Sensor Spots on 1Ds Mark III

[MarkAlsop]

This should be a good brain teaser…

During the AF and DOF testing on some new lenses I noticed that the onset of visible diffraction was at smaller apertures when using the 1.4x and 2x Extenders with an EF 400mm f/5.6L lens …so I took a look at how far down (small) I could push the aperture with the extenders on a 400mm f/5.6L lens.

That was when I noticed sensor spots that only show up when the aperture is small enough to see diffraction at 100% on the computer screen.

I checked the 400mm without and with both the 1.4x and 2x extenders. Then I checked with a 300mm f/4L IS lens, no extender.

In all cases the spots were in exactly the same place in the image area. I performed a sensor cleaning just to rule that out.

When the aperture is large enough that there is no visible diffraction at 100% in ACR or PS the spots disappear! The worse the diffraction the more pronounced the spots.

This is not a ‘long exposure’ issue since I am using a 580EX to illuminate the test target and the shutter speed is set to 1/125th for all exposures.

I am not particularly concerned as the spots only appear when diffraction has already rendered the image unusable …but I am curious!

Anyone have any good idea what causes these spots?

The following photo illustrates the spots. The images are a 100% crop from raw images no sharpening or anything except for exposure and white balance ...then conversion to jpeg. Diffraction is way over the top, which is why the image is so soft.

[sjprg]

Mine was even dirtyer. Send it to Canon they will clean it under waranty.

[MarkAlsop]

You are right!

At first I didn’t think these were dust spots because of the effect of aperture/diffraction and some blue-sky shots but it turns out you are right!

I can clean the sensor but something always remains …always in a different location, which proves your point. The 1Ds Mk III internal sensor cleaner is useless for this type of debris.

Apparently the front of the sensor filter is just far enough in front of the sensor sites that extremely small apertures bring the smallest bits of debris into sharp focus whereas larger apertures manage to get enough light under the debris so that the shadows are not obvious unless you are really looking for them.

Did Canon actually get rid of all debris on your lens?

[sjprg]

Cleaned it up perfectly. YMMV. I beleive it is actualy lubcation from the shutter. I could clean it partialy but could never get all of it myself. Mine had huge spots all over when delivered, like the assembler over lubed.

[MarkAlsop]

I hate sending things in for repair. But like you I can only clean it partially; there are always several dozen of these little bits to debris, never in the same place, way too small to be real ‘dust’ particles. I guess I’ll have to call Canon.

Thanks

[MarkAlsop]

The 1Ds Mark III came back from Canon service center in Irving CA today after sending it in for a sensor cleaning.

The sensor is cleaner than it was …that is there are fewer of the very small spots on the sensor and none of them are in the same place they were before sending the body in for cleaning.

This tells me the sensor can be cleaned and that Canon is not really much better than I am at doing the job. Most of the spots are near one of the short sides, which is the same issue I was having with my efforts at cleaning the sensor.

To be fair these spots are very small and can only be seen clearly by stopping down to f/64 so the cone of light reaching each sensor site is very, very narrow.

In testing my lenses I found that f/22 on all lenses, while still usable, already shows the signs of diffraction in the 1Ds Mark III body. At f/16 and above the cone of light is wide enough that these sensor spots are very hard to detect shooting a white board and impossible to see in any real image.

I guess this is one of the prices I’ll have to pay for using a camera with a full-frame 35mm, 21 mega pixel sensor.