More than dust: anyone know what this is? End of sensor?

[DanK]

Noticed a few dust bunnies when I edited a shot taken with my 50D, so I did a test shot (uniform surface, f/32). It showed numerous dust bunnies, along with the big blob that you can see toward the right. A cleaning (air, then a brush, then wet cleaning) removed some dust bunnies, although a few remain in the image below (e.g., toward the top left). Cleaning had no effect whatever on the blob, which looks like nothing I have ever seen before. Examining the sensor (well, the AA cover, anyway) with a loupe showed nothing.

Anyone know what this is? I have a bad feeling about this, and it happened only two days before a trip to the mountains.

thanks for any ideas.

[Colin Southern]

Are you sure it's not on the rear element of the lens?

[DanK]

Colin,

Thanks. Problem solved. The rear element looks fine, but I just tried several other lenses, including a second copy of the one in question, and no blob, so something else is wrong with the lens. it has a month left on warranty, so in it goes.

Dan

[Colin Southern]

No worries Dan.

Just shine some light through it - you should be able to see the obstruction.

[Inkanyezi]

The obstruction could as well be at the front element of the lens. At f/32 on a 100 mm lens, dust on the sensor should be much sharper. This spot might be on the front or rear element of the lens or on the filter if there is one.

[gcowan]

It looks like oil or a drop of some other fluid on the sensor. It is a job for a pro.
I speak from bitter experience.
Graham

[DanK]

It looks like oil or a drop of some other fluid on the sensor. It is a job for a pro.
I speak from bitter experience.
Graham

That was my first thought, but it can't be, because it only happens with this lens--not even with a second copy of the same lens, which I happened to have on hand.

The blob is there with and without a filter.

[gcowan]

Dan,

Hmmm... Looking at it on a monitor rather than a phone it is a little too out of focus.

Can't really help any further except that the repairer that fixed my 5D had really good optical gear to find the dirt etc. and special cleaning gear. I used a static charged brush, which ended up spreading the muck all over the sensor. It didn't really occur to me that there might have been too much lubricant in the works. If it had been dust, then the brush would have worked.

It might be something similar in the lens, and if it is clear fluid it might need UV light to show it up. There are lots of possibilities. I have had mould growing in a lens before; but in my memory it just gave a general and really bad softness.

Graham

[DanK]

Graham,

The lens is now all packed and double boxed, so I can't look further, but my guess is that it is something on the forward surface of the rear element or on one of the other elements near the back. The puzzle is that this is an L prime, so it is well sealed, and I have really babied it during the 11 months I have owned it, so I have no idea how muck could have gotten into it. Given that is under warranty, I'll let Canon disassemble it and figure it out. It costs me only one-way shipping.

Dan