Changing the distance between elements in a lens

[Greg]

I just purchased a Wollensak Portrait Lens Series A f-5 6 1/2 x 8 1/2 lens. Per Vade Mecum the lens is a RR. I love the image it throws on my GG, but at infinity it doesn't quite cover the Whole Plate format. Hope to have it mounted in a modern shutter in the future.

Question is: The distance between the lens groups is now over 4 inches. If the lens groups are mounted closer together, would that increase the coverage of the lens? Or if by increasing the distance between them, would that increase the coverage? I know the image quality will change a bit, but I am willing to accept that as long as the lens will fully cover the Whole Plate format.

thanks

[LabRat]

Maybe increase the FL by adapting a -1 Telek or other - lens (like a round lens blank from an optician) to the front of the lens??? (It is a portrait lens, anyhow...)

Steve K

[Luis-F-S]

Maybe you should consider getting the right lens in the first place

[Xipho]

Hello Greg, what lens design has the lens? RR???

If you have a double anastigmat, then when you increase the space between two groups the focal length stays the same, but the coverage decreases...

I have a postwar Meyer WW-Aristostigmat in a barrel, and a fitting shutter that has a few mm more spacing.
Still have to compare it with the prewar uncoated Aristostigmat with the right shutter....

[desertrat]

I would be the last to argue with the Vade Mecum, but I was under the impression that the Portrait Series A was a Petzval. Hopefully someone who owns one and has disassembled it for cleaning could let us know.

If it is an RR, the front and rear cells should be cemented doublets and identical with each other. If it is a Petzval, the front cell should be a cemented doublet and the rear cell should have two single airspaced elements with a spacer ring between them.

It wouldn't hurt to check.

[Jim Noel]

I just purchased a Wollensak Portrait Lens Series A f-5 6 1/2 x 8 1/2 lens. Per Vade Mecum the lens is a RR. I love the image it throws on my GG, but at infinity it doesn't quite cover the Whole Plate format. Hope to have it mounted in a modern shutter in the future.

Question is: The distance between the lens groups is now over 4 inches. If the lens groups are mounted closer together, would that increase the coverage of the lens? Or if by increasing the distance between them, would that increase the coverage? I know the image quality will change a bit, but I am willing to accept that as long as the lens will fully cover the Whole Plate format.

thanks

This "portrait" lens is not meant to cover the format at infinity. it is designed for use at portrait distances.

[Greg]

I would be the last to argue with the Vade Mecum, but I was under the impression that the Portrait Series A was a Petzval. Hopefully someone who owns one and has disassembled it for cleaning could let us know.

If it is an RR, the front and rear cells should be cemented doublets and identical with each other. If it is a Petzval, the front cell should be a cemented doublet and the rear cell should have two single airspaced elements with a spacer ring between them.

It wouldn't hurt to check.

Initially when I got the lens, couldn't unscrew the rear elements so just assumed that it was a RR as per stated by the Vade Mecum. Finally using a jar top opener was able to remove the group of rear elements. Well we have a Petzval lens here.

Taped the lens to a lens board and shot an 8x10 negative outside. Results were absolutely terrible. Being about 20 degrees outside, hastily shot the exposure not noticing that I had forgotten to tape over the 6 screw holes in the flange. Turns out that from the time I removed the dark slide to when I put it back in, I had 6 pinholes imaging on the film which completely over exposed the exposure I had made with the lens' Studio shutter. Reshot 2 exposures with 6 screw holes now taped over, one at f/5 and the other at f/22. Negatives are gorgeous.

Still am inquisitive as to if the coverage can be increased just a bit by changing the distance of the front group to the rear group.

If that does not work, will make up a mask with WP proportions. Then when I scan the negative to make a digital negative, will just crop the image as per the mask and enlarge it a bit to 6.5x8.5 inches.

[Greg]

Took the better part of this afternoon to jury rig this: The front lens group is roughly 5" away from the rear lens group. If that distance is cut down to approximately 3", the focal length of the lens is not measurably changed but the covering power in substantially increased. Decreasing the distance by about 1/3 and the lens actually covers the 8x10" format with a little room for movement. Visually on the GG, I could not see a difference between the normal 5" spacing and the shorter 3" spacing, but of course I'm sure it's there. But since it's a "portrait" lens, any minor differences in the images probably will not matter. Also since I am contact printing the imagery on Platinum/Palladium, I am not enhancing any differences by enlarging the image.

Now all to do is to shoot a negative at each of the two distances and compare them. Jury rigging the shorter distance in order to shoot a negative outdoors will be quite a challenge up here during wintertime.

A while back contacted S K Grimes about mounting a barrel lens in a shutter. Turned out not possible to insert a shutter between the front and rear groups because of clearance. They did offer to increase the spacing of the groups to fit a shutter, but told me the lenses image would change. But no way to tell how much it would change and if it would really matter to me since I contact print. FYI: I've had several barrel lenses mounted in shutters by them over the years. In each case they returned the brass parts making the whole adaptation reversible if I wished in the future.

[Maris Rusis]

If you want to look at the mathematics of altering the effective focal length of pairs of lenses by changing their spacing check out Gullstrand's Equation.

[Greg]

This afternoon was finally able to shoot a film test. 2 negatives, Both shot at f/22, and focusing on a stop sign (center of film) about 100 feet away with buildings in three corners of the frame. I always go to the same place in town to shoot test negatives, have one test negative shot at f/22 for each of my lenses to compare.

Lens with front and rear groups of elements in normal position (5" spacing): Portrait lens image quality as expected. Didn't quite cover the WP format.

Lens with front and rear groups of elements closer together (3" spacing): Again Portrait lens image quality as expected but resolution in the center was noticeably higher and the lens covered the 8x10 negative. Focal length of lens increased by maybe 5%. You have to lay the negatives on each other to see the difference.

Now to shoot a comparison with the subject being the distance away of a portrait and using the lens as it was intended to be used.

[djdister]

Feel free to post scans of the negatives - this is a show and tell forum...

[Greg]

Up here in the Northeast freezing rain so shooting outside out of the question, shot test negatives in the basement. Subject matter anything but 3 dimensional so Petzval Bokeh seemingly missing. Know this cause back a bit won a Darlot Petzval lens. Unpacked the lens in the evening and made one exposure with it of a similar 2 dimensional subject... negative was very disappointing. Next day shot a 3 dimensional subject outside and negatives looked gorgeous.

Made two exposures: both f/16. One with the lens as is and the other with less distance between the front and rear lens groups. Posted is the image as shot with the lens as is. The one with the element groups closer doesn't look all that different at arms length.

But when you place the front and rear lens groups closer together you notice the following:
1, loose some center sharpness but not enough to matter if negative is contact printed. On the GG, the image the lens projects actually looks sharper but isn't under a magnifier.
2. loose contrast, but very little
3. Focal length increases by less than 10%
4. At infinity the lens now covers the Whole Plate format.... only PLUS

When closing down the iris, it is anything but circular. Stopping down any further than f/8 and specular highlights will definitely be degraded in character. Shape of aperture only gets worse as you stop down further.

[Mark Sawyer]

Changing the spacing will most likely change the field curvature, introduce coma to the specular highlights, and if you're lucky, bring about a little spherical aberration. You may get a larger circle of illumination, (especially if you move the elements closer together and shorten the barrel), but the outer area will be a blur of light as the field curves.

Believe it or not, the optical engineers behind the lens knew what they were doing. Then again, sometimes what we're doing has nothing to do with what they were doing... :rolleyes: