Thumbprint on Rear Element Affected Performance?

[Guest leica_mage]

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Chaps, a very simple question. After a few months of using my second 35mm Summicron ASPH (the first I'd used for two years with no complaints but I wanted to go silver chrome) I suddenly discovered a thumbrint (not mine!) on the rear concave element, which I've now cleaned. It became properly visible only in oblique sunlight, which is why it had gone undetected for so long:

 

Could that thumbprint alone be responsible for a slight drop in microcontrast as well as some occasional veiling glare, according to your experience? I shall find out fairly soon (and shall report back), but I was wondering if anyone here had a quick answer based on personal - not theoretical - experience. I always had the impression that the SC lens underperformed with respect to my earlier BA one and was wondering whether that thumbprint (covering almost the entire suface of the element) could be the culprit.

 

Many thanks in advance for any hands-on responses (pun intended ).

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[dave_d]

A thumb print on the rear element is the cause of your problems. In fact a thumb print or dust on the rear element will be more noticeable in your photos than the same on the front element. Both will degrade your image quality.

[Guest leica_mage]

A thumb print on the rear element is the cause of your problems. In fact a thumb print or dust on the rear element will be more noticeable in your photos than the same on the front element. Both will degrade your image quality.

I thank thee, Sire. Could it have been responsible for that noticeable degree of degradation?

 

I hope so, but what worries me is this excerpt:

 

Fingerprints should be cleaned off immediately as the grease from fingers (or any skin) can eat into the coatings of a lens, destroying it for ever...

 

which I found on this site. How likely is this to have happened, if at all?

[lars_bergquist]

Depends on what the chap had on his fingers: Sweat, fluorine, peanut butter ... Just clean the surface and examine it in reflected light from different directions. If you can't see it anymore, it isn't there.

 

The old man from the Age B.C. (Before Coating)

[Guest leica_mage]

Just clean the surface and examine it in reflected light from different directions. If you can't see it anymore, it isn't there.

As I said, I did clean it. It is no longer there. Nada. However, I am still concerned that damage may have been done to the coating - or did you intend that had that been the case, the damage would still show up as a thumbprint pattern?

[budrichard]

I'm not an optical engineer but I am an engineer and did spend some time studying optics as an undergraduate. My understanding of the way a lens images is that linear defects such as thumb prints do not image as linear images. Every part of the lens is involved in imaging every part of the scene. This can be understood by the fact that when you close the diaphragm, the only thing that changes is the total amount of light reaching the recording media but the image is the same.

I would not expect a single thumbprint to measurably change the objective measured optical parameters of a lens.

Since you have removed the print, an objective before and after test is no longer possible.

If under magnification you can no longer view the print or any etching of the coating, there is no problem.

Frankly, 'A Tempest in a Teapot'! -Dick

[guywalder]

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I would not expect a single thumbprint to measurably change the objective measured optical parameters of a lens.

Dick

 

While I would agree that marks and indeed scratches can have a surprisingly small impact on resolution, the effect they can have on flare under adverse conditions is another matter....

[Guest leica_mage]

Frankly, 'A Tempest in a Teapot'! -Dick

That depends on the verdict based on distinct personal experience, and it's not out yet. Your teapot might be my swimming pool, and my teapots might be your oceans!

 

I'll rephrase the question, with emphasis:

 

Could that thumbprint alone be responsible for a slight drop in microcontrast as well as some occasional veiling glare, according to your experience?

 

In other words: did you ever experience a curious drop in a lens' performance, only to discover that a beefy fingerprint was the cause (and breathe a sigh of relief)?

[lars_bergquist]

As I said, I did clean it. It is no longer there. Nada. However, I am still concerned that damage may have been done to the coating - or did you intend that had that been the case, the damage would still show up as a thumbprint pattern?

 

Exactly.

 

The old man from the Age B.C.

[mhoersch]

I'll rephrase the question, with emphasis:

 

Could that thumbprint alone be responsible for a slight drop in microcontrast as well as some occasional veiling glare, according to your experience?

 

In other words: did you ever experience a curious drop in a lens' performance, only to discover that a beefy fingerprint was the cause (and breathe a sigh of relief)?

 

With emphasis: So far, I have tried to avoid that experience.

[johnnyj]

It’s of interest that you should ask this question because I just had a very similar occurrence, or rather my wife did. The other day I was reviewing some images that my wife had taken with her D-Lux 3 when, noticeably, the center of the images I was looking at became less contrasty. At first I thought that lens flare had caused the degradation of the center of the images. Then I thought, "I wonder if ..." and turned the camera around, took off the lens cap and there in the middle of the lens was a fingerprint. The image degradation was noticeable with loss of contrast and resolution compared with the edges of the image. I would imagine that a fingerprint on the lens element closest to the film or sensor may have even a greater impact on image quality since your image is being projected through a larger circumference front element to through a smaller circumference rear element.

[lct]

...Could that thumbprint alone be responsible for a slight drop in microcontrast as well as some occasional veiling glare, according to your experience?...

According to my long experience, it does nothing visible at all unless i've plunged my finger into marmelade, butter, motor grease or French cheese essentially. French cheese is the worst by far. The simple fact to breath on the lens after eating some Camembert, for instance, and you can get some stinking flare forever.

[Guest leica_mage]

It’s of interest that you should ask this question because I just had a very similar occurrence, or rather my wife did. The other day I was reviewing some images that my wife had taken with her D-Lux 3 when, noticeably, the center of the images I was looking at became less contrasty. At first I thought that lens flare had caused the degradation of the center of the images. Then I thought, "I wonder if ..." and turned the camera around, took off the lens cap and there in the middle of the lens was a fingerprint. The image degradation was noticeable with loss of contrast and resolution compared with the edges of the image. I would imagine that a fingerprint on the lens element closest to the film or sensor may have even a greater impact on image quality since your image is being projected through a larger circumference front element to through a smaller circumference rear element.

Very helpful, and I'm very grateful for this.

 

According to my long experience, it does nothing visible at all unless i've plunged my finger into marmelade, butter, motor grease or French cheese essentially. French cheese is the worst by far. The simple fact to breath on the lens after eating some Camembert, for instance, and you can get some stinking flare forever.

Highly unhelpful, vague and unempirical. Please refrain. Or do a remake of the Monty Python 'Cheeseshop'.

[lct]

...Highly unhelpful, vague and unempirical. Please refrain....

I promise. I'll try with Reblochon next time.