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Newbie: High speed photos coming as black

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Newbie: High speed photos coming as black
« on: 18 / October / 2010, 07:27:26 »
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Hi folks,

I have Canon powershot SD700 IS, and have loaded CHDK Full version on it. It works absolutely fine. I am able to take night photos with long exposure shots.

However, when I try to take high speed shots, the pictures come out very dark. even in very sunny/broad day light. The only way i have managed to take shots is by turning on the flash. This is a problem because re-charging the flash takes time (a couple of seconds) and it takes away the advantage of using CHDK for high speed photography.

What is very suprising is that my camera is capable of 1/1600s. the photos clicked using that (typically when i set to auto mode) are much brighter than 1/1600s set using CHDK. even when flash is used.

does anyone have a suggestion to avoid dark photos in high speed?
can i do it without using flash? i plan to click photos in sunny/broad day light (sports photography).. what is the best way to go about it.

thanks,
Adi

Re: Newbie: High speed photos coming as black
« Reply #1 on: 18 / October / 2010, 07:47:54 »
Hello Adi and welcome.


On the brightest of sunny days, Canon have determined that 1/1600s is the fastest speed that you need and will automatically create a correctly exposed image.

The speeds above that value have very limited use, without flash when will you use them ?

The pictures will get progressively darker, because your camera is already set to the  maximum aperture and ISO (sensor sensitivity).

The override shutter speeds in CHDK go to ridiculous values.



David

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Offline reyalp

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Re: Newbie: High speed photos coming as black
« Reply #2 on: 18 / October / 2010, 11:50:02 »
On the brightest of sunny days, Canon have determined that 1/1600s is the fastest speed that you need and will automatically create a correctly exposed image.
In my experience, Canon actually does a slightly poor job of this. The limit is low enough that you can hit it on a bright day. This makes the stock firmware put in the ND filter, which drops your exposure to something a bit too slow for fast moving subjects. Allowing your Tv override to go a stop or two higher than the Canon limit is one of the few places where Tv overrides shorter than canons limit actually have value, IMO.

Which brings us Adis question:
When you use overrides, the camera auto exposure is not aware of them. So the camera may still be putting in the ND filter. This would explain why identical exposure gives a much darker shot with CHDK.

You should be able to see this in the exif aperture value (exif values with CHDK overrides aren't always accurate, but in this case it would be the original firmware putting in the ND filter, so it should be recorded normally). You can try forcing ND filter off in overrides.
Don't forget what the H stands for.

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Offline yvesson

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Re: Newbie: High speed photos coming as black
« Reply #3 on: 18 / October / 2010, 11:56:48 »
Hej,
Reyalp, is that impossible to use the cam exposure with the overrides ? I need more than 1/1250s at F2.6 on my A540 for the motion detection. :/


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Offline reyalp

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Re: Newbie: High speed photos coming as black
« Reply #4 on: 18 / October / 2010, 12:12:56 »
Hej,
Reyalp, is that impossible to use the cam exposure with the overrides ? I need more than 1/1250s at F2.6 on my A540 for the motion detection. :/
As far as I know it is currently impossible. But you should be able to calculate all the components of the exposure yourself and set them in script.
Don't forget what the H stands for.

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Offline fudgey

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Re: Newbie: High speed photos coming as black
« Reply #5 on: 18 / October / 2010, 12:40:13 »
If you have enough light for autoexposure at high speed Tv, you can
- select Tv mode, 1/1250 s (or whatever is fastest)
- set chdk override Tv by -2 Ev
- set canon's exposure compensation to +2 Ev

and the camera should adjust ISO/Av/ND for correct exposure since your CHDK Tv override and Canon's exposure offset will cancel each other out.

(edit: premature submit...)
« Last Edit: 18 / October / 2010, 12:43:23 by fudgey »

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Offline yvesson

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Re: Newbie: High speed photos coming as black
« Reply #6 on: 18 / October / 2010, 13:07:45 »
Hej,
Except... With motion detection I let the cam alone for x hours, and sometimes I have enough light, then a cloud passes by etc. It is annoying that the few moments when 1/1250s isn't fast enough you get burnt shots. It's also the case when I shot in Av mode (I do most of the time), and I don't notice the cam shows a red 1/1250s (especially on sunny days, I don't see the screen so well...).
And well reyalp I'm not sure I can calculate all the component of the exposure myself. :/

Isn't there a possiblity to get the exposure the cam calculates right before the shot and use it ? Then it's possible to compensate with a script.
Would be easier to break canon's firmware limit though (easier not including the coding/hacking part).

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Offline fudgey

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Re: Newbie: High speed photos coming as black
« Reply #7 on: 18 / October / 2010, 13:56:49 »
Except... With motion detection I let the cam alone for x hours, and sometimes I have enough light, then a cloud passes by etc. It is annoying that the few moments when 1/1250s isn't fast enough you get burnt shots. It's also the case when I shot in Av mode (I do most of the time), and I don't notice the cam shows a red 1/1250s (especially on sunny days, I don't see the screen so well...).
And well reyalp I'm not sure I can calculate all the component of the exposure myself. :/

Isn't there a possiblity to get the exposure the cam calculates right before the shot and use it ? Then it's possible to compensate with a script.
Would be easier to break canon's firmware limit though (easier not including the coding/hacking part).

Ah you are describing a slightly different problem... you want to extend Canon's autoexposure range, not just change its preference of not ruining the photo by using a ridiculously narrow aperture.

If I understood you correctly, your problem is that ISO is already minimal, ND filter is active and/or aperture is as narrow as it goes without override and Tv is as short as it goes without override but you still have too much light.

If you find a way to decide how much to speed Tv up, things are easy since you are using a script. I'm assuming you use MDFB in fast mode (or similar script), meaning your camera sits waiting for motion with shutter half pressed. That means exposure is determined after each shot (and at start, and after timeout if there is no motion). This means any code you add to override Tv will not slow MD reaction time down at all and the override code needs to be put right before the md_detect_motion command.

Your problem is that the camera exposure is incorrect -- you can't just e.g. make tv faster and compensate with av and sv (ISO). If you're lucky the camera may how much it is overexposing. Read the bv (brightness value) propcase (34 or 74 depending on camera model). If it changes even though exposure doesn't, you should be able to calculate tv for your override from the apex formula tv=bv-av+sv. So your code could do something like this:

...
read tv96
read av96
read sv96
read bv96
if tv is as fast as it goes in your camera, then set tv96=bv96-av96+sv96
md_detect_motion ...


Re: Newbie: High speed photos coming as black
« Reply #8 on: 18 / October / 2010, 14:51:21 »
The limit is low enough that you can hit it on a bright day.

'Bright day' implies the ND filter is in which means that if the ND filter were out the required shutter speed would be about 1/12,800 sec (on most cameras).


Does not seem likely,  unless using maximum ISO (which would be rather strange).




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Offline yvesson

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Re: Newbie: High speed photos coming as black
« Reply #9 on: 18 / October / 2010, 14:57:11 »
Hej,
There is no ND filter in the A540, as far as I know.
Yeah I don't wanna set F8, I'd rather set F2.6 and have a faster shutter speed because for flying birds, 1/1250s isn't that fast. And you're there kinda often on a sunny day with 100 ISO and F2.6.  I wanna use all the light available for the fastest shutter time possible with the right exposure.
I guess 1/2000s would be hard to achieve but I would want my cam to choose 1/1500s id it's possible and at the same time not burn the pics where 1/1250s isn't enough. I would even try 200 ISO to see what it looks like with 1/2000s or more.

What I think is that obviously the sensor can do 1/30000s, so it is just canon having decided that 1/1250s was the limit. What I don't get is why at F4 it suddenly can reach 1/2000s, and who cares since I have never seen enough light to expose 1/2000s at F4, unless I raise ISOs (but then suddenly at 200ISO it could be too much light again).
Do canon thing that some crazy people would use 200ISO on a sunny day just to get more speed, when they own a cheap compact camera ? Oo

I guess what you are saying is doable with Av fixed as well ?

Anyway, I don't need this until next spring. :p

 

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