dust on sensor? lens? mirror?

[nartreb]

OK, basic question but I have not been able to solve this yet:

Take a look at the sky on the right side of this photo (click through to full size [warning full size is 3MB]): you'll see some faint spots, like badly-rinsed dishes. Since this is a panorama you can even see the same pattern of three spots repeated three times, once for each underlying frame.

http://www.summitpost.org/view_object.php?object_id=490310

These spots only show up at long focal lengths. They tend to be consistent for several frames in a row, but then show up in different places unpredictably.

Does anybody know which of the following I should clean (more thoroughly than I have been): lens, mirror, sensor?

 

[DougPaul]

A good way to hunt for dust is to take out-of-focus pictures of a clean blank sheet of white paper. Use a small aperture. (Dust is more visible (smaller and darker) with small apertures.) Use several lenses and focal lengths. (Note that some lenses have rotating elements and if so, dust patterns from them will rotate with the focal length.) By comparing the pictures, you should be able to figure out which surfaces need cleaning.

Your picture was taken at F/18 (from EXIF data).
I assume that the camera is a DSLR, but I don't remember which one you use.

The mirror is not in the optical path when the picture is taken so you needn't worry about it. (Except that if it has a lot of dust on it, it could be a source of dust for the sensor or back of the lens.)

The sensor in many cameras is protected by a cover which keeps any dust particles at least somewhat out of focus. The image size of sensor cover dust should be a function of the F-stop, but not the lens. (Dust directly on a sensor surface will tend to stay in focus at different F-stop settings.)

Dust on a lens will, of course, come and go with the lens. Its position may rotate with changing FL if the lens has rotating elements. Its position may also also change (along radials from the center of the image) with FL. Dust on a lens may also be so out-of-focus that it is not directly visible in the image, but in a high dynamic range image (or if there is sun on the lens) it may scatter light and fill in the dark areas somewhat.

My guess is dust on the sensor but, as noted above, you should be able to check that by taking test images with multiple lenses. (But if it moves around, either you have movable dust or it is not on the sensor.)


There is a prior thread on sensor cleaning: http://www.vftt.org/forums/showthread.php?t=17492&highlight=cleaning+sensor

Doug

 

[nartreb]

Wow, I was having a bad brain day when I wrote the above.

mirror not on the path - duh!!!

I wrote "long focal length" but I meant "high depth of field". In fact I see it most when the lens is zoomed wide. It shows up in landscape shots, usually against the sky: lens zoomed all the way out (28mm), small aperture (f/18 to f/25).

I've only noticed this with one lens, but I only own three lenses. One I almost never use (the kit 18-55 - oh yeah the camera is a Canon dRebel xT), one I use all the time including for the shot linked above (Tamron 28-300), one doesn't do landscapes (100-400 IS L - for wildlife shots I'm usually shooting close to 400mm with large apertures). I'll try shooting a white screen and see if it shows up on the other lenses, and whether the spots rotate with focal length.

 

[DougPaul]

It shows up in landscape shots, usually against the sky: lens zoomed all the way out (28mm), small aperture (f/18 to f/25).

Those small apertures will certainly bring out the dust... And it also shows up in areas of constant shading such as sky. (It is there, but you just don't tend to see it on top of fine detail.)

The FL is at most a minor factor if the dust is on the sensor.

Doug

 

[Brambor]

dust on sensor

 

[Grumpy]

My first answer was “dust or crud on the sensor, for sure.” I still look to the sensor as the primary junk collecting agent, but now that we know the photos were taken with a shorter lens at small aperture, there is a possibility the apparent dust spots are from crud on the lens.

I’d especially check the lens rear element, which you should be able to clean. (You always should be careful to keep lens surfaces clean, anyway.) If there is a lot of dust inside the lens, you are looking at a repair shop job.

One thing I found rather peculiar is the elongated off-horizontal spot at the top of the pattern. To me this looks like a possible “oops!” with a Photoshop cloning or spotting brush tool. Were any of the images in this panorama worked on in that way before the stitching was done?

G.

 

[nartreb]

Grumpy, that curved shape is visible in the original files (before stitching or any other manipulation). Similar, slightly longer, marks have shown up on other photos. I think it's a fragment of fiber, probably from clothing.

 

[Grumpy]

I'll buy the "fibre" explanation. Just haven't encountered it in my own digital un-"Photoshopped" images.

G.