Sensor Dust and Magnification

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rjlittlefield
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Post by rjlittlefield »

For amusement & edification, try it with a pinhole. If the color shifts are due to stuff inside the camera, then they should reproduce even when the "optics" consist of nothing but a pinhole placed at the exit pupil position of the current optics. So first, take off the camera, peer into the back of your current optics, and see where the aperture appears to be. Then take off all the lenses and physically place just a pinhole at that same position. Replace the camera, shoot, levels adjust, and post.

--Rik

elf
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Post by elf »

I think it shows
  • Your lens is not centered on the sensor
    The objective's image circle does not fully cover the sensor

    or something else is blocking the image circle
The vignetting is also visible in the first image on my monitor.

Craig Gerard
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Post by Craig Gerard »

I missed the initial comments on the second page of this thread...reading now.

In the meantime, in the image below I used a 10X finite objective.

Image


Craig
To use a classic quote from 'Antz' - "I almost know exactly what I'm doing!"

Chris S.
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Post by Chris S. »

Craig Gerard wrote:. . .cleaning. . . 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. . . .
I have a collection of old swabs, on which I simply replace the pads as described here. This reduces the cost of a wet cleaning--even using four or five swabs--to a few pennies.

Craig Gerard
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Post by Craig Gerard »

Chris,

Thanks for the link. Are you referring to this comment:
....snip... Save those used swabs (or buy a package of the ultra cheap ones), and buy a package of very inexpensive Pec Pads. Remove the pads from spent swabs, and cut pieces of Pec Pad to same approximate size and shape (you don’t need to be exact). Use the little rubber bands that came with the original swabs to hold the replacement pads on.
Ref:http://photomacrography.net/forum/viewt ... 2964#82964

I'm considering all options at this stage with regard to sensor cleaning. I've kept the sensor swab blanks and could use those with the Pec Pads; but no matter how clean you get the sensor, the dust within the camera body eventually finds its way onto undesirable locations.

Has anyone used the NRD FireFly gizmo? Currently USD$125.00 on the NRD website (for USA residents) or you can buy from Adorama or Amazon for $159.00.
http://www.youtube.com/watch?v=iHexaG--mkw

http://www.youtube.com/watch?v=ZCc8x0V9nCA



Craig


*edit: added quote and reference hyperlink and second video link
Last edited by Craig Gerard on Mon Oct 10, 2011 11:47 pm, edited 2 times in total.
To use a classic quote from 'Antz' - "I almost know exactly what I'm doing!"

Craig Gerard
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Post by Craig Gerard »

Rik,

In place of a 'pinhole' plate I used an iris assembly closed to minimum aperture which almost equates to a pinhole in diameter.

I cleaned off some dust specs in post-production, more than I care to mention; but they were much smaller with the decrease in magnification.

NR (post-production noise reduction) has been applied to both of the following images.

This image has the tube lens in place.
Image

This image is with the tube lens removed. There are no optics used in this image.
Image

Craig
To use a classic quote from 'Antz' - "I almost know exactly what I'm doing!"

rjlittlefield
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Post by rjlittlefield »

Interesting! So, your sensor sees slightly green on the right, magenta on the left, even when there's no plausible way for the light to be different colors.

My speculation is that it has to do with the angle that light hits the sensor, though I don't have a clue about the details of why that would matter for color balance. Maybe the colored filter dots are slightly asymmetric, with different asymmetries for different colors.

For what it's worth, I've seen similar but much more extreme color shifts on other lower cost sensors. Some of them are vivid enough to see without levels adjustment or even looking especially hard. My cell phone is one of those. The center of the image is obviously brown compared to the edges.

--Rik

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Post by Chris S. »

Craig,

While you were performing your iris test, I was testing my own Mitutoyo 10x M Plan apo using your approach, with the objective mounted on a Mitutoyo tube lens and Nikon D200. The results of this test are much less interesting, now that you've demonstrated that the effect you're observing occurs with no optics at all. But perhaps they are still worth posting. If you prefer I not post images to your thread, let me know and I'll take them down.

Here is a straight image of a white card, out of focus, with the light adjusted to record as gray.
Image

Here is the same image with Photoshop's Autocontrast applied.
Image

There is vignetting in the bottom corners, which makes me suspect that I might want to tweak lens alignment a bit. That said, I doubt this would be noticed in normal shooting. As an example, when I first purchased a Nikkor 24-70 f/2.8, I shot pictures of a white wall to check for vignetting. I was horrified at the result, as vignetting was quite pronounced. So I returned the lens and bought another copy. This gave pretty much the same results--a little bit better, but still with noticeable vignetting. But as you may know, this Nikon lens is widely considered to be excellent. So I took it out and shot some real world images with it, and loved it. Many shots later, I know that this lens delivers very high quality. Photography is, apparently, not about white walls. But you of course know this.

Anyway, in my 10 Mitty test, I do not see asymmetrical color weirdness, as is evident in your images. I suspect that a large part of what we're making evident, with this sort of over-processing, is sensor noise. So I wonder--does your sensor produce noise unevenly? I could easily see a manufacturer not worrying about such a "problem," as long as it is only evident under extreme pixel-peeping--which we are doing here--and does not affect real-world photography.

Craig, did you by chance use live view for your tests? If so, does switching out of live view make any difference? I've often wondered if the heat potentially produced by live view's continuous on-state has any effect on noise. Such a thing would not be hard to envision. And I could imagine that sensor noise might differ from one portion of the sensor to another according to local temperatures, based on which portion of it is closest to the battery or other source of heat or other emissions.

Cheers,

--Chris

Craig Gerard
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Post by Craig Gerard »

Chris,

Thanks for the additional images and report. 8)

I did use Live View, always do when the DSLR is tethered; but it was initiated and then turned off after each shot, so probably not much chance of heat affecting the outcome. When shooting a focus stack sequence, Live View is 'active' throughout the course of the sequence and then switched off.

To soothe my concerns, I placed a 'real' subject in front of the lens and as expected the vignetting has no significant impact under 'normal' conditions as far as I can determine. There are ways to correct for light falloff if required. It was just a passing observation whilst confronting the dust acummulation problem.

......................................................

Keeping things in perspective, here is an image I posted in another thread taken with the same setup. The vignetting is visible, it was in taking such images I first began to notice the 'anomaly' when using microscope objectives as macro lenses, which in itself is an anomaly when you consider what we do with such 'objectives' :)

Image



Craig
To use a classic quote from 'Antz' - "I almost know exactly what I'm doing!"

Craig Gerard
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Post by Craig Gerard »

A quick single exposure.

Light Falloff compensation has been adjusted to +10% in RAW post-production. I'll keep a close eye on this setting in future images and stacks and adjust further if it presents an improvement.

I'm happy with the 'dust' issue and was pleased to see this result of a 'real' subject. I have an arsenal of cost-effective products on the way from various companies, many of which (or similar approaches) have been mentioned elsewhere on the forum.

I was also further impressed by the USMC 20X M Plan Apo (but I would still like a Mitutoyo 20X M Plan Apo :wink: )

Canon 50D, USMC 20X @ 20X, Nikon MXA20696 tube lens, 1/15s, ISO 100, fiber optic illumination, EFSC, Rosco diffusion cylinder
Image


Craig

*edit: added the word 'compensation' in second paragraph
Last edited by Craig Gerard on Mon Oct 17, 2011 6:01 pm, edited 1 time in total.
To use a classic quote from 'Antz' - "I almost know exactly what I'm doing!"

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Post by ChrisLilley »

rjlittlefield wrote:So it sticks to the sensor? Now that is interesting. I don't know what it means, but it sure is interesting!
Might it mean that the sensor plane is not exactly parallel to the lens mount, but is slightly tilted?

Edit: no, because photos with no optics also show colour shifts.

Craig Gerard
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Post by Craig Gerard »

ChrisLilley wrote:
rjlittlefield wrote:So it sticks to the sensor? Now that is interesting. I don't know what it means, but it sure is interesting!
Might it mean that the sensor plane is not exactly parallel to the lens mount, but is slightly tilted?

Edit: no, because photos with no optics also show colour shifts.
Chris, it's a possibility, another reason to double check my setup. I will also play with the 'swing' on the PB4 and see what happens :)


Craig
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Craig Gerard
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Post by Craig Gerard »

I'm including some additional information from another thread which is related to the title and content of this thread.
Ref:http://www.photomacrography.net/forum/v ... hp?t=14689
Harold Gough wrote:
pbertner wrote:Especially pertinent to Macro work, I haven't had a problem with sensor dust until I've started shooting at 10X.
Why should that be so? The sensor doesn't know what magnification you are using. Surely, at the sensor, an image is an image, whatever the subject, lens, etc.

Harold
Chris S. wrote:
Harold Gough wrote:
pbertner wrote:Especially pertinent to Macro work, I haven't had a problem with sensor dust until I've started shooting at 10X.
Why should that be so? The sensor doesn't know what magnification you are using. Surely, at the sensor, an image is an image, whatever the subject, lens, etc.
Harold, I was hoping that one of the many members of this forum whose grasp of optics exceeds mine would respond. But in the meantime, I'll give it a shot.

Empirically, as magnification increases, so does the the clarity of sensor dust in our images. I've definitely seen this--a 4x lens is vastly more tolerant of sensor grunge than, say, a 60x or 100x objective. As I understand it, as we increase magnification, we tend to decrease nominal aperture--effectively, we move closer to having a point light source. And a point light source is very effective at imaging sensor dust.

I once attended a lecture by the late, and truly great, Harold Edgerton--aka "Papa Flash." I don't recall if it came up during his lecture, or in our conversation afterward, that he had made useful images of bacteria by placing them on a piece of film and hitting them with a flash placed far enough away to act as a point light source. It struck me as a very cool way of imaging bacteria. But as we increase magnification, we are unfortunately doing much the same thing with sensor dust--moving toward a point light source that images the dust very acutely.

Those more erudite in optics will hopefully correct my errors in this explanation. But this may be a decent "horse sense" way of looking at it.
ChrisR wrote:
pbertner wrote:Especially pertinent to Macro work, I haven't had a problem with sensor dust until I've started shooting at 10X.

Harold quite reasonably replied:
Why should that be so? The sensor doesn't know what magnification you are using. Surely, at the sensor, an image is an image, whatever the subject, lens, etc.

I don't know, but my impression is that it's often true. Perhaps it depends on the angle of the cone of light hitting the dust. Do eg complex zoom lenses, do odd things with exit pupil position?

rjlittlefield wrote:
Harold Gough wrote:
pbertner wrote:Especially pertinent to Macro work, I haven't had a problem with sensor dust until I've started shooting at 10X.
Why should that be so? The sensor doesn't know what magnification you are using. Surely, at the sensor, an image is an image, whatever the subject, lens, etc.
Two reasons: effective aperture and stacking. At 10X, a typical NA 0.25 objective operates at effective f/20; higher magnifications give even larger effective f-numbers. As Chris S. says, these narrow cones of light make dust a lot more visible. Each speck of dust produces a sharper and darker spot than it would with the wider cones typical of low magnification and street shots. In addition the shallow DOF at higher magnifications encourages stacking, which typically expands a single dust dot into some sort of dust trail, either a family of dots or a connected line. Worse, any trails get repeated in some similar form for each speck of dust, which produces a repeating pattern that is very obvious to human eyes.
To use a classic quote from 'Antz' - "I almost know exactly what I'm doing!"

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