Leica User Forum - View Single Post - Leica D2 and Nikon SB-28

hongtoh · 09/01/06, 07:05 PM

Jerry wrote:

<<I'd appreciate any camera/flash web site where true TTL is touted as a capability where the flash reads reflections off the digital sensor to calulate pulse/exposure time as happens with film. To date I, Leica and Metz are unaware of any.>>

No single website that explains it all but gathered this from various forums and sites:

You are correct that digital sensors do not reflect light to the TTL sensor. The digital TTL systems use a very short (low power) preflash so that the TTL sensor would measure the light reflected off the shutter (often white) before it opened. This preflash coincides with some redeye reduction schemes where multiple short bursts are fired from the flash so that the subject's eye pupils will close and reduce the red eye effect. The exposure off the first shutter is momentarily stored in memory to control the flash during actual sensor exposure. The preflash occurs in the order of 1/10,000 sec prior to final flash so it is imperceptable to your eyes.

Due to the low intensity of this preflash it was not always reliable for subjects further away. Nikon in their "D" designated lenses sent distance information based on the lens focus position as well as aperture information to the flash which computed the correct flash output. Then during the preflash measurement, this computed flash output is verified to be correct or otherwise and then readjusted before the final flash output for sensor exposure. Any flash compensation selected is also adjusted prior to the final flash output.

That would explain why you would use "TTL" for the M7 which has a focal plane shutter and "A" for the Digilux 2 which has in lens shutter and therefore no TTL sensor.

09/01/06, 07:05 PM	#8 (permalink)
hongtoh Neuer Benutzer Join Date: 09/01/06 Posts: 6	Re: Leica D2 and Nikon SB-28 Jerry wrote: <<I'd appreciate any camera/flash web site where true TTL is touted as a capability where the flash reads reflections off the digital sensor to calulate pulse/exposure time as happens with film. To date I, Leica and Metz are unaware of any.>> No single website that explains it all but gathered this from various forums and sites: You are correct that digital sensors do not reflect light to the TTL sensor. The digital TTL systems use a very short (low power) preflash so that the TTL sensor would measure the light reflected off the shutter (often white) before it opened. This preflash coincides with some redeye reduction schemes where multiple short bursts are fired from the flash so that the subject's eye pupils will close and reduce the red eye effect. The exposure off the first shutter is momentarily stored in memory to control the flash during actual sensor exposure. The preflash occurs in the order of 1/10,000 sec prior to final flash so it is imperceptable to your eyes. Due to the low intensity of this preflash it was not always reliable for subjects further away. Nikon in their "D" designated lenses sent distance information based on the lens focus position as well as aperture information to the flash which computed the correct flash output. Then during the preflash measurement, this computed flash output is verified to be correct or otherwise and then readjusted before the final flash output for sensor exposure. Any flash compensation selected is also adjusted prior to the final flash output. That would explain why you would use "TTL" for the M7 which has a focal plane shutter and "A" for the Digilux 2 which has in lens shutter and therefore no TTL sensor.