Nikon 90mm f/4.5 and copal 0 shutters

For all you lens gurus out there:<br><br>

 

I recently purchased a Nikon 90mm f/4.5, which had (to my surprise)

a copal 0 shutter made and marked for the 135mm f/5.6 lens. With the

shutter fully open (a bit before 5.6) there is no "closing down" on

the elements themselves; it is free and clear. I'd like to know

if:<br> a) the lens is still working at 4.5 wide open<br>

b) the aperture scale is still valid, and<br>

c) there are any significant structural/load bearing differences

between these two copal 0 shutters? They are spec'ed the same in

terms of hole size.

<br><br>

I'm most interested in "b". Since wide open (basically f/5.6)

yields full lens elements, wouldn't that be f/4.5 on the proper

shutter thus invalidating the marks on this shutter by 1 stop, e.g.

f/5.6 is really f/4.5, and f/8 is really f/11?

<br><br>

thanks,

Aaron

The shutter should function and open sufficiently wide to give you f4.5, but the aperture scale is wrong -- there is no obvious relation between the aperture markings of the two lenses. The situation is not as simple as reasoning that f5.6 is the widest aperture on the 135 mm lens, and f4.5 is the widest aperture on the 90 mm lens, so to get f4.5 on your 90 mm lens you should set the aperture lever at the f5.6 mark of the 135 lens scale.

 

The simple equation for apertures is the definition of f-number: f-number = focal length / aperture diameter. But the trick is the aperture diameter really should be the entrance pupil diameter, and this differs from the physical diameter of the stop by the optical pupil magnfication. This may differ between the two lens designs, so the conversion between the two aperture scales is not obvious.

 

Your observation that the lens appears to be open at the fully open position of the aperture lever probably tells you the true location of the f4.5 mark.

 

If the seller didn't disclose this aspect of the used lens, then you have been mislead and have paid too much. If I were you, I would ask to return the lens for a refund. There are plenty of other used lenses out there -- unless the price is really low, why bother.

First and foremost, I agree completely with Mr. Briggs.<p>But If you are happy with the

images that the lens projects and you want to keep it, get out your calculator and and divide

the focal length, here it would be 90mm, by the f:stops you'd like to use, 5.6, 8, 11, 16 and

so on, and use the measurements THROUGH THE FRONT ELEMENT to re-mark the f:stops on

a piece of white tape covering the original aperture scale.<p>The photo I'm attaching here

demonstrates how to measure f:11 on a 150mm Symmar. It's going to be trickier for you to

do it on a 90mm lens. You have to hold the lens at arms length to do it properly.<p>I'd

return the lens if I were you, though. There could be many more problems with it down the

road.<div></div>

You can get the correct scale for a Copal #0 with a 90mm focal length

lens from any of the LF lens manufacturers that use the #0 shutter for that focal length. The scales are all the same for apertures used to take photos.

 

Your scale is off by 90/135 or 2/3. If you set it at f/22 on a scale marked for 135 mm, the actual aperture with the 90 mm lens is 90/135 x 22 = 14.66. If you want to use an aperture of f/22 with the 90 mm lens, set the 135 mm scale at 135/90 x 22 or f/33.

Michael G., neither part of your answer is correct. Aaron can't get a replacement scale from any manufacturer. Nikon Copal shutters are different in minor ways from those sold seperate from lenses or used by by Rodenstock and Schneider. Instead of having separate aperture scales that are screwed onto the lens, the aperture scale is printed onto the exterior of the shutter. The entire outside barrel surface would need to be replaced, and this would be available only from Nikon, if at all.

 

Your calculation about the correction between the apertures of the two lenses assumes that they have the same pupil magnification. This may be true, or close enough, but it is an assumption. It may not be true since the two lenses are of completely different designs, one a biogen-style super-wide-coverage type, the other a plasmat.

 

Noah's procedure of measuring the pupil by sighting through the front element is correct -- this takes care of the magnification of the aperture by the front cell.

 

But why should Aaron have to deal an unsatisfactory lens with this "surprise"? Let the seller deal with it, or some buyer who has paid a bargin price because of this feature.

Because it's not the original shutter, is there a shim between the front cell and the shutter? Many nikkor SW lenses use shim to optimize the optic spacing. I would return it.

Yeah, and that's another thing, Robijanto Soetedjo is absolutely correct.<p>I optically tested

a pallet-full of Nikkor lenses in the early 1980's and most of them, and I mean the vast

majority of them, were full of unacceptable aberrations, ones that couldn't be corrected even

with shims. Just to muddy the waters, a few were very sharp. So I'd avoid used Nikkor lenses

at all, (unless you can have them optically tested by someone like myself before you buy,)

and find yourself an old Super-Angulon or Grandagon, and you'll be much impressed with

the results.

Robijanto's point is important. The Biogen-style super-wide design is very sensitive to the spacing of the cells. The manufacturers adjust each lens to correct for manufacturing variations. Nikon uses a thin washer between the front cell and the shutter on the SW lenses. If whoever moved the lens cells to the shutter of the 135 mm lens lost the shim washer, then the performance of the lens will suffer.

 

My guess is that the manufacturing variations are in the optics and not in the thickness of the shutter. If I am wrong, then switching shutters might require adjusting the thickness of the shim. This is another reason to return the lens.

I currently have a 90mm f5.6 Super Angulon. It was fitted to a Sinar board that had no shutter. Fitting this lens to a Copal 0 gave pinsharp images in the centre but terrible results in the corners. Measurement showed that the elements were 0.8mm further away from each other than when fitted to the original Sinar assembly. I was able to machine the rear thread of the shutter down by this amount so the elements are now the correct distance apart. End result is a razor sharp lens from corner to corner. If your lens renders unsharp results, you`ll have to get this dimension correct. Check with someone who has a similar setup, only in the correct shutter. The aperture is not so important to me as I shoot everything at f22 to assure good coverage, (6x17 format). Alternatively bracket your shots using three different apertures. After a while you`ll know the lens`s performance and sweet spot, if it has one.... Dean.

Thanks guys!

 

It seems I've gotten myself into a bit of a pickle. I will definately be contacting the seller.

 

Aaron