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  #11  
Old 23rd June 2022, 12:21 PM
Stocky Stocky is offline
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Quote:
Originally Posted by Martin Aislabie View Post
All this is true - however, the taking lens will also suffer from light fall off towards the edge of the frame.

The problem of light fall off is particularly true for wide/very wide angle lenses - large format wide angle lenses had centre filters available, even if the prices are rather high.

The X-Pan had a Centre Filter for the 30 mm.

So, the light fall off issue can be something of a red hearing/non-issue.

If you are using a longer than normal enlarging lens, it is important that you mask the unused negative area within the negative mask - all that excess white light bouncing around is bound to produce some level of internal flare. Even the very best lenses suffer from flare - even if their flare suppression reduces the problem to almost immeasurable levels.

Martin
So to some degree it's self correcting, loss of density towards the edge of the negative compensated by the falloff of the enlarging lens (as long as one isn't extreme, like a very wide taking lens.
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  #12  
Old 24th June 2022, 02:01 PM
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Martin Aislabie Martin Aislabie is offline
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Originally Posted by Stocky View Post
So to some degree it's self correcting, loss of density towards the edge of the negative compensated by the falloff of the enlarging lens (as long as one isn't extreme, like a very wide taking lens.
Yes ~ more or less.

It isn't precise because the fall off for each focal length differs.

Remember, the fall is controlled by Cos^4.

All lenses are bound by this physical law - so the Cos^4 curve should be quoted in the illumination fall off test results.

Martin
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  #13  
Old 24th June 2022, 05:40 PM
Nat Polton Nat Polton is offline
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Neil mentioned earlier, good optics being an investment.

I talked with a chap many years ago when I was buying some gear from him. The old boy told me when he popped off, due to her lack of knowledge, his wife would probably get a dealer in, and nowhere near the value of his expensive glass.

I wonder how many members here have made suitable arrangements for their cherished, expensive kit?

I know I haven't, and should do something about it now.

Cheers.
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  #14  
Old 24th June 2022, 08:50 PM
John King John King is online now
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Originally Posted by Nat Polton View Post
Neil mentioned earlier, good optics being an investment.
I wonder how many members here have made suitable arrangements for their cherished, expensive kit?

I know I haven't, and should do something about it now.

Cheers.
All catered for in my will.
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  #15  
Old 27th February 2023, 04:48 PM
Mark J Mark J is offline
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Just noticed this old thread, forgive me adding a couple of extra points, might be of use to someone.
I have an El-Nikkor 105 N for 6x9 printing. I don't know if there was a change to the optical design, from the earlier lens, however the 'N' version almost certainly has better coatings. This will be most relevant for colour work though.

Quote:
Originally Posted by Martin Aislabie View Post
Remember, the fall is controlled by Cos^4.

All lenses are bound by this physical law - so the Cos^4 curve should be quoted in the illumination fall off test results.
Martin
This is not generally correct, Martin.
It is approx correct however for Plasmats and Double-Gauss lenses, and some simpler, symmetric lenses. This Nikkor lens ( like Symmars ) is a Plasmat.
The fall-off rate is affected by the angle the off-axis principal rays come through the stop, and also the angle they ultimately make when they hit the image plane. These parameters are construction-dependent.

With Super-Angulon types, and symmetric wides like the Leica M ones, you get about cos^3 (Ө) falloff . With inverse-telephoto wides for SLR's, it's as low as cos^2(Ө) .
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  #16  
Old 18th March 2023, 10:49 AM
JOReynolds JOReynolds is offline
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Originally Posted by Mark J View Post
...for Plasmats and Double-Gauss lenses, and some simpler, symmetric lenses....
Suddenly, on reading this post, I felt as if I was back at college (optics). So I will put my current dilemma to you, Mark.
I am considering buying a very expensive lens for architectural photography - the 40mm Hasselblad Distagon. It is roughly the same price as a SWC with a 38mm Biogon. Reviews suggest that the Distagon suffers less vignetting compared with the much simpler Biogon. Can you explain this? Obviously the angle of rays subtended at the film surface is less extreme on the Distagon because the design must allow for mirror movement - the SWC has no mirror. But is that it?
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  #17  
Old 18th March 2023, 01:36 PM
Mark J Mark J is offline
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Where did you go to college ?
I did the Imperial 'Applied Optics' course.

Good question. You can find all the data on the older lenses here :
https://www.zeiss.co.uk/consumer-pro...otography.html

From what I can see, the 40/4 is not fully free of mechanical vignetting at f/8, but it ought to be by f/11, then it will be better - as you say - vs. the Biogon. The distortion is quite low for a retrofocus wide - only max 1%. The 'T' field resolution at higher frequencies isn't great - as with a lot of 1970's/80's Zeiss designs. This is due to secondary lateral colour. If you can use the lens with a yellow or orange filter, the fine detail sharpness will improve quite a lot.

Both lenses have a negative front group, which decreases the off-axis angles through the stop. However the Distagon has a positive (only) rear section, and a longer back focus, which keeps the principal ray angles lower at the film plane. In the Biogon, those angles increase again through the rear negative group, so the obliquity is greater for the off-axis fields.

So, the answer to "is that it ?" is yes. That makes the difference of one Cos factor.

Hope this makes some sense.
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