120 Apo-Symmar L question

[Noah A]

I just bought a second-hand 120 Apo-Symmar L and it seems to be exhibiting strange behavior. From Schneider's own specs, the lens is supposed to have 189mm of image circle which they say allows for 25mm of rise at f/22. I mounted the lens on both of my cameras, a Linhof TK45S and MT2000, and if I check the clipped corners of the groundglass the lens appears to mechanically vignette after only a one or two millimeters of rise at F/22. Of course I checked that all of the other movements are zeroed and I checked this on two cameras that I know are aligned and working properly. I'm focused at 15 feet or so, so less than infinity which should give me more coverage.

If I stop all the way down to f/45 I get about 11mm of rise, which is still far less than I should be getting.

Can someone help me out here, I'm ashamed to say I'm stumped. I always thought that checking the groundglass corners for a round aperture instead of a cat's-eye or oval shape was a good way to check coverage. Does this not always apply? Or does it sound like the lens is out of whack somehow?

[Leigh]

From Schneider's own specs, the lens is supposed to have 189mm of image circle which they say allows for 25mm of rise at f/22.
I don't know how you get 25mm of rise in a 189mm circle on a film format with a 163mm diagonal.

To determine the usable rise you must find the intersection of a film corner with the image circle.

- Leigh

[Noah A]

Leigh--you must have magical full-frame 4x5 film holders ;). Mine crop into the film a bit, which is nominally 4x5 inches but really a tad smaller. I was under the impression that 153mm was a more realistic diagonal dimension for 4x5, although to be honest I've never actually measured the size of my 4x5 images on film.

In any event--Schneider's site says: "It has a 75 degree angle of coverage at f22. This results in an image circle of 189mm at f22, which allows shift of up to 25mm vertically and 21mm horizontally with 4x5 inch film."

And from looking at the lens itself, it appears to barely cover 4x5, which makes me wonder if checking the groundglass clipped corners is not always accurate, or if it sounds like there is something wrong with the lens. Of course I can shoot some test film but it if sounds like the lens is really screwy I'll just send it back.

Even with the 162mm diagonal I should be getting what, around 19mm of vertical rise for a horizontal-format photo? As it is the lens appears to mechanically vignette at around 2mm of rise.

[Leigh]

Leigh--you must have magical full-frame 4x5 film holders ;). Mine crop into the film a bit, which is nominally 4x5 inches but really a tad smaller. I was under the impression that 153mm was a more realistic diagonal dimension for 4x5, although to be honest I've never actually measured the size of my 4x5 images on film.I always use the full diagonal dimension of a nominal 4"x5" rectangle for evaluating coverage.

This provides a small safety margin that compensates for slight image degradation near the perimeter of the IC.


Schneider's site says: "It has a 75 degree angle of coverage at f22. This results in an image circle of 189mm at f22, which allows shift of up to 25mm vertically and 21mm horizontally with 4x5 inch film."
I just plugged 189mm into my shift calculator and came up with 19.2 and 16.2, slightly less than the Schneider numbers.
Of course I'm using the somewhat longer film diagonal, which probably accounts for the discrepancy.

Certainly if you're getting significantly less than that, there's an issue of some sort.

- Leigh

[Noah A]

Thanks leigh, it's certainly good practice to err on the side of caution by using the full diagonal. Sorry about my sarcasm, I'm kind of frustrated and that frustration got the best of me.

My manual calculation based on the full diagonal came up with 19.2mm, so as you say less than Schneider's numbers, which are probably based on a smaller real-world diagonal. But I'm not getting anything near that, it's closer to 1 or 2 millimeters.

So--anyone with real-world experience with this lens? The elements appear to be screwed properly into the shutter, the shutter is mounted normally on a genuine Linhof lensboard, and both of my cameras have been checked for alignment in the past few years.

[EdSawyer]

Does it have a hood or filter on it? I have experience with an 120 apo symmar, as I have one but it is on a chamonix saber (no rise). As an aside, the non-L apo symmars are actually better than the newer L models. Schneider had to reformulate them to deal with lower caliber glass than had been previously available when the non-L ones were designed. Around 2006 or so I think, the universe of glass choices decreased significantly due to restrictions on leaded glass. Thus, earlier non L ones were made with the better glass....mtf diagrams bear this out.

[Noah A]

No hoods or filters are attached. I like the regular Apo Symmar lenses but at 120 I picked the L version for the increased coverage.

[Noah A]

I'll shoot some test film. I was under the impression that when using the clipped groundglass corners to check for coverage that one is supposed to see a round aperture if the lens covers a given amount of rise. With the 120L I'm getting an (american) football shaped aperture, the front lens barrel appears to be causing the mechanical vignetting. Obviously some light gets to the corner since there is image on the GG and I can see some light through the aperture. But I guess what I'm asking is this--do I always need to be able to see a round aperture or is an oblong one ok?

Lots of folks say that Schneider is actually conservative when listing specs for image circle. But mine seems to barely cover 4x5 if I look only for a round aperture.

I'm tempted to suspect the lens since I bought it on ebay, however it looks like it has not been tampered with, and the front and rear cells appear to be correct. The A-S-L lens cells do have a different shape from, say, the older normal A-S lenses.

[EdSawyer]

Will check my non L 120 tonight if possible.

[Noah A]

Ed--that would be a huge help, thanks.

My test film won't be back for a few more days.