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A (sort of) noobish question about Av override

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Offline TMHKR

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A (sort of) noobish question about Av override
« on: 23 / March / 2015, 16:10:56 »
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I've been looking through the forum, but couldn't find a reliable answer on this one. It's about the usefulness of the Av override on cameras that already have manual modes.

- If I want to set both Tv and Av, I use M mode. I can set the ISO manually.
- If I want to set Tv only, I use Tv mode. Camera determines the Av. I can set the ISO manually.
- If I want to set Av only, I use Av mode. Camera determines the Tv. I can set the ISO manually.

So:
  M = can set Tv + Av + ISO + flash settings
  Av = can set Av + ISO + exp. compensation + flash settings
  Tv = can set Tv + ISO + exp. compensation + flash settings
  P = can set exp. compensation + flash settings
  AUTO = can set flash settings

With that said, it would seem that Av override can be useful in P and AUTO modes, but in that case I could simply use the Av or M mode instead. Can someone give me a real-life example where I could use the Av override, since Av and M modes provide the same functionality? Thanks. :)
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Offline reyalp

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Re: A (sort of) noobish question about Av override
« Reply #1 on: 23 / March / 2015, 16:42:35 »
Quote from: TMHKR on 23 / March / 2015, 16:10:56
With that said, it would seem that Av override can be useful in P and AUTO modes, but in that case I could simply use the Av or M mode instead. Can someone give me a real-life example where I could use the Av override, since Av and M modes provide the same functionality? Thanks. :)
As you've noticed, using the Canon Av or M is a better option in most cases.  Some exceptions might be
1) If you want to override beyond canon limits. This will generally only allow higher F numbers the high end of the zoom.
2) If you want to use Av control in some mode that normally doesn't allow it.

The ability to control from script is still worthwhile.
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Offline waterwingz

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Re: A (sort of) noobish question about Av override
« Reply #2 on: 23 / March / 2015, 17:02:27 »
Quote from: TMHKR on 23 / March / 2015, 16:10:56
Can someone give me a real-life example where I could use the Av override, since Av and M modes provide the same functionality?
A simple answer here is that if your camera supports Tv, Av, & M modes then you don't need CHDK overrides for anything.  Unless of course you are trying to go beyond the ranges of settings that Canon provides.

But what you might be missing is that a big percentage of Canon P&S cameras do not have Tv, Av, & M modes available.  So without CHDK overrides, people with those cameras are stuck with Auto modes, scenes, and maybe the ability to set ISO.

It's kind of the same question about needing CHDK RAW/DNG on Canon P&S cameras that have native RAW capability.

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Offline TMHKR

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Re: A (sort of) noobish question about Av override
« Reply #3 on: 23 / March / 2015, 17:40:08 »
Quote from: waterwingz on 23 / March / 2015, 17:02:27
A simple answer here is that if your camera supports Tv, Av, & M modes then you don't need CHDK overrides for anything.  Unless of course you are trying to go beyond the ranges of settings that Canon provides.

Yes, I'm using CHDK for extra long shutters, bracketing, and ND / dark frame control. I was just asking for the Av override.

Quote
But what you might be missing is that a big percentage of Canon P&S cameras do not have Tv, Av, & M modes available.

If I remember correctly, Av is not useful on those cameras because they don't have adjustable iris (correct me if I'm wrong). As for the Tv override, it's extremely useful since Canon limit of 15 seconds isn't enough in some cases (many Panasonic cameras can go to 60 seconds out of the box).
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Offline waterwingz

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Re: A (sort of) noobish question about Av override
« Reply #4 on: 23 / March / 2015, 19:35:49 »
Quote from: TMHKR on 23 / March / 2015, 17:40:08
I was just asking for the Av override.
So why is having an Av override useful?   Well one thing that often gets overlooked when people use a CHDK override (e.g. for very long or very short Tv settings) is that they also need to override the other exposure settings  (e.g. Av & Sv).  If you don't,  the camera will pick a values for those based its own exposure calculation - the Canon firmware is not aware of the CHDK override and does not correct for it.   This means your actual exposure will be somewhat random as you are not in control of the other two settings.  Being able to override Av (for cameras with a variable aperture) is particularly useful here.

Edit : typo correction
« Last Edit: 23 / March / 2015, 20:10:43 by waterwingz »
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Offline reyalp

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Re: A (sort of) noobish question about Av override
« Reply #5 on: 02 / April / 2015, 01:55:30 »
One use for Av override is to get more depth of field at the long end of the zoom. Here's an example from sx160

9875 is F/8, the camera default maximum. Shutter is 1/128
9878 is CHDK override set to F/16. Shutter is 1/51

There is clearly more depth of field in the "F/16" shot. The detail on the moon doesn't seem to have suffered, though there is more noise since I added 1 stop additional exposure compensation in raw therapee, to give the sky roughly equal exposure in both shots. Combined with the ~1 stop shutter difference, this shows the aperture really was something close to F/16.

This is also an example of a case where raw allowed me to recover quite a bit more form an under-exposed shot than the jpeg would have.

Originals DNGs and camera jpegs at https://app.box.com/s/07nyqlj9v3q4g6jqu31yvbwl8nqgqbkj
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Offline TMHKR

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Re: A (sort of) noobish question about Av override
« Reply #6 on: 02 / April / 2015, 06:05:53 »
Quote from: reyalp on 02 / April / 2015, 01:55:30
One use for Av override is to get more depth of field at the long end of the zoom. Here's an example from sx160

Heh, nice. What I'm aiming for is more shallow depth of field, which I can only obtain using full zoom with background distance of several hundred meters. If only I could get more of it... But the widest aperture at maximal zoom is f/5.9, so it's tricky.

Since most SX1xx cameras have manual zoom, I was wondering if I could get away with this procedure:

- Take sharp shot of the subject and close background (which will also be sharp, because of deep DoF)
- Take another shot without subject, but with manual de-focus of the entire scene
- Somehow combine the two shots (similar to DoF stacking, but with the opposite effect)
- Resulting image would have DSLR-like sharp subject and out-of-focus background (I guess)

There's no problem taking those two photos, the question is if there's some method of post-processing to accomplish the effect, other than layering two photos in Photoshop and masking out the subject (which would look amateurish).
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Offline waterwingz

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Re: A (sort of) noobish question about Av override
« Reply #7 on: 02 / April / 2015, 08:33:49 »
I know it's not exactly what you are looking for, but given the constraints of the small len & image sensor on P&S cameras then the Brenizer method might be of some interest to you.

http://en.wikipedia.org/wiki/Brenizer_Method
http://blog.buiphotos.com/2009/07/10/the-brenizer-method-explained-with-directions/
http://ryanbrenizer.com/category/brenizer-method/
http://ryanbrenizer.com/2011/05/brenizer-method-instructions/

If you google it,  there are a lot more examples and "how to" pages.

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