Filters for Large Format Photography

By Q.-Tuan Luong for the Large Format Page

Summary: a market survey and collection of tips on filters from the perspective of the large format photographer, including rear mounted filters, center filters, polarizers, and rectangular filter systems.

Attaching filters at the rear of the lens

With LF, you have the option to attach filters at the rear of the lens. This works for color correction filters which don't need to be adjusted. The advantage is that a filter inside the camera is less likely to get dust, dirt, smudges, fingerprints, and will cause less flare.

There are two precautions to take:

Center filter

Principles

For most lenses, the illumination will go as cos^4 theta where theta is the angle between the center of the lens and the image point on the film. This is called Light fall-off. It's basically the result of the law of optics. For wide-angle lenses, this can result in the corners being two or three stops darker than the center.

Two powers come from the light traveling a greater distance and the inverse square law, one power comes from the exit pupil of the lens being tilted and looking like an ellipse rather than the full circe, another power comes from the rays striking the film at an angle [the same reason for winter: the sun's rays strike the earth at an increased angle]. The best aperture re light fall-off is any aperture more than about 2 stops (maybe 3) down from wide open. Further stopping down should not change the relative illumination, center to corners. This is well explained in the book Applied Photographic Optics by Sidney F. Ray, published by the Focal Press. Excellent but rather expensive. It is a big book, more than 500 pages. Michael Briggs

This is different from vignetting, caused by mechanical obstruction. At wide apertures, lenses will vignete because the glass elements aren't big enough (if they were made larger, the lens would be more expensive and heavier).

Most wideangle lenses from SLR cameras are retrofocus (retro-telephoto) designs, meaning that the nodal point is somewhere towards the front of the lens, in some cases in front of the lens. This is necessary so that the lens does not protrude into the mirror box area. Retrofocus designs decrease fall-off, which is why those lenses don't require a center filter. On the other hand, Mamiya suggests a center filter for their 43mm M7 lens, which has the distinction of being symmetrical thanks to the RF camera. I don't know of a retrofocus lens for LF. Retrofocus designs are asymmetric and require floating elements for good performance at close range.

The center filter is a simple solution to the light fall-off problem. It is darker in the center and gradually brighter at the edges. Center filters always are mounted in front. They must be screwed directly into the lens without using a step up ring, so you need one per filter size of lens. This is because center filters are designed to be used a certain distance from the lens. You need to remember that it can not be seen by the film until the lens has been stopped down at least 2 stops.

Do I need one ?

The wider the lens, the more of the full coverage of the lens you use (ie using 5x7 with a 110mm vs using 4x5, or using 8x10 with the 150mm vs using 4x5 with a plasmat 150/5.6 ), or the more movements you make, the worse the effect. Most people find that light fall-off is neglectible unless you start using a 90mm on 4x5. Even though, few shooters would use a CF at 90mm on 4x5. On the other hand, most find it necessary at 90mm on 5x7 or at 58mm on 4x5.

Whether you feel the need for correction is an esthetic decision. Some people actually like the fall-off. It's less noticeable for dark scenes than for bright scenes. It's less noticeable if your scene has dark areas in the corners (like trees). For landscapes, it is most noticable in skys and tends to pull the eye toward the center. It seems to be more bothersome in interiors, particularly white walls.

For the more even image, you pay quite a price. Not only these filters tend to be expensive, they eat at least two f-stops and add complexity to calculating exposures. Since they are mounted directly on the lens, and have larger front elements (for example a 67 center filter has an outer threading of 86mm) they can make difficult to use of other filters without vignetting. You could correct for the fall off in the darkroom.

Which density and brand to use ?

The filters are not matched to a particular lens, so at least you can interchange them between two lenses if they have the same filter size. The difference between having a CF and not having one is quite relative (some photographers are happy without, while some think that they are necessary), thus you'd guess the difference between two different brands is extremely subtle. Center filters are made by B+W/Schneider, Heliopan, Rodenstock, Hoya and Fuji. Prices can vary considerably depending where you buy them.

" We stock some of the denser Heliopan CF and have sold virtually none. You would lose so much light (3 stops) + you must stop a lens down at least 2 stops for the filter to work that the denser version is not too useable for most people. Virtually every CF sold is either .40 (Rodenstock) or .45 (Heliopan and others). I don't think you will find many people, if any, who have actually used all 3 [B+W/Schneider, Heliopan, Rodenstocks] on the same lens with the same film at the same time [and can compare them]. We are in a unique position. We are the Heliopan and the Rodenstock importer and also the Linhof importer. As such we import the Heliopan and Rodenstock ceter filters as well as Schneider ones for the 58 through 90mm XL lenses we sell. An extremely knowlgeable photographer named Lief Erickson (tragically died too early) too had this question. So he took the Linhof Technorama 617 and the old Fuji 617 with a Schneider, Rodenstock and heliopan center filter for a test. He found no difference on film between them. However that was before Rodenstock redesigned their center filters. No center filter is totally neutral. They tend to shift torwards green under cetain conditions. The latest version of the Rodenstock ones are the most neutral of all center filters. " Bob Salomon

Polarizer

The main point of using a circular polarizer instead of a linear polarizer is to ensure that autofocus and TTL metering would work properly with your reflex camera (although this might not always be necessary). If you intend to use your polarizer also on a reflex camera, it's probably safer to get the circular variety. Since a LF camera has no mirror, you can use either a linear or a circular polarizer on your LF camera. Linear polarizers are cheaper, and some people think they are better at removing reflections than circular polarizers.

A Kaeseman polarizer is sealed, and therefore more durable (the foil on other polarizers tends to degrade over time). It is also said they are better at removing reflections.

To avoid vignetting, you can use a slim polarizer (no threads for a clip-on lens cap or an additional filter), or a wide-angle polarizer which has a larger front element.

Many polarizers tend to have a cold color bias. It's generally better to have a warm color bias than a cold color bias, hence the interest of the warm tone polarizers. In a C&D article, Englander compared ten linear and circular polarizers. His densitometer readings are (total,red,green,blue): 

 Wratten  (.6,.62,.62,.62)
 B+W Warm  (.56,.57,.57,.58)
 B+W circular  (.51,.55,.52,.48)
 Heliopan circular  (.58,.64,.58,.53)
 Heliopan linear  (.62,.69,.63,.57) 
 Heliopan Warm  (.60,.60,.61,.56)
 Tiffen linear  (.50,.52,.49,.50)
 Tiffen circular  (.51,.50,.52,.47)
 Hoya circular  (.54,.56,.55,.53)
 Hoya linear  (.45,.47,.45,.43).
 Tiffen Warm  (.62,.58,.62,.62)
He concluded that the most neutral filter was the B+W Warm. The Tiffen Warm is linear. Hoya has subsequently introduced a Warm polarizer (called the Moose filter, after the so-named wildlife photographer) in circular version, which is relatively economical.

Englender also found the Wratten .6ND filter to match the density of the polarizers well. This confirms the rule to use a filter factor of two stops for a polarizer.

UV filter

Some folks like to use them permanently for protection of the front element of the lens, just like in 35mm. Image degradation would be neglectible, especially considering the enlargement ratios needed in LF. Others find that in LF, accidental damage is unlikely. They are definitively useful at high altitudes to remove excess UV.

Step up rings

You can buy circular filters (glass) to fit your largest lens, and then a set of step-up rings to fit your other lenses. The main problem with this approach is that step-up rings put the filter further from the lens, which might actually increase vignetting on wide-angle lenses. For instance, I find I get more vignetting on my 110 with a step-up ring from 67 to 82 than with a plain 67 filter. Filter systems (see next) work on the same principle (one set of filters, one ring for each lens), but are more versatile. The only advantage of using step-up rings would be that they are cheaper than the adaptor rings used in most filter systems.

Filter systems

Filter systems are by far the most versatile way to use filters for a set of different lenses. You only need to get a different adaptor ring for each lens. You have a huge choice of filters, can adjust them independently in rotation and translation (making this kind of system almost mandatory if you use graduated filters) and combine them easily. Some systems also include a lens shade. Their main drawbacks are the bulk and set-up time required for the system. Both factors are not a significant aggravation in LF, hence their popularity with LF users.

A filter system has three components:

There are three widths of holders that are usable with LF:

Cokin

The 100mm (4 inch) standard: Hitech, Lee/Calumet, Sinar

The filters are extremely similar in optical quality, the only real difference being the holder system and the thickness of the filter. The quality is very high. They might be actually made at the same plant. Hi-Tech has the best selection and the best prices (notably cheaper than Lee) for filters. The holders are well built, however they are heavier, bulkier, and considerably more expensive than the Cokin. The advantage is having larger filters, and a more configurable holder.

Other manufacturers

Singh Ray produces high quality and expensive resin filters to fit other manufacturer's holders. Their hard-step GND has a sharper transition than other manufacturer's equivalent. They also offer a few specialty filters, like a reverse GND and a strip, and will consider making custom filters. Not all their filters are listed on their web site. Their filters fit both the Cokin P system (grad: $100, Pol: $160), and the 100mm system (grad: $150, Pol:$340). Their polarizer for the 100mm system is a 100mm x 100mm square (like the Lee).

Wide angle lenses

Using additional filters on wide angle lenses present some unique problems, because of the wide angle of coverage, large front element, and necessity (for some) to have a center filter. SK Grimes adapter can help and is much cheaper than custom adaptors made by Lee. Larry Huppert is developing a custom system based on 100mm filters.

More information

Rodenstock has a 2-page brochure about the use of center filters. In the US, you can request it from info@hpmarketingcorp.com

View or add comments