Need help calculating gude numbers for 800 ASA film.

[ricardo12458]

I have a Honeywell Tilt-A-Mite bulb flashgun (satin reflector FTW) with 3 different types of bulbs:

• Sylvania FP26B
• M3 (clear)
• M3B
• AG1B

I happen to have Kodak UltraMAX 800 film. The highest GN was 500 for M3, 400 for M3B, none listed for AG1B, and the guide numbers for the FP26B lamps were too tattered to read. For a maximum ASA rating of 500.

What do you think?

-R

[polyglot]

GN are generally defined as being at ISO100 so if you shoot at EI800, you get 3 more stops or 2.8x as much range at the same aperture. However, they may be quoting GN at ISO500, which means the real GN is 220' at ISO100.

Check out this thread for more definitive numbers and a note that you may want to derate by 0.7 using the satin reflector. And that thread says to expect a GN of about 190' from an M3.

So at ISO800, your effective GN is probably about 530' without taking the 0.7 factor into account. Burn speed is also a factor; you can get more light by dragging the shutter a bit.

[Mike Wilde]

Guide numbers for flash bulbs are a bit more tricky than electronic flash.

At fast shutter speeds you don't get the full output of the bulb, even when set up to M synch.

FP-26B I think are designed for focal plane shutters, and burn really quickly. Look at them, and if all there is is two wires coated in flash paste, it is maen to be used with handheld brownies and the like.

AG-1B's are about as powerful as the M2B's they replaced.

Look at the documentations for the M3B's. They will likely say 'for polished 5" reflector', or something like that. I find I have to open up 1 to 1.5 stops to compensate for the folding fan refector of a tilt a mite. It also depends on whenter you latch the fan at the first (wide angle) or second (more focussed) latch.

Pick the guide number suited to 400iso film, and then open up to 1 stop less for iso 800 film.

Then test. Some things can only be confirmed by testing.

I test using a reflective patch from an old Kodak darkroom photoguide that has 6-8 patches from white to black.

I know the effective guide number of my flash when there is some denisty in the second from blackest patch, and then can also fine tune developing time for the contrast range the flash produces, by developing to make sure that the neg has a denisty range of no more than Dlog 1.2. with this neg range I can use a #2 or 3 contrast paper to print it in b&w.
Otherwise it will fit comfortably onto colour paper, if I am working with C-41/RA-4 for colour, with minor trims of c-41 developer time.

Using b&w or colour neg film, the overexposure latitude is much easier to deal with than under exposure.
Overexposure mostly yields a denser neg with slightly longer print times, unless things are severe.

[ricardo12458]

Guide numbers for flash bulbs are a bit more tricky than electronic flash.

At fast shutter speeds you don't get the full output of the bulb, even when set up to M synch.

FP-26B I think are designed for focal plane shutters, and burn really quickly. Look at them, and if all there is is two wires coated in flash paste, it is maen to be used with handheld brownies and the like.

My Canon AE-1 has an X contact @ 1/60; M sync @ <=1/30 s. I plan to use it @ 1/30s or 1/15s, whichever freezes motion better.

The FP26B bulbs have the thin wire filaments, like the other bulbs.

-R