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flash range metering

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Offline larrycafe

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flash range metering
« on: 31 / October / 2013, 05:24:15 »
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I have been playing with flash and the 1/1500 flash sync really helps a lots.

Now I am having problem deciding the exposure in some situation.

Usually it works best to shorten the exposure time as much as possible, the 1/1500 on my Ixus 115 suppress the environmental lighting very well. And it provide the best reach of the flash light.

However, sometimes it will become too dark to work with ISO 100. I will move to ISO 200 and 400 if needed.

when I am zooming in to the tele end, the aperture will be 5.9 and the flash can reach 2m (according to canon spec).

now the problem is, I can definitely use customer auto ISO, so that it sticks to 1/1500, and metering will adjust the ISO, flash metering will then determine the flash power to be used. this usually give a very high ISO like 1600, because the exposure time is too short.

in such case, for a 1/1500 ISO1600 situation, the exposure could be just right with 1/100 and ISO 100, but then the flash might not have enough reach (for example to 4m). 1/400 and ISO400 will work in this case. As the increase of ISO will double the flash range (2m X 2), and the shorter exposure keep the environmental lighting the same level.

the flash might be good if I choose these combination
1/1500 ISO 1600 flash low power
1/800 ISO 800 flash mid power
1/400 ISO 400 flash full power

but certainly ISO400 will give the best picture.

I know it is a very special idea, not even any manufacturer adjust the exposure according to the flash power. Traditionally, flash metering is always done at the end (after the aperture/shutter/ISO is decided). Just want to see if we can do anything to change this and could benefit in flash photography.
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Offline waterwingz

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Re: flash range metering
« Reply #1 on: 31 / October / 2013, 10:53:48 »
It might be possible to script this but I'm not really sure I completely understand what you are doing? For example,  internal or external flash unit?   And are you using a script of some type already? Are you using CHDK override / menu settings?

There are a few propcases that affect / report flash performance :

PROPCASE_FLASH_SYNC_CURTAIN             
PROPCASE_FLASH_ADJUST_MODE               
PROPCASE_FLASH_FIRE                   
PROPCASE_FLASH_EXP_COMP                 
PROPCASE_FLASH_MANUAL_OUTPUT             
PROPCASE_FLASH_MODE                   
PROPCASE_IS_FLASH_READY                 

Using those and the exposure & focus information gathered during a "half_shoot" allows a lot of choices to be made by a script.

Would you please describe exactly what you are doing when you shoot - camera settings,  CHDK override settings, other CHDK settings,  shooting sequence etc ?
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Offline larrycafe

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Re: flash range metering
« Reply #2 on: 31 / October / 2013, 11:17:18 »
thanks for you reply waterwingz

what I am doing currently is just using custom auto ISO. set the highest shutter as much as possible, in Ixus 115, it is 1/1500

with this I sometimes get an picture taken at ISO 1600. the flash is giving good enough reach, as it is ISO 1600.

however, this is not optimal in most case. when I check the photo in computer screen, it seems the ISO is unnecessarily high. Use a lower ISO could give less noise and better color.

as in my first post. the 1/1500 ISO 1600 combination might give the same exposure as 1/100 ISO 100. the different is the flash reach.

at 1/100 ISO 100, the flash might not be powerful enough to light a person 4m away. ISO400 1/400 will increase the flash reach, but keep the noise at acceptable level.

I don't need ISO1600 if the person is 4m away, but if the person is 8m away, ISO 1600 will be needed.

let me try to describe the steps need to be done to make the proper setting:
1. preflash and do flash metering
2. adjust ISO to make a full power flash give proper exposure to the object (since aperture is fixed and flash can sync at all shutter)
3. adjust shutter to cater for the ambient exposure
4. take the picture and flash fires

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Offline waterwingz

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Re: flash range metering
« Reply #3 on: 31 / October / 2013, 11:55:16 »
Bear with me - I'm gradually getting this.  Its a question of autoISO almost doing what you want but not quite (seems to be a theme here recently).  I've got some more questions / observations that might help me understand further.

Quote from: larrycafe on 31 / October / 2013, 11:17:18
what I am doing currently is just using custom auto ISO. set the highest shutter as much as possible, in Ixus 115, it is 1/1500
Can I assume you are doing this to limit the effect of the ambient lighting on the overall picture?

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with this I sometimes get an picture taken at ISO 1600. the flash is giving good enough reach, as it is ISO 1600. However, this is not optimal in most case. when I check the photo in computer screen, it seems the ISO is unnecessarily high. Use a lower ISO could give less noise and better color.
So autoISO forces the ISO up in order to honor the 1/1500 shutter speed that you have requested? How is it supposed to know that its okay to lower the ISO and shutter speed?

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as in my first post. the 1/1500 ISO 1600 combination might give the same exposure as 1/100 ISO 100. the different is the flash reach.  At 1/100 ISO 100, the flash might not be powerful enough to light a person 4m away. ISO400 1/400 will increase the flash reach, but keep the noise at acceptable level.
Not sure I follow this. As long as the Tv and Sv values are adjusted in balance,  the exposure should remain constant.  Are you perhaps saying that the camera also changes the flash intensity and the autoISO adjustment does not realize that and so makes the wrong adjustment?

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I don't need ISO1600 if the person is 4m away, but if the person is 8m away, ISO 1600 will be needed.
So the required ISO setting is a function of distance,  once the camera sets the flash to the max power setting?  That makes sense (given your f-stop is fixed).  But the ISO setting should adjust automatically for the change in exposure caused by the distance change.  What am I missing?

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let me try to describe the steps need to be done to make the proper setting:
1. preflash and do flash metering
- this happens automatically when you "half-press" - the camera makes the decision to arm the flash and adjusts the Tv & ISO appropriately for the power range selected. So far so good.
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2. adjust ISO to make a full power flash give proper exposure to the object (since aperture is fixed and flash can sync at all shutter)
- if the camera has decided to use full flash, then the ISO value should be correct.  However, if you override the flash power to a different value, the ISO needs to be adjusted?
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3. adjust shutter to cater for the ambient exposure
- the actual exposure will be determined mostly by the flash duration (i..e the shutter normally stays open much longer than the flast) but you want to use different shorted shutter speeds to adjust for how much the ambient exposure affects the image?  How do you decide what value to use?
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4. take the picture and flash fires 
ok
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Offline Microfunguy

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Re: flash range metering
« Reply #4 on: 31 / October / 2013, 12:52:56 »
Quote from: larrycafe on 31 / October / 2013, 05:24:15
I have been playing with flash and the 1/1500 flash sync really helps a lots.

In what way ?

It is a very unusual requirement, especially for photographing people.

Why would you want to completely eliminate the effect of ambient lighting ?

Do you have any example photos you can post ?

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Offline larrycafe

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Re: flash range metering
« Reply #5 on: 01 / November / 2013, 05:15:21 »
I have shoot some photos to illustrate it.

here is the link to the photos
https://picasaweb.google.com/104666660786836150238/November12013?authuser=0&feat=directlink

the first photo is ISO 100 1/100 no flash

and then the remainings are all force to use full power flash
ISO100 1/100 flash
ISO400 1/400 flash
ISO1600 1/1500 flash (in the exif it says aperture is 17, that's the camera think ISO 1600 is too high, and kick in the ND. however, I forced out the ND actually)

you can see the background exposure level are almost the same.

while the flash effect on the color checker is increasing as the ISO increase.

at ISO 100, even at full power, the color checker is not well lit
at ISO 400, at full power flash, it is almost properly lit
at ISO 1600, at full power flash, it looks a bit too bright

looking at the 3 flash photos, the optimal setting could be ISO 640 1/640 with full flash power. I may not find this setting before I shoot several photos to know it.
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Offline larrycafe

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Re: flash range metering
« Reply #6 on: 01 / November / 2013, 05:18:16 »
and at the same environment, if I am stepping back by 2m, I might really need ISO 1600 so the flash can have enough power to light up the color checker
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Offline Microfunguy

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Re: flash range metering
« Reply #7 on: 01 / November / 2013, 07:01:45 »
How does that compare with simply reducing the flash intensity and using a 'normal' shutter speed ?

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Offline larrycafe

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Re: flash range metering
« Reply #8 on: 01 / November / 2013, 07:43:06 »
Quote from: Microfunguy on 01 / November / 2013, 07:01:45
How does that compare with simply reducing the flash intensity and using a 'normal' shutter speed ?



for a normal shutter speed, like ISO 100 1/100s, you cannot lit the color checker even at full power flash. the flash will not make any effect on the color checker at reduced power
« Last Edit: 01 / November / 2013, 07:45:34 by larrycafe »
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Offline Microfunguy

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Re: flash range metering
« Reply #9 on: 01 / November / 2013, 11:03:04 »
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« Last Edit: 01 / November / 2013, 12:22:04 by Microfunguy »
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