Flashes and Guide Numbers

Gerald Undone usually focuses on Video content on their channel. But yesterday I stumbled upon a more photography related video

The Video about lux vs lumen vs nits is also good.


I found the first few minutes useful, but he then gets into trying to estimate (or calculate) flash power based on guide number, which is like trying to calculate vehicle speed based on engine RPM.

Guide Number is (as the name suggests) a guide - it lets you know how powerful two flashes are when comparing them. It’s not something you would use in the field. Think of it like engine horsepower - you can use it as part of the decision when buying a car, but you don’t think about it as an absolute value when driving. I don’t know any photographers, of any skill level that even mention GN unless they’re buying a flash.

The proper way to calculate flash power is to use a light meter. With digital cameras you can chimp as well, and as you become more familiar with your flashes you will need fewer and fewer attempts to get it right (nowadays this is how I do it - I only pull out the meter if I’m balancing multiple flashes.)

It is a head spinner. I guess this guy had an “A” in math. But the information is useful (if you can digest it).

Don’t knock this. The speedo cable on my motorbike broke a couple of weeks ago and this method served me well (with no speeding tickets) until the replacement arrived in the post. :slightly_smiling_face:

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Some old tractors had multiple scales in different colours on the tachometer (rev meter), corresponding to road speed in the different gears. Saves on having two gauges. :slightly_smiling_face:

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I am riding a 30 year old bike, so I am surprise this old tractor (with two wheels) doesn’t have the same markings on the tacho.

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He was vague about why shutter speed doesn’t matter: “The light from the flash is provided at the same interval that the photo was taken, so the shutter speed is kind of irrelevant.”

As he probably knows but didn’t say: electronic flash illuminates for a very short period (eg less than 1/1000 second), and it goes off when the shutter is fully open. Provided the ambient light is trivial compared to the flash intensity, and the shutter speed is slow enough all that sensels are exposed at the same time, then shutter speed is irrelevant.

On guide numbers: in the bad old days, cheap flashes didn’t have variable power, and very few flashes had “zoom”, and unless we had a Polaroid back, we couldn’t see results until at least a couple of hours after the shoot. A guide number provided the “best guess”. Life became complicated when we used multiple flash units, or one unit multiple times. Then spotmeters came out that could fire the flash unit(s), and that was amazing: point the meter at the face or clothing or gray card or anything, press the trigger on the meter, and quickly discover the dynamic range of the scene, hence the optimum exposure, etc.

I’m an old fogey, and I think for photographers who use flash, it is useful to know about the inverse square law, and how apertures work, and so on. If I was setting up a studio, I would want a tethered solution – like the Polaroids of old, but much better.

he mentioned why shutter speed matters actually. it decides how much ambient light enters your sensor. rewatch it if you dont believe me :slight_smile:

Yes, he explained why shutter speed matters for the light that comes from ambient light. But he didn’t explain why it doesn’t matter for the light that comes from the flash. He just glossed over that. Perhaps because the reason is so obvious (to him).