Why do we need center filters?

[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

[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.

[Jim Andrada]

Thanks - I thought it was something along these lines but as a marketing guy instead of a technical guy these last 40 years or so I only deal in high level generalities. Or put another way, as a former Chem/Physics major and software developer from the late 50"s (yes we had computers!) I have a fundamental understanding of optical principles but don't know sh-t about practical real-life details. This of course may be one of the defining differences between physicists and engineers or between, say, Harvard and MIT.

Education is what remains when you've forgotten everything you learned in college. Can't remember who said it, but then again I don't remember much anymore anyhow.

Better to have forgotten what happened than to have remembered what didn't.

[Drew Wiley]

Some of what is or is not avail in center filters is really related to marketing. Serious architectural photographers of the past were most likely to use LF, so the appropriate view
lenses had center filters made for them, and appropriately marketed. Lots of MF and 35mm
lenses have serious falloff issues too, but it simply wasn't a traditional custom to use CF's
for journalist work etc. You certainly can if needed, if you find something appropriate. Now that panoramic formats are a bit popular, filters get specified.

[Bob Salomon - HP Marketing]

"Some of what is or is not avail in center filters is really related to marketing. "

Since I am involved in the sales of Rodenstock, Schneider and Heliopan center filters and Rodenstock and Schneider (for Linhof) lenses I will add my two cents.
Hogwash!
The vast majority of wide angle lenses from 23mm up are sold without a center filter and center filter sales are far from equaling lens sales. And that includes the sales of the 58, 72 and 90 SA XL and 80mm XL on Linhof Technoramas.

When we do sell center filters it is usually after the shooter has used the lens for what they intend doing. When a shooter orders a center filter at the same time as they buy the lens it is usually an experienced shooter who knows and appreciates the difference with and without a CF.

Modern wide angle lenses have fall off. That fall off is controllable with a center filter. Fall off is also correctable with lighting as well as with dodging and burning. What you shoot, how you shoot, what you shoot on will all play a part in how, or if, you use a center filter or another correction technique. Other lenses also fall off. Just not enough to require or need a CF. And I believe even Leica also offered a center filter for some of their 35mm lenses in the past and in the further past I think they even had a lens with a propeller to even out the results.

[Leigh]

Hi Guys,

Thanks for all the replies. Some interesting information.

Other lenses also fall off. Just not enough to require or need a CF.

This is really the thrust of my question.

For a given angle of view, the ratio of the peripheral ray length to the focal length (i.e. the cosine) will be the same.

So why does the effect appear more pronounced for LF lenses vis-a-vis those for smaller formats?

- Leigh

[rdenney]

For a given angle of view, the ratio of the peripheral ray length to the focal length (i.e. the cosine) will be the same.

Not so. There is nothing that demands that a lens have the same angle of view on both sides of the glass. That's what I meant by marginal rays not being straight (or even parallel), except through a simple lens. With complex lens designs, it's completely possible for the angle subtended by the subject in front of the lens to be different from the angle subtended by the image of that subject behind the lens.

That's why I used the fisheye as an example--it's extreme enough to easily visualize. I have a 30mm fisheye that provides a 180-degree angle of view in front of the lens. That range of subject material makes an 80mm circle on the film. If it subtended the same angle as the subject (180 degrees), the angle of light approaching the film would have to be zero, which is impossible, if the marginal rays were straight (or parallel). That's why a fisheye lens is a modern invention--it's a derivative of the retrofocus design.

In fact, a fisheye uses the same general design strategy of a retrofocus lens, with de-magnifying elements in front of something like a gaussian lens. It is just isn't designed to magnifiy the edges to provide rectilinear correction as rectilinear lenses do.

The rear of that 30mm fisheye is a 30mm filter. And that rear surface is at least 60 mm in front of the film. The crossing point of the rays is in front of that rear filter, but even if it was between the rear of the lens and the film, it could not be closer than about 40mm to the film, unless light rays bend. Thus, the light is approaching the film at most roughly 45 degrees, even though the subject material forming those rays is at right angles to the lens axis.

Example image:

There is darkening in the upper corners of this image, mostly the result of the veiling flare in the middle of the image and the natural polarization of the sky at right angles to the sun. But there is no darkening in the lower corners. The field of view of this image is 180 degrees on the diagonal. There was no center filter used (obviously), but also no correction for falloff. A center filter was not needed here.


Dirty Devil River, Inundated, Glen Canyon, 2001

In contrast, this image shows much less field of view (although it is still extremely wide--a 47 on 6x12), but the approximately symmetrical Super Angulon did have nearly parallel marginal rays, and the light approached the film at a much shallower angle. The Super Angulon minimizes this, by the way, by presenting a very round aperture image even at the edges of coverage, assuming a small enough aperture to eliminate mechanical vignetting. This image goes right to the edge of coverage, and the falloff was about four stops. I did not use a center filter, and applied very strong post-processing after scanning to correct it as best I could, but a significant chunk of the subject brightness range fell off the bottom of the Velvia characteristic curve and the contrast just increased. I had to leave the corners dark just to keep it from getting any weirder.


Fort Niagara, 2009

Rick "who learned this the hard way in an argument several years ago over lenses designed for use with digital sensors" Denney

[Leigh]

Not so. There is nothing that demands that a lens have the same angle of view on both sides of the glass. That's what I meant by marginal rays not being straight (or even parallel), except through a simple lens. With complex lens designs, it's completely possible for the angle subtended by the subject in front of the lens to be different from the angle subtended by the image of that subject behind the lens.

Hi Rick,

Yes, of course the subject side and the image side can be different.

The parameters I gave in the post that you quoted are "peripheral ray length" and "focal length",
both of which only apply to the image side of the lens.

Perhaps my choice of "angle of view" to describe the image-side geometry is imprecise or incorrect.

- Leigh

[Bob Salomon - HP Marketing]

Hi Guys,

Thanks for all the replies. Some interesting information.


This is really the thrust of my question.

For a given angle of view, the ratio of the peripheral ray length to the focal length (i.e. the cosine) will be the same.

So why does the effect appear more pronounced for LF lenses vis-a-vis those for smaller formats?

- Leigh

This link will take you a brochure that has the fall-off graphs for every current Rodenstock large format lens from 23mm to 360mm.

http://www.rodenstock-photo.com/medi...1-75__8347.pdf

This link will give an explanation of center filters

http://www.rodenstock-photo.com/medi...Engl_10184.pdf

[Oren Grad]

This link will take you a brochure that has the fall-off graphs for every current Rodenstock large format lens from 23mm to 360mm.

http://www.rodenstock-photo.com/medi...1-75__8347.pdf

Sorry for a tangential nitpick, but this catalog, very useful though it is, doesn't have detailed data for anywhere near the entire line-up. There's full data for at least one lens for each lens series, but not for evey focal length. The digital lens series are fairly well covered, but few of the LF film-oriented focal lengths are included.