You'll need to do some math to figure that out. Flashes are rated by Guide Number at a particular zoom setting and ISO (Guide Number divided by aperture determines the distance the flash will illuminate a subject correctly at that ISO. This is actually the most accurate method for determining flash exposure. For example, a flash with a GN of 100 feet at ISO 100 will correctly illuminate a subject 50' away at f2).
-Adam Rebekah wrote: > Can anyone tell me how bright these flashes are, or where I can find > information telling me what illuminance they produce (fc or lux is > fine, or f-stops) ? > > rg2 > > > On 8/28/07, Adam Maas <[EMAIL PROTECTED]> wrote: >> William Robb wrote: >>> ----- Original Message ----- >>> From: "Toralf Lund" >>> Subject: Re: TTL, P-TTL, other *TTL's and DSLRs (Was: Wedding photography) >>> >>> >>> >>>> Something I've been wondering about lately, is which is better of a >>>> traditional auto-mode and P-TTL. I mean, which is greater of the error >>>> in the pre-flash based prediction and the inaccuracy resulting from >>>> using a sensor not measuring through the actual lens (and making >>>> calculations based on a "theoretical" aperture)? Any thoughts? >>>> >>>> Maybe the best result might be achieved if you used an external sensor >>>> to control the actual flash operation, but included a through-the-lens >>>> pre-flash measurement to collect knowledge about the actual exposure >>>> conditions? Is anyone doing that? >>> Once I left Nikon, I discovered that basic auto flash was more accurate than >>> TTL flash. I happen to have a very accurate auto flash though, I expect a >>> cheap auto flash will be as inaccurate as the rather useless Pentax in body >>> flash control. >>> I suspect that the best way to get body controlled flash would be to >>> transmit distance information from the lens rather than use reflective >>> information from the sensor. >>> >>> William Robb >>> >>> >> Which is how Canon E-TTL II works(E-TTL primarily used reflectivity under >> the selected AF point), and is also used by P-TTL and Nikon's D-TTL and >> i-TTL systems. >> >> -Adam >> >> >> -- >> PDML Pentax-Discuss Mail List >> [email protected] >> http://pdml.net/mailman/listinfo/pdml_pdml.net >> > -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
