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07-14-2010, 10:59 AM   #16
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What I'm getting at is that it is possible that the image transmitted through the separation area of the image is outside the blue rectangle and at least in part maybe the red rectangle as well. This circle is equivalent to the green line in the diagram.






Last edited by Blue; 07-14-2010 at 11:11 AM.
07-14-2010, 11:18 AM   #17
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QuoteOriginally posted by Blue Quote
What I'm getting at is that it is possible that the image transmitted through the separation area of the image is outside the blue rectangle and at least in part maybe the red rectangle as well.

That damage will affect the captured image even though it is outside the APS zone.

If the lens was undamaged any light traveling through the outer portion, near the periphery, will not be refracted into the APS zone.

However, if the periphery is damaged, the light refracted through that zone would be unpredictably scattered everywhere including into the APS zone. This would present itself as veiling flare, reducing the contrast and sharpness. The amount of that is only a guess.

Obviously the lens is damaged enough to see the cement separation. That is enough to cause some veiling flare.
07-14-2010, 11:35 AM   #18
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QuoteOriginally posted by lmd91343 Quote
That damage will affect the captured image even though it is outside the APS zone.

If the lens was undamaged any light traveling through the outer portion, near the periphery, will not be refracted into the APS zone.

However, if the periphery is damaged, the light refracted through that zone would be unpredictably scattered everywhere including into the APS zone. This would present itself as veiling flare, reducing the contrast and sharpness. The amount of that is only a guess.

Obviously the lens is damaged enough to see the cement separation. That is enough to cause some veiling flare.
That is a good point that hasn't been discussed yet. It is also different than what drougge was talking about. The "obvious" effects are the important ones in this context which can only be determined by trial and error.
07-14-2010, 12:56 PM - 1 Like   #19
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QuoteOriginally posted by Blue Quote
What I'm getting at is that it is possible that the image transmitted through the separation area of the image is outside the blue rectangle and at least in part maybe the red rectangle as well. This circle is equivalent to the green line in the diagram.
And what I'm getting at is that all parts of the image are transmitted through that area.

Let's try this again, from the top: A point of light in the middle of the produced image will pass through all parts of at least some lens elements.

Proof: Stopping down (which only blocks the edges at the diaphragm) will reduce the intensity of this point (and not reduce the image circle).

What you quote about vignetting seems true and all, but it still does not seem relevant to what I'm saying.

It's still possible that I'm wrong, and if so I'd like to know, but crop factors and the effects of hoods are really completely irrelevant. They're relevant to all sorts of things of course, but not to this.

And let's restate what I'm saying yet again, in the hope that some version of it will actually be understandable:

In my image, the left green arrow is an object being photographed. The right green arrow is the image of that object. The red lines are some of the pathways light from the tip of the arrow travels. The blue lines (and the dotted line) are some of the pathways light from the bottom of the arrow (middle of the image) travels.



And again: All parts of the lens are used for all parts of the image.

This may not be true for all elements in a complex lens, and may therefore not be true for the element with the problem, but it should be true for some elements and I don't see any way to know which without detailed knowledge of the lens in question.

07-14-2010, 06:53 PM   #20
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QuoteOriginally posted by drougge Quote
And what I'm getting at is that all parts of the image are transmitted through that area.

Let's try this again, from the top: A point of light in the middle of the produced image will pass through all parts of at least some lens elements.

Proof: Stopping down (which only blocks the edges at the diaphragm) will reduce the intensity of this point (and not reduce the image circle).

What you quote about vignetting seems true and all, but it still does not seem relevant to what I'm saying.

It's still possible that I'm wrong, and if so I'd like to know, but crop factors and the effects of hoods are really completely irrelevant. They're relevant to all sorts of things of course, but not to this.

And let's restate what I'm saying yet again, in the hope that some version of it will actually be understandable:

In my image, the left green arrow is an object being photographed. The right green arrow is the image of that object. The red lines are some of the pathways light from the tip of the arrow travels. The blue lines (and the dotted line) are some of the pathways light from the bottom of the arrow (middle of the image) travels.



And again: All parts of the lens are used for all parts of the image.

This may not be true for all elements in a complex lens, and may therefore not be true for the element with the problem, but it should be true for some elements and I don't see any way to know which without detailed knowledge of the lens in question.
Great stuff! I'm learning a lot from you guys while enjoying my new problematic M85/2
07-14-2010, 08:19 PM   #21
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My line of thought was similar to Blue's, with an unexpressed recognition that what lmd91343 is describing could well be a (likely fairly small) problem visible under some lighting conditions but probably not a major factor in day to day use.

Obviously ducdao now has to go out and buy a pristine M85 f/2 so that he can test the two lenses against each other under a wide variety of lighting conditions to determine just how significant the optical degradation caused by the element separation really is (I'm too lazy to pull out my textbook that covers lens theory to see if it has anything worthwhile to add to the discussion; I have an interest in applied optics but find the theory kind of boring).

I'm happy to hear that Eric thinks he can separate and re-glue the elements. Presumably the cost of the repair is less than the cost of replacing the lens?
07-14-2010, 08:28 PM   #22
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There's even a chance that the cement Erik would use to replace that old cement is better than the original stuff. There has been some very good advances in that area over the past 25-30 years. That would also be interesting to see the results from.
07-14-2010, 09:31 PM   #23
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snip~
QuoteOriginally posted by drougge Quote
And let's restate what I'm saying yet again, in the hope that some version of it will actually be understandable:


And again: All parts of the lens are used for all parts of the image.

This may not be true for all elements in a complex lens, and may therefore not be true for the element with the problem, but it should be true for some elements and I don't see any way to know which without detailed knowledge of the lens in question.
I think you have part of that backwards.

Every element in a lens (simple or complex) is used.

07-14-2010, 10:23 PM   #24
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QuoteOriginally posted by lmd91343 Quote
I think you have part of that backwards.

Every element in a lens (simple or complex) is used.
Part of the problem is that the English word "lens" means both that which we mount on a camera and the elements that make this up. I should have avoided this ambiguity.

In this case I'm talking about the elements. My claim is that for at least some elements in a typical complex lens design, every part (that is the full surface) is used for all parts of the image.

I should perhaps also clarify that "simple lens" doesn't mean "lens with a simple design", it means a single element. (And a complex lens then is any lens used on a camera, except some very crappy ones.)
07-14-2010, 10:37 PM   #25
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QuoteOriginally posted by drougge Quote
. . .

In this case I'm talking about the elements. My claim is that for at least some elements in a typical complex lens design, every part (that is the full surface) is used for all parts of the image.

. . .
This I will agree with and is actually different from what I gathered from your earlier posts. They way I interpreted your early posts was that you were saying that every part of the image went through every part of the elements.

I never said only part of the elements and groups were used. Obviously in this case, the project at least a 43.3mm circle diameter. What I was saying is only the part that covers the rectangle will be recorded on film or by the sensor.
07-14-2010, 11:46 PM   #26
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QuoteOriginally posted by Steinback Quote
My line of thought was similar to Blue's, with an unexpressed recognition that what lmd91343 is describing could well be a (likely fairly small) problem visible under some lighting conditions but probably not a major factor in day to day use.

Obviously ducdao now has to go out and buy a pristine M85 f/2 so that he can test the two lenses against each other under a wide variety of lighting conditions to determine just how significant the optical degradation caused by the element separation really is (I'm too lazy to pull out my textbook that covers lens theory to see if it has anything worthwhile to add to the discussion; I have an interest in applied optics but find the theory kind of boring).

I'm happy to hear that Eric thinks he can separate and re-glue the elements. Presumably the cost of the repair is less than the cost of replacing the lens?
Good suggestion. Now I have a valid reason to buy another M85 f/2 lens

Eric's fee would be around $100
07-15-2010, 01:39 AM   #27
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Not a bad price for the repair. An M85 must be worth $250 + at current market prices and if the lens is repaired well its value is probably increased by at least $100 from what it is worth in its damaged state.

Probably fodder for a new thread, but the concept of "upgrading" to a better cement reminded me of a theoretical problem I've considered a few times. If somebody were to take an old uncoated or poorly coated lens and have the elements professionally stripped and coated with the most advanced modern coatings available, how much would the lens properties change? Considering the lens wasn't designed with the new coating in mind, how big is the risk of introducing undesirable side effects (possibly changing UV hotspot characteristics or the amount of vignetting wide open). I know there are a number of factors involved, but you would be changing how much light is passed through each element and between elements, which impacts ghosting, flare, overall light transmission through the lens, and other characteristics. I also realize that some Pentax lenses have had their coatings upgraded through different generations, though not in such an extreme way (say the M 50mm f/1.7 through to the FA 50mm f/1.7, or the Super Takumar 50mm through to the various SMC versions).
07-15-2010, 02:05 AM   #28
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QuoteOriginally posted by drougge Quote
Part of the problem is that the English word "lens" means both that which we mount on a camera and the elements that make this up. I should have avoided this ambiguity.

In this case I'm talking about the elements. My claim is that for at least some elements in a typical complex lens design, every part (that is the full surface) is used for all parts of the image.

I should perhaps also clarify that "simple lens" doesn't mean "lens with a simple design", it means a single element. (And a complex lens then is any lens used on a camera, except some very crappy ones.)
I would think that as you stop down a lens, less of the outer part of the elements will be used. Just like the rear element of a 50mm F2 lens has a much smaller diameter than the rear element of a 50mm F1.2 lens. If I'm correct then stopping down OP's 85/2 lens would eliminate any effect of the damange.
07-15-2010, 04:20 AM   #29
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few test shots

I finally got few minutes to try the lens after work. These are all shot at either wide open of stepped down half click. Shot with K100D at ISO 400. I remove the noise a bit with LR 3 and no other PP made.

Sorry in advance for boring and redundant subject.

What do you guys think?









07-15-2010, 02:45 PM   #30
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QuoteOriginally posted by Steinback Quote
Not a bad price for the repair. An M85 must be worth $250 + at current market prices and if the lens is repaired well its value is probably increased by at least $100 from what it is worth in its damaged state.

Probably fodder for a new thread, but the concept of "upgrading" to a better cement reminded me of a theoretical problem I've considered a few times. If somebody were to take an old uncoated or poorly coated lens and have the elements professionally stripped and coated with the most advanced modern coatings available, how much would the lens properties change? Considering the lens wasn't designed with the new coating in mind, how big is the risk of introducing undesirable side effects (possibly changing UV hotspot characteristics or the amount of vignetting wide open). I know there are a number of factors involved, but you would be changing how much light is passed through each element and between elements, which impacts ghosting, flare, overall light transmission through the lens, and other characteristics. I also realize that some Pentax lenses have had their coatings upgraded through different generations, though not in such an extreme way (say the M 50mm f/1.7 through to the FA 50mm f/1.7, or the Super Takumar 50mm through to the various SMC versions).
.

Have you heard of someone who will re-coat a lens? I didn't know anyone or any manufacturer did that, I thought that if you had a damaged coat, the 'fix' is to replace the whole element (if they're still stocked.)

I thought the coatings were applied at the point of element manufacture.




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