90mm for 6x17

[michael9793]

Again Ole, you amaze me with your knowledge.

 

[SMBooth]

Yes, I meant to thank Ole for his clear explanation.

 

[polyglot]

What about strongly retrofocus lenses? For example, I have a 20/2.8 designed for a 35mm SLR and it exhibits very little falloff despite covering about 95 degrees. Assuming they have the strongly-negative front element (one less cos) and a rear node that's about as far from the film as a film-diagonal, would I be right in guessing that these have closer to cos^2(phi)*cos(phi/k) falloff, where k is the pupil ratio?

If so, is there any particular reason other than cost that we don't see such lenses being manufactured and sold for large format? There are tele lenses sold for LF that mean you can use a shorter bellows, why not retrofocus lenses that allow you to go stupidly-wide without recessed lens plates and centre filters?

Is it that smaller formats tend to have much larger exit pupils compared to the film size than larger formats, which further reduces the falloff? And that we wouldn't get that benefit with LF?

 

[David A. Goldfarb]

Retrofocus lenses do exhibit less falloff of illumination, and modern LF wide lenses tandem to be slightly retrofocus mainly to provide more working room, without incurring too much of a penalty in distortion or field curvature.

 

[Ole]

To make an LF lens retrofocus enough to make a significant difference to the evenness of illumination, it would be impractically big and heavy. Compare a retrofocus and a non-retrofocus 20mm lens for 35mm format, and remember that the mounting/barrel is a much greater part of the total weight in that format than in LF lenses.

A good LF retrofocus WA lens could easily be made, but how many would like a 75m f:8 lens weighing 4 kg?

 

[michael9793]

Hell, My 90mm works on my 5x7 camera. So the coverage for that is good. therefore if I use it on my 6x17mm camera, which is 2.25x6.69" it is still inside the circle. Sorry Ole, Once I got out of dental School I tried not to get to involved with math since I use it everyday at work the last thing i want to do is squirrel over math instead of photographing. But impressive it is.

 

[Ezzie]

I too use a 90 on my DIY 6x17, the Fuji f8 in fact. I can't say the fall off is at all worth correcting for on regular silver negative film, i.e. I do not use centre filter. It's a pukka lens.

 

[Klainmeister]

I have the Linhof Schneider 90mm f/8. Covers beautifully.

 

[Jeff Searust]

I use a WIDE Field Caltar 90mm. same thing as the rodenstock for a heck of a lot less money. works fine.