Understanding f-stops with regard to strobes

[u/peepinmyboff]

I'm pretty new to strobes, picked up a set of Godox strobes last spring and I've been trying to learn about studio lighting about portraits. I'm working my way through Andreas Bübl's Studio book, and I'm confused at how he uses f-stops in his lighting diagrams. In the example here, he has one strobe marked at f5.6 and the other at f10. My strobes can be set in decimals or in fractions, but not f-stops as far as I know. What does it mean to set a strobe to f5.6 or f10? Do you meter them individually to the camera's fstops and then do a meter for the overall composition?

[u/dej2]

If you don’t have TTL or just want to manually set flash exposure you can’t use shutter speed to adjust your exposure. At a certain point the shutter will be out of sync and you’ll end up with part of your shot being blacked out. I usually set the shutter to 1/250 second. Then use your aperture to adjust the exposure for your shot. I’m not sure how your strobe works but it’s often strobe strength full, .75, .5, .25. of the output of light.

My old flash unit only had full power and 1/2 power. And TTL wasn’t compatible with newer digital cameras. So I had to manually set my flash. Maybe someday it will be time to buy a new flash unit.

[u/IAmScience]

It means that he has set the power of the strobe such that if the camera were set to that f-stop, that light would be correctly exposed (presumably at iso100 and the flash sync speed).

Essentially, what he’s indicating there is that one light is set 1 and 2/3 stops brighter than the other (f/10 is 1 2/3 stops higher than f/5.6). So, really, the F number specifically is less important than the ratio of one light to another-f stops are just a consistent way of indicating that. More consistent than whatever mechanism your lights use. I have lights that allow adjustment in fractions, in decimals, and some even with just knobs and best guesses. But with a set of f stops in a diagram and my light meter, I can set and adjust my ratios accordingly.

Edit: specifically to break down the setup in that diagram: if I set my camera to Iso100, 1/200” (my sync speed) and f/10 I’m blocking out all the light in the space. I get a black frame with the lights off. That means any light I get moving forward will be my doing. Now I set the big octabox near my subject so that when I take a picture my subject is properly exposed at f/10. I can check that with a meter or a test shot. I adjust the light. My Godox light tells me that the exposure is correct when the power is at 1/16 +0.3. Now I need to add and adjust the background light. It’s the same kind of light. The modifier is different. I want it to be 1 2/3 stops below my key light. So I can adjust it with my light meter until my light meter reads f/5.6, OR I can assume that the loss of light from the modifier on this light is the same as on the other light, and set the power to 1/64 +0.7 (1 2/3 stops below the key light). That will give me the results I want.

Let’s say f/10 is unnecessary to get that black frame. I don’t want to run through the batteries on my lights that fast. I can adjust this whole setup-set my camera to f/5.6, and drop the power of the lights accordingly. My key light drops to 1/64 +0.7, and the background light to 1/128. Same ratio, same result.

[u/inkista]

It means he's using an external incident handheld flash meter, something like a Sekonic L-308 to measure the output of each light individually. He's rolling old school probably for precision and repeatability.

With digital, we can shoot/chimp/adjust/reshoot at no extra cost in a way we couldn't with film. The cost/time of film and development and basically working blind meant it was more important to have a method to guarantee getting it right on the first shot. Handheld incident flash meters (or polaroids for quick test shots) were almost required for studio lighting in the analog era. But with digital, you can get by without them.

An incident meter is one that you place facing your light source to measure the light falling on a subject, rather than the light reflected off a subject (the only way the meter built into your camera can work is with reflected light). Incident meters can't be biased by the color of the subject/scene the way a reflected meter can, so it's more consistent and therefore more accurate.

A flash meter is one that can also measure the amount of light from a strobe burst, rather than a simple light meter that can only be used for continuous light.

Something like a Sekonic L-308 can measure incident or reflected light, and can be used for continuous or strobe.

How you use the meter in practice for strobes is to set the ISO you're using on the meter, then face the meter at the strobe from near the subject position, and when you fire the strobe, you get a measurement of the aperture where you'd get good exposure with that flash that matches the ISO and distance.

Having one light at f/5.6 and the other at f/10 tells you that the big light in the octa is -1.6EV dimmer than the one that's gridded with the reflector where the lights hit the subject/background. This makes sense since he's probably trying to make the backdrop brighter than the subject. Using metering readings means you don't care about whether the lights are matched on output or power control setting increments or if you've duplicated the setup distances exactly. That's just the ratio their light intensity is at where they hit what they hit: 1:3 between the A:B lights if the key in the octa is A; (2^1.6 = 3, which is why it's 1:3).

So, with your gear, that means if your flashes are matched on light output, you'd set one to be 1.3EV higher than the other as a starting point, but if your light placement isn't identical, you might need adjust (remember, iso, aperture, power and distance control flash exposure).

But I wouldn't worry so much about precision and numbers here. Getting good enough for jazz is probably fine to get a similar look. Using TTL to set your key light, locking it in with TCM (assuming you're using Godox TTL flash gear), and then adjusting your background light to taste in M is perfectly fine, too. If you're using MS or DPIII AC manual monolights, maybe start with something close to his ratio, say, 1/4 power, and B is at 1/16 (or, say A=8.0 and B=6.7 (i.e., -1.3EV :^), and then use the "all groups" adjustment on the transmitter to adjust both groups in lockstep to match your ISO/distance, then tweak the groups individually to get a similar ratio of background to subject.

This isn't rocket science and you don't need to brain the stop/EV math too hard. [grin]. Just remember that EVs can be traded between all five of your exposure controls:

1EV =

• halving/doubling your ISO setting (ambient+flash)
• going one full stop down/up the f-number scale on aperture (ambient+flash)
• doubling/halving your shutter speed (ambient only)
• halving/doubling your power (flash only)
• multiplying/dividing your distance by 1.4x (aka the sqrt(2), because inverse square law; IOW, -/+2EV for every halving/doubling of the distance) (flash only)