Is Custom Auto ISO working for the A640 under 1.2?

waterwingz

12121

Re: Is Custom Auto ISO working for the A640 under 1.2?

« Reply #10 on: 03 / October / 2013, 19:56:15 »

Advertisements

Quote from: peabody on 03 / October / 2013, 14:34:30

My bias in this is what I need to have for KAP. The biggest problem is that the camera is constantly moving, so a fast shutter speed is the most important thing. At the same time, in low light sometimes you really do need to slow down the shutter just to get the shot at all. So the Custom Auto ISO function is really useful. But the problem is that it lets the shutter slow down even though the aperture hasn't been maxed. Would it be possible to change the code to include aperture control, at least in P mode?

It might be easier, faster and more flexible to do this with a short Lua script. I can take a look if you are interested in testing it out.

Logged

Ported : A1200 SD940 G10 Powershot N G16

peabody

Re: Is Custom Auto ISO working for the A640 under 1.2?

« Reply #11 on: 03 / October / 2013, 23:09:49 »

I hadn't thought of using a script. I guess my impression was that scripts operate pretty slowly, and by the time it could run, the camera would already be pointing at something different. But if that's not the case, then a script would be great, and I would appreciate anything you could come up with. Thanks very much. And yes, I'd be happy to test it.

Actually, everybody who uses CHDK for KAP would appreciate it.

Logged

waterwingz

12121

Re: Is Custom Auto ISO working for the A640 under 1.2?

« Reply #12 on: 03 / October / 2013, 23:52:34 »

Quote from: peabody on 03 / October / 2013, 23:09:49

I hadn't thought of using a script. I guess my impression was that scripts operate pretty slowly, and by the time it could run, the camera would already be pointing at something different.

Lua scripts can work with millisecond precision. The gating factor in speed is how fast the Canon camera returns exposure information when the shutter button is half-pressed. This time is the same with or without a script .. and I'd venture that a Lua script will out run anything else once that information is available.

Quote

But if that's not the case, then a script would be great, and I would appreciate anything you could come up with. Thanks very much. And yes, I'd be happy to test it. Actually, everybody who uses CHDK for KAP would appreciate it.

Thought so. It the huge riches from working with CHDK through a pseudonym that keep me in this game

That and the international fame!

Give me a day or two to work something up?

Logged

Ported : A1200 SD940 G10 Powershot N G16

peabody

Re: Is Custom Auto ISO working for the A640 under 1.2?

« Reply #13 on: 04 / October / 2013, 11:34:13 »

Thanks very much. Take as long as you like. :-)

Logged

waterwingz

12121

Re: Is Custom Auto ISO working for the A640 under 1.2?

« Reply #14 on: 05 / October / 2013, 10:00:03 »

So I had a few minutes to think about how a script version of autoISO for KAP should maybe work. This pseudo code is a little rough but you should get the idea.

1) Get user entered target Tv (shutter speed) setting and Av (aperature) setting. For example, 1/1000 sec at f5.6
2) Measure the current scene brightness level.
3) Calculate required Sv setting (ISO) for the target Tv & Av settings
3a) If the resulting Sv is greater than a maximum ISO setting (e.g. 400) then decrease the Av value until the required ISO <= MaxISO or Av = minAV. If Av=minAV then decrease TV until ISO<=secondarySv setting (eg 800).
3b) If the resulting Sv is less than a minimum ISO then increase Av until required ISO >= MinISO or Av = maxAV. If Av=maxAV then increase Tv until ISO=minISO.

Somewhere in there I'll figure out what to do with the ND filter but I just want to be sure I have the right idea to start out with.

Logged

Ported : A1200 SD940 G10 Powershot N G16

peabody

Re: Is Custom Auto ISO working for the A640 under 1.2?

« Reply #15 on: 05 / October / 2013, 22:32:11 »

In 3a, I'm confused by the secondary Sv target of 800. I know that Custom Auto ISO has fallback settings, but I never quite understood why they are useful. Perhaps the idea is that if you can reduce ISO by increasing aperture, you would do so all the way down to ISO 400, but if you also have to start reducing shutter speed to get ISO down, you would only go to 800. I suppose that would make sense. I guess it really doesn't matter because I can just set them both to 400 if I want.

In 3b, I think my preference would be to increase shutter speed first until you reach the camera's fastest shutter speed, and only then start changing aperture. There is no downside to using a faster shutter speed, higher than your target (1/1000), but making the aperture smaller might get you into diffraction effects that could make the image softer.

I don't know what to tell you about the ND filter cameras.

I think this is looking great. Thanks very much for your help. I look forward to a trial script to test.

Logged

waterwingz

12121

Re: Is Custom Auto ISO working for the A640 under 1.2?

« Reply #16 on: 06 / October / 2013, 09:25:53 »

Quote from: peabody on 05 / October / 2013, 22:32:11

In 3a, I'm confused by the secondary Sv target of 800. I know that Custom Auto ISO has fallback settings, but I never quite understood why they are useful. Perhaps the idea is that if you can reduce ISO by increasing aperture, you would do so all the way down to ISO 400, but if you also have to start reducing shutter speed to get ISO down, you would only go to 800. I suppose that would make sense.

Close. Don't forget that, as I posted earlier, the CHDK autoISO routine ignores the Av setting. It just assumes it stays fixed and adjusts Tv & Sv ratiometrically. So the second ISO setting is used when it can't get the correct exposure using the first ISO setting and the Tv low limit.

Quote

I guess it really doesn't matter because I can just set them both to 400 if I want.

True.

Quote

In 3b, I think my preference would be to increase shutter speed first until you reach the camera's fastest shutter speed, and only then start changing aperture. There is no downside to using a faster shutter speed, higher than your target (1/1000), but making the aperture smaller might get you into diffraction effects that could make the image softer.

Good idea. With CHDK, you can get really fast shutter speeds so maybe there is never a reason to go to a smaller aperture. I guess we will need a user entered upper shutter speed limit - it seems to vary between camera models. Same comment for the ND filter I guess - increasing the shutter speed removes the need to use it. The only application I've heard of is taking fire works, where you don't want the shutter speed to be too fast but the subject is quite bright.

Quote

I don't know what to tell you about the ND filter cameras.

As a minimum, it needs to be locked out. If found out early on with the YASS sunset script that the camera would automatically insert the filter at times (shooting directly into the setting sun) and totally mess up my exposure settings.

Logged

Ported : A1200 SD940 G10 Powershot N G16

peabody

Re: Is Custom Auto ISO working for the A640 under 1.2?

« Reply #17 on: 06 / October / 2013, 15:18:43 »

Will the script require that the camera be in a particular shooting mode, such as Program? How about the ISO setting - will the script just ignore it?

Logged

waterwingz

12121

Re: Is Custom Auto ISO working for the A640 under 1.2?

« Reply #18 on: 06 / October / 2013, 15:41:45 »

Quote from: peabody on 06 / October / 2013, 15:18:43

Will the script require that the camera be in a particular shooting mode, such as Program? How about the ISO setting - will the script just ignore it?

The script will override the camera's Tv, Sv, Av, ND filter settngs according to the exposure rules we've been showing above. It won't care what mode the camera is or if ISO in set to auto or fixed value.

Logged

Ported : A1200 SD940 G10 Powershot N G16

waterwingz

12121

Re: Is Custom Auto ISO working for the A640 under 1.2?

« Reply #19 on: 06 / October / 2013, 16:44:31 »

Okay .. I have a working BETA version of an autoISO KAP script.

Screen shot of script parameters below :

I'll write some instructions at some point. For now its mostly self explanitory, except that you press Menu to exit the script and any other key to disable the backlight "screensaver" for 30 seconds.

PM me if you would like to test a copy - I'd rather not post it for general release pending a little testing.

« Last Edit: 06 / October / 2013, 16:52:28 by waterwingz »

Logged

Ported : A1200 SD940 G10 Powershot N G16