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bakerbang
12-Feb-2016, 09:14
Hi I have an exposure problem that I think I have figured out but would like to run past everyone.

I am using 5x4 and I need to photograph at F/16. With my bellows extension and Infrared filter I need to factor in 10 extra stops in total. My subject is delicate and prone to imperceptible movements of which I cannot control so I need to try and capture the image as fast as possible to eliminate movement.

My flash heads will fire at F/128 giving me 6 extra stops (from F/16) in one flash. If I flash once, then reduce the power on the flash heads to F/64 giving me 4 extra stops (from F/16) and flash again will this give me the 10 extra stops I need in 2 flash pops?

Jon

pau3
12-Feb-2016, 09:52
Each stop means that you double the quantity of light. If with one pop you are at 6 stops, with two, you are doubling the light, you are at 7. Four will leave you at 8. Eight at 9. Sixteen at 10.

Best,
Pau

photog_ed
12-Feb-2016, 09:53
Sorry, posted this before seeing the first reply, but here goes.

It's easiest to think of it in terms of exposure times. Suppose the exposure time for F/16 is 1 second. You need 10 more stops, which is 2^10 = 1024x more exposure, or 1024 seconds. Your F/128 setting is giving you 6 more stops, or 2^6 = 64s total. To get to 1024s exposure, you 1024/64 = 16 exposures at your F/128 setting to get to 1024s equivalent exposure.

HTH

Ed Freniere

bakerbang
12-Feb-2016, 10:26
Each stop means that you double the quantity of light. If with one pop you are at 6 stops, with two, you are doubling the light, you are at 7. Four will leave you at 8. Eight at 9. Sixteen at 10.

Best,
Pau

Thank you for your replies. I understand that each flash pop doubles the amount of light but if with one flash pop I am 6 stops over the intended exposure (and I need to be 10 stops over) then with 2 flash pops will I not be 12 stops over? I cant see how with one pop I will gain 6 stops but with 2 I will only gain one more stop making 7?

Sorry if its obvious, I just need to be sure.

Jac@stafford.net
12-Feb-2016, 11:30
Thank you for your replies. I understand that each flash pop doubles the amount of light but if with one flash pop I am 6 stops over the intended exposure (and I need to be 10 stops over) then with 2 flash pops will I not be 12 stops over? I cant see how with one pop I will gain 6 stops but with 2 I will only gain one more stop making 7?

Read Ed's post again, or consider the exposure as the number of flashes to give ten greater f-stops (ignoring reciprocity failure of using multiple pops instead on one huge flash from an impossible source.)

Begin at 1, then add 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024

That gives you ten stops.

A smaller number, real life example. A flash gun that adds one stop.

146463

These guys knew what they were doing. O. Winston Link with lights and assistant.
146464

photog_ed
12-Feb-2016, 12:03
Your second 6-stop-over exposure adds 64s to the exposure in my example above, giving a total of 128s. You need to add another 14 of them to get to 1024s.

bakerbang
12-Feb-2016, 12:23
Ok I got it! thank you everyone for your help.

I would need 1024 pops of the flash to achieve F/16 (if I need an extra 10 stops) but I already have 6 extra stops with the power of my lights so If originally I needed
1024 then going backwards 512 256 128 64 32 16. I need 16 pops of the flash to achieve 10 extra stops if my lights are giving me an extra 6 stops.

Thank you everyone I appreciate your time

JB

Jac@stafford.net
12-Feb-2016, 12:30
My flash heads will fire at F/128 giving me 6 extra stops (from F/16) in one flash.

Three stops, not six.

photog_ed
12-Feb-2016, 13:01
Jac,

He's right, six extra stops.

Jac@stafford.net
12-Feb-2016, 13:27
Jac,

He's right, six extra stops.

Thanks, Ed. I must be mucking up powers of two.

paricpo
8-Apr-2016, 23:53
Instead of bend the knowns, fold a penny over and place it in there and you are sensible with years.

Kevin Harding
9-Apr-2016, 14:04
Are you photographing with infrared film? Will your flashes output IR?

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