C100: Spots on image

T

TCollings

Member
On my C100 I am seeing two blurry round dark spots on the image when I close down to f16-22 and point the camera at a very bright light source. It does not seem to be a lens issue, as it happens with any lens. Could it be dirt or dust on the sensor? Do I have to send the C100 in to Canon for cleaning/repair?

Here is what it looks like (spot just below middle center and another spot above and to the right of it):
image.tiff
 
You should never be stopping down to that range unless it's an extreme emergency or some sort of lens technique you know about and I don't. A lens is sharpest around two stops down from wide open and when you get into the maxed stopped down range you're asking for problems, especially when you're focusing close. That's why dust or a speck will show up when it wouldn't at wide open focused far.

Many of the lenses that were made for stills, as compared for film & ENG, have places where dust can get in the barrel, especially if you bought it used. Turn all the lights off in the room, stand a flashlight on it's end facing up, place the lens over it in different positions and rotate a bit, use a magnifying glass. Unless it is a lens of the highest quality and brand new you will see LOTS of specks. These will be unnoticeable until you stop all the way down and focus at the MOD: Minimum Object Distance. Also, if you point at the clear sky stopped down to 11, 16 or 22 and the sun flares you, you will see every damn speck. I have owned lenses with nicks on the front elements from shooting grinding, welding, etc. without a clear protective filter on the front. Stupid I know. So I made a mental note never to stop down far and focus close especially against a plain background with that lens issue. I now always have a clear UV high quality filter on all my lenses and only take them off when doing teleprompter shooting, and that is based on superstition rather than fact.

Also, always buy a used lens with a 48 hour return window so you can do the flashlight inspection mentioned above.

In sum, start using the ND wheel and lower your ISO so you don't have to stop down that far. If Depth of Field is not a concern and you prefer not to be off on your focus, then stay mid range on a lens, avoid flare. When shooting a plain area such as a cyc in a studio, seamless paper for product photography, clear blue sky, etc. that's when dust and such is most noticeable to the viewer. Conversely, you should avoid wide open unless on the most exquisitely built lenses. See:

http://www.lensrentals.com/blog/2011/11/stop-it-down-just-a-bit
 
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