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Unusual vignetting in corner in afocal setup
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rjlittlefield
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Joined: 01 Aug 2006
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Location: Richland, Washington State, USA

PostPosted: Sat Mar 17, 2018 11:18 pm    Post subject: Reply with quote

Can you clarify, please? When you say "Even when the camera was not centered at all - shifted over to the right - there was still vignetting in the top right corner", are you talking about the top right corner of the camera frame? That is, the top right corner of the camera frame stays dark no matter how you slide the camera around with respect to the eyepiece?

Have you tried tilting (not shifting) the camera left/right with respect to the eyepiece?

--Rik
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dragonblade



Joined: 18 Oct 2014
Posts: 154

PostPosted: Sun Mar 18, 2018 12:37 am    Post subject: Reply with quote

rjlittlefield wrote:
Can you clarify, please? When you say "Even when the camera was not centered at all - shifted over to the right - there was still vignetting in the top right corner", are you talking about the top right corner of the camera frame? That is, the top right corner of the camera frame stays dark no matter how you slide the camera around with respect to the eyepiece?


Correct. That is exactly right.

rjlittlefield wrote:
Have you tried tilting (not shifting) the camera left/right with respect to the eyepiece?

--Rik


I was going to rotate the camera left and right but the battery went flat. I'll have to try again.

By the way, I'm beginning to suspect that it's not the camera lens aperture that's causing the vignetting. I notice that I have to stop down the aperture a fair bit before I see vignetting appear on all sides of the frame (and that particular kind of vignetting is a bit soft.) However, the problematic vignetting in the top right corner is sharp.
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rjlittlefield
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PostPosted: Sun Mar 18, 2018 1:05 am    Post subject: Reply with quote

dragonblade wrote:
However, the problematic vignetting in the top right corner is sharp.

1. Can you post an image, please, showing this vignetting?
2. Silly question, but have you confirmed that the vignette does not occur when you use the camera off the microscope, just pointed across the room?

--Rik
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rjlittlefield
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PostPosted: Sun Mar 18, 2018 1:10 am    Post subject: Reply with quote

Another silly question, probably redundant...

Have you looked for the vignette by eye, through the same eyepiece that you're pointing the camera into?

--Rik
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dragonblade



Joined: 18 Oct 2014
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PostPosted: Sun Mar 18, 2018 1:30 am    Post subject: Reply with quote

Yea I'll try and post a photo. The vignetting is not visible when the camera is used away from the microscope. When I look through the eyepiece with my eye, everything looks normal.
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dragonblade



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PostPosted: Thu Mar 22, 2018 3:29 am    Post subject: Reply with quote

I tilted the camera left and right. Throughout the rotation, the vignetting stayed in the same spot. Though it looked like it may have changed in size slightly.

This is a photo taken further away from the eyepiece.

https://imgur.com/a/aCbTN

Note that the vignetting in the top right corner appears sharper than the vignetting in the other corners. There's also less blue fringing in the top right corner compared to the other corners. The top right corner also has yellow fringing - this is not visible anywhere else in the frame.

This photo was taken closer to the eyepiece.

https://imgur.com/a/SqLcG

Looks like a bit of vignetting has appeared on the bottom right as well as the top right. And they both show yellow fringing.

And here is a photo taken off center.

https://imgur.com/a/jXu0m

Despite the camera being moved sideways, there is still vignetting in the top right corner. And once again, it looks different to the vignetting on the left. Right is sharper and left is softer. Right has a yellow fringe and left has a blue fringe.

All photos were taken wide open at f2.8.
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Pau
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PostPosted: Thu Mar 22, 2018 5:37 am    Post subject: Reply with quote

You have two kinds of vignette, this is clearly shown at your third picture: The upper right is the eyepiece field limit; sharp and with a slightly orange halo while the others are due to other components, not sure but I suspect it's just due to placing the camera lens too far from the eyepiece

Quote:
When I look through the eyepiece with my eye, everything looks normal.

The first kind of vignette clearly means that the camera is decentered related to the eyepiece, if not it will be symmetrical, and you do see it when looking through the eyepiece: it's just the round limit of the eyepiece field. If you manage to have it centered you get it, if eventually you need to crop a bit the corners it's not tragical, in any case your optics will not be so good at the limits of the field.

Is much easier to understand the issues with images...we would had saved lots of time and words.
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dragonblade



Joined: 18 Oct 2014
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PostPosted: Thu Mar 22, 2018 6:44 am    Post subject: Reply with quote

Pau wrote:
I suspect it's just due to placing the camera lens too far from the eyepiece



When I did my first afocal video recording session, the lens was further away from the eyepiece and vignetting was nowhere to be seen. With these latest tests, the lens is closer to the eyepiece and vignetting is definitely a problem.
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rjlittlefield
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PostPosted: Thu Mar 22, 2018 9:57 am    Post subject: Reply with quote

rjlittlefield wrote:
Have you tried tilting (not shifting) the camera left/right with respect to the eyepiece?

I'm asking this question again, because the images that you're showing are consistent with the camera being tilted slightly to the upper right. That tilt -- not shift -- could allow the camera to see the field stop only in the upper right corner.

--Rik
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dragonblade



Joined: 18 Oct 2014
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PostPosted: Thu Mar 22, 2018 1:38 pm    Post subject: Reply with quote

Like I said before, I tilted the camera in both directions and the vignetting is always in the same corner. Is a field stop related to a field iris? There is no field iris on this microscope.
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rjlittlefield
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PostPosted: Fri Mar 23, 2018 10:48 pm    Post subject: Reply with quote

Quote:
Like I said before, I tilted the camera in both directions and the vignetting is always in the same corner.

I don't want to start a testy interaction, but I strongly suspect that whatever you did is not what I intended to suggest.

To help isolate your problem, please do this:
1. Place a slide with recognizable content in the microscope and focus the microscope so that the content is clear.
2. Shoot a picture showing the vignette in the upper right corner.
3. Tilt the camera to the upper right.
4. Shoot another picture, showing the vignette in the same place.
5. Post both pictures.

When the tilt is done correctly, the slide content will shift toward the lower left when you tilt the camera.

If the vignette is due to the field stop in the eyepiece, as Pau and I suspect, then the vignette will shift along with the slide content, maintaining a fixed relationship to it.

But as I'm reading your description, both images will show the vignette in the same place, with the slide content moving relative to it.

I will be very interested to see what the pictures show.

Quote:
Is a field stop related to a field iris? There is no field iris on this microscope.

The one I'm talking about is inside the eyepiece, what Pau describes as "eyepiece field limit". Physically it is a thin plate of metal with a round hole in it, located in or very near the focus plane of the eyepiece. This stop is related to a field iris in the condenser in the sense that they are both focused in the same plane as the image. "Conjugates" would be the formal term for that relationship. But they serve different purposes. A field iris in the condenser serves to block stray light from entering the objective, for the main purpose of increasing contrast by preventing glare. The stop in the eyepiece provides a hard edge to the visual field, at a point where the image quality is still pretty high. It prevents the visual ugliness of having the image just go very bad toward the edges.

Again, I will be very interested to see the two pictures as described above.

--Rik
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dragonblade



Joined: 18 Oct 2014
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PostPosted: Fri Apr 06, 2018 11:15 pm    Post subject: Reply with quote

Ive just done another afocal photography session. When I was setting up and trying to get the camera lens centered over the eyepiece, I could see the dreaded vignetting in the top right corner. After adjusting the position of the camera (which took quite a while) the vignetting disappeared. That may not mean that the problem is fixed however. The vignetting could very well return in the future. In previous sessions, I moved the camera all over the place and experimented with different distances from the eyepiece but could not get rid of the vignetting then.

I got that weird flickering again in the LCD screen where extreme vignetting occurs about every second, accompanied by a clicking sound. And so far, it only happens in still photo modes. I tried both Manual and Aperture Priority and it occurs in both. Aperture was wide open at f2.8. When I switched to Manual Video mode, the flickering stopped but then resumed when I went back to still photo mode. The flickering eventually stopped but the extreme vignetting remained (a different kind of vignetting to what I had experienced in the top right corner.) The extreme vignetting was occupying the majority of the left side of the frame. Though when I took photos, there was no vignettting to be seen in the images.

I wonder if there could be something whacky going on with this Sigma 30mm f2.8 lens. It was a very recent purchase - I bought it second hand. I haven't really used it hardly at all away from the microscope. When I was outside today, focusing the lens on infinity with some far away hills in the distance, I wasn't getting the flickering / clicking. So far, that only seems to occur in an afocal configuration.

I'm going to look at a water sample this afternoon and hopefully do another video recording session of aquatic organisms. Fingers crossed that the vignetting in the corner won't return.
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dragonblade



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PostPosted: Sat Apr 07, 2018 1:17 am    Post subject: Reply with quote

#####, I spoke too soon. The vignetting has returned. Even though the camera is in the same position as last time. Though I did remove the camera from the tripod temporarily in order to change objectives on the microscope. I'll take those photos now with the different tilt positions.
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dragonblade



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PostPosted: Sat Apr 07, 2018 2:21 am    Post subject: Reply with quote

Ive just come back from another photo session. The vignetting wasn't as severe this time. And it was in a different place. This time, it was in the top left corner. At one stage, vignetting was in the top left and right corner. And one corner had a soft vignette with a blue fringe and the other corner had a sharp vignette with a yellow fringe. I will post photos soon with different tilt positions showing vignetting in the top left corner.
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dragonblade



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PostPosted: Sat Apr 07, 2018 6:20 am    Post subject: Reply with quote

Here are three photos from tonight. For each photo, the camera was tilted by varying degrees.

https://imgur.com/a/vXqOk

https://imgur.com/a/bUDHD

https://imgur.com/a/Oa0SQ
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