#3. "RE: Slave with SB800" In response to In response to 2 Tue 01-Apr-08 02:29 AM by Wayne
>So I am ok with manual mode.
Me too, I use Manual flash mode a lot too. It offers unlimited control that automation sort of skips over. I refer to the fixed setup studio situations where it is possible to take the time to do it, and the subject is not moving. And of course, a flash meter really helps too. But many people get glazed-over eyes when you say manual.
I do also often use iTTL on the camera hot shoe for walk-around bounce work too, so I dont have to worry with it. And sometimes two SB-800 using Nikons remote wireless system. It is all good, and it seems a shame to miss out on that, but the price is higher.
There are always some ifs and buts.. There are some "digital" optical slave triggers which know how to ignore the preflash from the digital iTTL flash. That means they could trigger in sync, but they themselves would not be metered or automatic, and so that their light will mix in unknown ways with the metered iTTL flash.
>1) Also could you please tell me some pointers to read about >how to calculate the relation between flash power and >ISO/shutter speed/Aperture?? > >2) What does 52' at ISO 100 (for example) mean??
The flash units have Guide Numbers to represent their power capability. This guide number depends on their reflector zoom, that is, what angular field width they can cover, therefore they will have a different GN for every lens zoom setting that they might zoom, like from 24mm zoom is a lower GN, and 85 mm zoom is higher GN (concentrated into a narrow beam). The power is the same, but the idea is that the power per unit area covered is different.
Such Guide numbers might refer to feet or meters, and they will say feet or meters. There are 3.05 feet per meter, so the feet value will always be 3.05x the meters value. And such published GN are always for ISO 100 (unless otherwise stated). If you use ISO 200, then multiply ISO 100 GN value by 1.414. Or for ISO 400, multiply ISO 100 value by 2.
So a number for GN 100/33 surely will specify it as 100/133 (feet/meters, ISO 100) to make it clear. Meaning, use GN 100 if computing in feet, and use GN 33 if computing in meters. Multiply either by 1.414 if using ISO 200.
GN is the simple product of aperture number times distance (flash to subject distance).
That is... if the flash (at this zoom value used) is seen to give the correct exposure at f/11 at 10 feet, then the GN = 11 x 10 = 110.
Knowing that, or from the GN chart in the manual or on the rear of the flash, if the GN is 110 and we are shooting something at 15 feet, then we know the correct aperture is GN 110 / 15 feet = f/7.3 - or thereabouts.
The GN values are never very precise, often actual will be a bit less, but still they are a fine starting point, and give a quick realization of what is possible. For example, if GN 100, outdoors at 50 feet, then expect f/2. We just know.
Shutter speed does NOT affect flash, only the ISO and aperture does.
However shutter speed does affect how much continuous ambient light comes through, like from daylight or tungsten lights, same as we always understood exposure to work.
Flash is simply much faster than the shutter speed, so it simply does not matter how much longer the shutter might hang open. The flash already finished.
So there are always two exposures to balance with flash, one for the flash itself, and one for the continuous ambient light (but about which we may or may not care, for example indoors at night, any ambient is probably insignificant). If ambient is low and insignificant, then it really does not matter what shutter speed is (so long as the shutter is open for the flash, meaning, the shutter speed setting does not exceed the cameras maximum sync speed)... the flash exposure will be the same at any shutter speed. Flash exposure depends only on aperture and ISO. But again, any continuous ambient will be affected by shutter speed. So if we using fill flash outdoors in the daylight, the ambient is of very great importance to us.
So the flash picture indoors at night, a fast shutter 1/250 may show the table lamp to appear to be dark, but a slow shutter 1/60 probably shows it was lighted. It wont contribute much though, and the overall flash exposure of the room wont matter either way if the aperture says the same.
>3) Do you have some experience about these cheap flashes?? >Which one would you suggest?? Sunpak/Vivitar/Bower/Opteka?? As >I said I have read good reviews about Sunpak 383 but could not >find much info on other brands. Just that the Vivitar flashes >have some decent repo too.
No, sorry, not in many years. Other than the iTTL incompatibility, the one thing I do know is that the new Vivitar 285HV is created to have a low and safe sync voltage (see next section). I know no more about it though, I do not know if it is good like the early ones or not.
>4) Am I right in assuming that I could use the cheap flash on >hot-shoe in almost any point-and-shoot camera??
Yes, but... two things... the point&shoot must have a manual mode, but probably does if it has a hot shoe.
And there is an issue about using old manual flash units with digital cameara. The flash sync trigger from the flash puts a certain amount of voltage on the camera hot shoe contact or on the PC cord contacts. Todays digital cameras want that voltage to be low, like 5 or 6 volts. Modern flash units are low, or I suppose all are now. Nikon flashes have been low sync voltage for 20 years. Old flash units, for one example, like a 20 year old Vivitar 283 might be up near 200V. The old days were like that (not digital). Nikon DSLR are rated for up to 250V -IF- the polarity is not reversed. Some Canon DSLR models say 250V too, but a few say no more than 6V.