Sensor Dust and Magnification

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Craig Gerard
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Sensor Dust and Magnification

Post by Craig Gerard »

I have been cleaning the sensor on one of my DSLRs and noticed an anomaly.

Subject was a grey (gray) card.

It is the second image which requires an explanation. What is the cause of the weird vignetting? I have only noticed it when using microscope objectives.

The first image is taken @ 10X with an infinity-corrected Mitutoyo M Plan Apo. This is what the sensor of a Canon 50D looks like after a liquid swab or two and a casual glance.

I took the first image into Photoshop and adjusted the levels; this tends to increase noise and contrast, in so doing, it also reveals dust on the sensor which may not have been immediately obvious.

It seems to me that such a proceedure provides a clear indication of how effective the sensor cleaning has been; but don't do this on a bad day, sometimes the results are shocking :shock:

Image

However, I'm looking for some explanation regarding the vignetting? It is also evident, to some degree, in the first image, the levels adjustment makes it more pronounced and easier to see.

Image
...................................................................

Here is what the sensor looked like before cleaning .....almost two years since I last cleaned it and many, many lens changes and bellows exercises during that time. I know some of those visible spots intimately.

Sensor before cleaning and levels adjustment.
Image


Here is what was revealed when the levels were adjusted. Applying 'Auto Contrast' in Photoshop has the same effect. :shock:

Image





Craig
Last edited by Craig Gerard on Mon Oct 10, 2011 12:42 am, edited 1 time in total.
To use a classic quote from 'Antz' - "I almost know exactly what I'm doing!"

canonian
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Post by canonian »

Horrifying, Auto Contrast also reveals some purple/green CA.
I often wonder if adjusting a bellows pumpes more dust on the sensor.
Isn't there a way to place a filter between the body and the lens, like the UV you put on the front of a lens?

Harold Gough
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Post by Harold Gough »

canonian wrote:Isn't there a way to place a filter between the body and the lens, like the UV you put on the front of a lens?
It might reduce the performance of the lens.

I have some MF film telephotos which were designed to have a rear filter always in place and should not be used without one. For lenses not designed to work with a rear filter using one might create problems. If one is to be used it might be preferable for it to be one of the high quality, thinner types.

Harold
My images are a medium for sharing some of my experiences: they are not me.

Craig Gerard
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Post by Craig Gerard »

Horrifying
This may be classed as an 'extreme' example. As mentioned in the initial thread, the DSLR has been used for many MF lens and bellows projects over the course of almost two years and the sensor has not been cleaned in that time. :oops:

Another liquid swab or two should add some respectability.

I am about to implement a new sensor cleaning regime that will be performed on a regular basis. I know of some professional photographers who clean their sensor between each photo shoot (using more than just a rocket blower) followed by a regular wet clean at specific time intervals or as required. Currently in the process of deciding which products to use for these proceedures.


Craig
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seta666
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Re: Sensor Dust and Magnification

Post by seta666 »

Craig Gerard wrote: It is the second image which requires an explanation. What is the cause of the weird vignetting? I have only noticed it when using microscope objectives.
Craig
I might be wrong but normally Vigneting is asociated with fast lenses used wide open, As we use fast lenses some vigneting is expected.
From the lenses I own I would say that the Nikon BD plan 40/0.65 has quite a pronounced vigneting, but it also has a NA of 0.65
I use a Full frame camera, this vignetimng would not be much problem on an APS-C camera

Regarding dust, the higher the effective aperture the worst is the effect. With the mitutoyos dust is problem but an easy one to deal with, as Effective aperture stays between f16-f25 deppending on the magnification.
When working with lenses like the M plan 100/0.80 ELWD the problem is much worst as I use an effective aperture of f62
Regards
Javier

Harold Gough
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Re: Sensor Dust and Magnification

Post by Harold Gough »

seta666 wrote: I might be wrong but normally Vigneting is asociated with fast lenses used wide open, As we use fast lenses some vigneting is expected.
Wide open, yes, but fast? I have 3 lenses with my X-Pan, an f4 90mm, an f4 45mm and an f5.6 30mm. The 30mm needs a graduated centre spot filter to correct for vignetting at all apertures, the 45mm needs it at f 5.6. and f4 and the 90mm doesn't need correction at any aperture.

I believe that the angle of view is the major factor, which is sort of related to aperture.

Harold
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rjlittlefield
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Post by rjlittlefield »

Craig, can you be more specific about what's "weird" with the vignetting? Are you talking about the fact that it's not centered, that there's a color shift for left versus right, or something else?

--Rik

Pau
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Post by Pau »

In my modest opinion the only issue I can see here that really matters is the small dust/dirt one. Pushing levels at that extreme allways will maximize any image defect, not only vignette but also noise... in images that with normal processing are OK.
Microscope objectives aren't fast in absolute terms but are really fast for its magnification, no surprising for me to get some vignette.
Pau

Craig Gerard
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Post by Craig Gerard »

Rik wrote:Craig, can you be more specific about what's "weird" with the vignetting? Are you talking about the fact that it's not centered, that there's a color shift for left versus right, or something else?
I was wondering if the particular colours, left and right of frame, displayed an obvious, recognisable signature?
Pau wrote:Pushing levels at that extreme always will maximize any image defect, not only vignette but also noise... in images that with normal processing are OK.
I found that by over adjusting the levels, it revealed dust spots that may at first go unnoticed.

I should have termed the levels adjustment as an 'over adjustment'. The sensor cleaning exercise should also be referred to as 'low pass filter cleaning'.

I'm looking at the low pass filter cleaning process, granted, this particular cleaning was long overdue and took a number of liquid swabs (too many) to remove the majority of the dust; but I am finding in less severe cases that more than one liquid swab is required. The clean room prepared swabs are expensive. An effective 'dry' product is required in between application of liquid solutions via the swabs.

An anti-static blower seems a likely candidate; but I am only aware of one such product made for cleaning DSLRs and customer reviews are inconsistent.



Craig
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rjlittlefield
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Post by rjlittlefield »

Craig Gerard wrote:I was wondering if the particular colours, left and right of frame, displayed an obvious, recognisable signature?
Not to me. An interesting experiment would be to rotate as many optics as you can, and see if the colors stay stuck to the rotating parts, stay stuck to the fixed parts, or do something else.

--Rik

Craig Gerard
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Post by Craig Gerard »

I could also rotate the camera body, yes? 8) The DSLR is currently attached to a bellows which permits such rotation.


Craig
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rjlittlefield
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Post by rjlittlefield »

Indeed. I assume results will be posted real soon now...

--Rik

Craig Gerard
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Post by Craig Gerard »

Image after levels adjustment. (Tube lens Nikon MXA20696)
Image


Craig


*edit: adjusted size of sample image and added tube lens details
Last edited by Craig Gerard on Mon Oct 10, 2011 6:34 pm, edited 2 times in total.
To use a classic quote from 'Antz' - "I almost know exactly what I'm doing!"

rjlittlefield
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Post by rjlittlefield »

So it sticks to the sensor? Now that is interesting. I don't know what it means, but it sure is interesting!

--Rik

Craig Gerard
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Post by Craig Gerard »

I replaced the tube lens with an alternative for comparison. (Apo-Gerogon 150/9)

Image


Same image after post-production NR (noise reduction)

Image




Craig
Last edited by Craig Gerard on Mon Oct 10, 2011 7:01 pm, edited 2 times in total.
To use a classic quote from 'Antz' - "I almost know exactly what I'm doing!"

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