what does this mean for M6 TTL flash metering (or for any camera applicable for that matter)? i remember i've read somewhere here that it's not exactly center-weighted or spot metering. and how can "off-the-film" metering be better (or worse)?

thanks.

-- Dexter Legaspi (dexter@secomm.com), January 15, 2002

Answers

TTL stands for Through The Lens (metering).

Sometimes TTL is simply used to describe cameras which mesures the light through the lens as opposed from an external cell (integrated or not in the camera). In this regard the TTL light cell of Leica's M6 is a kind of big spot metering. The big spot is exactly the size of the white circle you can see on the curtain after winding the camera.

In the M6 TTL, there are 2 light cells. Both mesure light through the lens from the white spot on the curtain. One is use to drive the little red leds telling you if the exposure is right. The other one is use to direct the amount of light emited by the flash (I think it's done directing the length of the flash burst).

I hope I didn't overdo my answer.

-- Xavier Colmant (xcolmant@powerir.com), January 15, 2002.

no, no, no...i was not asking about TTL at all...i was asking about the "off-the-film" FLASH metering.

-- Dexter Legaspi (dexter@secomm.com), January 15, 2002.

An OTF flash sensor measures the light that is literally reflected off of the film emulsion itself during exposure. In this fashion, in an SLR with the mirror up during exposure, the body can still detect the exact amount of light that has reached the film, and turn the flash off at the appropriate moment.

-- Jack Flesher (jbflesher@msn.com), January 15, 2002.

I should have said in an SLR with the mirror up or an RF with the shutter curtain open!

-- Jack Flesher (jbflesher@msn.com), January 15, 2002.

My Minolta CLE meters off the film emulsion for both flash and ambient light. It has proven to be a good meter, as long as you realize its not a Matrix type of meter and can be fooled by scenes that have extrem lighting. I remember reading that some film emulsions are off the standard in reflectance, and this could result in minor errors in this type of meter.

-- Andrew Schank (aschank@flash.net), January 15, 2002.

Andrew:

Not to step on your comment, but I believe your CLE meters normally off the shutter curtain -- hence that interesting speckeled appearance on the curtain. I also understand it was one of the first cameras to meter flash OTF however.

-- Jack Flesher (jbflesher@msn.com), January 15, 2002.

The CLE meters off the actual film emulsion during the actual exposure. The pattern on the shutter is just to give a close approximation in the finder of what speed it will pick when reading off the film. If you ever try to test the CLE with no film in it, the shutter speed in the finder (that is picked from reading the curtain pattern) may say 1/60, but when you fire the camera with no film in it, it will give a very slow speed trying to read the reflectance of the dark pressure plate.

-- Andrew Schank (aschank@flash.net), January 15, 2002.

Andy: Okay, I'll take your word for it!

-- Jack Flesher (jbflesher@msn.com), January 15, 2002.

TTL flash metering HAS to be off the film (except for pre-metering systems like the R8). The flash is only 'on' while the shutter is open.

-- Andy Piper (apidens@denver.infi.net), January 15, 2002.

OTF is not a magic bullet, though. Not all emulsions reflect the same amount of light. Still, I have no complaints with my Nikons that use OTF.

-- Bob Fleischman (RFXMAIL@prodigy.net), January 16, 2002.

"In the M6 TTL, there are 2 light cells. Both mesure light through the lens from the white spot on the curtain. One is use to drive the little red leds telling you if the exposure is right. The other one is use to direct the amount of light emited by the flash (I think it's done directing the length of the flash burst)."

Xavier, you're partly right but the second ligtht cell doesn't measure from the white spot - it can't, as Jack has pointed out, because, by the time the flash fires, the first shutter curtain has already opened and the white spot has already disappeared. Instead, the second light cell measures flash light reflected off the film (that's what OTF means) and the metering pattern is similar to the full-field centre-weighted pattern used for flash metering in the R cameras, AFAIK.

It's true that different film types have varying reflectances but the difference this makes to exposure measurement in practice is very small. Olympus has been using OTF for all exposure measurement of 1/60 second or slower in its OM cameras, not just for flash, for decades and has experienced no problems due to this.

-- Ray Moth (ray_moth@yahoo.com), January 16, 2002.