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Easy to overexpose when using darkfield illumination?

 
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dragonblade



Joined: 18 Oct 2014
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PostPosted: Sun Nov 05, 2017 9:05 pm    Post subject: Easy to overexpose when using darkfield illumination? Reply with quote

Ive been reading up on darkfield illumination and there's one bit of information someone mentioned in a youtube video which I haven't heard before. According to the video's uploader, it's very easy to get overexposure in darkfield images while shooting through a microscope. And sure enough, there were a few cases of overexposure in the darkfield video.

Is this likely due to the increase in contrast in darkfield imagery? I did note in the video that there was a water flea with some parts of the animal being correctly exposed but a large proportion of the 'stomach' area being overexposed. So it looked like there was a lot of contrast in the imagery. During another point in the video, there were a group of eggs that were very overexposed.

It could be possible too that the person who shot the video was using auto exposure. If that's the case, then all that large expanse of black would easily cause overexposure in many cases. As for myself, I plan to use full manual exposure when I get my equipment set up. As I'll be shooting video, I'll be forced to keep my shutter speed at 50th and the aperture wide open. I guess I'll have to adjust my iso setting to change exposure. If I get overexposure at the lowest iso setting, I'll have to use an ND filter to compensate.
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rjlittlefield
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PostPosted: Sun Nov 05, 2017 9:59 pm    Post subject: Re: Easy to overexpose when using darkfield illumination? Reply with quote

dragonblade wrote:
It could be possible too that the person who shot the video was using auto exposure. If that's the case, then all that large expanse of black would easily cause overexposure in many cases.

The same thing happens if you do manual exposure but fall into the trap of believing a metering system that relies on averaging. The only safe approach is to check for blown brights in the captured images, preferably by loading them into a computer where software can zoom in as required. Histograms in camera give a pretty good indication, but if you have small bright spots, the blowouts may not be evident in histogram either.

--Rik
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dragonblade



Joined: 18 Oct 2014
Posts: 140

PostPosted: Sun Nov 05, 2017 10:27 pm    Post subject: Re: Easy to overexpose when using darkfield illumination? Reply with quote

rjlittlefield wrote:

The same thing happens if you do manual exposure but fall into the trap of believing a metering system that relies on averaging. The only safe approach is to check for blown brights in the captured images
--Rik


Exactly. With my Panasonic G6 Micro 4/3 camera in video mode, I judge exposure in live view and then adjust accordingly (in real world situations.) I'll try the same technique when recording through a microscope.
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dragonblade



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PostPosted: Tue Nov 07, 2017 2:07 am    Post subject: Reply with quote

As noted above, adjusting on the fly is what I do for video (prior to pressing the record button.) Though for still photography using darkfield illumination, I would like to try HDR (if I can get my hands on some prepared samples / static subjects.) I think a darkfield HDR portrait of Daphnia could look awesome.
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Beatsy



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PostPosted: Tue Nov 07, 2017 2:24 am    Post subject: Reply with quote

Overall dynamic range of a DF scene is usually much greater than in brightfield too. The Sony A9 has a handy "protect highlights" metering mode that works wonderfully for DF images. It generally yields a very dark image overall but the high dynamic range of the sensor allows you to pull the shadows up by miles in post processing too. A pretty expensive option, I know, but if you have the camera for other uses anyway it's a handy feature to have in the toolbox. I presume there are other cameras with this option...?
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dragonblade



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PostPosted: Tue Mar 13, 2018 1:02 am    Post subject: Reply with quote

My Panasonic G6 doesn't have a an equivalent 'protect highlights' option. Sounds like a handy feature for high contrast scenes.

Last night, I did some video recording through a microscope for the first time (using brightfield illumination.) Normally when I use manual video mode, I can see changes in exposure in real time - when adjusting the aperture, shutter speed and iso. However last night, I could not see any change in exposure when adjusting the aperture in an afocal setup which I thought was strange. I did see the image circle in the eyepiece get smaller when I reduced the aperture size. When I changed the iso setting, I could see changes in exposure.

When I have a darkfield setup, I'm thinking of using an ND filter to underexpose the image initially. Then increase the iso gradually until I'm at a point just before the highlights begin to blow out. That's the plan anyway!
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Pau
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PostPosted: Tue Mar 13, 2018 1:54 am    Post subject: Reply with quote

Quote:
Normally when I use manual video mode, I can see changes in exposure in real time - when adjusting the aperture, shutter speed and iso. However last night, I could not see any change in exposure when adjusting the aperture in an afocal setup which I thought was strange.

This is normal, to understand it you need to be aware that afocally the camera lens is stopped down by the microscope optics, not by its own diaphragm, when you close the lens aperture what you will get past some point is vignette. This is true in most real life cases, the only exception will be when the exit pupil of the eyepiece pupil exactly matches the optical position of the lens aperture. To regulate the light you can play with the expossure time, ISO and the microscope light.
Quote:
When I have a darkfield setup, I'm thinking of using an ND filter to underexpose the image initially. Then increase the iso gradually until I'm at a point just before the highlights begin to blow out. That's the plan anyway!

You only want high ISO if you don't have enough light and you only want ND filters if you have an excess of light. If you can avoid overexpossure without high ISO and ND filters the results will be better. Combining ND with high ISO in principle makes no sense. Be aware that at high ISO the image has more noise and lower dynamic range.
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dragonblade



Joined: 18 Oct 2014
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PostPosted: Tue Mar 13, 2018 3:13 am    Post subject: Reply with quote

Pau wrote:
To regulate the light you can play with the expossure time, ISO and the microscope light.


True. Though when I'm shooting video, I prefer to keep the shutter speed unchanged / fixed otherwise it can lead to additional blurring or jerky motion.

Pau wrote:
If you can avoid overexpossure without high ISO and ND filters the results will be better. Combining ND with high ISO in principle makes no sense. Be aware that at high ISO the image has more noise and lower dynamic range.


Oh yea I'll see how much light there is with darkfield first and check just how bright the brightest parts of the image are. Having said, my Panasonic G6 produces pretty clean video at 1600 iso. Noise isn't much of an issue at that setting. Though I doubt I would be needing anywhere near 1600 iso for microscope videos regardless.
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