Why is there speckling on the underside of my lens glass?

[pavlos]

My lens has developed speckling on the underside of the front glass; it resembles dust in pictures, especially when the aperture is narrowed, but is inside the lens and doesnt appear to shift. I've had it for only 3 years and always used a UV filter. What exactly is this, and is it repairable?

[RobertTheFat]

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@pavlos wrote:

My lens has developed speckling on the underside of the front glass; it resembles dust in pictures, especially when the aperture is narrowed, but is inside the lens and doesnt appear to shift. I've had it for only 3 years and always used a UV filter. What exactly is this, and is it repairable?

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Dust. Yes, but not (obviously) without partially disassembling the lens, a task best left to professional repairers.

While dust on the front element of a lens can be very annoying, the good news is that it takes quite a lot of it to noticeably degrade the image. And when that does happen, the degradation takes the form of a slight softening of the image, not visible specks. That's because dust on the front element is, almost by definition, nowhere near in focus. Where you do want to worry about dust is on the sensor. There it is in focus, and a little can cause a lot of problems, depending on just where and what it is.

BTW, a UV filter, whatever else it does, doesn't keep out dust. Dust doesn't come in around the front element, which is usually pretty well sealed. It comes in where one exposed part of the lens barrel moves relative to another exposed part. The less movement of that sort a lens requires, the better for keeping dust out.

[ebiggs1]

Boston, Massachusetts USA is correct.  The dust on your lens is not the problem.  If you are seeing specks in your photos it is probably on the sensor.  But there again specks can be from other issues with the sensor besides dust.

[jrhoffman75]

"dust in pictures, especially when the aperture is narrowed"

This would almost certainly indicate sensor dust.

http://www.the-digital-picture.com/Photography-Tips/sensor-cleaning.aspx

[TCampbell]

Dust has to be on (or very close to) a focus point in the image path in order to show up in the photo.  I've shown people that I can take a post-it note, cut it down to about 1/4" x 1/4" square, stick it somewhere on the front of the lens, snap a photo... and defy you to figure out where I put the post-it note... it wont show up in the image (not even something that big.)

If it were on the sensor (or extremely close) then it would show up.  I wouldn't worry about dust "in" the lens unless it's getting pretty bad (but then it would require sending in for cleaning.)

Some zooms that "extend" have a bigger problem with dust because as they extend, air obviously has to be pulled in to fill that void and, with the air... comes dust.

To test for dust on the sensor (which you can clean and is a completely different issue) set the camera to a very high f-stop and take a photo of something with absolutely no contrast... a plain white wall... a plain white sheet of paper... the blue sky above, etc.  It's actually better if the camera isn't focused when you take the photo.   Dust specs will show up more easily when there's nothing with distracting contrast in the photo.