>From: "Jay D. Washington" <[EMAIL PROTECTED]> > >Don Weiss wrote... >>...If lens designers went around calling their lenses by the >>geometrically defined aperture, and those lenses >>reported the same to their camera bodies, how >>could the designers of the exposure electronics know >>how much transmission loss to factor in? > >Wouldn't the basic exposure information going to the camera be based on the >actual light going in with the aperatue held full open and going through the >lens? This would factor in any light loss due to transmission loss, just as >if you had placed a filter over the lens. Of course for hand held >metering, it would be off, but I'm assuming even a bad lens has no more than >1/2 stop of light loss due to the glass. I gave that argument some thought after I posted it, and your comment has given me some more occasion to do so. So let me amplify its refutation @--<)B^). Suppose a lens reports an 'optimistic' f-stop to the camera during metering. That will be its maximum aperture, because the metering occurs with the lens wide open. Because the report is optimistic, the camera isn't getting as much light as a true lossless lens of that aperture would be giving it, but the camera doesn't know that. The camera calculates the exposure based on what it has available: The scene brightness (measured these days through numerous segments), the indicated film speed, and the indicated (viewing/metering) f-stop. It calculates the taking f-stop and sends it to the lens before the exposure. The taking aperture thus calculated is _larger_ (i.e. the f-stop is numerically smaller) than it would be if the taking lens were lossless. And how much larger? Just enough to make up for the light loss, i.e., the 'optimism' of the actual lens. The lens loss is, in effect, in the forward path of a feedback loop where it is automatically compensated for. For the same reason, filters in front of the lens on a TTL metering camera do not normally require exposure compensation. Lens loss is equivalent to a light ND (one hopes the D is N!) filter in front of the lens. And, as you say, this automatic compensation cannot occur if you're using a handheld meter. There, it might be important for the lens' f-stops to represent its true light-transmission capabilities. DGW * **** ******* *********************************************************** * For list instructions, including unsubscribe, see: * http://www.a1.nl/phomepag/markerink/eos_list.htm ***********************************************************
