Custom Center Filters Printed On Clear Gelatin

Old-N-Feeble · Oct 12, 2014

Jim Jones said:
I exposed film through a wide angle pinhole camera and developed it for a gamma of about one. Mounted near the film holder it was good enough for pinhole photography.

Are you saying you made a center filter by exposing film to a continuous tone image then processed the film then you use that as a center filter near the film holder? The scattered light through the film base and emulsion doesn't affect image quality?

OldBikerPete · Oct 13, 2014

If anyone wants to print a centre filter on an inkjet printer, some time ago I wrote a program to create a BMP image file with the appropriate density fall-off curve from centre to edges. There are two versions, one to compensate for lenses and one to compensate for pinholes.
Both are written to be compiled and used as command-line programs on Linux. I attach them here for your use if interested. I have had to rename them from '.c' to '.doc' to get then through the input filters here. I wanted to include a couple of masks I prepared earlier but they are all for 5x4 and about 10x the allowed size limit for BMP's.

Jim Jones · Oct 13, 2014

Old-N-Feeble said:
Are you saying you made a center filter by exposing film to a continuous tone image then processed the film then you use that as a center filter near the film holder? The scattered light through the film base and emulsion doesn't affect image quality?

T-Max film has a smoother emulsion surface than some older films. It seemed to work fine in a pinhole camera. I used it in a full frame 35mm camera with a 16mm pinhole. In that camera the filter was about 16mm in front of the pinhole. If the film center filter is quite close to the negative, scattering might not be conspicuous. It could even be loaded in contact with the raw film in some film holders, although this might lead to Newton's rings unless the center filter is made on film with a less glossy emulsion than T-Max.

Old-N-Feeble · Oct 13, 2014

OldBikerPete said:
If anyone wants to print a centre filter on an inkjet printer, some time ago I wrote a program to create a BMP image file with the appropriate density fall-off curve from centre to edges. There are two versions, one to compensate for lenses and one to compensate for pinholes.
Both are written to be compiled and used as command-line programs on Linux. I attach them here for your use if interested. I have had to rename them from '.c' to '.doc' to get then through the input filters here. I wanted to include a couple of masks I prepared earlier but they are all for 5x4 and about 10x the allowed size limit for BMP's.

That's amazing, Pete. I could install MS Office but, unfortunately, I don't run Linux.

How much, if any, is the image quality affected with those filters?

Old-N-Feeble · Oct 13, 2014

Jim Jones said:
T-Max film has a smoother emulsion surface than some older films. It seemed to work fine in a pinhole camera. I used it in a full frame 35mm camera with a 16mm pinhole. In that camera the filter was about 16mm in front of the pinhole. If the film center filter is quite close to the negative, scattering might not be conspicuous. It could even be loaded in contact with the raw film in some film holders, although this might lead to Newton's rings unless the center filter is made on film with a less glossy emulsion than T-Max.

That's very interest, Jim. I wouldn't have thought that would work at all. I learn something new every day. Unfortunately, I nearly always forget the following day.

DREW WILEY · Oct 13, 2014

I'm wondering if anyone has learned to do this precisely yet, or just in some fun artsy/craftsy sense. I mean something color-neutral and durable. My own range of potential application would be to even out light in an enlarging diffuser, rather than attempting to use in actual image-formation applications relative to lens position, which is something I'm very very skeptical about in terms of DIY.

Old-N-Feeble · Oct 13, 2014

DREW WILEY said:
I'm wondering if anyone has learned to do this precisely yet, or just in some fun artsy/craftsy sense. I mean something color-neutral and durable. My own range of potential application would be to even out light in an enlarging diffuser, rather than attempting to use in actual image-formation applications relative to lens position, which is something I'm very very skeptical about in terms of DIY.

Understood... but if one can get a sharp evenly exposed image on film it's a better procedure because the entire frame is exposed in the film's central "sweet spot". This is very important for films with very limited exposure latitude. Gels are not known for their durability though. The latter stated, many gel filters can be made on an inkjet vs. buying one factory-made glass CF.

DREW WILEY · Oct 13, 2014

True gels are easily kinked and scratched and do affect image quality a bit, at least compared to modern multicoated glass filters. Polyester "gels"
affect sharpness even more, as would virtually all dimensionally-stable transparent inkjet substrates, while acrylic resin gels can be pretty good in that respect but are prone to reflections. I don't know if there are any appropriate inks that will stick on acrylic; but there would be a dot pattern
regardless. Using something like this in proximity to the film at full size if of course going to have less an over the lens for purposes when the film must be enlarged. There are such things as customized true center-grad filters, but not something exactly affordable unless you happen to have a NASA or NSA credit card.

Old-N-Feeble · Oct 13, 2014

Pro-made optical quality gels, if undamaged, are still almost completely optically invisible. They're still the very best you can buy. What I don't know is how much, if any amount, printing CF on an inkjet affects optical quality.

DREW WILEY · Oct 13, 2014

Contact printing is one thing, enlargement another. Like I said, any polyester film base meeting the requirements of being both transparent and
suitable for ink printing is going to be less than ideal optically. Acetate is even less desirable. Large true gels are infamously expensive, even when consistent ND. I can't imagine the cost of doing it custom. Might as well do it with dyes in acrylic resin, or via vacuum deposition on glass. But maybe some optical supplier already has something like this laying on the shelf, just gathering dust, left over from some military project or whatever, willing to part with it cheap. Never hurts to ask. Over the years I have gotten some ridiculously expensive things (to make in the first place) downright free that way. Do some sleuthing.

David A. Goldfarb · Oct 13, 2014

Has anyone just tried it? It wouldn't be that hard to print up a few 4x4" filters with different gradients and densities on a sheet of transparency film and see what works. Of course there will be optical imperfections, but maybe they won't be bad enough to worry about.

Putting a dot pattern in front of the lens, for instance, is one way of making a diffusion filter, but those dots are fairly coarse relative to an inkjet printed transparency. That close to the front of the lens, the dots are going to be way out of focus, so they might not matter, just as bubbles in the glass or light lens scratches have very little effect on the image.

One issue is that OHP tends to be a little milky, so that the printer can detect it. Perfectly clear OHP sometimes needs a backing paper for the printer to recognize that it's there, and it's OK to print.

Old-N-Feeble · Oct 13, 2014

Right, and what about using optical grade gelatin sheets? If unavailable just use an existing Wratten clear or UV gel filter... and at 1400dpi or better. Had I an inkjet and proper software I'd try it myself and report my findings. I just don't want to spend $1000+ just to experiment.

OldBikerPete · Oct 14, 2014

Old-N-Feeble said:
That's amazing, Pete. I could install MS Office but, unfortunately, I don't run Linux.

How much, if any, is the image quality affected with those filters?

It depends on how high-res you print them at and where you fit them in your camera:

If you assume 60lp/mm res from your lens and film combo, that requires 15240x12192 pixels, or 3048x3048 DPI for 5x4 film if you are fitting the filter at the film plane, because the filter will be in focus and any pixellation in the filter will be faithfully reproduced on the film.

If, however, you are fitting the filter at the normal place - the filter ring - it will be way out of focus and a relatively low DPI - say 600 or even 300 - suffices with no pixellation - none - visible in your image.

In a pinhole camera, I'm not entirely sure what you'd do as I haven't tried this out. I'd begin by arranging to have the filter stand between the pinhole and the subject somehow in the hope that it will, again, be out of focus.

The program generates an image which goes from transparent at the corners to totally black at the centre, your graphics printing program has to fiddle with the print density to get the effect you need.

Peter.

Dan Fromm · Oct 14, 2014

OldBikerPete said:
The program generates an image which goes from transparent at the corners to totally black at the centre, your graphics printing program has to fiddle with the print density to get the effect you need.

Interesting. Center filters from, e.g., Rodenstock and Schneider are clear at the edges, typically two or so stops denser at the center. They aren't totally black at the center.

DREW WILEY · Oct 14, 2014

This is kinda like duck hunting with a fly swatter. If you need a real center filter, why not just buy one? I have no idea what they'll do with a
pinhole aperture, but you'd have to measure your density falloff at the film plane anyway. Or make one of those little spinning propeller devices like they once used for hyper-wide angle lenses.

Old-N-Feeble · Oct 14, 2014

DREW WILEY said:
This is kinda like duck hunting with a fly swatter. If you need a real center filter, why not just buy one? I have no idea what they'll do with a
pinhole aperture, but you'd have to measure your density falloff at the film plane anyway. Or make one of those little spinning propeller devices like they once used for hyper-wide angle lenses.

I already have matching CF for the lenses for which they're made. This thread is about making CF for which none are made and for those people who don't want to, or can't, spend the funds on factory CF. Also, this isn't about pinhole photography... that was just mentioned in a couple of posts.

I'll add this though: Factory CF are designed to work properly within a very limited aperture range. I suspect custom filters could be made to allow use with smaller and slightly larger apertures.

DREW WILEY · Oct 14, 2014

But a relatively large sheet of true Wratten gel material will probably cost as much as an official center filter! And I'm certain you'd need more
than one piece for the sake of the learning curve. It's not like some of us haven't tried all this before!

Old-N-Feeble · Oct 14, 2014

Drew, one can buy a 3x3 or 4x4 inch clear or UV Wratten gel filter for as little as $10US (more likely $20) and black inkjet ink is cheap. A genuine Schneider CF for a 90mm SA will cost at least $200 on the used market. Others cost far more.

John Koehrer · Oct 14, 2014

Quick & dirty. UV filter with random spots from a Sharpie.

Obviously the Sharpest of the lot.

DREW WILEY · Oct 14, 2014

If anyone knows it would be the airbrush crowd, but that would still involve opaque paint or ink. It would be easier to use scrim material, but that
sounds like a task for a jeweler, or at least, don't drink coffee that day! If it were me I'd look up one of the local hi-tech optical lab and try to trade some service with them, cause no way I could ever afford their official services. So that leaves just two realistic choices, either put on a
black ski mask and rob a bank for funds sufficient for a project like this, or stretch the mask fabric over the lens itself and hope it provides the
correct neutral density!

Old-N-Feeble · Oct 14, 2014

Thanks, Drew, but airbrushing would be far too difficult and imprecise. Anyway, what makes airbrush ink any less optically damaging than inkjet ink? By ski mask I can only assume you mean pantyhose because a true ski mask is opaque. Stretching pantyhose over a lens just makes a SF filter.. okay for female portraits... sometimes.

BetterSense · Oct 14, 2014

Why the fascination with ink?

Why not litho film? Is tmax 100 base not clear enough in 120 or 4x5?

Old-N-Feeble · Oct 14, 2014

BetterSense said:
Why the fascination with ink?

Why not litho film? Is tmax 100 base not clear enough in 120 or 4x5?

It's not the just the ink... it's the optically superior gelatin base. Gel filters, in pristine condition, are superior to any other type of filter. What we don't know is how inkjet ink affects gel filter optical qualities.

BetterSense · Oct 14, 2014

Is gel really that much better than Estar or cellulose acetate film base?

Old-N-Feeble · Oct 14, 2014

BetterSense said:
Is gel really that much better than Estar or cellulose acetate film base?

Yes, it's at least somewhat better but more importantly, the gelatin might absorb the ink better.

Custom Center Filters Printed On Clear Gelatin

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