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Is it the lens or is it the camera producing a black blob on every photo? How can I tell which is at fault? It is a used Nikon camera kit I have just purchased and there is only one lens so I can't do a comparison test.

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7  
Depends. What does the black blob look like? Can you post an example? –  mattdm May 14 '13 at 19:08
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It's unlikely to be the lens - see kurtmunger.com/dirty_lens_articleid35.html –  Nick Miners May 14 '13 at 21:38
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Apply the scientific method. –  Phil May 15 '13 at 6:57
    
@mattdm Can he post an example if he has only one point of reputation? @ BILL upload a test image to some other place and edit your question with the link. –  Unapiedra May 15 '13 at 7:33
    
Many thanx for all those answers. I have sorted the problem by contacting the seller who is sending me a replacement camera body so he might have known about it beforehand if he is confident it is the body at fault. –  BILL LOUGHLIN May 15 '13 at 12:26
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2 Answers

i suspect you have a dust spec on your digital sensor (assuming here that this is a DSLR).

Fit a lens and set the aperture to be small ~f16 and take a photo of a light surface, you should see your blob. Change lens and repeat.

If the blob is still there then you have a dirty sensor that needs to be cleaned. If is not then you have a dirty lens and that needs to be cleaned.

[edit] Since it is a recent purchase, is it new or s/h ? either way the most likely culprit will be sensor dust bunnies - can you go back to the retailer ? [edit over]

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2  
Not helpful if he only has one lens. With one lens, maybe different focal length change the appearance. –  Unapiedra May 14 '13 at 20:05
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the op didn't say and I admit to the assumption of them having more than one lens ........ but the question was how to determine which is at fault. My answer gives the solution; therefore I deem it to be helpful. If he has a single lens, well then the trial can be done with lens fitted and repeated without. Not sure that changing the focal length would offer any solution, of course you're making the assumption of a zoom and not a prime lens ! –  Tony May 14 '13 at 22:55
    
You are right, he didn't say. Now he does say so. I suspected this, because most people do not have more than one lens and his question and user account give the impression that he might be new to photography. –  Unapiedra May 15 '13 at 7:31
    
good spot :o) well done –  Tony May 15 '13 at 7:44
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Especially if your camera is old, the most likely candidate is that your sensor is dirty.

You can find this out, if you

  • put your camera in A-Mode (on the dial).
  • Then turn the wheel so that Aperture is f/16.
  • Face a white wall.
  • In manual focus, focus very far away (infinity). This way you won't see the structure of the wall.
  • Take a picture.
  • Inspect the picture for specs of dirt (it might be easiest if you do this on a computer).

If you can see the blob clearly with this test, then the blob is on your sensor. Look on this site for questions on how to clean your sensor, or have it done in a camera shop.

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