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Unusual vignetting in corner in afocal setup
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rjlittlefield
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PostPosted: Sat Apr 07, 2018 12:18 pm    Post subject: Reply with quote

dragonblade wrote:
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

Thank you for posting the images.

First, let me annotate them:



Note that the slide contents and the hard vignette due to the field stop stay in a nearly fixed position in all three frames. That tells me that you did not change what I'm calling "tilt".

Attempting to illustrate more clearly what I'm talking about, I have shot some images and prepared what I hope is an understandable explanation.

First, here is the setup -- simply a cell phone held carefully over the eyepiece of a binocular microscope:



Then I misaligned the camera in various ways, shot the images that resulted from those misalignments, and put them together into this explanation:



In words...

The light coming from the eyepiece forms a sort of hourglass shape. It is important that the camera is positioned so that the narrow part of that hourglass lines up with the aperture of the lens, so that all the light from the eyepiece can get through the lens.

The camera also needs to be pointed straight into the eyepiece, so that the center of the field goes to the center of the camera sensor.

If the camera is perfectly aligned but zoomed too wide, it will see part or all of the field stop that is built into the eyepiece (top left image).

Holding the camera still perfectly aligned, and zooming it narrower, the image expands so that the entire field stop lies outside the captured frame. This is the ideal situation (bottom left image).

On the other hand, if the alignment is not correct, then I get various forms of vignetting. Hold the camera too close, and I get a big fuzzy vignette. Hold it too far, and again I get a fuzzy vignette, though maybe smaller and sharper. Tilt the camera to the left (or up, down, or right), and the camera sees the field stop in the eyepiece, at the same time that the image shifts within the frame. Shift the camera sideways, but keep it pointed straight down the eyepiece, and again I get a big fuzzy vignette but this time it's not centered. Of course I can commit multiple sins at the same time, such as simultaneously tilting and shifting the camera to both see the field stop (sharp vignette) and block part of the light cone (big fuzzy vignette), as shown in the lower right.

So now, analyzing your images, it looks to me like they show just a slight tilt that is almost constant, with varying amounts of mostly shift that causes a fuzzy vignette to march across the frame.

All in all, there's a lot that can go wrong. You need to have the camera located at the correct place (X,Y,Z), pointed straight down the eyepiece (pitch, yaw), and zoomed enough to miss the field stop. That is six things to get right, all at the same time, and errors of even 1 millimeter can produce nasty effects. If somebody doesn't understand what's going on, this can all look completely random and incomprehensible.

So, I'm highly sympathetic to your confusion!

That said, all this is neither black magic nor rocket science. What you really need to do is to calm your frustration, stop thrashing around, and start paying very close attention to how the image changes as you move the camera around by small amounts.

Quote:
I got that weird flickering again in the LCD screen where extreme vignetting occurs about every second, accompanied by a clicking sound.

I'm not at all surprised.

I would place a heavy bet that when you get the alignment under control, the weird flickering will start to make more sense. The extreme vignetting accompanied by a clicking sound is caused by the lens iris suddenly stopping down. That's a transient effect triggered by sudden big changes in the amount of light getting through the lens, which is caused by shifts in alignment, which you don't have under control yet, so the problem seems random. It doesn't occur away from the microscope because then the camera doesn't see those sudden shifts in brightness. With my cell phone the effect manifests as overshoot and hunting in the automatic brightness control, unless the alignment is near perfect. (Yes, I understand that you have intended to set the camera on "full manual". But it seems that the camera's view of what that means is a little different from yours.)

I hope this helps.

--Rik
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Charles Krebs



Joined: 01 Aug 2006
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PostPosted: Sat Apr 07, 2018 2:13 pm    Post subject: Reply with quote

Rik and Pau and others have done a yeoman's job in explaining the problem. Their advice is sound IMO, and it is an "alignment" issue.

I'll add a few other thoughts based on some things you said.

Quote:
I used this same combination several days ago for an afocal video shoot and there was no vignetting at all then.

Quote:
When I did my first afocal video recording session, the lens was further away from the eyepiece and vignetting was nowhere to be seen.

Quote:
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

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


All makes sense. In video mode your aspect ratio is different, You are, in essence, "cropping" out the corners. And while I am not familiar with your camera, many (most?) digital cameras don't use the full sensor width when recording video. So in addition to the top and bottom (and thus corner) crop from the aspect ratio change this is yet an additional "crop" that would remove corner vignetting.

Quote:
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.

Now this does not make any sense! Sad .... unless... Are you saying that you move the rail (camera) closer then stop at a point where the vignetting looks smaller, and then the vignetting suddenly returns "larger" not having moved any of the physical set-up? Do you hear that little "click" sound you've mentioned several times? It sure sounds like the lens is stopping down, even though you say everything is set to manual (and lens should always focused at infinity... no autofocus).

Quote:

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

It isn't at delicate as brain surgery, but sometimes the best possible match-up in an afocal arrangement requires a very precise alignment to avoid vignetting. (And in quite a few cases it can't be completely avoided, but it should at least be symmetrical in appearance). If your arrangement needs such precise alignment it will be very frustrating and time consuming to get it "dead on" with the method you are using, especially if you need to share the same eyepiece (monofocal) for viewing.

You might want to check for an inexpensive used eyepiece and make up a "permanent" adapter with an eyepiece attached that can simply be screwed on to the lens. Then all you need to do to switch from viewing to photography is pull the viewing eyepiece out, and insert the eyepiece /camera combination.
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dragonblade



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PostPosted: Sat Apr 07, 2018 6:26 pm    Post subject: Reply with quote

Rik, thankyou for the help. I'm glad to know that you've worked out what is causing the hard vignette - the field stop in the eyepiece.

And with the three photos I posted, it certainly does look like I shifted the camera sideways but I swear I didn't. All I did was rotational tilt. I used the adjustment on my tripod that is used for vertical shots. That's the only movement I used and I did so in very small increments.

And yes I do realise there is another way you can tilt the camera - so that the lens is no longer parallel to the eyepiece. I did not do that kind of tilt during the tests. Good illustrations by the way.

Actually, when you recommended tilting in a previous post, I was wondering if you meant 'that other kind of tilt' but I wasn't really sure. The thought had crossed my mind! With the illustrations, it's really clear now.

rjlittlefield wrote:

All in all, there's a lot that can go wrong. You need to have the camera located at the correct place (X,Y,Z), pointed straight down the eyepiece (pitch, yaw), and zoomed enough to miss the field stop. That is six things to get right, all at the same time, and errors of even 1 millimeter can produce nasty effects.


It takes me an incredibly long time to get the alignment right. And it looks like most of the time, I don't end up getting the alignment quite right. I'm constantly adjusting and readjusting.

rjlittlefield wrote:
That said, all this is neither black magic nor rocket science. What you really need to do is to calm your frustration, stop thrashing around, and start paying very close attention to how the image changes as you move the camera around by small amounts.


It always has been straight forward what happens to the fuzzy vignette when I change the position of the camera. If I shift the camera sideways, the fuzzy vignette will be off center etc. That is logical. It was the hard vignette in the corner that was perplexing to me - which you have identified as the field stop - thankyou once again for that. So, unless Ive misunderstood you, you reckon the appearance of the field stop is caused by the camera lens not being quite parallel to the microscope's eyepiece? In other words, the camera is tilted slightly away from the eyepiece?

rjlittlefield wrote:

The extreme vignetting accompanied by a clicking sound is caused by the lens iris suddenly stopping down. That's a transient effect triggered by sudden big changes in the amount of light getting through the lens, which is caused by shifts in alignment


That would only happen in automatic exposure mode.

rjlittlefield wrote:
(Yes, I understand that you have intended to set the camera on "full manual".


It wasn't just my intention to set the camera to manual exposure mode. I actually did set it to manual exposure mode. It was set to M (which I use 98% of the time with the Panasonic G6.) In this mode, I have full control of the aperture, shutter speed and iso. The flickering / vignetting also occurred in aperture priority mode where I have control of the aperture and the camera selects the shutter speed.

rjlittlefield wrote:
But it seems that the camera's view of what that means is a little different from yours.)


Actually, the camera shares the same view as me of what manual exposure means. And that's been the case for the past 20+ years with all manner of cameras that Ive used which have included 35mm film SLRs, medium format film cameras, MiniDv and DVCam camcorders, super 8 movie cameras and a 16mm movie camera. The basic principle of setting the exposure is the same though some of these cameras do have limits on what exposure parameters can be set. Regardless, the aperture can be set manually with all of those cameras. That is no different to the G6.

By the way, Ive just come up with a theory of what might be causing that flickering / vignetting and why it doesn't occur in video mode. In the still photo modes, the view through the EVF and the LCD screen is kind of like automatic exposure, even if the exposure settings are set to manual. The camera will respond to changes in light and adjust the brightness of the onscreen image accordingly though that will not necessarily reflect or indicate what the finished photo will look like. The photo you take may very well look very different to what is shown on the EVF / LCD screen (with regards to exposure.) Though I haven’t heard that clicking noise before. In manual video mode, it’s very different. In this mode, what I see is what I get – regarding exposure. I see changes in exposure only when I change them myself.

In my previous post, I mentioned that I took a photo during the extreme vignetting of the left side of the LCD screen while in manual exposure mode. And the resulting photo was nothing like that. The image showed none of that massive vignetting on the left. The photo was correctly exposed. I determined the correct exposure by bracketing a series of exposures previously by adjusting the shutter speed while keeping the aperture and iso values the same - f2.8 and 160 iso. The shutter speed I ended up using was 1/15th.

Charles Krebs wrote:

All makes sense. In video mode your aspect ratio is different, You are, in essence, "cropping" out the corners. And while I am not familiar with your camera, many (most?) digital cameras don't use the full sensor width when recording video. So in addition to the top and bottom (and thus corner) crop from the aspect ratio change this is yet an additional "crop" that would remove corner vignetting.


To keep things consistent, I used the same aspect ratio between the video and the photos the vast majority of the time - 16:9. Did you note that the photos I posted were 16:9? Yes, there was more a lot more vignetting in the 3:2 photos I took on one occasion though I was expecting that. No surprise there. So considering that the aspect ratio was the same most of the time (for both photos and videos) I was surprised by the different results I got after the initial afocal session.


Charles Krebs wrote:

Now this does not make any sense! Sad .... unless... Are you saying that you move the rail (camera) closer then stop at a point where the vignetting looks smaller, and then the vignetting suddenly returns "larger" not having moved any of the physical set-up? Do you hear that little "click" sound you've mentioned several times?


Yes but only slightly. Not a big change. And no I don't hear the clicking sound. It's not the same as that rapid flickering accompanied by the clicking sound - nothing like that. This is a really cheap rail slider I'm using and it is very bumpy when I move the camera forward on it. Don't suppose it's the bumps that cause this?


Charles Krebs wrote:

You might want to check for an inexpensive used eyepiece and make up a "permanent" adapter with an eyepiece attached that can simply be screwed on to the lens. Then all you need to do to switch from viewing to photography is pull the viewing eyepiece out, and insert the eyepiece /camera combination.


I admit it takes me so long to set my camera up. I will consider the above advice. Not sure how accurate I would be though if I had to make it myself! I don't suppose there are already products like that on the market? It would certainly save time though I guess it would also increase the risk of vibrations passing from the camera to the microscope.
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rjlittlefield
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PostPosted: Sat Apr 07, 2018 8:07 pm    Post subject: Reply with quote

dragonblade wrote:
So, unless Ive misunderstood you, you reckon the appearance of the field stop is caused by the camera lens not being quite parallel to the microscope's eyepiece? In other words, the camera is tilted slightly away from the eyepiece?

Yes, that's correct.

Quote:
rjlittlefield wrote:
(Yes, I understand that you have intended to set the camera on "full manual".

It wasn't just my intention to set the camera to manual exposure mode. I actually did set it to manual exposure mode. It was set to M (which I use 98% of the time with the Panasonic G6.)

I chose my words carefully.

In all your arguments prior to your latest reply, you seemed to think that because your camera is set to M mode, it never closes the aperture at any time. Even now you say "that would only happen in automatic exposure mode." And yet the sudden extreme vignette combined with clicking noise does not admit any other interpretation. The aperture is closing, regardless of whether you think it ought to be.

Quote:
By the way, Ive just come up with a theory of what might be causing that flickering / vignetting and why it doesn't occur in video mode. In the still photo modes, the view through the EVF and the LCD screen is kind of like automatic exposure, even if the exposure settings are set to manual. The camera will respond to changes in light and adjust the brightness of the onscreen image accordingly though that will not necessarily reflect or indicate what the finished photo will look like. The photo you take may very well look very different to what is shown on the EVF / LCD screen (with regards to exposure.) Though I haven’t heard that clicking noise before.

This sounds like you've come to some way of understanding why the aperture might be moving even though the camera is set on M. If so, that will be a very helpful advance.

Quote:
In my previous post, I mentioned that I took a photo during the extreme vignetting of the left side of the LCD screen while in manual exposure mode. And the resulting photo was nothing like that. The image showed none of that massive vignetting on the left.

Yes, I noticed that. The report indicates that the lens was doing what you expected, when the exposure was taken. Before the exposure was taken, the lens was stopping down. Whether it was doing that because the lens is whacky or it's a normal but previously unnoticed aspect of operation for the camera, I cannot know. Your observation that it only occurs in afocal configuration, when I know that sudden light changes often occur as I discussed, suggests to me that the lens is fine and the stopping down is triggered by the camera. But I could be wrong about that, and in any case you'll have to figure out that bit for yourself.

--Rik
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dragonblade



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PostPosted: Sat Apr 07, 2018 9:33 pm    Post subject: Reply with quote

rjlittlefield wrote:

Yes, that's correct.


Great to confirm that it's the tilting that causes the field stop to appear. I'll have to pay extra close attention to that next time.

rjlittlefield wrote:

In all your arguments prior to your latest reply, you seemed to think that because your camera is set to M mode, it never closes the aperture at any time.


Not really. The aperture closes down when I take the photo. The aperture blades close down to the f stop setting I selected during manual exposure or aperture priority mode. Unless I'm shooting with the aperture wide open in which case the aperture blades won't close at all.

rjlittlefield wrote:
Even now you say "that would only happen in automatic exposure mode." And yet the sudden extreme vignette combined with clicking noise does not admit any other interpretation.


As I explained previously, there is a difference between what is seen in the EVF / LCD and the actual exposure that is made to the sensor when a photograph is taken. In a sense, the view through the EVF and LCD does appear like it's in auto exposure when the still photo modes are engaged. For example, if you tilt the camera up to a bright sky, the EVF will darken etc. Though that is totally independent to the exposure changes that the photographer makes. Regardless of how dark that sky now looks in the EVF, you can make it as light as you want in the resulting photograph by decreasing the shutter speed, opening up the aperture or increasing the iso. And after that photograph is taken, the sky will still look dark through the EVF, even though the photograph of the sky (taken at the same angle) will be lighter due to your exposure adjustments.

I don't know the technical reason behind that difference. I could be wrong but perhaps there might be some kind of virtual or secondary aperture or something that is used exclusively for the view through the EVF and LCD and is independent of the lens aperture.

rjlittlefield wrote:
The aperture is closing, regardless of whether you think it ought to be.


Not really. The lens aperture was wide open at f2.8 when that extremely dark vignetting occurred on the left side of the LCD screen. And it was still at f2.8 when the photo was taken. And the resulting photograph was correctly exposed with no extreme vignetting. The exposure was identical to a previous photograph I took during bracketing (when I was determining which exposure to use.) And just to clarify, a number of photos were taken during the extreme vignetting (not just before) and showed no such vignetting in the images. In other words, the left side of the LCD screen was mostly dark at the same time that the shutter was released. Though the resulting photographs are nothing like that. At times, the extreme vignetting remained on the screen for several seconds (during which time a number of photos were taken including the ones I posted.) And as you see, the left half of the image is not dark.

Additionally, the exif data on the digital files in Windows 7 confirms that the f stop was f2.8. I took many more photos during that session than just those three. Though despite some of the extreme vignetting / flickering that occurred during some of the shooting session, every exposure turned out the way I had predicted (after bracketing) with no extreme vignetting. And once again, the exif data confirms that every single photo (all 56 of them) were taken at f2.8.


rjlittlefield wrote:
Your observation that it only occurs in afocal configuration


Like I mentioned earlier, the EVF and LCD screen do darken in response to areas of brightness away from the microscope (such as in outdoor scenes.) But I don't get the flickering / vignetting in outdoor scenes or most indoor scenes. That only occurs in an afocal setup. I don't get the clicking sound in normal use either.


Last edited by dragonblade on Sun Apr 08, 2018 2:47 pm; edited 1 time in total
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Charles Krebs



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PostPosted: Sun Apr 08, 2018 2:01 pm    Post subject: Reply with quote

Some cameras allow you to choose an "exposure simulation" mode for electronic viewing instead of always trying to show the scene as it would look if "properly" exposed.

Apparently this is not a choice on the G6, but there seems to be some type of "work-around". See this thread:
https://www.dpreview.com/forums/thread/3944530

Is the light source of your microscope incandescent or LED? (Some camera/led combinations can be prone to flickering).
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dragonblade



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PostPosted: Sun Apr 08, 2018 3:06 pm    Post subject: Reply with quote

Charles Krebs wrote:


Is the light source of your microscope incandescent or LED? (Some camera/led combinations can be prone to flickering).


Mine is LED.
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dragonblade



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PostPosted: Mon Apr 09, 2018 2:45 am    Post subject: Reply with quote

It took a long time but today, I managed to set the camera up with no vignetting. I had to rotate / tilt the camera, shift, rotate / tilt, shift many times over and over until the vignetting disappeared. Then I temporarily removed the camera and rail slide from the tripod and took a sample from a jar full of spring water with grass, leaves and dirt from a dry creek bed that had been sitting for over a week. Not one single organism to be found under the scope. All that effort for nothing lol. I'll take another sample from the jar after dinner and hopefully I might find something living to record.
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zzffnn



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PostPosted: Wed Jul 11, 2018 11:32 am    Post subject: Reply with quote

dragonblade,

I managed to replicate your vignetting problem, with m4/3 camera E-M10 II and Sigma 30mm F/2.8.

For me, the vignetting only occurs when camera (sometimes mistakenly) thinks there is too much light hitting the sensor at any moment. That occurs even when lens is set to manual mode with manual focus and MF assist/magnify turned off. Reducing shutter speed to reduce total exposure does not reduce vignetting. Only reducing actual light level + activating magnified focus preview will remove vignetting, if vignetting (iris closing down) already occurrs.

I think the solution is:

1) use a fully manual lens that is not controlled by camera. A 28mm lens is better than 24mm or 35mm lens, I think.

2) simply avoid giving the camera too much light.

I don't know exactly why vignetting occurs, but I am guessing it is caused by iris closing command from the camera and/or floating element (which is also controlled "by wire") of the Sigma 30mm lens.
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