Question for Nikon 68mm enlarging lens owners

[Marco Buonocore]

If you have this lens, could you do me a favour and observe the aperture blades when the lens is wide open at f3.5? Do you have a perfectly round opening, or do the blades create protrude a little bit?

Thanks!

 

[john_s]

That is a very rare lens! I thought you might have mistyped 63mm, but I found that the 68mm does exist!

For what it's worth, on my 63mm f/3.5 the iris at full aperture is a perfect octagon, and on my 63mm f/2.8 it's almost a perfect circle with the slightest hint of an octagon, barely noticeable.

 

[Marco Buonocore]

Hi john_s, thanks for this info. It is the 68, and not the 63. Truth be told, I'd rather have the 63. Right now, wide open, I can see the aperture blades. Normally this wouldn't be a problem, but with a point light source you can see the shadow of the blades - it causes a vignette. All my other lenses are a perfect circle when wide open - and that includes a Nikon 50 and 105.

I might need to find someone to open in up. Probably cheaper to get a new one, which breaks my heart.

 

[bernard_L]

but with a point light source you can see the shadow of the blades - it causes a vignette

Can you confirm that you see the shadows of the blades on the easel, with the negative properly focused? Normally, this should absolutely not occur. The iris blades are in a pupil plane. The area of the pupil controls the illumination in the image plane; the shape of the pupil is is irrelevant to the image itself (except if approaching the diffraction limit, in which case you don't see the shape of the aperture, but its Fourier transform); the shape of the pupil becomes gradually important for the image as one moves away from best focus.

I just start to understand what is happening: with a point light source, you start to see a small but near-perfect image of the pupil shape as soon as you move away from best focus. Technically, that is because you have a beam of zero étendue. That is perfectly normal. Remember, you are supposed to use your enlarger with the film focused on the easel.

Put a film in the negative stage. Focus. Look at the image plane carefully. Use a grain focuser if you have one. Do you see "the shadow of the blades"?

 

[Marco Buonocore]

Can you confirm that you see the shadows of the blades on the easel, with the negative properly focused? Normally, this should absolutely not occur. The iris blades are in a pupil plane. The area of the pupil controls the illumination in the image plane; the shape of the pupil is is irrelevant to the image itself (except if approaching the diffraction limit, in which case you don't see the shape of the aperture, but its Fourier transform); the shape of the pupil becomes gradually important for the image as one moves away from best focus.

I just start to understand what is happening: with a point light source, you start to see a small but near-perfect image of the pupil shape as soon as you move away from best focus. Technically, that is because you have a beam of zero étendue. That is perfectly normal. Remember, you are supposed to use your enlarger with the film focused on the easel.

Put a film in the negative stage. Focus. Look at the image plane carefully. Use a grain focuser if you have one. Do you see "the shadow of the blades"?

Hello bernard_L - the blades are quite visible even with a negative in place. Because the 68mm lens is quite long for 35mm negatives, it doesn't quite interfere with the enlargement, but it is close. With a point light, it's very evident if you close down more than one stop. Not only do you see the iris, it creates a very colourful flare and wildly uneven light. From what I understand, the intensity of the light can damage a lens by drying out the oil on the blades. This is why you use the system with the lens wide open.

So it was for this reason that I was trying to determine if my 68mm was a bit damaged, you know? All my other lenses, when stopped down, have no evidence of the iris blades. I thought if another member had the same lens, I could get to the root of the problem pretty quick.