Dirty Lens or Dirty Sensor?

[pasoviennese]

Attached are two photos taken with my new Tamron 18-270mm lens fitted with UV filter.

One was taken at f/8.0 and the other one taken at f/40, both at 270mm and on Canon 30D body. With the f/8.0, the photo is clean; however with f/40, there're lots of "dust". I'm not sure if this is my sensor or my lens or the filter?

I don't have any cleaning equipment at the moment. However, I brought home some cleaning stuff we use in the lab (I work in medical research). Basically wipe we use to clean microscope lens and 70% pure ethanol diluted in distilled water.

What do you suggest I do?

Thank you!

f/40


f/8

[dcclark]

Yup, that looks just like sensor dust. Notice how you can't see it at all at f/8? How often do you actually shoot at f/40? I generally wouldn't worry unless it starts to show up at reasonable apertures. (Side note: what's the scratch on the left? That looks like something actually in the image to me.)

That said, there are a lot of threads here about sensor cleaning, if you're interested in actually doing that.

[zona5101]

help me out... how does the lens aperture affect dust on the sensor? I would think magnification would make dust on the sensor more visable not the aperture. Aperture would make dust on or in the lens more visible as the dof increases enough to bring the dust into focus. But I don't know, there maybe an element of the process that escapes me...

perhaps i should have quoted this

Originally Posted by dcclark

Yup, that looks just like sensor dust. .

[candleman]

my understanding is that a narow aperture F/32 etc etc puts more things in focus at greater distances ... right?

so its putting the specks of dust in focus.

imho, wider apertures would cause it to essentially "bokeh" away..

i could be wrong.. thats just how i figure it could work.

[OsmosisStudios]

Originally Posted by candleman

my understanding is that a narow aperture F/32 etc etc puts more things in focus at greater distances ... right?

so its putting the specks of dust in focus.

imho, wider apertures would cause it to essentially "bokeh" away..

i could be wrong.. thats just how i figure it could work.

Pretty much it. Thats why I always stop down as much as possible.

[pasoviennese]

But if the dust is on the sensor, why would it matter if the lens gets stopped down?

[Aegea]

AFAIK, there is usually a filter over the sensor, so the dust is actually on the filter, a small distance above the actual pixels on the sensor. I guess the wider apertures have light hitting the sensor from more directions, as it were, hence the dust shadows on the sensor get blurred.

[zona5101]

Originally Posted by Aegea

AFAIK, there is usually a filter over the sensor, so the dust is actually on the filter, a small distance above the actual pixels on the sensor. I guess the wider apertures have light hitting the sensor from more directions, as it were, hence the dust shadows on the sensor get blurred.

This still isn't clear to me (I am admittedly thick sometimes). All the modification to the light are happening way ahead of the sensor - out in the elements of the lens and the diaphragm blades. What the lenes and blades do to the light is of no consequence to the sensor as long as the light converges in focus on the film plane. (right? or no?)

Getting back to this issue, I do see how the extremely small aperture could bring into focus dust inside the lens or dust trapped between the lens glass and the filter.

[Aegea]

Um, it seems to me that with a very small aperture all the light hitting a particular pixel is coming from just one direction (through that small aperture), so it's easy for a piece of dust on the sensor filter to block it. With a wide aperture, some of the light that focuses on just that pixel comes from one part of the lens, some from others, so a small piece of dust on the filter above the sensor can only block some of it - some kind of sneaks by the edges. Does that make sense? I wish I could draw a diagram ...

[zona5101]

Originally Posted by Aegea

Um, it seems to me that with a very small aperture all the light hitting a particular pixel is coming from just one direction (through that small aperture), so it's easy for a piece of dust on the sensor filter to block it. With a wide aperture, some of the light that focuses on just that pixel comes from one part of the lens, some from others, so a small piece of dust on the filter above the sensor can only block some of it - some kind of sneaks by the edges. Does that make sense? I wish I could draw a diagram ...

thanks.
I did some more research and it is a depth of field related issue. I just couldn't "get it" that dust behind the lens would be affected. Thanks for helping me get it straight.