>>3621946Not all flashes do ttl, and when they do they're usually "locked" to a specific manufacturer. TTL protocols are proprietary, for instance a canon speedlight won't do ttl on a nikon body and vice versa.
In any case, if you have TTL you don't have to worry about metering, that's the point of TTL in the first place.
If you don't have TTL, like in your example, and you doubled the distance, you need to quadruple exposure, i.e. give it two stops of extra exposure. You do that by either opening the aperture 2 stops, or upping the ISO 2 stops, or increasing flash power by 2 stops (i.e. from 1/125 to 1/30). Shutter speed doesn't matter for flash.
If you moved the flash to 1m, i.e. half the distance, you need to *reduce* exposure by 2 stops. By closing the aperture down 2 stops, or reducing flash power, or iso.
That's the inverse square law thing. For double the distance of the light (to the subject), you need 4 times the exposure. For 4 times the distance, 16 times the exposure. Half the distance, 1/4 the exposure. ⅓ the distance, 1/9 the exposure, and so on.
And yes it's possible to get out of range of your equipment. For instance, with big enough distance, you can run out of aperture (and iso) to compensate for the increase of distance. And the same for very close distances.
That's why flash meters are convenient, it's much quicker to take a reading than trying to guesstimate the distance every time you move a light around. With digital you can just take test shots until you get it right, but a meter is still faster I'd say and more convenient, even if only to give you a good starting point. With film, it's an absolute must.
>so why not use auto flash instead ?As I said, maybe it's not compatible with your camera. Also with multi-light setups, you want to dial in specific ratios for key and fill, maybe have a hairlight on etc. TTL doesn't know any of that and can only give you an average exposure for what the camera metering area is pointed at.
