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 Post subject: 50mm 1.8D Issue
PostPosted: Sun Dec 18, 2011 4:45 pm 
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There i was looking for dust on the sensor on my D90 and noticed this round lighter circle in the centre of the image.
I thought the sensor was damaged but its only a problem with the 50mm.
Here's a couple of pics. Might be a bit hard too see so ive changed the levels on the second pic so it stands out. These are centre crops of the images.
Camera was pointed at the sky, sun was behind me.
Its noticeable from f16, at f22 its worst.
Any ideas?

Image

Image


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 Post subject:
PostPosted: Sun Dec 18, 2011 6:47 pm 
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Hmm, do you see this only in this one shot or do you have other examples?
What do you see when you look through the lens? Anything inside?
Perhaps that is an UV-hotspot of that lens, that is mostly visible when shooting against clear blue sky???

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 Post subject:
PostPosted: Sun Dec 18, 2011 7:25 pm 
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Forgive me for asking this... but did you try cleaning the lens?

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Trevor Harris

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 Post subject:
PostPosted: Sun Dec 18, 2011 9:44 pm 
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Thomas, i took about 5 pictures in row and they all had this effect. This picture was the worst of those, or where it was most evident.
I was really only looking for dust, which i found plenty of as well...
Lens looks completely clear, i tried with the UV filter removed as well with the same result.
Also tried taking a picture of the wall inside (with a flash) which also showed the issue.

Trevor, lens is very clean though i did clean it anyway thinking there must be something on the glass..


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 Post subject:
PostPosted: Mon Dec 19, 2011 5:13 am 
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I can only hazard a guess - I like guessing about things I know little about!

The 50/1.8 comes from film days. Here, it is OK for the light to hit the film at any angle. In digital terms, the sensors are designed much more for light coming in at 90 degrees.

I wonder if the central portion of the photo is benefiting from the more direct light path when compared with the edges, though I would have thought that it would have been more gradual "fade-out".

The effect is like dust on the sensor - more noticeable at smaller apertures, but I couldn't postulate whether this might prove or disprove the theory.

With digital, I rarely go smaller than f8 because of the diffraction effect, but there are certainly times that one has to.

It would be interesting to see if the 35/1.8, new 50/1.8 and zooms do the same thing. I think Gonz is saying that this isn't the case. Have to go and try myself.

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 Post subject:
PostPosted: Mon Dec 19, 2011 6:52 am 
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Can you reproduce the effect with your 50/1.8D, Hilary?
Btw the spot is slightly to the left and above center (and I assume that the image is not cropped).
One question: does the diameter change noticeably (+40%) when using f16 instead of f22? Or does the edge only become a bit blurred - and contrast lower? If it's the first that hole might simply be a ghost-image of the aperture.
Other explanation could be a defect in coating.
Oh well, I'm only guessing here...

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 Post subject:
PostPosted: Mon Dec 19, 2011 9:45 pm 
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HilaryC
Interesting theory, that's not something i really know about.
Ive got an older Tokina 28mm manual prime that i tried and that was fine.
Also went through all my other lenses at this point to see if it was camera, but only the Nikon 50mm did this.

Thomas
The image is a crop from the centre of the image, i wasn't very careful about centring it so the hole is actually in the centre, sorry that's a bit misleading...
Diameter seems to stay the same between f16~f22, just gets more defined.
Not sure about the contrast changing as ive only really pointed it at the sky, i should take some more test shots and see what happens.
Something else i noticed was when i took a picture of inside the room there was an angle poise lamp on. As i rotated the focus ring i got a diffraction grating effect from the lamp through the viewfinder that rotated when focusing. Very strange, i think i need to get the lens back on the camera and look at all this again..
I had a bit of a read up on the UV hotspot of lenses, but only read about the infra red side as that's what the web pages were really about.


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 Post subject:
PostPosted: Tue Dec 20, 2011 5:10 am 
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A wet clean of the sensor may help


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 Post subject:
PostPosted: Tue Dec 20, 2011 11:07 pm 
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Dear Thomas and others.

Finally, the skies cleared a bit and I was able to do a test.

On my D40, the 18-55 and 35/1.8 exhibit no symptoms, but my 50/1.8D performs exactly as per Gonz's photo. Noticeable at f16, quite obvious at f22.

Obviously a characteristic of the lens, but I have nothing more to add from the guesses.

Maybe a test on other sensors would be interesting, but the solution is reasonably simple - avoid f16 and f22 on the 50/1.8D.

I wonder if the 50/1.8G and 50/1.4D perform differently? That might give a clue to the film/digital theory.

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 Post subject:
PostPosted: Wed Dec 21, 2011 1:09 am 
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Hi HilaryC
I guess that's great news and not at the same time!!
Thanks for taking time out to test it.
I must admit i wouldn't normally have it set much above f8~f11, just had it set higher looking for the dust.

dubaiphil
That will be happening as soon as the cleaning kit turns up, my sensor is filthy :roll:

Many thanks for all your help, just need to find someone with the Tokina 11-16mm to test what their aperture does when zooming...


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 Post subject: reflection
PostPosted: Thu Dec 22, 2011 1:31 pm 
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It is a reflection of the back lens to the sensor, visible at small apertures.


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 Post subject:
PostPosted: Thu Dec 22, 2011 5:47 pm 
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Hello komet, and welcome to the friendly Camera Labs forum!
To enjoy your stay here please have a look at the house-rules!
----
As to your remark: Did you encounter this effect with other gear too?

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 Post subject: 28-300 VC
PostPosted: Fri Dec 23, 2011 10:14 am 
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The Tamron 28-300 VC is als infected.


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 Post subject:
PostPosted: Fri Dec 23, 2011 3:05 pm 
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Here's a link which explains a few things:

http://forums.dpreview.com/forums/readf ... d=40090412


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 Post subject:
PostPosted: Fri Dec 23, 2011 7:15 pm 
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Reflections then, i think i can maybe live with that.
We tried the 50mm D on a Sony A390 with a conversion adapter, it does a similar thing though not such a defined edge, it just gets lighter in the centre.
Thanks for all your input!


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