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06-04-2010, 11:24 AM - 1 Like   #1
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Is sensor reflection a myth?

I ask this question to all those who have a lens or two that exhibits severe loss of contrast, usually in a circular pattern when the lens is stopped down.

I got prompted on this issue, because I have such a lens, a Rikenon 135mm F2.8 screw mount, and because there was a post a few weeks ago with a complaint that their 645 pentax lenses took poor photos on their K20.

https://www.pentaxforums.com/forums/pentax-dslr-discussion/102680-my-k20d-tak...enses-why.html

The results looked the same as mine, and I assumed, wrongly, that it was sensor reflection off the elements.

When reviewing the thread as it developed, and photos of the adaptor were shown, it was obvious there were shinny black surfaces all around the optical path. The OP used part of an old sock to absorb the light reflecting off these surfaces and posted before and after shots with the same lens.

This got me thinking about my Rikenon 135, so I looked at it, and the retaining ring for the rear element was indeed very shinny black.

I painted it with flat black acrylic craft paint, and the difference is amazing, as shown by the shots below, before and after shots of my "lens calibration wall" where I check exposure consistendy, and also a more real world subject.

While I am pleased that I fixed this lens, the issue was more principle than cost, I hate buying anything that does not work properly, it opens up a bigger question. Is the issue of sensor reflection off the rear element a myth, and the issue really is poorly finished internal lens parts.

What do you think?

One thing I know, I have to go back and edit my lens review, and perhaps upgrade the rating based on what I now know

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06-04-2010, 12:58 PM   #2
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It really is amazing how much of an difference some matte paint makes. In normal use none of my lenses have this problem but when reversed I got a pretty serious hot spot in the centre with the M 3.5/28. So it has to be the reversing adapter which indeed is shiny coated black. Must have to try if un-shinying does the trick.
06-04-2010, 12:58 PM   #3
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QuoteOriginally posted by Lowell Goudge Quote
This got me thinking about my Rikenon 135, so I looked at it, and the retaining ring for the rear element was indeed very shinny black.

I painted it with flat black acrylic craft paint, and the difference is amazing, as shown by the shots below, before and after shots of my "lens calibration wall" where I check exposure consistendy, and also a more real world subject.

While I am pleased that I fixed this lens, the issue was more principle than cost, I hate buying anything that does not work properly, it opens up a bigger question. Is the issue of sensor reflection off the rear element a myth, and the issue really is poorly finished internal lens parts.
The light reflecting off that element has to come from somewhere, and one presumes much of it got there by reflecting off the sensor first. So no, it's not a myth, you're just debating whether the light the sensor reflects is returned by the lens element, or the rest of the lens internals. I'd guess the answer is that it's returned by both, and the ratio to which each contributes to the problem varies with the specific lens design.

As for why this is an issue, remember that you're talking about a lens that predates digital. Likely their finish, while shiny, was perfectly fine when used with film, because less light was being reflected onto it in the first place. Sensors are significantly more reflective than film, so they are more likely to cause the issue to rear its head. So it isn't really a poorly finished part, just one that wasn't designed with your current use in mind.

It's likely that if the same lens was made today, a different finish would've been chosen for just this reason. I'd be a bit concerned about using craft paint myself though, as it seems likely to degrade with age and start to shed dust specks, which could cause problems depending on where they settle.

The important thing, though, is that you're happy with the result. Sensor reflection assuredly is an issue, and one that lens manufacturers take into account with their current for-digital lens designs. It's good to hear you've found a solution for your own issue with a pre-digital lens, and others may be able to learn from it too...
06-04-2010, 01:02 PM   #4
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QuoteOriginally posted by knoxploration Quote
The light reflecting off that element has to come from somewhere, and one presumes much of it got there by reflecting off the sensor first. So no, it's not a myth, you're just debating whether the light the sensor reflects is returned by the lens element, or the rest of the lens internals. I'd guess the answer is that it's returned by both, and the ratio to which each contributes to the problem varies with the specific lens design.

As for why this is an issue, remember that you're talking about a lens that predates digital. Likely their finish, while shiny, was perfectly fine when used with film, because less light was being reflected onto it in the first place. Sensors are significantly more reflective than film, so they are more likely to cause the issue to rear its head.

It's likely that if the same lens was made today, a different finish would've been chosen for just this reason. I'd be a bit concerned about using craft paint myself though, as it seems likely to degrade with age and start to shed dust specks, which could cause problems depending on where they settle.

The important thing, though, is that you're happy with the result. Sensor reflection assuredly is an issue, and one that lens manufacturers take into account with their current for-digital lens designs. It's good to hear you've found a solution for your own issue with a pre-digital lens, and others may be able to learn from it too...
I don't dispute that the light comes from somewhere, and the sensor is a possible candidate, and in my case specifically this is true because the issue is not visible in the viewfinder, but in the post I reference, with the 645 adaptor, the OP of that post could see it in the viewfinder also, meaning it was not sensor reflection because the mirror and shutte blocks the sensor.

I think it is a case by case issue, BUT the discussion dor Digital lenses has always centered around the coating of the rear element, and not the mechanical parts. I think that is where the myth is.

as for craft paint, this is not the same as children's paint, acrylic craft paint is waterproof and durable. and this is not going to be a high abrasion hogh impact area, after al it is inside the lens barrel.


Last edited by Lowell Goudge; 06-04-2010 at 01:59 PM.
06-04-2010, 08:34 PM   #5
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The obvious answer is that you have solved the problem very cheap for your own lens. It well known (or at least claimed) that digital sensors reflect some light in ways that film do not which is why newer "made for digital" lenses have different coatings. I can understand easily how there could be all kinds of lens flare problems from glossy paint on the internal parts of the lens. All one has to do is stroll through the lot of a body shop on a sunny day and observe the differences in reflection from the areas of origional paint and the partially repaired areas with just primer.
06-04-2010, 09:49 PM   #6
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QuoteOriginally posted by Lowell Goudge Quote
I don't dispute that the light comes from somewhere, and the sensor is a possible candidate, and in my case specifically this is true because the issue is not visible in the viewfinder, but in the post I reference, with the 645 adaptor, the OP of that post could see it in the viewfinder also, meaning it was not sensor reflection because the mirror and shutte blocks the sensor.

I think it is a case by case issue, BUT the discussion dor Digital lenses has always centered around the coating of the rear element, and not the mechanical parts. I think that is where the myth is.
Have to admit I didn't have time to read the linked post... But yeah, it's certainly a case by case basis. Just noting that the phenomenon is real, even if it isn't the cause in every case.

QuoteOriginally posted by Lowell Goudge Quote
as for craft paint, this is not the same as children's paint, acrylic craft paint is waterproof and durable. and this is not going to be a high abrasion hogh impact area, after al it is inside the lens barrel.
Don't worry, wasn't imagining you daubing kids' paint in there. ;-) What I was referring to was simply that degradation over time can be a problem, as the paint ages. It doesn't necessarily need mechanical abrasion to happen, just the surface of the paint to break down with exposure to the atmosphere, light, changes in temperature, shocks and vibrations, etc. This potentially could free very small specks of paint, which become dust inside the camera body, lens, or both (depending on where the paint was applied). Manufacturers have to take this into account in their choice of materials and coatings too, and even more so for digital than for film.

No idea for sure, but I would imagine that given matte paint needs a rough surface at a microscopic level so as to scatter what light it reflects, it could likely be at least somewhat more prone to these effects than glossier paints.

Which isn't to say tha this will necessarily prove to be an issue -- just that it's something to bear in mind. You may well have considerable knowledge and experience in the long-term durability of the paint you've chosen, and whether it is likely to cause a dust problem to arise. Others following your lead might not know, though, which is the only reason I mentioned it as something to consider. :-)
06-05-2010, 08:47 PM   #7
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This thread has reminded me of something I've been trying to figure out... my Tokina AT-X AF 28-70mm f/2.6-2.8 isn't crisp at f/2.8 and 70mm, and looking closely at (for instance) white text on a dark background in those shots the text has a glow all around it... the glow is pretty much gone by 50mm, where shots are looking really good even fully open.

Looking at the lens off the camera, the rear elements of the lens are farthest away from the camera sensor at 70mm, and the lens significantly predates digital cameras, so I'm wondering if the lens isn't coping with sensor reflection well out at 70mm? Or is what I'm seeing some sort of design limitation of the lens at 70mm and f/2.8?
06-05-2010, 09:50 PM   #8
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This is indirectly related. However, Pentax developed their "Ghostless Coating" during the film era and announced it in 1992. It was implemented in the ltd lens with the FA 43mm ltd being the first to get it. Supposedly all ltd lenses get it and certain premium lenses. This implies that this "ghosting" was also of concern to lens designers for film cameras. Its just the sensors are more ghost prone. Trying to find much out about this "Ghostless Coating" is like chasing an actual ghost. Its hard to know for sure other than the ltd lenses as to which ones got it. Now we have the Aero Bright Coating used on the DA* 55mm which is even more mysterious.

06-05-2010, 09:50 PM   #9
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I've only really noticed (or seen) sensor reflections on macro or closer than normal pictures. I suspect that generally, sensor reflection just robs a little bit of contrast, if it does anything.
If it is coming off the mechanics of the camera or coming off the glass should be easy to figure out.

For example, I'm pretty sure this is off the back element of my 100/2.8 macro (this really pisses me off because this is a lens that I truly love).

https://www.pentaxforums.com/forums/attachments/pentax-slr-lens-discussion/46...-a100flare-jpg
06-06-2010, 04:16 AM   #10
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I get those bright sort fo glowy, fuzzy circle in the middle when I stacked a bunch of ext. tubes and tried to take shots. I was wondering whether it has to do with light reflecting off of the inner bright tube and had considered painting it black.

I think it might help, after seeing these results. Dang! wish I didn't sell all those 'auto' tubes (modded TC's) though
06-06-2010, 06:13 AM   #11
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This is why I made this post.

I have seen a few posts lately with similar flair problems to my 135.

It occurred to me that we may be chasing "ghosts" here with respect to coatings, and that a few glossy parts in the light path are the real culprit.

Since fixing my 135 I have gained new respect for that particular peice of " junk"

it is actually a fine lens.

My only problem is due to reflections I
bought a surperb tele lentar 135 f2.8 preset with 16 blade aperture and fell upon a 135 f3.5 smc tak as well, so I have 3 excellent 135mm M 42 lenses.

What to do???
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