Hi, I just picked up a Sigma mini wide angle/macro lens. The numbers on the front read 1:2.8 f=28mm.

2 questions.

1. What do those numbers mean?

2. When the lens is mounted on the camera body (a Canon AV-1) there is a small black push button on the lens, closest to the camera body. Does anyone know what that button is for?

I thought it was some sort of release button, but pushing it dosen't seem to do anything. I know, never push a button unless you know what it's for, but I couldn't resist. :)

Thanks all

-- Greg Kamer (gkamer@bingoware.com), September 27, 2001

Answers

I'm not sure what the button is for unless it is the release for the lens mount. Canon used two different schemes for mounting lenses to the same FD camera mount. The earlier one used a ring around the base of the lens that rotated and locked it into place. Later they just turned the whole lens to lock it into place. As I recall, some of the first type had a button on the ring for a release. Been a while since I played with them though, so I may have it all wrong.

The "f=28mm" means the focal length of the lens is equal to 28mm. That means the lens acts like its optical center is just over 1 inch from the film plane. It's probably a retro-focus design so its actual center will probably be a bit farther out than that. In any case 28mm is a moderately wide angle lens by todays standards.

The "1:2.8" means the lens has a maximum aperture opening of f/2.8. This is a reciprical statement so consider it to mean that the diameter of the opening of the lens is 1/2.8 (or nearly 1/3) as wide as its focal length. So in this case the physical aperture is probably close to 10mm. All that means is the lens is fairly "fast" optically, or that it lets quite a bit of light through. A smaller number here is better, and f/2.8 is on the good side of average.

-- Jim Strutz (j.strutz@gci.net), September 28, 2001.

Hi Jim, Thanks for the response. So from what you've told me, this is about an average lens.... not the best and not the worst?

I have taken some test pic's with it, switching between it and my normal 50mm to see what anle of view I get taking a picture of the same scene. Now all I have to do is wait for the film to come back... :)

Appreciate the info....

-- Geg Kamer (gkamer@bingoware.com), September 28, 2001.

Well, the specs are about average, but the quality is unknown to me.

-- Jim Strutz (j.strutz@gci.net), September 28, 2001.

The button is likely to be a catch that you press before turning the aperture ring to auto aperture mode for use on the Canon AE-1 shutter priority cameras. The AE-1,AV-1, and AT-1 were marketed about the same time.

-- Fred Sun (redsky3@yahoo.com), September 29, 2001.