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03-08-2022, 05:05 PM - 2 Likes   #1
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Finger print = Bad

Having a discussion elsewhere about effect of having a smudge on the front of your lens and thought I would do a rough and ready experiment.
I suspected something like a fingerprint, being translucent, would have a greater effect than something opaque on your front element.
Rather than foul up my front element I chose to use filters. I half covered the filter with the different materials and used the lens wide open, figuring this would maximise the comparison.
I had two types of cellotape to mimic a fingerprint, one a little milkier than the other but not as bad as "invisible tape" As you can see in the master shot both are all but invisible.
So yes I think a fingerprint punches above its weight in mucking up your image.







03-08-2022, 06:15 PM   #2
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Bravo for testing. I am surprised by the outcome.

Are all of the impairments in the same place - bottom half?
03-08-2022, 10:02 PM   #3
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QuoteOriginally posted by UncleVanya Quote
Bravo for testing. I am surprised by the outcome.

Are all of the impairments in the same place - bottom half?
Naah my master illustration confuses that. They are on the right hand side. Of course with a wide open 50 that effects all of the image circle pretty well equally.
My first attempt consisted of a square of tape in the middle of the filter. But that meant with the black only the flawed outer edge was providing the light lowering quality unfairly.
03-09-2022, 03:37 AM   #4
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I believe by shooting the lens wide open you quite minimize the effect of dirt on the front element.

But the results are what I expect. The effect of dirt, dust, obstacles is greatly exaggerated.

03-09-2022, 03:58 AM   #5
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QuoteOriginally posted by m96 Quote
I believe by shooting the lens wide open you quite minimize the effect of dirt on the front element.
If you think of the black tape as a piece of dirt then agreed. But the point here is something close to transparent as in a finger print behaves differently because rather than stopping light it is bending it into random directions that the lens can't correct.
And this also applies deep inside a lens. So if you (like me) have a dangerous amount of knowledge and decide to delve into a lens to remove a tiny opaque speck and end up leaving a tiny fingerprint in there then you are worse off. That speck of dirt is optically invisible but not so a fingerprint.
03-09-2022, 06:17 AM   #6
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After reading your experiment it reminded me of an article I read a few years ago.
Lens Rentals | Blog
Just goes to show, worrying about every speck on the lens is a waste of time.
03-09-2022, 07:20 AM - 1 Like   #7
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QuoteOriginally posted by DeKay Quote
After reading your experiment it reminded me of an article I read a few years ago.
Lens Rentals | Blog
Just goes to show, worrying about every speck on the lens is a waste of time.
The single biggest source of damaged lenses when I was in the game of selling camera gear was frontvelements ruined by people trying to keep their lenses spotless.
The quickest way to wreck a lens is to clean it.

03-09-2022, 12:48 PM   #8
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QuoteOriginally posted by GUB Quote
Of course with a wide open 50 that effects all of the image circle pretty well equally.
You'll see the effect gets significantly worse at f/8 and ludicrous at f/16. Shooting wide open minimizes the filtration.
03-09-2022, 01:28 PM - 1 Like   #9
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QuoteOriginally posted by Alex645 Quote
You'll see the effect gets significantly worse at f/8 and ludicrous at f/16. Shooting wide open minimizes the filtration.
Yes I was just outside demonstrating this.
I put a 6 mm dot on the filters. And shot at f8.
The point is if you have a tiny fingerprint \ grease mark on your lens that happens to line up with your critical subject matter then you have a problem. It looks similar here with the black dot but it is really just a loss of exposure vs blurred.
If the dots were actually on the element then the black dot effect would be lessened (spread over a wider area) but the fingerprint would stay relatively unchanged.
I would rather slowly wear my lens out rather than tolerate a fingerprint on my lens.
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03-09-2022, 05:16 PM - 2 Likes   #10
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What I find interesting is my rear element being cracked on my 85 1.4



Outside of shooting directly into light there's no discernible effect on the image.

Single In - October 2021 Cracked FA* 851.4 | Flickr
03-09-2022, 05:29 PM   #11
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Yes I have seen this poor lens before. Amazing results considering.
I guess the damaged bits are spreading the light so widely it is more like an opaque obstacle rather than a translucent one.
03-09-2022, 06:00 PM   #12
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I'm not sure that obstructions on a filter give a true representation when compared to being on the front of the lens itself. Why? Because the light hitting the lens is already affected and is being processed as impaired light coming from the filter in front. When it is on the lens, it is focused in a normal way and that doesn't include what is on the front of the lens.
https://www.diyphotography.net/can-you-still-take-photos-even-with-a-cracked-lens/
03-09-2022, 06:55 PM   #13
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QuoteOriginally posted by arnold Quote
I'm not sure that obstructions on a filter give a true representation when compared to being on the front of the lens itself. Why? Because the light hitting the lens is already affected and is being processed as impaired light coming from the filter in front. When it is on the lens, it is focused in a normal way and that doesn't include what is on the front of the lens.
https://www.diyphotography.net/can-you-still-take-photos-even-with-a-cracked-lens/
Ahaaa -waiting for someone to say that!
I couldn't face putting the stickies on my 8 element 50 1.4. So here they are on the surface of a Chinon 50 1.4.
I took the shots here at f5.6 because at f8 the black dot was still eclipsing the f8 iris - they were about the same size. And I wanting to present that the black dot doesn't degrade the image like the translucent one - just darkens it.
Note the sickle shaped bokeh in the black one.
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03-09-2022, 07:28 PM - 1 Like   #14
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QuoteOriginally posted by GUB Quote
Ahaaa -waiting for someone to say that!
I couldn't face putting the stickies on my 8 element 50 1.4. So here they are on the surface of a Chinon 50 1.4.
I took the shots here at f5.6 because at f8 the black dot was still eclipsing the f8 iris - they were about the same size. And I wanting to present that the black dot doesn't degrade the image like the translucent one - just darkens it.
Note the sickle shaped bokeh in the black one.
In for a penny… now put a fingerprint on that lens. Lol.
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