View Full Version : Light fall-off

16-Mar-2004, 08:02
Is it logical to think that a lens with a wider field coverage reduces light fall-off at the corners and therefore less edge burning during printing? Thanks.

Michael S. Briggs
16-Mar-2004, 08:47
Designing a lens to have wider coverage doesn't necessarily improve the uniformity of illumination. There are optical design approaches which do improve the uniformity of illumination. For SLRs, the mirror forces retro-focus designs, which also improve the uniformity of illumination. In LF, the usual approach is to use large outer elements shaped to tilt the enterence and exit pupils. In theory this improves the illumination from the fourth power of cosine to the third power.

So there are wide-coverage lenses without improved illumination, e.g., the Schneider Super-Symmar-XL series, and there are wide-coverage lenses with improved illumination, e.g., Schneider Super-Angulon types, Rodenstock Grandagon types, Nikkor-SW, Fuji-SW. The two German manufacturers publish graphs showing the relative illumination of their lenses, so you can verify what I say by studying the graphs supplied by the maker of the lenses.

Tim Curry
16-Mar-2004, 10:49
To add to Michael's answer, take a 65mm lens with greater coverage as an example. If there is greater coverage, there is still light fall-off at the corners due to the relative difference from the axis of the lens at the center of the film plane and the axis of the lens to the corner of the film plane. The fact that greater coverage exists will not change this relationship, and in fact may tend to increase the fall-off if movement (shift, rise or fall) is employed to move the axis of the lens nearer to one edge of the film. This will increase the fall-off at the far corner.

Paul Metcalf
16-Mar-2004, 12:34
I'm not an expert by any means, but my experience using a Wollensak 159mm WA on my 8x10 has resulted in actually requiring dodging the corners when making contact prints from B&W negative. So far I haven't figured out what's up with all of the edge burning "needs" I read about...

Mark Sampson
16-Mar-2004, 12:55
two reasons for 'edge-burning' when making prints: 1) camera flare and 2) marginal illumination in the enlarger.

Michael S. Briggs
17-Mar-2004, 00:29
Very similar questions elsewhere on the internet: Wide Angle lenses, Image Circles and Light Falloff (http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=004fzV) and Image circle and fall-off (http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=005gK2).

17-Mar-2004, 00:42
Thanks for the replies. Also thanks Michael for pointing me to the other sites. I have better understanding now.

17-Mar-2004, 01:41
Marks comment about camera flare did ring a bell. I have recent 5x7 negs that require very little edge burning and 8x10 negs that require lots of edge burning. Both formats were made using the same 8x10 camera (5x7 on reducing back). I'm quite conclusive that camera flare is at play. Thanks.