mauney (03/24/83)
Q: Why do two different lenses (specifically a f1.8 normal lens and a f3.5 zoom) both set at F4, cause a through-the-lens automatic meter to choose different shutter speeds? A: First, when you set F4 on your lens, you don't get exactly f4. There is considerable tolerance, especially in zooms, since the f-number tends to change somewhat as you zoom. More importantly, the f-number is the ratio of focal length to aperture, and is not a measure of light transmittance. There is another measure, the T number, that tells you how much light gets through. The F and T numbers are usually close, but a zoom lens has lots of glass, which absorbs light, so the transmittance is decreased. The worst offenders are mirror lenses, since the secondary mirror blocks a significant fraction of the light. Q: How does the camera know the f-stop of the lens? A: Examine the lens mount. You will find frobs that communicate info from lens to camera and vice-versa. The actual mechanism varies from mount to mount. Jon Mauney duke!mcnc!ncsu!mauney