Toronto Photography Meetup Group

So I picked myself up an sb600 and decided Im going to try and teach myself the math of correct exposures using manual mode using flash guide numbers aperture and shooting distances.. I've been reading the manual and I' not fully understanding everything .. So far what I have is ..

Aperture = GN (Guide Number) divided by Shooting distance

Now to determine your guide number you look at the chart in the booklet and see your flash output power (for this sake Ill choose 1/4 power)
Now I take 1/4 power and look across under 24mm zoom head and determine the number is 42.7 ft

Gn = 42.7 ft?

Now shooting distance is wherever I place my flash?

( Ill place it at 10 feet for arguments sake)

So I take my guide number and divide it by my flash distance from the subject?

42.7 divided by 10 = 4.2

Therfore set my aperture on my camera to 4.2 to get a correct exposure?? Am I right ? Hopefully this will help others aswell ..

Now I understand a light meter tells me all this but I want to learn the math to it all.

Thanks

- Tyler

Your math is assuming ISO100...to add ISO to the equation the formula is on the following page:
http://knol.google.com/k/understanding- ... de-number#

That's why it's so much easier to use the built-in chart/calculator on the flash, or a flash meter.

So I have a small area to do a shoot with 2 sb 600s... and I want a lighting ratio of say 1:6 or so (for dramatic lighting) .. I would in steps do what?

1 - Pick the power I want my flash at?
2- pick the zoom head distance
3- find the GN
4- determine the distance of my key lighting
5-divide my GN by the Distance
6- set Aperture
7 set second flash (fill) to 1/6th of the inital power I chose for my fill light?

Or get a light meter with flash metering capabilities and have your setup done in minutes.

Keep in mind the inverse square law of light when calculating light fall off from your subject to your light source ie.

Double the distance is 1/4 as bright, and 1/2 the distance is 4x brighter (2 stops)
Triple the distance is 1/9 as bright, and 1/3 the distance is 9x brighter (8x is 3 stops)
Quadruple the distance is 1/16 as bright, and 1/4 the distance is 16x brighter (4 stops)

So in essence, if you wanted to change your lighting ratio, instead of dialing down the light, you can actually move the light away or closer to the subject but keep in mind that you might need a modifier to tighten or flag the light if it starts to spill over on areas you don't want it to etc.

the equation works fine, but there are problems.

the GN is based on the manufacturer's definition of correct exposure, can vary across manufacturers, and is often measured with the zoom head at the extreme end of the zoom, so that light is focussed and the figures can be inflated.

If you're in a studio environment, or using the flash off camera for that matter, you are unlikely to be using the flash at full zoom. And if you were to be using a modifier, would you know how much the GN changes with the effect of the modifier?

As for Carlton's tip of moving the light source, that works, and I use it too. But there is also the quality of light to contend with

There are a bunch of posts on strobist that deal with this subject.

Do a search and you'll find all the answers you're looking for, but be warned that DH's methods are not based on using flash meters and they work perfectly fine, once you get a good understanding of the DOF of light and how distance to subject and flash power ratios work.

don't you lose some light, when diffused? I think manufacturers meant bare bulb flash in their charts

Yes. The GN numbers etc are based on bare bulb pointed at the subject. If you shoot through a softbox or use and umbrella or bounce off the ceiling etc. you lose light output.

Personally, the only true method to get your lighting ratios bang on is with a meter. If you don't have a meter, as Mike pointed out, you will have to chimp it and get it close or something to your liking.

error. deleted.

It's a pretty time-honoured technique. A friend of mine described how he had to take polaroids to check the light levels before taking the actual shot.

But honestly, good on Tyler for posting and trying pretty much EVERYTHING

Couldn't you also determine the aperture you need to get the DOF you want then determine the flash power ratio or GN (and maybe the ISO) you need from there?

Back in the "old days" when flashes fired at full power aperture was used to control exposure. But with digital and adjustable flash output why not use ISO and flash power to get the aperture you need for the shot?

Yes, Tyler your calculation is correct but I would throw it out the window. Folks have already expressed why using the manufacturer's guide numbers are not ideal. As others have said you can guestimate and chimp to a close approximation of the proper exposure. To get it right on you should get a light meter imho. After a while you will get proficient at getting your initial set up close to where you want it - this comes with experience and practice.

and a specific zoom value for flashes that can zoom.
Canon flash specifications are for 105 mm (max zoom) at 100 ISO.

With a shorter lens and shorter zoom on the flash the GN will be lower because the light is spread a little more.

A light meter is worth it's weight in gold, especially when ambient is dropping fast.

Mr.Walczak,
Tie a string to your flash and and measure the light output bang on the fstops at distance and you will easily see what Carlton was talking about with distance, # of stops, and the laws of light. Again, this is easier to do with a bought, borrowed, or rented meter. After visualizing the f-stops and distance you will now have a rough estimate on the math when the time comes.

Also, if you are manually shooting start with your flash at 1/4 or 1/8th, instead of full power, or you will be moving your stands a lot. Not easy to do in smaller spaces or with crap in the way.

The next easiest thing to do, after the light meter, is make sure you use the same flashes. Ratios are simple after that. I know how hard it is to pass by a great deal on a one off flash..but it ain't worth the misery.

Some things I've learned along the way...

I think I posted a GN chart for a Nikon flash here...can't recall which one it was...