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Identify issue with lens
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PostPosted: Mon Sep 22, 2014 4:00 pm    Post subject: Identify issue with lens Reply with quote
I bought a 50mm lens, one of the very best. Sadly it is faulty, and I'm wondering if, by the image samples if anyone knows what the issue is. I'm wondering if it's a simple case of one element being flipped (double gauss/planar type).

f1.7

f2.8

f4

f5.6

f8


Last edited by iangreenhalgh1 on Mon Sep 22, 2014 4:20 pm; edited 1 time in total


PostPosted: Mon Sep 22, 2014 4:05 pm    Post subject: Reply with quote
Which very best lens 50mm you bought Question


PostPosted: Mon Sep 22, 2014 5:01 pm    Post subject: Reply with quote
It's not entirely impossible. Sometimes curvatures on the rear element are so similar it's next to impossible to tell which side should go in.


PostPosted: Mon Sep 22, 2014 5:24 pm    Post subject: Reply with quote
It seems to be a problem that is connected to the lens edges, thus the improvement when stopping down.
And it can be the case that an internal element is flipped which only shows at open aperture settings.
The problem is to find out which one is the flipped one.


PostPosted: Mon Sep 22, 2014 8:32 pm    Post subject: Reply with quote
That does look like the results I got when I took my Tamron 90 2.5 macro apart, I laid everything out carefully on the table, got up to go for a pee, and kicked the table. One of the three rear elements that I'd got on the table must have flipped over when I kicked the table, that's eight possible combinations. I got it right on the third I think. But each time I tried it the results were like yours, getting better as it closed down.


PostPosted: Mon Sep 22, 2014 8:38 pm    Post subject: Reply with quote
I think I'll return it for a refund. Most annoying.


PostPosted: Mon Sep 22, 2014 9:45 pm    Post subject: Reply with quote
Out of interest, what lens is it?
OH


PostPosted: Mon Sep 22, 2014 10:51 pm    Post subject: Re: Identify issue with lens Reply with quote
iangreenhalgh1 wrote:
I bought a 50mm lens, one of the very best. Sadly it is faulty, and I'm wondering if, by the image samples if anyone knows what the issue is. I'm wondering if it's a simple case of one element being flipped (double gauss/planar type).


Are you sure you didn't somehow get ahold of my Tessar? It performs exactly like that. It was made in the 50's and I'm sure some high quality repairman along the way (I'm wildly speculating on February 9, 1965) is responsible.


PostPosted: Tue Sep 23, 2014 12:28 am    Post subject: Reply with quote
Based on the shape of the turn signal light in the first image and how the effect decreases then disappears, I'm guessing one of the elements is resting at an angle, but the lens' center is still properly aligned. That would explain why light coming through most of the lens' optical space exhibits image-wide coma-like aberrations. Then, stopped down, when only the central portion of the glass is used, it disappears.

Who knows if it was reassembled incorrectly at some point or if it's from a drop or the like. My guess is that if you took everything out and re-assembled it that the error may go away. If not, then there's probably some clip or rest for one of the elements that's missing or broken.


PostPosted: Tue Sep 23, 2014 1:32 am    Post subject: Reply with quote
Strong coma near the center of field ---> severe decentering.



PostPosted: Tue Sep 23, 2014 3:16 pm    Post subject: Reply with quote
Oldhand wrote:
Out of interest, what lens is it?
OH


He says "one of the very best" and it starts from 1.7 so I would guess it's a Planar T*.
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