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11-19-2016, 08:59 AM   #1
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Puzzling glow in inverted SMC-M 28mm/3.5

Hi,
I have a 52mm thread to K mount adapter, which I (rarely) use with my SMC-M 50mm/1.7.
Today I tried it with my SMC-M 28/3.5 to get that extra magnification, with a step-down ring to get to the correct 48mm thread size.

Big surprise, there was a glow right in the middle of the image.
It changed size and intensity by varying the aperture (more strong and concentrated when stopping down, obviously), and wasn't related to the angle of the light used to illuminate the subject (diffused room light, flash camera left... same thing), nor the camera angle (I was shooting a coin so I thought it could be a reflection).

I know that the 28/3.5 isn't known for good flare resistance, and that by inverting I completely forfeit the SMC, but if it was a flare I could have controlled it by just changing the position of the light, right?
So what the heck is that?

RAW all default:
strange flare by Lens Beginner, su Flickr

Thanks in advance

11-19-2016, 09:50 AM - 1 Like   #2
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The colour suggests to me that it is a reflection of the sensor on the front element...
11-19-2016, 09:56 AM - 1 Like   #3
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pretty sure that's flare off the rear element,
your working distant must be very short, just a few cm?
to get light on the subject you are probably skimming the light between the lens and the coin, that skimmed light is also across the rear (now front) element. I'm too lazy to dig out mine, but that lens has a protruding rear element if I remember right.


i'd see if there is a way to get a hood or shade the lens from direct light, possibly set the focus helicoid to the closest focus position, that should pull the element as far into the lens as possible, not going to change your magnification much either position.
it's going to be a challenge to light the subject without lighting the rear element too.

one of the reasons longer focal lengths are more popular for macro, greater working distance for equal magnification.

Last edited by K-Three; 11-19-2016 at 10:02 AM.
11-19-2016, 09:59 AM - 1 Like   #4
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That's what I was thinking. A couple of years ago I did some testing with this lens reversed, and didn't have any problems, so I'm a bit surprised to see this. This is with the adapter mounted directly to the camera (i.e., no extension), is that right?

11-19-2016, 10:11 AM   #5
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@K-Three
Yes, very close... but flare is still visible with just ambient light... a ceiling lamp with my body and the camera blocking any direct light.
Focus ring has next to no effect on focusing, with this setup, I'll try and see what it does to the back element.
I'd use a longer lens, but when inverted you get the highest magnification with shorter FL.

@baro-nite
Yes: camera, K-to-52mm ring, 52mm-to-49mm ring, lens (inverted).
11-19-2016, 10:33 AM - 1 Like   #6
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It does look like light reflected from the sensor, back into the lens, and the bouncing off the coating of some slightly concave element. If you change the sensor-to-lens distance, you'll change the size of the spot. You might find that an extension tube can make the spot so large and diffuse that it disappears.
11-19-2016, 10:34 AM - 1 Like   #7
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Afraid I don't have anything to suggest, other than you might try a "macro coupling" adapter to reverse mount the M28/3.5 onto another lens. As a rule I find this gives better results (especially at the edges and corners of the image) than just using the one inverted lens, with or without extra extension. If you have an M100 handy that would be a good candidate for a primary lens, to get a similar magnification.

11-19-2016, 10:47 AM   #8
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QuoteOriginally posted by photoptimist Quote
It does look like light reflected from the sensor, back into the lens, and the bouncing off the coating of some slightly concave element. If you change the sensor-to-lens distance, you'll change the size of the spot. You might find that an extension tube can make the spot so large and diffuse that it disappears.
I could try an UV filter, perhaps... :-)
I'll see tomorrow, thanks for the input!

@baro-nite
Yes, I have the SMC-M 100/2.8 (non macro), I just need the coupler...
11-19-2016, 11:33 AM - 1 Like   #9
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I get exactly the same phenomenon with my 8 element Super Takumar 50mm 1.4 in normal orientation, both on K30 and on K1 with bright shots.
11-19-2016, 01:43 PM - 1 Like   #10
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i get this problem when i stack 4+ extension tubes for super extreme macro... the best explanations i could find are here:

Ghosting with reversed lenses | Poor-Man's Macro (PLEASE, read the description carefully!) | Flickr
Modified cheap Chinese extension tubes part 6/6 | The finish? | Flickr

light baffles work, but it takes some monkeying around
11-19-2016, 04:00 PM   #11
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QuoteOriginally posted by sculptor666 Quote
i get this problem when i stack 4+ extension tubes for super extreme macro... the best explanations i could find are here:

Ghosting with reversed lenses | Poor-Man's Macro (PLEASE, read the description carefully!) | Flickr
Modified cheap Chinese extension tubes part 6/6 | The finish? | Flickr

light baffles work, but it takes some monkeying around
I don't know... seems a different issue... in his case the phenomenon disappears when stopping down, in my case the exrra light just gets "focused" and more concentrated...
11-19-2016, 05:29 PM - 1 Like   #12
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is it visible through the optical viewfinder?
does it appear in live view?

that should tell you if it's the sensor or not.
if it's the sensor, it should not be visible through the viewfinder.
11-20-2016, 02:06 AM   #13
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QuoteOriginally posted by K-Three Quote
is it visible through the optical viewfinder?
does it appear in live view?

that should tell you if it's the sensor or not.
if it's the sensor, it should not be visible through the viewfinder.
Sorry, K-01, no optical anything... but I can do that test with the K100Ds!
...as soon as I get some time I have quite the battery of tests to run, thanks everyone, I'll report back!
11-20-2016, 07:42 AM - 1 Like   #14
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A couple observations & suggestions.
1) with a reverse mounted unit-focusing lens, twisting the focus ring has no effect on focus or magnification because the lenses don't move, only the outer barrel and focus ring move.
2) ghosting/flare is pretty much always a consequence of light bouncing around on lens elements and/or shiny metal;
3) twisting the focus ring to closest distance may shield the lens to some extent, but the bright metal of the exposed bayonet mount can be a problem;
5) a well-baffled extension tube placed on the exposed bayonet mount will act as a lens hood and might solve the problem. A simple, plain tube with no linkage for AF or aperture is perfect.
6) A sheet of black paper or a piece of black velvet wrapped around the lens and held in place with a rubber band might make an effective lens hood.
11-20-2016, 03:14 PM   #15
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Well, it's quite the mistery...
I did some quick tests because I'm a little strapped for time this month...

1. Took out my trusty K100Ds and replicated the original setup:
camera, K->52, 52->49, lens (inverted).
The glow was completely absent in the viewfinder but was there in the pictures (no live view), it was however different, and less marked than it was on the K-01.
If it's a reflection of the sensor, the quality and thickness of the glass covering the sensor (protection plus all the filters) play a significant role, methinks...

2. popped out the K-01 again, since being the issue quite camera-specific I wanted to solve it on the camera I'm planning to use fpr the trick.
camera, K->52, 52mm UV filter, 52->49, lens (inverted).
I was hoping to fudge things a bit, but the UV filter (both the addition of a layer of cheap glass and the added distance) did little to improve the situation.

3. I played another card, trying strange things
camera, K->52, 52mm polarizer filter, 52->49, lens (inverted).
Rotating the filter did almost nothing, but the spot changed into an oblique slash of light... while fiddling with my smartphone (using the led torch because I didn't want to pull out the flash again) I noticed that careful placement of the light made the phenomenon almost disappear.
Key wad to place the smartphone level with the rear (now front) element, slightly angled away, so that the light hit the coin and then bounced away (for the most part at least).

Edit: on careful examination the glow is still there... a slash of light (I guess because it's polarized now...) going from top left to bottom right... gotta think about something else.
Trying to shield the lens did nothing... I bet it's the sensor at this point... but that's harder to solve... probably need some tubes (which I haven't got... can deglass some filters and stack them, though! :-P )

Last edited by LensBeginner; 11-20-2016 at 03:22 PM.
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