View Full Version : flash math for macro and general use

Murray@uptowngallery

06-02-2007, 03:01 AM

Can anyone suggest a link to read about how to determine how to adjust exposure calculations based on GN or flash meter readings.

I borrowed a flash meter and took measurements on my flashes at 1 meter (seemed like a useful distance at the time), bare & with diffuser, for future reference.

The more general the better. Math is OK.

Thanks

Murray

eddym

06-02-2007, 05:27 AM

I'm not sure I understand what you are asking. Are you asking how to determine your own guide number? If so, and you prefer to work in metric, then your distance of 1 meter makes it easy. If your flash meter exposure at 1 meter was say, f11, then your metric guide number for the flash is 11. If you move the flash to 2 meters, your aperture will be 11/2= f5.6. If you move the flash to 0.5 meters, then your aperture will be 11/0.5= f22.

Is this what you are asking, or am I misunderstanding?

Lee L

06-02-2007, 06:51 AM

Eddy's method (and all guide number systems) relies on the fact that light per unit area increases and decreases as a function of the distance squared. If you double the flash to subject distance, you get 1/4 the original light per square unit of area. If you cut the distance in half, you get 4 times the amount of light per square unit of area. That's two stops of adjustment for doubling or cutting the distance in half. For 4 times the distance, or 1/4 the flash to subject distance you need to adjust by 4^2 or 16 times (or 1/16), which is 4 stops of adjustment in the appropriate direction. So with your 1 meter reference settings, you should be able to work that out.

Remember that it's the flash to subject distance that counts, not film or lens to subject. This can make a difference at macro distances. You'll also need to account for light "loss" from extra lens extension at macro distances. George Lepp and John Shaw (esp. Closeups in Nature) have published useful hints on using flash for macro work.

Lee

Nick Zentena

06-02-2007, 07:14 AM

Do you understand bellows extension for LF cameras? It's the same idea when using tubes. IIRC there is an article on bellows factor on the LF website.

Murray@uptowngallery

06-02-2007, 07:59 AM

Yep, understand the BF and inverse square concepts.

I'm just directionally dyslexic or something when it comes to mechanical stuff...I would probably apply the correction factor in the opposite way with my luck.

If I can do the math, I for some reason understand better and screw up less.

I want to explore a single step correction for distance change with flash analogous to bellows factor so I don't have to think about how many factors of two.

Flash for pinhole is one path here, and since the typical f-stop and distances one might move a flash to/from are unlikely to be 'clean' factors of 2 or square root of 2, I'm going to scratch my head some more. I use EV too so I don't care how where the math goes.

Thank you for the jump-start.

Murray

Lee L

06-02-2007, 08:19 AM

If you're moving the flash independently of the camera, one option you might consider is moving the flash in f-stop increments. Moving it from 8 feet away from your subject to 11 feet, or 8 inches to 11 inches, or .8 meters to 1.1 meters would mean you get one stop less light from it. Moving it from 8 feet to 5.6 feet, or 8 inches to 5.6 inches, or .8 meters to .56 meters would mean you get a stop more light. In other words, move the flash in f-stop numbered distances relative to the subject and you'll know how many stops to adjust.

Lee

Dan Fromm

06-02-2007, 01:03 PM

Murray, when I get back to work on Monday -- home PC is kaput, I'm in the public library -- I'll send you a copy of my handy dandy macro flash spreadsheet. And then you can tear it apart and see the magic formulas etc.

By the way, in it I allow for pupillary magnifications other than 1.0, which is what all of the rules of thumb people throw around assume. If you think that PM can safely be neglected, take a look at the little data sheets Nikon provided with the PB-4.

But on the whole the easiest way for idiots like me to do flash closeup is to build a simple non-adjustable flash bracket and take some test shots with it. I've always done 1:1, 1:2, 1:4, sometimes 1:6 also. Each magnification at every aperture from largest to smallest, usually in 1 stop steps. The last calibration runs I did were in 1/2 stop steps, I don't think that bought me anything useful. For other magnifications, linear interpolation works just fine. Been there since 1971, see no need to change.

Murray@uptowngallery

06-03-2007, 11:07 AM

Cool thanks Dan

Jim Jones

06-04-2007, 09:02 AM

In the old days, before exposure compensating macro lenses and autoexposure flashes, I went through the math of flash macro photography. Over a modest macro range the bellows factor and the square law light fall-off compensate for each other. I don't remember the details, but the flash was an anemic 1960s model, and the indicated aperture on the lens was f/22 for Kodachrome 25 over a macro range typical for photographing flowers. The flash was mounted fairly far from the lens for better molding of the subject.

Dan Fromm

06-04-2007, 12:48 PM

Jim, there are two possible geometries. (A) flash bracket attaches to the camera, (B) flash bracket attaches to the front of the lens.

With the right layout your claim is true, +/- perhaps half a stop, from around 1:5 to not quite 1:1 with setup (A), from around 1:2 to 2:1 for setup (B). But only for lenses with fixed focal length. Remember that some lenses, e.g., 105/2.8 and 200/5 MicroNikkor AIS, shorten focal length as they focus closer.