Lens Shimming Experience

[ic-racer]

I just shimmed my 8x10 lens today and thought I would share the experience.

INDICATIONS:
1) Ever since I started using the lens I could tell that the edges, at infinity, did not focus exactly on the same plane as the center. The error was small and easily subdued with f45, but I wanted to 'fine tune' the lens for optimum performance.
2) The lens was an e-bay special, where the lens came in a shutter with a non-matching f-stop scale. There were no shims.

DEFINITIVE TEST:
First I set the back swing to zero (no front swing available). I chose a distant, easily focused, object and rotated the camera on the leveled tripod so the object fell on either edge of the ground glass. I adjusted the back swing until the object was in focus when placed on either side of the ground glass.

Then I rotated the camera to place the object in the center and confirmed that it was slightly out of focus. Indicating a curved field.

I loosened the front element and progressively unscrewed it and re-checked edge and center focus of the distant object. Eventually I got to the point where I got good simultaneous focus at the edges and the center.

FINDING THE SHIM THICKNESS:
I measured the depth of my threads and divided by the number of threads and got 0.714 mm per thread. I needed to rotate the lens about two revolutions to get the flat field and this translated to about 1.4 mm (I was expecting less, but it is what it is...)

[Erik Larsen]

Dang it, now you are gonna make me test my lenses Ignorance was bliss!!
erik

[ic-racer]

I had a washer that was about the correct thickness but the hole was too small. After some work with the Dremel, I achieved a 40mm opening.

[Skorzen]

Hmm I have a 150mm Kowa in a shutter with no shims, but I believe it is supposed to have some.... might give this a try.

[ic-racer]

After some fine shaping with hand tools and sandpaper I painted the shim semigloss black. There is a place in the front of the shutter where the shim fits nicely.

This picture of a similar Symmar-S 210mm from the internet shows either a trim ring or spacer near the base of the lens that was missing on my lens.

[ic-racer]

Put it all back together and it looks great. Too dark to try it today, but I'm anxious to shoot some pictures with it tomorrow.

[ic-racer]

Thanks to all the previous posters on this topic for giving me the idea to try it. With this 8x10 format and this lens, the effect was clearly visible on the ground glass.

Doing this with my 4x5 would be more difficult. One reason is most of my 4x5 lenses have big image circles, so the 'edge' of the field is not going to be in view, so some more complicated camera gymnastics would probably be required to see an effect.

[Don Hutton]

I've never seen a shim on a factory shuttered lens which was remotely close to that thick - every one I have ever measured has been between 40 and 70 thousandth. I have seen thicker shims on barrel mounted lenses like Kowas and even those are considerably thinner than 1.4mm.

I presume you did this "testing" at shooting apertures, not wide open? Wide open, you'd have all sorts of issues going on at the edge of the image circle due to lens design which would explain why you need a monster shim to "correct" what you're seeing. I'd strongly advise that you make test images at shooting apertures (say f16 to 32) and very closely examine the resulting negs with a 10X loupe.

[Don Hutton]

After some fine shaping with hand tools and sandpaper I painted the shim semigloss black. There is a place in the front of the shutter where the shim fits nicely.

This picture of a similar Symmar-S 210mm from the internet shows either a trim ring or spacer near the base of the lens that was missing on my lens.

That "trim" ring is purely cosmetic - it does not even contact the barrel of the front element. They are often missing as they are easily broken when you try to remove them. Spacers fit right between the barrel and front of the shutter (or between the rear element and shutter) and are pretty much never visible until you unscrew the elements. I'd seriously doubt that you would ever need a shim remotely close to that thick on a lens which was sold shutter mounted from the factory. FWIW, if you're also trying to squeeze 8x10 coverage from this lens, you will substantially reduce the coverage of the lens with a "slice of toast" shim stuck in it - check it out with and without... You want the edge of the front barrel and the edge of the rear barrel as close togther as possible for least mechanical vignetting.

[Paul Fitzgerald]

ic-racer,

"Then I rotated the camera to place the object in the center and confirmed that it was slightly out of focus. Indicating a curved field."

Why did you not use rear shift? Rotating around on the tripod only indicated rotating around on the tripod. I agree with Don, I have never seen any shims that thick, something is wrong.

just a thought.