Pentax 6x7, 45mm lens - template for rear mounted gel filter

[Neil Grant]

...so, now i have a super-wide 45mm lens for my inherited P67. The 82mm front filters would be very expensive so I shall try out the rear mount. I' m thinking of just cutting the gel to a rectangle shape so it will slip under the circlip with an excess sticking out to aid removal. Not very elegant, any better ideas?

 

[DREW WILEY]

These aren't even optically designed for rear filters. Only a few really long P67 lenses are. Even if you could figure this out without ruining something inside, the image quality would be significantly degraded. Any 82mm filter is going to be a lot less expensive than a repair or another camera body when something inevitably comes flying off and gets stuck where it doesn't belong. Just bite the bullet and start with a couple of good multicoated round glass ones like Hoya makes. They'll be the right size for the superb 75/4.5 and 300 teles too, and via a step ring can be used for the 55 focal length.

 

[abruzzi]

I believe there is a clip on the back on the lens to hold a gel filter. You can see in this image the ring, held on by two screws on one side:

there is a raised portion on one side that you can lift up to slide the gel under. I don’t know how it optically affects the lens, but it was something Pentax designed for.

 

[DREW WILEY]

I'd think twice about that. Doesn't make sense. That looks more like part of the dust-seal system, something you don't want to tamper with. Or else it's a retrofit; why is there a mismatch with all the other screws? (shiny hardware store pan head rather than black recessed - egads! - potential reflections in the image). Any any hypothetical gel would have to be cut down to fit. Nowhere in their lens manuals do they mention internal gel usage. They did market a gel holder for the FRONT of their lenses. By the time the 45 was designed, even using gel filters was getting as arcane a selling leashes for a pet Triceratops. But if someone wants to risk their camera trying that, so be it.

New gels are themselves by no means cheap. There are a lot of cheap used ones or "new old stock" out just a few flavors, many quite possibly faded due to age or improper storage. They're fragile, attract dust, and are hard to clean. Substitute polyester filters are optically inferior, and acrylic resin filters are thick and expensive. You could end up spending more on a flat tire than a brand new one, if you get my drift. Real glass threaded 82's aren't terribly expensive in the overall scheme of things. Deals pop up on those too. There's a fellow across town from me who is a filter liquidator, and has all kinds of odds and ends in stock at reasonable pricing, including 82's. He's at filterfind.net.

You also have to think about it often being harder to focus a WA lens like this with a contrast filter already in position, rather than screwing it in right before the shot. I own all kinds of gels relative to studio and lab application, but never in the field.
There are logical reasons for that. Why turn something simple,basic, and proven into a what-if nightmare?

 

[abruzzi]

thats definitely not a hommade retrofit. Its on all the 45's I've seen. Just go to ebay and look at the dozens for sale, most have a shot of the rear, and most give you a good view, and the filter holder is clearly visible in all of them. Again, I'm not saying its a good or bad idea, just that it is highly unlikely to damage the camera unless you put a 4" square gel in there and don't bother to cut it down.

 

[cramej]

 

[DREW WILEY]

Oh, the fisheye is a very early design, and perhaps their worst optically, but does have built-in contrast filters.The 45 is one of their latest. But thanks for posting the details and correcting me. Not a single P 67 lens of my own, or that I've ever personally seen, has anything like that. But I don't own a 45. Otherwise, no, you're not going to get a 4 inch square gel inside, not even a 2 inch square one. The lens rear simply isn't wide enough. You'd have to cut something way down. Hokey, nonetheless. Seeing that, I'm glad I never bought a 45. If the screws are shiny, a simple black Sharpie pen will solve that for awhile. What their reasoning must have been, is that because of the extreme angle of view in both these instances, the front thread would have been ideally used for a step ring to an even larger sized filter, like a 95, to prevent mechanical vignetting at the edges of the field wide open, just like the wording implies, but somewhat ambiguously, referring just to polarizers, which are not consistent across wide fields of view anyway! I would prefer just to use an 82 front filter and never shoot relatively wide open, and never even bother with a polarizer.

And you've got quite a bit of illumination falloff to contend with too. With my 55, sometimes I'd borrow my Schneider 82 mm center filter from my large format kit, which itself is considerably wider than the rear thread. Falloff is always worst at wider apertures anyway. Pretty dim viewing with any of the wides if a contrast filter is already in place; but the wider they are, the greater the issue. Only the expensive 75/2.8 offers a relatively bright maximum aperture. Not too big a deal if just shooting at infinity; but I always use a tripod for anything closer, and use the clip-on magnifier.

 

[btaylor]

In the rear mounted gel holders I’ve seen and used the gel is cut to fit the holder, careful work with a pair of scissors all that is needed. I believe Lee makes them these days, the last ones I got were from Freestyle. I was getting 4 filters out of one 100mm gel (they don’t last) on the back of the OPF18 20-120mm.

 

[Neil Grant]

...I've got tons of 'wratten' gels 76mm square in all sorts of colours, many unopened. They are ultra-thin and probably what Pentax had in mind with the rear filter circlip on the back of the 45mm lens. For me, it would be an inexpensive way of adding a contrast filter until a glass one can be afforded. The Sigma 12-24 zoom had a similar rear filter clip - and also a handy template for cutting down 'wrattens'.

 

[macfred]

There were P67 filter holders back in the days designed for Kodak Wratten gel filters. Here's one in 82mm for the 45mm. This one is described as 'screw-in' - there were bayonet filter mounts too.
Not sure about those will work with your lens.

https://www.ebay.com/itm/2029999761...DefaultOrganic&_trksid=p2563228.c101113.m2108

 

[DREW WILEY]

True gels are still Kodak Wratten. The thin version of Lee is polyester, a cheap substitute, and more appropriate for lighting rather than lens gels. The better Lee filters are acrylic resin, and thicker. I have a large set of Lee polys for sake of lighting mockups, from back in my architectural shooting days. But I'd never use either poly or true gelatin gels for anything significantly enlarged. Most stock, book, and advertising photography gets printed relatively small, making precise imagery less of an issue for those kinds of commercial photographers than to someone making large detailed prints, especially from a relatively small original like 6X7. Gels also trap dirt and grime fairly easily, and are hard to clean; so image quality itself tends to degrade over time. I now use them only for specialized lab applications when glass alternatives are not available. Certain colors of them are distinctly more expensive than glass filters. But the classic old Wratten handbook is unsurpassed for specifics, so still highly useful for scientific approaches.

I figured out my confusion over the shiny pan head screws in certain examples of the 45. They probably were blackened originally, but that gradually wore off due to fingers fooling around back there, placing or removing gels.

I still have one of the Pentax front-mount gel holders. The old seal material went bad and sticky, and had to be removed with citrus-based solvent. But otherwise, their version is similar to generic ones by Lee and others. I repurposed it, as well as a few gels, for sake of a diffused flashing attachment, allowing me to selectively warm deep shadows in Ektar shots without affecting midtones or highlights.

 

[doctorpepe]

Wratten gels are still a good option. They do not degrade the image at all (as long as they are clean and fingerprint free!). I have even used them in the optical center of many lenses (near the diaphragm) when i plan on using a filter for a lot of shots (such as a #8 when shooting b&w). Having said that, i always keel a uv on the front of my lenses to protect from elements, rain and random bb’s (easier to replace a filter than the front element of your lens).

 

[doctorpepe]

Sample disaster averted with front filter (not mine, image borrowed from web). In a real life story, i was shooting stage photos for an acting troupe with a 20mm lens close to the director. He gesticulated and when his arm came back, his large jade and silver ring hit sqarely on the front of my lens. Shattered the UV, made a mess, but the front element was unscathed. Rest of the shoot went fine and the Director replaced my filter.