I have a first generation digital Canon Rebel, and I started noticing a shadow in some of my pictures recently. I have narrowed it down to a single lens, but I cannot find the problem when I inspect it. The shadow is shaped like a worm (approximately 2 inches long on a 11x17 photo), but it isn't always in the way. After I found the shadow, which I was fortunately able to edit out in some of the photos, I took some other shots as a test, and it had disappeared (possibly due to moving from the shoot to the house). Any ideas on what this might be and/or how to resolve this problem without trashing the lens?
Problem with lens
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purpleclouds
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Some sample photos may be of use 
HoppyUK
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Probably dirt on the sensor, which only shows when you shoot at a high f/number when high depth of focus pulls it into view. Sounds like you could blow this little sucker away with a rocket blower or similar. See your camera manual re sensor cleaning.
TriggerHappy
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I had a hair in one of my lenses lodged between the first rear lens element at the aperture blades that produced a similar result. It puzzled me for some time before a quick blow of air with the blades held open (using the catch, not direct contact!) dislodged the hair and solved my problems.
HoppyUK
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Whatever you cleaned from the lens would not have been visible on the sensor. It's not possible.
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Agreed. Sounds like a small hair on the sensor which has possibly now dropped off.
TriggerHappy
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Must have been a coincidental fix in that case. I was only offering a suggestion and make no claim to be an expert...yet!
HoppyUK
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That is a spec of dirt on the sensor. It gets into the camera usually when you change lenses and when the mirror flips up and down it is blown around and eventually ends up on the sensor. What you are seeing is the shadow it casts, which is usually only visible at high f/numbers.
Set the camera to sensor cleaning mode (see handbook) and get a blower to it. Don't touch the sensor except with a special sensor cleaning kit. The spec is probably too small to see easily.
Set the camera to sensor cleaning mode (see handbook) and get a blower to it. Don't touch the sensor except with a special sensor cleaning kit. The spec is probably too small to see easily.
StewartR
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If it is contamination on the sensor, then it suggests his testing procedure was faulty in some subtle way.
One thing that strikes me about that sample image is that it was shot at f/25. (Look how sharp that structure in the background is!!!) Sensor contamination shows up much more effectively at small apertures. If he tested each of his lenses systematically at a range of apertures, he might have found the problem on more lenses.
StewartR
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Actually, I'm not 100% sure about this. If there's something on or just inside the rear element of the lens, and you shoot with a small aperture, it might show up.
I agree that this sort of problem is almost always on the sensor. But I wouldn't want to entirely rule out the lens, especially if the OP has tested all his lenses at small apertures.
HoppyUK
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Hi Stewart. It's not really possible. Depth of focus with short focal length lenses is miniscule - measured in fractions of a mm. It increases with high f/numbers and with very long focal length, but to all intents and purposes with 'miniature' cameras like DSLRs it is effectively zero. It's the angle of the imaging light cone which is always relatively obtuse except on something like a 10x8in technical camera (standard lens 300mm) shooting at f/64.
I once spoke to a Leica lens designer who told me that one of the problems with their 50mm f/1 Noctilux was that it was impossible to guarnatee the film being perfectly flat within the acceptable zone of depth of focus!
I've just has a quick google to find some actual numbers but nothing comes up immediately. Presumably because it is not relevant to regular photography, but there's no way anything remotely near the rear element would ever be drawn into discernable sharpness.
For the same reason, we only ever get to see dust on the sensor when shooting at very narrow apertures, like f/22. Of course, any dirt is not actually on the sensor surface itself, but on the filter/s just a tiny distance (like less than 1mm) in front. Even that distance is enough the throw it out of focus most of the time.
StewartR
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Hmmm. You might be interested by this, then.
Sigma 4.5mm fisheye, f/16.
What you're looking at (amongst all the sensor dust elsewhere in the picture) are two small marks on the FRONT element of the lens.
HoppyUK
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LOL Stewart. Yes, that is very interesting And equally bewildering 
I really would like to know what's going on there. All I can say is that fish-eyes distort the normal laws of optical physics in much the same way as they distort the image, but I have to confess I've not seen anything like that before.
What do you think? Things in focus on the sensor and on the front element? That is unconventional to say the least!
And equally bewildering I really would like to know what's going on there. All I can say is that fish-eyes distort the normal laws of optical physics in much the same way as they distort the image, but I have to confess I've not seen anything like that before.
What do you think? Things in focus on the sensor and on the front element? That is unconventional to say the least!
StewartR
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I didn't say they were in focus. Just that they were visible.
leaky5
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I had exactly the same thing, I took a Minolta lens apart to clean some fungus out and in doing so left a small hair or the like on the inside. It did show up on a photo and I know it was this as you could see it if you looked through the lens and when I took the lens apart again I found and removed it.
Afterwards it did not show on any pictures.
I must admit that I tend to only use that lens for for the outdoor/high f/number shots, and I use a different one for my interior/controlled lighting shots.
I have inspected the exterior parts of the lens, and I never saw anything that seemed out of the ordinary. I also used an air blaster and a lens cleaning cloth to make sure that nothing was on the lens. Unfortunately, the shadow was still there.
I will check tonight to see if there is something on the sensor, and I will post the results.
leaky5
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In fact I still have the lens, although I don't know if it takes pictures anymore, I broke the zoom encoder after the 3rd clean. :thumbsdown:
If it does take pictures I will dismantle it again and insert a few foriegn bodies on one of the internal elements and then see if they show up.