Manual flash calculations with accounting existing light

radiant · Dec 21, 2020

I have figured out how manual flash calculations work. For example my FlashQ has GN 20 at ISO 100 with 1 meter distance and 1/8 power I should use f8. OK.

But how ambient / existing light is calculated / accounted? I mean if I have ambient light congitions EV(100) = 8. How is this calculated so that the flash lit target is exposed correctly?

This might be really basic and stupid question but I cannot find any answer to this by google, all I find is DSLR TTL automatic yadayada..

BrianShaw · Dec 21, 2020

Search in “fill flash”. This is one hit:

https://strobist.blogspot.com/2006/03/lighting-101-balancing-flash-and.html?m=1

BrianShaw · Dec 21, 2020

I also think that the old Kodak Professional Photoguide have a calculator for that but would have to verify.

radiant · Dec 21, 2020

BrianShaw said:
Search in “fill flash”. This is one hit:

https://strobist.blogspot.com/2006/03/lighting-101-balancing-flash-and.html?m=1

Thanks for the link but it doesn't explain it because (quoted from the article) the method is "Take a test shot. If the flash is too dim, I'll turn up the power." - I cannot develop and print between every shot

I've searched fill flash term and I always end up with DSLR articles which are using trial-and-error method..

radiant · Dec 21, 2020

BrianShaw said:
I also think that the old Kodak Professional Photoguide have a calculator for that but would have to verify.

If anyone has one, please share some examples from it!

radiant · Dec 21, 2020

Hey now I found some info, this strange Pandora's box called photrio has an thread about same issue:

https://www.photrio.com/forum/threads/how-do-you-calculate-fill-flash.34303/

.. oh wait, it's the same place

I think the idea is: calculate flash normally. Measure exposure with camera normally. Cut down the camera exposure by two stops. Did I get this right?

Chan Tran · Dec 21, 2020

Let say that with no ambient light your correct exposure is f/8.
With no flash and using a shutter speed of 1/60 your correct exposure is f/2.
With both the correct exposure is square root of (f/8 ^2) + (f/2 ^2)= f8.25.

Now if you choose to use a shutter speed of 1/4 then the ambient exposure is f/8 and the combination is f/11.

MattKing · Dec 21, 2020

This link is reasonably informative: https://www.shootproof.com/blog/fill-flash-outdoors/
I used to use the exposure determined by the ambient light reading which included an aperture that, with just light from the flash, would have resulted in a one stop under-exposure.
I generally used it when the subject was not fully front illuminated - i.e. their faces were at least partially shadowed.

radiant · Dec 21, 2020

Chan Tran said:
Let say that with no ambient light your correct exposure is f/8.
With no flash and using a shutter speed of 1/60 your correct exposure is f/2.
With both the correct exposure is square root of (f/8 ^2) + (f/2 ^2)= f8.25.

Now if you choose to use a shutter speed of 1/4 then the ambient exposure is f/8 and the combination is f/11.

Sorry but I couldn't follow your calculations. Could you open up what happens here? I don't understand why the shutter speeds should set to 1/4.

If I understood something here: flash calculation is f/8. Expose at one stop faster when combined? So the basic rule is here same 1 (to 2) stops from measured? But if we keep speed 1/60 and f2 the exposure should be done at f8.25?

MattKing said:
I used to use the exposure determined by the ambient light reading which included an aperture that, with just light from the flash, would have resulted in a one stop under-exposure.

Also vote for one stop from ambient reading

radiant · Dec 21, 2020

BTW the link Matt posted had interesting list of "Flash ratio". Can this calculated with Chan's formula so for example 1:1 ratio at both flash+ambient @ f8 would mean exposure at f11?

Or ratio 1:2 would be for example f8 and 5.6 which would mean f9.7 ?
Ratio 1:3 f8 and f4 would be f8.9 ?

ic-racer · Dec 21, 2020

Intensity of a manual flash is related to subject distance. Intensity of the background is related to shutter speed. The aperture affects both equally. Good fill flash is a balance of those three.

Helge · Dec 21, 2020

If you are doing bounced or off camera flash at an oblique angle, you’ll need to wing it anyway.

With a regular battery powered flash it’s very rare to get catastrophic overexposure with indirect flash, so better too much than too little.
Remember you can always bring the exposure down to a more atmospheric darker one if you have too much.
Any negative film will pick up a lot of slack.

BrianShaw · Dec 21, 2020

Flash meter helps to confirm the flash-related exposure aspect.

BrianShaw · Dec 21, 2020

BrianShaw said:
I also think that the old Kodak Professional Photoguide have a calculator for that but would have to verify.

This should be easy to find at cheap price. Four pages of how-to plus a calculator to help.

Chan Tran · Dec 21, 2020

vedostuu said:
Sorry but I couldn't follow your calculations. Could you open up what happens here? I don't understand why the shutter speeds should set to 1/4.

If I understood something here: flash calculation is f/8. Expose at one stop faster when combined? So the basic rule is here same 1 (to 2) stops from measured? But if we keep speed 1/60 and f2 the exposure should be done at f8.25?

Also vote for one stop from ambient reading

Your original example said that the ambient light is EV10@ISO100 is that correct? If so the correct exposure for ambient light alone is f/2@1/60 or f/8@1/4 is that correct? At higher shutter speed your ambient light exposure is a lot less.

radiant · Dec 22, 2020

Chan Tran said:
Your original example said that the ambient light is EV10@ISO100 is that correct? If so the correct exposure for ambient light alone is f/2@1/60 or f/8@1/4 is that correct? At higher shutter speed your ambient light exposure is a lot less.

But I didn't understand why you chose to slow the shutter speed while the total exposure stays same?

Chan Tran · Dec 22, 2020

vedostuu said:
But I didn't understand why you chose to slow the shutter speed while the total exposure stays same?

Same exposure but you use a different aperture. f/11 for 1/4 sec and f/8.2 for 1/60 sec. Depending on how much ambient light you want to contribute to your picture.
In the case you use 1/60 and f/8.2 the ratio of flash to ambient is only 16:1 and in the case you use 1/4 and f/11 the ratio is 1:1.

Helge · Dec 22, 2020

For all practical purposes flash is instantaneous.
For the flash exposure alone, shutter speed can be taken completely out of the equation (that is unless it’s a focal plane shutter in which case you will be limited by sync speed).

If you want to have more ambient light contribution set the shutter as slow as you can to not have motion blur (if you do want motion blur, or it’s not a factor, you might consider not using flash, unless for effect).

radiant · Dec 22, 2020

Chan Tran said:
In the case you use 1/60 and f/8.2 the ratio of flash to ambient is only 16:1 and in the case you use 1/4 and f/11 the ratio is 1:1.

This is a good point! I think this thing can be solved through flash ratio!

radiant · Dec 22, 2020

Chan Tran said:
In the case you use 1/60 and f/8.2 the ratio of flash to ambient is only 16:1 and in the case you use 1/4 and f/11 the ratio is 1:1.

One more question; maybe stupid one.

Why the ratio is 16:1? .. f2 and f8.2 diff is ~2 stops. Or f2--f11 is 2,46 stops ..

RalphLambrecht · Dec 22, 2020

vedostuu said:
I have figured out how manual flash calculations work. For example my FlashQ has GN 20 at ISO 100 with 1 meter distance and 1/8 power I should use f8. OK.

But how ambient / existing light is calculated / accounted? I mean if I have ambient light conditions EV(100) = 8. How is this calculated so that the flash lit target is exposed correctly?

This might be really basic and stupid question but I cannot find any answer to this by google, all I find is DSLR TTL automatic yadayada..

I have come to the conclusion that a flash meter is a necessity to get correct exposures using flash and ambient.

Chan Tran · Dec 22, 2020

vedostuu said:
One more question; maybe stupid one.

Why the ratio is 16:1? .. f2 and f8.2 diff is ~2 stops. Or f2--f11 is 2,46 stops ..

16:1 because the flash exposure is f/8 and the ambient is f/2 and it's 4 stops apart.
However, there are many problems with the approach of using calculations and GN. If you shoot film it's best to get a flash meter. If you shoot digital you may want to take test shot and adjust.
The problem with GN are several.
1. It only works with direct flash. When you bounce or use an umbrella it no longer works.
2. The published GN is not correct. Manufacturers always inflate the GN.
3. While the flash output is very consistent but the power ratio isn't always correct. For example if you set the flash at 1/2 power the power is like not 1/2 although it would put out the same amount of light each time. So if you want to use GN you would have to do a lot of testing to determine the true GN at full power as well as the reduced power settings. You would need a flash meter to do this any way.
It's the reason why you don't find the how to on the internet to do it with GN.

radiant · Dec 22, 2020

Chan Tran said:
16:1 because the flash exposure is f/8 and the ambient is f/2 and it's 4 stops apart.

Of course.

Chan Tran said:
You would need a flash meter to do this any way.
It's the reason why you don't find the how to on the internet to do it with GN.

Ok, I see. I've only used manual flash with my digital so I've probably adjusted exposure after preview and never used the calculated GN as is. I need to do some tests how well direct flash GN calculation work with digital!

This has been an interesting thread!

MattKing · Dec 22, 2020

Guide numbers often include assumptions - for example that the flash is used indoors and there are floors and ceilings that help reflect some light on to the subject.
Your working guide number when using flash outside will usually be less than a published guide number.
You can easily determine your own "outside" guide number - just set up your camera and flash a useful distance from a test subject and expose several shots at apertures 1/2 stop apart.
The test is best done at night, without walls or ceilings or reflective floors - bring a flashlight so you can see to change the settings.
Develop the film, and pick the shot that is best exposed. Then take the aperture you used and multiply it by the distance from flash to subject you used. The result is your own personal guide number for that flash and that speed of film for outdoor use.
In days gone by I used ten feet as the distance, whereas you may prefer 3 meters.
Watch out for flash functions that affect output - things like variable reflectors and auto exposure and reduced output settings.

BrianShaw · Dec 22, 2020

Matt... flashmeter (you know this, I’m sure): one or two pops results in guide number.

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