Resolving Chromatic Abberations

[JG Motamedi]

I recently purchased a Vademecum-type Periskopic (two single-element meniscus lenses around a central stop) casket set from a fellow APUGger (thanks Richard!) and have been experimenting with it. It is a beautiful, small, and flexible set. However it has pretty severe chromatic aberrations. Some of these sets, notably the Busch Vademecum II, were sold with small auxiliary focusing lenses which were inserted into a waterhouse type slot, enabling the user to correctly focus both the visible and invisible (UV) spectrums. The set I have however was never supplied with either waterhouse slot or auxiliary lenses.

As is, it produces beautiful soft-focus images. The images are sharper stopped down, but still not quite as sharp as I think they could be if I could figure out how to correct for the difference between the visual and chemical foci.

How should or can I correct for chromatic aberrations where the visual and chemical foci do not correspond? I am sure that adding a strong filter will help a bit, but not completely. Can I "defocus" the camera by 2%, as suggested by the Lens Collectors Vade Mecum (under "meniscus") or can anyone think of a way of using some sort of commonly available diopters?

Thanks as always,

jason

 

[Ole]

The "discrepancy between Visual and Chemical Foci" stems from the days when the plates (daguerreotype or wet-plate collodion) were only sensitive to UV and a little of the blue, whereas the eye is practically insensitive to those wavelengths.

The "Corrector" in the Vade Mecum sets were intended to impart a focus shift countering that difference (yellow - UV focal length), and won't help much with most panchromatic films.

So to correct for the aberration, you must narrow down the spectrum as much as possible - by adding strong filters, or using a film with narrow-spectrum response (and a filter transmitting in the same narrow band so that you see what you're doing).

I'll know more when I've played around with my own set some more

 

[Ed Sukach]

I recently purchased a Vademecum-type Periskopic... ( ...enabling the user to correctly focus both the visible and invisible (UV) spectrums.

Say whut? Ultra-violet focusing? The glasses commonly used in camera lenses attenuate out MOST of light in the UV spectrum anyway. Note the special Calcite-element Hasselblad lens ( ~ US$ 10,000) for ultraviolet applications.

Chromatic abberation exhibits itself in the visible spectrum - "rainbow edges" -and has a detrimental effect on black and white imaging as well as color (loss of sharpness).

As is, it produces beautiful soft-focus images. The images are sharper stopped down, but still not quite as sharp as I think they could be if I could figure out how to correct for the difference between the visual and chemical foci...
... How should or can I correct for chromatic aberrations where the visual and chemical foci do not correspond? I am sure that adding a strong filter will help a bit, but not completely.

?? I don't understand what is meant by "chemical" focus. The use of a "strong" filter will decrease overall chromatic abberations by making the light more monochromatic .. eliminating the other wavelengths ... but not without a (great) price. Place a strong red (87 or so) fliter over any lens and observe what happens to the tonal "balance".

(Will it help) If I "defocus" the camera by 2%, as suggested by the Lens Collectors Vade Mecum (under "meniscus") or can anyone think of a way of using some sort of commonly available diopters?

I doubt if defocusing will do anything else to the overall image except softening the edges even more. A "diopter" (I cringe every time I hear the mis-use of that term - "Diopter" refers to a measuring system for the curvature of lenses - they should properly be called "Auxilliary Close-up Lenses") will have the effect of changing the focal length of the lens. That has as much chance of making chormatic abberation worse as it does of improving it; at 2% I don't think either effect will be great.

 

[David A. Goldfarb]

A medium yellow filter is the usual solution I've seen for this problem.

 

[Photo Engineer]

I've had this discusssion before regarding UV and lenses.

It seems that enough UV gets through a modern day lens that there is some degree of UV exposure. Most color films have UV filtration built into them and many color and B&W shots benefit from UV filtration.

Kodak makes a set of 3 UV filters for high altitude photography that are graded for use as you go up in altitude. These filters are (or were) made to counter the small but annoyingly real amount of UV that gets through even the best glass.

In any event, David has it spot on, but I would suggest adding a UV filter if you want still less UV in the exposure. Some yellow filters do not absorb sufficiently in the UV region.

PE

 

[Ed Sukach]

Hah! I'm noticing the "times" - Ole posted 31 miniutes before I did... adding valuable historical information.

So ... "Wet plate" emulsions more or less required UV for exposure. Considering how effectively ordinary optical glass filters out UV, it is no wonder why exposure times were so great!

Thanks, Ole, for explaining what was meant by "chemical focus" - I wouldn't have figured that out in my lifetime!

I was struggling with "spell check" - for something like the better part of 31 minutes...

 

[David A. Goldfarb]

In addition to what PE says, it seems that many "UV filters" do not really filter that much UV. Bob Atkins wrote a pretty good article on this subject a while back on photo.net, where he compared various brands of UV filters for UV absorption. As I recall, Heliopan and B+W were the most effective (unsurprisingly).

 

[JG Motamedi]

Comments and another Question

Thanks very much for the responses; they are quite helpful. As David and PE suggest, I will try a Yellow with a good UV filter and see.

Ole, as you have one of these sets (I assume), what exactly is the correction lens? Is it a medium yellow, or is it, as the Lens Collectors Vade Mecum describes, a "real" lens which affects FL?

Ed, I take it you understand that this lens was probably made in the 19th century, and is a very different lens from that $10,000 Hasselblad UV lens.

I also want to use this lens for ortho-sensitive materials. Any suggestions how this might be done? The UV and blue spectrum are necessary, so yellow or UV filters can't be used. Should I just use a blue filter and hope for the best?

 

[Ole]

Jason, my set doesn't have the "corrector". But from what I understand its purpose is to shift the focal point by a small amount, just enough to compensate for the distance between visual and chemical focus. I believe that could have been done with something as simple as a glass plate of the correct thickness, or perhaps a very low-power negative lens. A yellow filter would be no good with UV-sensitive emulsions...

With orthochromatic material I would (I will) try a strong green filter, or a strong blue filter. The green filter is probably better, since there's no telling (yes there is, but it's difficult to find out) just how far into the UV the film's sensitivity stretches. The blue filter may not block far UV.

Another way is to use a blue-green (=cyan) filter while focussing, to mimic the film's sensitivity.

I happen to have a set of Lee colour separation filters which should do the job just fine.

 

[Photo Engineer]

With a yellow filter and an ortho material, you will only record green light striking the film, and any UV not removed by the glass in the lens and the filter(s). If the film is pan sensitive then you would record the red light as well.

All blue information (anti - yellow) will be removed.

PE

 

[JG Motamedi]

Thanks, as always. Looks like I have some experimentation to do this weekend.

 

[smieglitz]

... The blue filter may not block far UV.

Another way is to use a blue-green (=cyan) filter while focussing, to mimic the film's sensitivity...

I'd try a #44 (cyan) wratten gel to mimic ortho response.

Jason, Are you trying to utilize the UV for wetplate/dags or eliminate it for ortho film? If the latter, somewhere I have a HF3 filter such as PE mentions. I'll see if I can find it and pass it along.

FWIW, I also have a Dallmeyer "Adon" lens that came with a yellow-green filter, presumably to correct for the difference in foci you are concerned about. I believe a similar filter is also packaged with Rodenstock Imagon lenses.

Joe

 

[sanderx1]

With a yellow filter and an ortho material, you will only record green light striking the film, and any UV not removed by the glass in the lens and the filter(s). If the film is pan sensitive then you would record the red light as well.

All blue information (anti - yellow) will be removed.

PE

Actually, most non-UV (or non-UV/skylight/haze) filters these days don't remove all that much UV, and the includes most coloured filters for B&W that are around. Making the wrong filters choie is unfortunaely way too easy

 

[Photo Engineer]

Actually, most non-UV (or non-UV/skylight/haze) filters these days don't remove all that much UV, and the includes most coloured filters for B&W that are around. Making the wrong filters choie is unfortunaely way too easy

See David's post.

PE

 

[JG Motamedi]

Joe,

The real goal is to be able to use the casket set with Wetplate and Dags, but would also like be able to use it for pan film. Asking too much? Probably. As for your kind offer of the UV filter, if you send it my way I will find something suitable to send back...

jason