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Unusual vignetting in corner in afocal setup
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dragonblade



Joined: 18 Oct 2014
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PostPosted: Fri Mar 16, 2018 6:43 am    Post subject: Unusual vignetting in corner in afocal setup Reply with quote

Several days ago, I tried some afocal photography for the first time. Things went really well and there was no vignetting to be seen. I used a Nikon CFW 10x eyepiece and a Sigma 30mm f2.8 lens on my Panasonic G6 M4/3 camera.

I tried again tonight with the same setup but for some weird reason, there is some vignetting in the top right hand corner of the camera's LCD screen. And no matter what I do, I cannot get rid of it. I moved the camera closer to the eyepiece than usual but the vignetting is still there. I then experimented a little. The camera lens was centered over the eyepiece for the photography but I shifted it to the left and there is still vignetting in the top right hand corner. Even when the eyepiece occupies half of the camera's LCD screen, the vignetting is still there in the same corner. Aperture is wide open as usual.


Last edited by dragonblade on Fri Mar 16, 2018 9:08 pm; edited 1 time in total
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enricosavazzi



Joined: 21 Nov 2009
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PostPosted: Fri Mar 16, 2018 10:44 am    Post subject: Re: Unusual vignetting in corner in afocal setup Reply with quote

dragonblade wrote:
Several days ago, I tried some afocal photography for the first time. Things went really well and there was no vignetting to be seen. I used a Nikon 10x high eyepoint eyepiece and a Sigma 30mm f2.8 lens on my Panasonic G6 M4/3 camera.

I tried again tonight with the same setup but for some weird reason, there is some vignetting in the top right hand corner of the camera's LCD screen. And no matter what I do, I cannot get rid of it. I moved the camera closer to the eyepiece than usual but the vignetting is still there. I then experimented a little. The camera lens was centered over the eyepiece for the photography but I shifted it to the left and there is still vignetting in the top right hand corner. Even when the eyepiece occupies half of the camera's LCD screen, the vignetting is still there in the same corner. Aperture is wide open as usual.

Just to eliminate all possible factors one at a time -
You probably already tried, but is there any vignetting when the lens is attached to the camera and to the microscope adapter, but not mounted on the microscope photo tube?
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Pau
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PostPosted: Fri Mar 16, 2018 10:46 am    Post subject: Reply with quote

Check:
- Is the camera lens aperture wide open?
- Play with the field aperture at the illuminator
- Play with condenser centering
- Look through the eyepiece at the photoport removing the camera
- turn the camera to see if the obstruction changes its position

...and the most stupid thing, although sometimes happening to expert but dumb microscopists like me: is the objective at its right position, at the nosepiece click or the nosepiece at its right position if removable?
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zzffnn



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PostPosted: Fri Mar 16, 2018 11:39 am    Post subject: Reply with quote

Pau wrote:

...and the most stupid thing, although sometimes happening to expert but dumb microscopists like me: is the objective at its right position, at the nosepiece click or the nosepiece at its right position if removable?


That was the first possibility I thought of. It happens to every microscopist, I think Twisted Evil

Another common possibility was that your light source or condenser was knocked out of center.

Change/move around things one by one and you should be able to find the cause.
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Ichthyophthirius



Joined: 07 Mar 2013
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PostPosted: Fri Mar 16, 2018 2:42 pm    Post subject: Reply with quote

Occasionally, the camera lens aperture is the problem. For example, Canon lenses have an open aperture when you take the lens off the camera but the lens aperture closes down when you take an image.

What does the Sigma lens do? Can you fix the lens aperture wide open (by changes in the camera menue or by simply tapeing off the lens electrical contacts)?
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dragonblade



Joined: 18 Oct 2014
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PostPosted: Fri Mar 16, 2018 6:34 pm    Post subject: Reply with quote

Enrico, the camera is not physically attached to the tube. And there is no microscope adapter in my setup. There is no vignetting when the camera is completely removed from the microscope.

Pau and Ichthy, the camera lens aperture is wide open. There is no field aperture with this microscope. The condenser setting is unchanged from last time. The view through the tube without the camera looks normal. Ive shifted the camera position left and right but I haven’t turned / rotated it. That could be worth a try. Good point about the objective not being in the right position – I’ll make sure of that now.

Zzffnn, the light source is rotatable. I’ll play around with that.
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rjlittlefield
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PostPosted: Fri Mar 16, 2018 8:46 pm    Post subject: Reply with quote

Have you tried moving the camera closer/farther from the eyepiece? Does the vignette change at all when you move the camera with respect to the microscope? If so, how?

--Rik
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dragonblade



Joined: 18 Oct 2014
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PostPosted: Fri Mar 16, 2018 9:12 pm    Post subject: Reply with quote

Rik yea like I mentioned before, I moved the camera closer and further away from the eyepiece. The vignetting is always there in the top right corner of the camera's LCD screen. I think it may have changed size a little when moving the camera. Can't recall if it got bigger or smaller when I moved the camera closer.
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dragonblade



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PostPosted: Sat Mar 17, 2018 12:20 am    Post subject: Reply with quote

I played around some more just now. One thing's for sure - the vignetting is not part of the outer rim of the eyepiece. When I move the camera closer, I can see the outer perimeter of the eyepiece disappear from the sides of the LCD screen. Also, the edges of the eyepiece look soft. Though the vignetting in the top right hand corner is sharp.

I move the camera forward in small increments with a rail slide. Each time I inch it closer, it looks like the vignetting gets smaller (temporarily) and then returns to the same size.

I switched objectives but this didn't get rid of it. The vignetting is still there.

Something very strange happened when I switched the G6 camera from video mode to photo mode. I had just returned from outside where I focused on infinity and positioned the camera in front of the microscope again. All of a sudden on the LCD screen, I got extreme vignetting covering most of the image (though dominating one side more than the other.) Then the view quickly returned to normal. Then it happened again and then quickly returned to normal again. Each time it did this, it made a clicking sound. The G6 has never done this before. It certainly didn't do it the last time I did afocal photography. It was in full manual mode by the way - manual exposure and manual focus.
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rjlittlefield
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PostPosted: Sat Mar 17, 2018 12:37 am    Post subject: Reply with quote

Well, obviously I don't know what your problem is. But what I'm thinking is that vignettes are always caused by two limiting apertures fighting with each other, so that for some part of the sensor, light that would get through one aperture is blocked by the other. The classic fight is between some aperture in the microscope and some aperture in the lens, essentially a mismatch between exit pupil of the eyepiece and entrance pupil of the lens. But in that case there should be obvious changes as you move the camera around. If there's no change in the vignette as you move the camera around, then the problem should be entirely within the microscope. But then you should see it by eye also. At least, you should see it by eye if you're looking through the same port that the camera is. Is the camera looking through a trinoc port, and possibly there's something wrong with say a photo/visual slider?

Quote:
All of a sudden on the LCD screen, I got extreme vignetting covering most of the image (though dominating one side more than the other.) Then the view quickly returned to normal. Then it happened again and then quickly returned to normal again. Each time it did this, it made a clicking sound.

This is typical of a camera that is automatically adjusting its aperture. I see "full manual mode", but I have to wonder...

--Rik
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dragonblade



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PostPosted: Sat Mar 17, 2018 12:52 am    Post subject: Reply with quote

Rik, thanks for your detailed reply. I'm using a monocular microscope so when the camera is positioned in front of the eyepiece, I don't have a spare eyepiece for my own eyes. When I view through the eyepiece with my eye, everything looks normal. The condenser iris was closed a lot more than usual so I opened it up wide open but that didn't stop the vignetting.

I admit that it is strange that this is happening with a high eyepoint eyepiece and a recommended prime lens. I used this same combination several days ago for an afocal video shoot and there was no vignetting at all then.

Yea the weird thing with the extreme vignetting fluctuating on the LCD screen did look like it was the result of some kind of automatic setting on the camera. I double checked (thinking maybe I accidentally put it on Auto.) Though the dial was clearly set to M.
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Pau
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PostPosted: Sat Mar 17, 2018 4:03 am    Post subject: Reply with quote

Quote:
Yea the weird thing with the extreme vignetting fluctuating on the LCD screen did look like it was the result of some kind of automatic setting on the camera. I double checked (thinking maybe I accidentally put it on Auto.) Though the dial was clearly set to M.


Here you have it: the camera is closing the diaphragm automatically despite the M set at the dial. Aside from M, is the aperture set at its maximum value (lower number)?. Dive into the camera menu and instruction booklet to find the cause, maybe it behaves differently in video than in still photo mode. Also try AV mode. There must be also a minor centering issue, maybe due to he angle of the camera respect to the eyepiece, how do you hold the camera?
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dragonblade



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PostPosted: Sat Mar 17, 2018 5:50 am    Post subject: Reply with quote

Pau wrote:

Here you have it: the camera is closing the diaphragm automatically despite the M set at the dial.


It does seem like that's the case. But that's impossible. In photo mode, M gives you full manual control over the aperture, shutter speed and iso. None of those parameters should be operating automatically.

Pau wrote:
Aside from M, is the aperture set at its maximum value?


Indeed it was - f2.8. Actually, when I shot the microscope video several days ago, I had it set at f3.2 and there was no vignetting then.

Pau wrote:
maybe it behaves differently in video than in still photo mode. Also try AV mode.


I use manual photo mode and manual video mode all the time and they are exactly the same. Both allow manual control of the aperture, shutter speed and iso. But yea I'll try AV mode as well and see if that makes a difference. I actually wonder if the camera has developed a fault.

Pau wrote:

There must be also a minor centering issue, maybe due to he angle of the camera respect to the eyepiece


I don't have the camera perfectly centered 100% but it's very, very close. When the outer rim of the eyepiece is visible and close to the edge of the frame, it looks fairly symmetrical to my eye. Though the vignetting issue appears on the right hand side. Actually, when I changed the aspect ratio to 3:2 for the photo mode, I saw vignetting on the bottom right corner as well as the top right.

Pau wrote:
how do you hold the camera?


I'm not holding it against the microscope. It's attached to a cheap slide bar which is mounted to a tripod.


Last edited by dragonblade on Sat Mar 17, 2018 6:25 am; edited 1 time in total
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ChrisR
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PostPosted: Sat Mar 17, 2018 6:09 am    Post subject: Reply with quote

It sounds as though the camera has moved - tilted maybe, relative to the microscope.
If you make a small movement like that, does the problem get worse?
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dragonblade



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PostPosted: Sat Mar 17, 2018 6:18 am    Post subject: Reply with quote

ChrisR wrote:
It sounds as though the camera has moved - tilted maybe, relative to the microscope.


I guess that's a possibility.

ChrisR wrote:
If you make a small movement like that, does the problem get worse?


Both last night and today, it took a very long time to get the camera lens centered over the eyepiece. A few times I started all over from scratch. And each time, the vignetting was in the same place. Even when the camera was not centered at all - shifted over to the right - there was still vignetting in the top right corner. And roughly the same amount too. So yea it doesn't get worse. It stays the same.

I know I mentioned this before but I tried an experiment. I deliberately set it up so that the view through the eyepiece occupied half of the camera's LCD screen. And the other half was black. And yet I still got the same amount of vignetting in the top right corner.
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