Looking for some help with flash + ambient math

**bobbotron** · 06-14-2015, 06:06 PM

I want to experiment a bit with some Agfa Rollei IR film I have in my camera at the moment, mixing flash in with ambient for a silly outdoor portrait idea. The film is ISO 400, but I'm using a R72 filter, which in theory is a 5 stop reduction in exposure. I'm planning on doing the photo on a bright summer day, and underexposing the background by 2 stops, but filling in my subject with a flash. So I the overall scene exposure (exposed 2 stops under) can be done with F8, 1/60. But I'm not sure about flash distance to get my subject properly exposed.

If I had a GN 115 flash, what distance do I need to roughly have it from my subject to gain those last two stops... I think it's about 14 feet, but could be mistaken.

**maxfield_photo** · 06-14-2015, 07:27 PM

I get about 16.5 feet, though I would "bracket" your flash-to-subject distance if the shot is important. Try 13, 16, and 19 feet. You might also want to "bracket" your flash/ambient balance, in case you decide you want a more flashy or more natural look, or possibly, in case the flash's IR contribution is different than natural daylight. To do that, try 1/45th @ f/9.5, and move the light to 13 feet, or 1/30th @ f/11 and move the light to 10 feet. Good luck!

06-14-2015, 06:06 PM	#1
bobbotron Site Supporter Join Date: Jan 2015 Location: Ottawa, ON Photos: Gallery Posts: 3,349	Looking for some help with flash + ambient math I want to experiment a bit with some Agfa Rollei IR film I have in my camera at the moment, mixing flash in with ambient for a silly outdoor portrait idea. The film is ISO 400, but I'm using a R72 filter, which in theory is a 5 stop reduction in exposure. I'm planning on doing the photo on a bright summer day, and underexposing the background by 2 stops, but filling in my subject with a flash. So I the overall scene exposure (exposed 2 stops under) can be done with F8, 1/60. But I'm not sure about flash distance to get my subject properly exposed. If I had a GN 115 flash, what distance do I need to roughly have it from my subject to gain those last two stops... I think it's about 14 feet, but could be mistaken.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Weird mirror issue... looking for some insight/ideas/help...	mrNewt	Film SLRs and Compact Film Cameras	10	01-08-2022 03:58 AM
Looking for nice lens with some range	Borislav	Pentax SLR Lens Discussion	7	07-22-2013 12:14 AM
Pentax K-7...help me with the math....	rubbertoy74	Pentax DSLR Discussion	7	11-29-2010 06:48 PM
New "Amateur" looking for some pic help.	MoparFreak69	Troubleshooting and Beginner Help	8	07-08-2009 02:42 PM
Looking for help with dance Recitals	vievetrick	Digital Processing, Software, and Printing	3	02-06-2007 01:35 PM

06-14-2015, 07:27 PM	#2
maxfield_photo Site Supporter Join Date: Mar 2011 Location: Cincinnati, OH Posts: 1,216	I get about 16.5 feet, though I would "bracket" your flash-to-subject distance if the shot is important. Try 13, 16, and 19 feet. You might also want to "bracket" your flash/ambient balance, in case you decide you want a more flashy or more natural look, or possibly, in case the flash's IR contribution is different than natural daylight. To do that, try 1/45th @ f/9.5, and move the light to 13 feet, or 1/30th @ f/11 and move the light to 10 feet. Good luck!