Originally posted by stevebrot Wow, it's midnight and I have just read through it. I don't think English is his first language, and he repeats himself over and over (I wanted to go back and count the number of times he said "
inverse square law" - yeah we got the point
), but it is good, and all you need to know is there. Looks like that site has a lot of other info too.
I think he is saying what I said earlier - that the GN
should go up
linearly with flash zoom "focal length", but it goes up less than that because of the crude optics of flash units. Here is the crucial part :-
Quote: My one regret about the GN calculator is that it doesn't also compute zoom. If the flash zooms, increasing the flash zoom mm number concentrates the power into a brighter smaller beam. We might imagine (if treating its field like focused lens zoom, which it's definitely not), doubling the flash zoom mm should theoretically cover half size dimensions, which is a 1/4 smaller area. The same flash power concentrated into 1/4 area would be 4x brighter intensity, which is +2 EV. Which you'd think the GN calculator could calculate for zoom too, but the flash zoom system does not actually refocus. Speedlight zoom does move the reflector back and forth, but does not change its curve to refocus it. Also the front fresnel lens is molded plastic, which does not refocus with zoom. Most of the range is not focused, and each reflector design may vary individually too.
He makes the point that doubling the zoom focal length (like doubling the camera lens focal length) should concentrate the same light energy on 1/4 of the subject area, raising its illumination by a factor of 4, ie two stops (he uses EV). Plus two stops of illumination would enable the lens f-number to be doubled, which is equivalent to a doubling of the GN.
But because of the crude flash unit optics, in practice, as he goes on to say, the GN at the various flash zoom settings can only be found by experiment, hopefully done by the maker and put in the instruction manual and/or the unit's software. The fact that many of these tables, like the one in my OP and in the Wikipedia article, seem to show a GN proportional to the square root of the flash zoom distance, rather than being linear, is just a co-incidence.
Incidentally I never understood what a "speedli[gh]t[e]" was. I thought it (and "strobe") were American words for "flash unit". But I've learned from these articles that they are brand names for Nikon and Canon hot-shoe flash units