flash range metering

waterwingz

12138

Re: flash range metering

« Reply #10 on: 01 / November / 2013, 11:06:06 »

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Quote from: larrycafe on 01 / November / 2013, 05:15:21

I have shoot some photos to illustrate it.

I had to look at those pictures for a while (maybe I need a better monitor?) before I started to understand what you are trying to do. If I focus on just one or two colors in the color checker I can see what is happening. I guess this is what people on the various camera forums call "pixel peeping" ?

In your example you have a brightly lit background (an outdoors water shot) and a color checker close by in the shade. So you want the exposure ( Tv & Sv) set correctly to get the background right, and you want to use "fill flash" for the foreground. And to get that exposure right, you need enough flash power for the distance to the subject and the Sv adjusted appropriately for both the background shot and the flash shot?

So simply put, if we take all the combinations of Tv & Sv that give "acceptable" exposure of the background, we want CHDK to automatically pick the one that gives the best fill flash results in the foreground. If there are multiple acceptable combinations over the adjustable range of the flash power, then pick the one with the lowest ISO.

e.g : if acceptable background exposures are :

Tv	Sv
1/30	ISO100
1/60	ISO200
...	...
1/800	ISO2400
1/1200	ISO3200

how do you decide which one gives the best fill flash ? (other than trial and error) Will forcing the flash ON and doing a test flash in half shoot give you the right answer ? Or can we look at the SD reading we get after the camera sets the focus?

I suppose one method would be a script to shoot enough images at the different setting to effectively bracket all the combinations?

« Last Edit: 01 / November / 2013, 11:37:42 by waterwingz »

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larrycafe

49

Re: flash range metering

« Reply #11 on: 01 / November / 2013, 12:18:52 »

yes waterwingz, you got my point correctly

e.g : if acceptable background exposures are :

Tv   Sv
1/30   ISO100
1/60   ISO200
...   ...
1/800   ISO2400
1/1200   ISO3200

I don't really care to use full power flash to lit the object, sometimes the object is people face. although ISO3200 allow less consumption of flash power, but I don't really care to save the battery power here.

ISO100 might be too low for the flash to lit the people.

I want an lowest ISO setting, so the full power flash is just right to lit the people face.

if I am to do it manually systematically, I might need these steps:
1. force shutter to 1/1500. force full power flash.
2. Make a test shot at ISO400. With this I can hopefully eliminate the ambient lighting (if it is not a bright sunny day).
3. check the face exposure, maybe checking the histogram. knowing how much to adjust the ISO so it give good looking face. if it is too dark, move up the ISO. if it is too bright, use a lower ISO.
4. adjust the ISO to the desire level. Allow the camera to choose the right shutter for proper background exposure.

It is not feasible when I am shooting people. With best luck and skill, I might get the correct combination in a minute.

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Microfunguy

4037

Re: flash range metering

« Reply #12 on: 01 / November / 2013, 12:23:09 »

[in case you missed my post at bottom of previous page I am moving it here]

If I understand correctly, do you simply want the ability to manually override shutter speed and ISO and set flash intensity to one of the three manual flash-mode settings ?

If so, you should be able to do that with CHDK as far as I know (I do not use CHDK).

Let me ask a question ..... if this is such a practical problem why has it never been requested on any cameras (with or without CHDK) by amateur or professional photographers ?

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larrycafe

49

Re: flash range metering

« Reply #13 on: 01 / November / 2013, 12:41:24 »

I know I can manually override shutter and ISO and set the flash intensity, that put me to some kind of M mode, I have to adjust everything.

What I am looking for is an automatic way to achieve the best combination, in changing environment and distance, that's why I tried to name it "flash range metering"

haha, and why there is no amateur or professional photographers asking for this?

this can simply be solved by a powerful flash gun for professional, with a camera having hotshoe.

For flash photography, the flash sync speed is so important, 1/1500 flash sync is really a treasure. The flash could be just too weak if you don't know how to get the best of it.

amateur might end up saying that it is the flash too weak in such situation. for me, I love my cheapy Ixus and I love CHDK for giving it so much functionality. I am just trying to squeeze the last bit of possibility in some real life situation where someone might think it is impossible.

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waterwingz

12138

Re: flash range metering

« Reply #14 on: 01 / November / 2013, 13:08:52 »

Quote from: larrycafe on 01 / November / 2013, 12:18:52

yes waterwingz, you got my point correctly

Good.

So, for those of us who remember when the only "electronics" in a camera was maybe a battery, CdS photocell and a galvanic meter movement, back then you used your manual flash unit with something called guide numbers. Guide numbers told you what f-stop to use for a given ISO film sensitivity and distance to subject. Feel free to google for more details.

The big thing to realize here is that this was a "open loop" exposure adjustment - no light meter or other reading was involved in the calculation. This kind of describes the problem we are facing here (i.e. we have no easy way to meter for the correct exposure from the flash unit). But if we can get the subject distance from the camera when it focuses (which we can in CHDK) and we can build a table of guide numbers for each of the three flash power values available to us ( low, med, high), then using CHDK we could be in business! We will let the camera set the Tv, Sv values based on the overall scene luminance, measure the distance to subject (you did say this is for portrait photography right?) and pick a flash power / Sv:Tv pair that makes sense. Almost seems too simple - other than the work to create the table ?

Other ideas : using "test flash" firings, we can use something like lapser's spot meter code (or the histogram code - which works from the LCD buffer and is less precise) to check for over exposure from the flash. This can get confused by bright objects in the background though - we would probably want to limit the measurement to a small area in the center of the frame. Based on that, offset the Tv, Sv and flash power but a guessed amount, change them and repeat the test firing. You get the idea .. better "closed loop" control but multipe test flash firings can be annoying and time consuming.

Thoughts?

« Last Edit: 01 / November / 2013, 13:13:25 by waterwingz »

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Microfunguy

4037

Re: flash range metering

« Reply #15 on: 01 / November / 2013, 14:47:49 »

Quote from: waterwingz on 01 / November / 2013, 13:08:52

Thoughts?

Use an inexpensive and far more powerful external flash like the classic Vivitar 283.

I have six of them.

You can set flash duration down to 1/30,000 second.

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waterwingz

12138

Re: flash range metering

« Reply #16 on: 01 / November / 2013, 15:04:20 »

Quote from: Microfunguy on 01 / November / 2013, 14:47:49

Use an inexpensive and far more powerful external flash like the classic Vivitar 283.

While that could work (I have several too), you would have to carry around a flash that is 8x the size and weight of your little IXUS P&S camera. The point of this thread was to find a way to do that with the basic camera. The OP demonstrated that its technically possible from an optics & sensor point of view - he just wants a way to automate it.

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