulb for your test shots of course  :D.
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