Color filters to correct focusing issues with meniscus lenses

[Petzval Paul]

I currently have a few (ok... five) lenses that are uncorrected for chromatic abberations and which, of course, make focusing quite difficult. I am aware of the focusing correcting meant to remedy this persistent problem, but it's still a bit of a crap-shoot, even on the Puyo lenses with the focusing scale (although they definitely help). Somewhere, I seem to remember hearing that using a yellow (I think) filter can bring the chemical and visual focus together.

The science here is somewhat over my head and I was hoping that someone (anyone) with any experience could share with me the best way to go about using colored filters to help save me some film (oh, I should mention that I'm using x ray and l ortho lith film).

Thanks everyone!

[xkaes]

I assume you are using B&W film.

The problem with lens uncorrected for chromatic aberrations is that the various colors are in focus at different distances. If using a panchromatic film, a strong yellow filter will minimize the shorter and longer spectrum of light for focusing purposes, so you end up focusing in the middle. But depending on your lens it's best to focus at the taking aperture -- on a highlight.

[nolindan]

A green or blue filter in conjunction with a UV or skylight filter might be the best bet. A green filter would make the visual and 'chemical' focus coincide and you should be able to focus without the filter. I find blue filters hard to focus through, though the focus shift is likely negligible.

[Drew Wiley]

Redundant. An additional UV filter won't add anything relevant. A slight focus shift sometimes occurs especially with respect to deep red filters, but only with lenses which aren't fully corrected apo-wise. In such cases, do final focus with the filter attached. I have only one such lens, a 300 mm P67 Takumar, where I've learned to set infinity focus between the normal infinity focus mark and the IR mark, or else not use a deep red filter at all, and rather a deep green or orange. With my better corrected 300EDIF, there is no such focus issue with any deep contrast filter,
Nor have I encountered the problem with any of my view camera lenses, which notably do not include any actual teles, but a number of long focal lengths per se, and are all modern.

That being said, IR film would be the most common example of an exception requiring special focus adjustment. But film sag alone in a typical 8x10 holder is an even bigger problem, especially with thinner than normal films or anything acetate based.

[ridax]

Actually, there are two topics here. The first is what filter to use while TAKING the picture and whether to focus through that filter or without it; the second is whether to focus through any (and which) filter for taking the picture WITHOUT any filter at all.

The above answers address the first topic. My own thoughts on it are in the thread about using double-anastigmat halves (which aren't well corrected for the chromatic aberration either):

https://www.largeformatphotography.i...80#post1652380
https://www.largeformatphotography.i...73#post1653073

And on the second topic, i.e. how to focus for a picture taken without any filter:

That certainly depends on the spectral sensitivity of the film used, but still the majority of BW emulsions, including panchromatic ones, are most sensitive to the blue-violet part of the spectrum. That means it is better to focus for those blue rays. So one can put some modern device with a blue LEDs display (my clock works fine) near the subject OR put a blue filter onto the lens or onto the focusing magnifier.

The problem is that human eyes have too low sensitivity to the blue and VERY low sensitivity to violet, and aren't sensitive to UV at all (while BW emulsions are pretty sensitive to the UV, and a lot of taking and enlarging lenses are considerably transparent in the near UV). That means that if the filter is not really blue but just bluish and also transmits some green, the human eye would concentrate on that longer part of the transmitted spectrum, and focusing would take place more or less in the greenish zone. The filters that effectively block the greens are just a few, worldwide - the 39 (3"x3" square only) glass Wratten, the 47 and 47b Wrattens (and the 47 and 47b Tiffens), and the CC-4 and the CC-8 glass filters made in the USSR (I don't mention those that are too dark to focus through). And nowadays, they all are available second-hand only. (There are other colored glasses like the Schott BG-25 but those are not commercially available filters, just glass out of which a filter can by custom made.)

But the most important part of the same problem is that in the dim for the eye blue light, the eye pupil widens a lot, and the UV an IR radiation present in the normal sun or artificial light that is not a problem when the eye pupil is small, enters the widened pupil in far excessive amounts. That's harmful for the eyes (yes it really is; I have experienced the fatigue and pain myself). So the viewing blue filter should also block the UV and IR. NO blue GLASS does, though. So from those listed above, only the 47 and 47b Wrattens/Tiffens are actually suitable for the viewing purposes (though the abovementioned glass blue filters have way less factors for film and thus are much better when one wants to achieve a blue-senvitive-film-look on a panchromatic emulsion). I prefer a 47 Tiffen as it not as dim as a 47b.

I also use my big enough to partially cover both eyes 82mm Tiffen #47 to evaluate how would my subjects look like on a blue-sensitive film.

(The top of the line Peak focusscope uses a glass blue filter. That comes from the era when there weren't much alternatives to incandescent lamps which are poor in UV. Don't use that blue glass with a cold light enlarger head unless you are positively sure the light source has a good anti-UV protector.)

[Petzval Paul]

Thanks, everyone. Ridax: so if I focus through a blue filter while using an Ortho emulsion, what I see on my GG should be the same as what I'll get on film?

[ridax]

Thanks, everyone. Ridax: so if I focus through a blue filter while using an Ortho emulsion, what I see on my GG should be the same as what I'll get on film?

Sorry it still would not be exactly the same as the visual ground glass picture would still differ from the film's spectral response. But it should be closer to the result you get on your film. Whether the method helps you or not would turn up only after you try it personally.

[Doremus Scudder]

Focusing and then making the negative with a blue filter like the #47 will likely be a bit difficult, since the filter is rather dense. The resulting image will be similar to one made with a blue-sensitive emulsion (not ortho). For the ortho look, a Wratten #44 is the "minus red" filter (passes both blue and green) and may be easier to focus through. They are only available as gels anymore and pretty spendy unless you find a used one somewhere. I use the #44 often for the ortho look.

However, a color-correcting filter like the 80A or 80B, although they appear blue, also pass a fair amount of green and can be used in place of a #44 for similar results. They would be easier to focus through than the #47, cut out most of the red and likely bring the visual and "chemical" focus close enough together to be practical in the field.

Do focus with the filter in place and at the taking aperture.

Best,

Doremus

[Drew Wiley]

rdax - even among pan films, there are somewhat different blue and UV responses; note the spectral sensitivity chart of TMax100, for example. Second, why would someone be using a blue filter outdoors? As simply a strong contrast filter relative to foreground coloration, or involving atmospheric effects. I have often admired the old blue-sensitive emulsion landscapes of pioneering photographers, with an enhanced sense of intervening atmosphere. That, perhaps by default in that era, accentuated intervening haze. But they learned how to compose the scene in relation to that effect. And in that kind of application, a degree of uncorrected light scatter might be deliberately employed creatively. Being large, with large format film, what might be an annoying discrepancy with the greater enlargement from smaller format can come out quite nuanced and subtle instead; likewise in contact printing (Watkins was a master of atmosphere in his mammoth plate work).

But as you already suggested, Ortho has its own range of characteristics, which differ somewhat film to film. Most Ortho Litho films, which some people experiment with, seem to be about three times more sensitive to blue than to green. Then there is the intermediate category of Orthopan, of which Fuji Acros is the only current example I can think of. The main point is that there is simply no substitute for experimenting and then printing the results, to find one's own comfort zone. I work in a variety of styles, so keep on hand quite a number of filters, but only carry a few at a time. After awhile, things get intuitive.

On does not necessarily need to go all in with blue filtration to have a nice atmospheric effect. Even something as a mild 82A tungsten conversion filter works well. Hard blue 47 and 47B color sep filters can have a harsh effect unless you are accustomed to them. And almost anything Tiffen needs to be frequently cleaned due to lack of coating; they collect condensation, haze, and grime quite easily.

[ridax]

I don't suggest to take pictures with a 47 Wratten/Tiffen to get a blue-sensitive look with pan- and ortho films (I've already mentioned a Wratten 39 would do the task better as its factor is much less). I suggest to view

(1) the subject through a 47 when shooting blue-sensitive film with any camera lens - to preview the tones, and

(2) to focus through a 47 when using a lens poorly corrected (or not corrected at all) for the chromatic aberration for taking the picture without any filter, on any film.

Lots of films, including panchromatic ones (yes there are exceptions) are sensitive to the UV even more then to the visible blue-violet. Most of the films are more sensitive to the blue then to the green and red. A lot of lenses are quite transparent in the UV - especially the thin menisci (unless they are made of the modern reading-glass lenses as the current standard requires all the spectacles to block UV). So what we get is a picture made by the radiation of at least 350nm (or shorter) to about 550nm for orthochromatic and to 650nm wavelength for panchromatic emulsions with the 350nm to 450nm wavelengths making about twice to 4 times more influence on the picture then the longer ones. At the same time, the chromatic aberration itself is much more prominent in the blue, violet and the UV zones of the spectrum.

As the human eye sensitivity rapidly drops in the blue-violet zone, looking through a 47 Wratten results in focusing with the light of about 480nm. In fact, that is still too long a wavelength to focus for a meniscus used with an ortho film. But that's certainly a lot closer to the film response in the combination with a meniscus lens.