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Center Filters
For Large Format Lenses

by Daniel W. Fromm

Introduction

While trying to understand how Horseman exposure meters would perform with wide angle lenses,
(please see www.galerie-photo.com/horseman-4x5-exposure-meter.html, English;
www.galerie-photo.com/posemetre-calculateur-horseman-pour-chambre.html, Français) I had to think about optical vignetting, the natural fall in illumination off-axis that afflicts all lenses. For most LF lenses illumination falls off with cos⁴ of the angle off axis. Some modern wide angle lenses have somewhat less falloff but even for them cos⁴ is a reasonable approximation.

I realized that my 58/5.6 Grandagon and 65/8 Fujinon, both usable, to my taste, on 2x3 without center filters, would need them when used on 6x12. I found much confusion and error in discussions on the ‘net about which center filter to use. The problem is especially acute for Fujinon and Nikon wide angle lenses; Fuji and Nikon didn’t make center filters or suggest which to use on their w/a lenses. There’s also no information about CFs for early Grandagons such as my 58/5.6.

Figure 1: An example of a center filter,
Rodenstock 4X filter for the Apo Grandagon Series

1  Schneider, Rodenstock and Heliopan
center filters

To find out which CFs might do for my lenses I made a list of Schneider and Rodenstock wide angle lenses and CFs. Schneider and Rodenstock agree that lenses which cover 100° to 105° should use + 1.5 center filters. They also agree that lenses which cover more than 110° need + 2. They disagree about 110° lenses. Schneider recommends + 1.5 for them, Rodenstock recommends + 2. The table below (table 1) summarizes what I found and puts Fujinon and Nikon wide angle lenses in the Schneider/Rodenstock context.

Table 1: cos⁴ loss of illumination, center to corner, for modern extreme w/a lenses

The tables below (tables 2, 3 and 4) list Schneider, Rodenstock and Heliopan CFs by the manufacturers’ designations, can be used to a find CF that will fit a lens and has the right exposure correction.

Table 2: Schneider center filters

Table 3: Rodenstock center filters

Schneider and Rodenstock center filters aren’t particularly lens-specific, their makers’ propaganda notwithstanding.

Table 4: Heliopan center filters

I’ve seen comments about center filters’ +1.5 and +2 markings to the effect that these are adjustments to exposure times. Schneider and Rodenstock literature on center filters make it very clear that the markings refer to f/stop adjustments to be made when using a center filter. The corresponding adjustments to exposure times are 3x (1.5 stops) and 4x (2 stops).

Some filters are marked ND 0.45 instead of + 1.5 or 2x. They’re equivalent. Similarly, ND 0.6 is the same as +2 and 4x and ND 0.9 is equivalent to +3 and 8x.

2  Suggestions for Nikon and Fuji wide-angle lenses

Here are my suggestions for Nikon (table 5) and Fuji (table 6) wide angle lenses that follow Schneider and Rodenstock recommendations for similar lenses:

Table 5: Suggestions for Nikon wide angle lenses

Table 6: Suggestions for Fuji wide angle lenses

My 58/5.6 Grandagon has 67 mm filter threads. It is reported to cover 135 mm, 140 mm and perhaps even 4x5 (150 mm) with absolutely no movements, i.e., no more than 105°, so should have a 1.5 stop center filter. Schneider’s Center Filter III, Rodenstock’s + 1.5 E67/86 Center filter and Heliopan’s 67mm ND Center Filter 3X should all work properly on it.

When I started this exercise I had none of these, did have a Rodenstock AR Center Filter + 2.5 E67/86. This is an early center filter for Apo Grandagons, is functionally equivalent to the more recent +2 E67/86. Some users say it performs worse than the newer + 2.

It fits my 58 Grandagon but the Schneider/Rodenstock rule suggests that it won’t give the best results. With a 58 mm lens 6x12’s corners are 2.2 stops down from the center. My +2.5 should give slight overexposure in the corners, the newer +2 should give slight underexposure.

My 65/8 Fujinon has 52 mm filter threads. It covers 105° so a 1.5 stop center filter is right for it. Schneider’s 52 mm Center Filter II and Heliopan’s 52 mm ND Center Filter 3X should work. My 47/5.6 Super Angulon’s 49 mm CF II would work too if it could be attached. Unfortunately off-the-shelf step-down rings are too long. SKGrimes advised me that a short custom threaded bushing is at best marginally possible, suggested that I get a 52 mm CF II. So I did. Mounting aside, it seems to be identical to the 49 mm CF II.

3  Historical note

That extreme wide angle lenses need center filters to even out exposure across the field has been known almost since the first such lens was made.

The best known early center filter is probably the “star,” also called spinner, supplied with f/18 Goerz Hypergons, which cover as much as 140°. The star is a mechanical center filter that is denser towards the center than at the edges.

For a fuller explanation of the Hypergon, see
http://web.archive.org/web/20161204154713/
http://www.cameraquest.com/hyper.htm

http://web.archive.org/web/20160506045622/http://matmarrash.com/blog/
2015/5/12/just-one-more-lens-i-promise
has a clearer image of the star in place, ready to be spun.

Rodenstock made an optical center filter “Enixantos” for their Pantogonal extreme wide angle lenses, which cover 125 - 130°.

A 1912 catalog (see www.pacificrimcamera.com/rl/00718/00718.pdf) says:

For time exposures, when the full angle is used, it is advisable to use the Enixantos Compensator which is furnished with the lens. This consists of a plano-convex lens of yellowish green glass which absorbs very strongly blue and violet light, and a plano-concave lens of a colorless, very transparent material. The optical constants of both these lenses is exactly equal so that the cemented lenses act as a plane parallel plate which is dark in the center and transparent at the sides. The effect of this is that the marginal rays which pass through the lens at great angle undergo only a slight absorption, and therefore any marked decrease of light toward the edges of the plate is prevented. Thus the necessity of bunglesome and complicated appliances usually found on extreme angle lenses is obviated. With orthochromatic plates the Compensator has the added advantage of giving excellent color values.

“Bunglesome and complicated appliances” refers to the Hypergon’s star.

Appendix: Rodenstock and Schneider Wide Angle Large Format Lenses With Recommended Center Filters

Table 8: Schneider Wide Angle Large Format Lenses With Recommended Center Filters

¹these may be errors in Schneider’s center filter list. They’re inconsistent with recommendations for all other lenses with similar coverage. Documentation of CFs for Apo Digitars is sparse. The information I present on them is certainly incomplete, may be incorrect.

Steve Goldstein pointed out to me that in addition to the CFs listed above (table 8) Schneider made two marked, respectively, 8/65 and 8/90. He believes that they are denser, probably +3, than the newer CF III and IV, both +1.5, for 65/8 and 90/8 Super Angulons.

Thanks to Steve Goldstein for bringing the existence of a 52/77 1.5x center filter to my attention. My source for information on Heliopan’s center filters,
https://static.bhphotovideo.com/FrameWork/Product_Resources/
SourceBookProPhoto/Section04LgFormatLenses.pdf
may be incomplete or incorrect.

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Dernière modification : 2017