Why no retrofocus wide angles?...

[Bob Fowler]

The recent discussion about short focal length l.f. lenses has got me to thinking: Wouldn't a retrofocus design reduce the need for a center filter? I would think that because the rear cell would be farther away from the film plane, the law of inverse squares would have less of an effect on the edge brightness. OK, I'm not an optics expert so I'm not sure how covering power would be affected, but I would think that for a situation where not a lot of movements are required, a retrofocus design would (could?) be a good compromise. Opinions?

[Leonard Evens]

Most wide angle lenses that I've looked at are of slight retrofocus design. For example, the Rodenstock f/4.5 75 mm Grandagon-N has a rear flange focal length of 82 mm, which is 7 mm, longer than its focal length. I imagine the reason this is not pushed too far is that it is difficult to design an assymetric lens with good defintion over a wide field.

[Sam Crater]

I think the law of inverse squares has already been invoked by the time the light reaches the aperture. That is, it's a fundamental problem with rectilinear perspective. Imagine taking wide-angle photo of a bank of round light bulbs all in a plane parallel to your film plane. The bulbs at the edges will be larger in the image (due to rectilinear projection) but they're not any brighter, so the intensity of the light in the image has to be less.

[ronald moravec]

Not an optical engineer either, but based on 35mm experience, the lenses start to get very large. The fall in light is less, but still present.

[Ernest Purdum]

The idea of a retrofocus lens getting us away from pesky recessed lensboards is very attractive but if one were available today I would expect it to be bigger, heavier, and either have lower performance or be much more expensive than a normal wide angle of the same aperture and focal length. Since SLR's have thick bodies, they need retrofocus lenses to clear their mirrors. Their users are happy with what the lens makers provide them, but picture one of their lenses scaled up to cover 4X5 or more.

[Darin Cozine]

Do the math. Use a 75mm lens. If the light hitting the center of the film has an intensity of I, then the light hitting the long edge (not the corner) would be at roughly .6 of I. Thats a little less than 1 stop, and I doubt you would notice the difference.

So why do wide angle lenses viginette so much?

The law of inverse squares is not the only factor in causing the viginetting of lenses. If you look at an aperture of a shutter, it looks round. When you look at it from an angle, the apeture becomes more oval. This reduces the amount of light travelling through it. At a really extreme angle the aperture looks more like a pancake, which really limits the light pasing through it.

One other thing to consider is that retro-focus designs require more elements and that increases the size, weight, and expense considerably.

[Paul Metcalf]

Darin- Doesn't the front element or group essentially "straighten" the light ray path through the shutter aperture (with the rear element turning the rays back toward the appropriate place on the film plane)? This is what I recall from (limited) ray tracing study, but I could be wrong. I thought vignetting was due to physical blockage from lens rings, etc.

Paul

[Kerry L. Thalmann]

Actually, there was a line of retrofocus large format wide angle lenses made by Komura back in the 1970s and early 1980s. I know they made at least two focal lengths, a 75mm f6.3 and a 90mm f6.3. In addition, these same lenses were briefly sold by Calumet under the Caltar Pro name (not to be confused with the later Caltar Pros that were made by Schneider).

As others have theorized, they were quite large and heavy. Performance was supposed to be excellent in the center of the field, but rumored to be not as good in the corners as the Super Angulons (the "gold standard" of the day).

Flange to focal distances were about 33% longer than the focal length (about 100mm for infinity focus with the 75mm lens). This reduced the need for a bag bellows and/or a recessed board. It was also supposed to reduce the need for a center filter.

All the samples I've sen were single coated. Not sure if any were made with multicoating.

I've seen several of these lenses over the years, but never actually shot with one. I'm not a big fan of overly large, heavy lenses. Still, I am curious how the illumination fall-off of the 75mm compares to other standard 75mm wide angles. I shoot 4x5 color transparencies and have yet to use a lens shorter than 90mm that did not require (for my needs) a center filter. The 75mm f6.3 Komura SW might be such a lens (but in the end, I'd no doubt still prefer the smaller size, lighter weight, greater coverage, and better performance of my 80mm Super Symmar XL - even with the center filter).

Kerry

[Darin Cozine]

Paul, lenses like the super angulon still suffer from this. In these cases i guess you could just look at the front element from an angle and see that less light can pass through it.

[Dan Fromm]

Um, er, ah, I understand how Biogons and other lenses with tilting pupils beat cos(theta)**4 down to cos(theta)**3 or so. Also how fisheye lenses get any illumination at all in the margins of the field..

Will someone please explain how retrofocus lenses beat cos(theta)**4, if indeed they do?

Cheers,

Dan