Why do we need center filters?

[Leigh]

This is not a question about optical geometry and light fall-off. I'm familiar with optical design.

My question is that rectilinear lenses for smaller formats, even those with wider Angles of View,
don't need center filters, and in fact such filters are not even available for them.

So why are they used for LF lenses?

- Leigh

[Jim Andrada]

This is just a guess but the geometry is very different - a DSLR wide angle lens is no closer to the film plane than a regular lens so there is no extreme difference in the distance of the corner of the image or the center of the image.

Might be full of S--t but it seems reasonable at this late hour. Maybe it won't seem reasonable in the morning.

[Leigh]

Hi Jim,

Not exactly true. The from the film to the rear node is equal to the focal length when focused at infinity.

This is true for all lenses, and is in fact the definition of focal length.

So for a given AoF, the LF lens will be a longer FL, and therefore farther from the film, than a 35mm lens.

- Leigh

[Bob Salomon - HP Marketing]

The rays that illuminate the edges and corners travel a greater distance then thos that illuminate the center. So theie is greater fall off then with regular lenses, But regular lenses also have less exposure at the edges and corners as well. just not enough to warrant correction.

If you download the relevent brochures from Rodenstock on lenses and on center filters it is all explained.

[David A. Goldfarb]

Wideangle lenses for SLRs are retrofocus designs, which inherently have less falloff of illumination, but greater distortion, which can be corrected in rectilinear lenses by adding more elements to the design or using aspheric elements. Some of the most recent wideangle lenses for LF are slightly retrofocus designs, leaving a bit more room for lens movements and reducing falloff a bit.

[rdenney]

This is not a question about optical geometry and light fall-off. I'm familiar with optical design.

My question is that rectilinear lenses for smaller formats, even those with wider Angles of View,
don't need center filters, and in fact such filters are not even available for them.

So why are they used for LF lenses?

How can this not be about optical geometry? Very short lenses on small format don't have the falloff because of their strong retrofocus design. In return, they have an assortment of geometric distortions (mustache distortion being common and annoying).

Remember that a retrofocus design is a reversed telephoto. The telephoto design uses a normal lens with a rear magnifier, and so the retrofocus design uses a front de-magnifier with a more normal lens behind it. The position of the rear node is an effective rather than an actual position in a compound design. Determining focal length is not as easy as it seems. The point is that you cannot draw a straight line of the marginal rays from subject to image surface as with a simple lens. (Consider, as an extreme example, a retrofocus fisheye. The marginal rays subtend a 180-degree angle to the subject, but only produce a circular image roughly three times the effective focal length at the image surface when focused at infinity.)

I own rectilinear lenses going down to 12mm for 24x36, 45mm for 6x7, 47mm for 6x9 and 6x12, and 65mm for 4x5. The latter two are typical biogon-type large-format lenses that are like two retrofocus lenses in opposition--this was, in fact, Schneider's breakthrough with the Super Angulon. Because of that near-symmetry, they lack any type of distortion, and their optical performance is excellent and fairly uniform across the frame. The opposing retrofocus lenses undermine the advantages of the retrofocus design, however, and that means the rear node position at the focal length is actual rather than effective, and therefore quite close to the image surface. I use center filters with these.

The former two are pronounced retrofocus designs that do not suffer from severe falloff, and center filters are not needed. But they lack corner performance that is as good as center peformance, and they suffer from various geometric distortions.

There are some lenses for small format that allegedly do not suffer from distortions, despite being strong retrofocus designs. They are also priced well outside what I can afford. (The Canon 17mm TSE lens is one example, but its corner performance relative to its center performance is still not quite as good as a later f/5.6 Super Angulon, near as I can tell.)

Rick "noting that designs targeted for digital sensors are also retrofocus designs, to increase the angle of the light rays approaching the sensor" Denney

[Dan Fromm]

I own rectilinear lenses going down to 12mm for 24x36, 45mm for 6x7, 47mm for 6x9 and 6x12, and 65mm for 4x5. The latter two are typical biogon-type large-format lenses that are like two retrofocus lenses in opposition--this was, in fact, Schneider's breakthrough with the Super Angulon. Because of that near-symmetry, they lack any type of distortion, and their optical performance is excellent and fairly uniform across the frame.

Thread drift strikes again. Sorry, Leigh.

Um, Rick, Schneider was quite late to that party. The general design type was first used by Roosinov (wide angle Russars), later by Bertele (Wild Aviogon and Super Aviogon, Zeiss f/4.5 Biogon). As far as I know f/8 and f/5.6 Super Angulons -- see, e.g., http://www.schneiderkreuznach.com/fo...R44907_1AE.PDF -- have cos^4 falloff, unlike Biogons which have cos^3. Russars and Bertele's designs take advantage of the Slussarev effect, f/8 and f/5.6 Super Angulons don't seem to.

[BetterSense]

I think the retrofocus may be part of it, but not all of it. What about wide-angle lenses for rangefinders?

My Olympus OM 28mm and 50mm lens have exactly the same physical focal length. When I use them for panoramic 'stitching', I don't even adjust the camera position; the exit pupil seems to be exactly the same distance from the film plane.

[rdenney]

I think the retrofocus may be part of it, but not all of it. What about wide-angle lenses for rangefinders?

My Olympus OM 28mm and 50mm lens have exactly the same physical focal length. When I use them for panoramic 'stitching', I don't even adjust the camera position; the exit pupil seems to be exactly the same distance from the film plane.

"Physical focal length" is a term that will attract confusion. You are saying that the rear of the physical lens is in the same position with both lenses. That's because the 28 is a stronger retrofocus design than the 50. Necessary, of course, to prevent interference with the reflex mirror.

I have a Russian rangefinder lens of 35mm focal length that gets quite close to the film. But the rear element is quite large, too. At that focal length, which is really only a little shorter than the 43mm format diameter, the falloff is not important. The rear lens cap is deeper than it is wide.

Very wide lenses for rangefinders still have somewhat of a retrofocus design. I've looked at the 12 and 15mm Voigtlaender lenses for the Bessa rangefinders, and they were both absolutely retrofocus designs. But they were not as extreme in that design as SLR lenses. You could tell by looking, though, that they had one or more negative meniscus demagnifiers on the front and a more normal gaussian lens behind it, like most retrofocus lenses. They probably did that so that the lens would not have to project too deeply into the box and fit through that smallish opening, past the rangefinder follower and the meter device. I haven't looked at, say, the 43mm lens for the Mamiya 7, but I'll bet it's similar. None of these are symmetrical in the way large-format wide-angle lenses usually are.

There is another point, too. If a wide-angle large-format lens has abundant coverage and is used without movements, it might not need the center filter. But it might well need it when used at the extremes of its coverage. I don't use a center filter for a 90, but I might need to if I really pushed its limits for critical work with narrow film. I absolutely use the center filter for the 65--I'm always close to the edge of its image circle. That's a problem for smaller formats only when using extremely short lenses and lenses with movements, neither of which dominate the topic.

Rick "who can attest to the three-stop dropoff at the edges of the Super Angulon coverage" Denney

[ic-racer]

In terms of retorfocus SLR lenses, the cosine of the rays hitting the film is less. Thus less need for center filter. But there usually is falloff in most all lenses in all formats, the degree depends on the aperture and focusing distance.