Quote Originally Posted by benjiboy View Post
Guide numbers don't work at closer than a meter.
Really? Why do you believe that bilge?

I ask because they work very well for me. Using them, however, requires understanding GN arithmetic and closeup photography arithmetic.

To do it, first set up and measure flash-to-subject distance. Then calculate the aperture required given distance and the flash's GN. f/# = GN/distance, where distance is in the same units that distance measured in and GN is for the speed of the film being used.

Next, calculate magnification. For most lenses, (film-to-subject distance/focal length) - 1 will get you close enough. If you're using a macro lens with focusing scale in distance and magnification, just read the magnification from the scale.

Next, adjust the aperture required for magnification. You have f/# = GN/distance. Illumination at the film plane lost because of magnification, in stops, is simply m + 1. Set the aperture accordingly, opening up from f/# = GN/distance by m + 1 stops.

That's what to do when the flash isn't fixed rigidly to the camera or lens or subject. Its what I do when shooting flowers with my little 2x3 Graphic, a lens that's good closeup and a hand-held flash. Except that I pick the aperture I want to shoot at and calculate flash-to-subject distance required.

But this isn't the OP's situation. The OP has a ring light, presumably fixed-output, that's rigidly attached to the lens. I have similar rigs.

The easy way to set up to use one of these rigs is simply to shoot a series of calibration shots with the film that will be used when shooting closeup in earnest. I take my calibration shots at each full stop from largest (f/2.8, in the case of my 105/2.8 MicroNikkor) to the smallest at 1:1, 1:2, 1:4 and, rarely, 1:6. I use reversal film for the calibration shots. Back when, KM. Nowadays, an ISO 100 E6 film. When the film comes back from the lab, I pick the best exposed shot for each magnification. If adjacent shots are a little lighter than I like and a little darker than I like, then the best aperture is a half stop between the two used for those two shots. And then I have a calibration table for the flash rig and the film. For magnifications between the ones at which I calibrated, I interpolate aperture linearly. One does have to open/close the diaphragm a little (1/2 - 1 stop) for subjects that are much darker/lighter than the calibration subject. This all works very well, eliminates the need for thinking about exposure. With my Nikons I just select magnification, look up the right aperture, set it, and there I am. Never fails.

In other words, by spending less than a roll of film to make a calibration table for a closeup flash rig, I completely elminate the need for Guide Numbers, TTL auto exposure with flash, ... I've been doing this since 1971. And practically no one believes that it works.