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03-04-2008, 07:10 AM   #1
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Green spot in the middle of the frame

Howdy,

I was testing a Pentax-A SMC 50mm/1.4 lens today and found a weird looking greenish spot that appears right in the middle of the frame when the aperture is stopped to f8 and down. The camera is Pentax *ist D. I did not see the phenomenon with any other lens I own.

Here are some photos. It is especially visible here at f16 and here at f22.

Any ideas, anyone?

Thanks,
Eli

03-04-2008, 07:20 AM   #2
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the spot almost looks like it is present on all frames, and becomes more focused as you stop down

Was this test done with or without a filter? It looks like flare, but seems to be almost centered.

The other possibility is a reflection (i know it seems unlikely) from the camera you are photographing.

Also, how many blades are in the diaphram on your lens, I think I can see that the spot is 8 sided. My SMC 50mm F1.4 has 8 blades.
03-04-2008, 07:24 AM   #3
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No visible marks on the lens. No filter was used. The spot is visible when I took pictures of the background only.

It does look like a very weird flare.
03-04-2008, 07:32 AM   #4
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take a picture without a lens on

03-04-2008, 08:37 AM   #5
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I have seen this before.
I think it is a reflection from the rear element back to the sensor.

Make more tests:
Has it to do with the position of the lighting source?

Try a lens hood.
I you have none that is fitting, use one made of paper.
Just to test the effect.


EDIT:
Here is the same effect:
Frage - Starke Reflexionen beim AF Nikkor 50mm F1,4D - Nikon Fotografie-Forum
(German, but note the image, conclusion: Sensor reflection)

EDIT 2:
The Digital Sensor Reflection Effect
(looks like your spot)

Last edited by blende8; 03-04-2008 at 08:51 AM.
03-04-2008, 08:45 AM   #6
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QuoteOriginally posted by blende8 Quote
I have seen this before.
I think it is a reflection from the rear element back to the sensor.

Make more tests:
Has it to do with the position of the lighting source?

Try a lens hood.
I you have none that is fitting, use one made of paper.
Just to test the effect.
Agreed.

The Tam 90 does this too. I did (an informal) test with it using a non-reflective subject and no light hitting the lens. Lighting source was a lamp shining on the subject, with the tripod above the light.

The spot was only visible at f/16 and smaller, but others have reported it at f/11.

Not an issue for me though, since it rarely sees f/11 or smaller...
03-05-2008, 06:52 PM   #7
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Were you looking through the viewfinder when you took these photos? Could this be light coming through the viewfinder?
03-05-2008, 07:26 PM   #8
Ole
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QuoteOriginally posted by ramshot Quote
Were you looking through the viewfinder when you took these photos? Could this be light coming through the viewfinder?
No, that's not possible. The mirror swings up and covers the focusing screen during exposure.

03-05-2008, 08:33 PM   #9
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I have many flare samples coming from various lens, first the green one that looks like yous and some red ones in sunset time.

From tokina 19-35 f/3.5-4.5 (without lens hood & filter)





Some become red




And these from my Vivitar 24mm f/2.8 manual,
I make it b&w to make it less pronounced





It is embarrassing to say the least. And it seems that I have less after getting a lens hood for my Tokina but I have not gotten the lens hood for all of my other manual lens. The green seems to come from internal reflection form the sensor plate and probably the digital designed lens will have less due to perhaps better coating on the rear end. Not all of my film lens have those and I think a lens hood will be first thing that I will attempt to test on the same lens in the same setting.

Thanks,
Hin

Last edited by hinman; 03-06-2008 at 11:06 AM.
03-06-2008, 10:49 AM   #10
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I have had the green spots with my 18-55 kit lens a couple of times, usually shooting into the sun or sunsets. I have been able to photoshop them out. I was using the lens hood the last time shooting a sunset scene and had a couple of green spots like the ones in hinman's first 2 photos. I also had a uv filter on. Maybe a polarizer would help in shots like that. I'm still learining.
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