How To Avoid Bright Spot In Middle Of Images

[georgedingwall]

Hi there,

I sometimes get a bright spot in the middle of my images when using my digital camera with my trinoc compund scope.

I can usually reduce this to some extent by adjusting the condenser and other things, but I find it difficult to get rid of it entirely, without heavy vignetting around the outside of the subject.

Can anyone offer any advice on dealing with this or point me to any online resources which might help.

Thanks for any help.

[Charles Krebs]

George,

Perhaps it would be best if you could post an example. These can be caused by some internal reflections or uneven lighting from the condenser.

Does it occur equally with different power objectives, or do you notice it more with the lowest power objective?

[georgedingwall]

George,

Perhaps it would be best if you could post an example. These can be caused by some internal reflections or uneven lighting from the condenser.

Does it occur equally with different power objectives, or do you notice it more with the lowest power objective?

Hi Charles,


I think the light problem seems most extreme at 400 X.

My scope has the following possible adjustments which seem to affect the lighting.

1. Rheostat to control brightness of light.
2. Collector with Field Diaphragm
3. Condenser with Aperture Diaphragm

I am somewhat unsure as to the most suitable combination of these adjustments. The condenser in particular has me a bit confused. It can be adjusted up and down, the diaphragm can be adjusted and the light path can be moved around with a couple of thumb screws. I'm never really sure what the correct adjusment of these should be.

Here is an image showing examples from all 5 objectives that my scope has. 40x, 100x, 400x 600xand 1000x. I've also included a shot of my scope.



I can correct this problem to some extent in photoshop, but would like to be able to get the best out of the scope. Any help with getting the lighting right would be greatly appreciated.

Bye for now.

[Ken Ramos]

Hi George

Allow me to put in my two cents here. You may want to try a diffusing filter. Diffusers will spead the light evenly over the field of view, reducing the extreamly bright area that appears centered in your normal field of view. I use blue filters (one clear light blue and a normal blue diffuser filter) which provide an illuminated field much like that of daylight. These filters are very inexpensive, depending on where you get them of course.

[georgedingwall]

Hi George

Allow me to put in my two cents here. You may want to try a diffusing filter.

Hi Ken,

I have a clear blue filter which came with the scope. I was told it was to correct the tungsten light coming from the illuminator into something near daylight white balance. I haven't really tried using it, as I shoot RAW images and then correct any white balance problems when I open the files.

There are 2 positions on my scope where this size of filter can be fitted. One is on the base directly on what my scope calls the Collector. The other is a swing out filter holder on the Condenser. Does it make any difference where you place a filter/diffuser.

Thanks for any help.

[Charles Krebs]

George,

The very first thing you want to do is set up the condenser properly. This should be done anyway, and may solve the problem.

1 ... use the 10X objective, and focus on the surface of a slide

2 ... close down the field diaphram and set a small aperture on the condenser diaphram

3 ... the aperture blades of the field diaphram should now be visible through the eyepieces. Don't change the microscope focus, but move the condenser up and down until the edges of the field diaphram are in sharpest focus.

4 ... now use those "thumb screws" to move the condenser around so as to center the field diaphram in your view through the eyepieces.

Leave the condenser in this position!

Open the field diaphram until it is just outside the field of view. (It is best to adjust the field diaphram size whenever you change objectives so that it is just outside the field of view... reduces flare)

To set the best condenser diaphram size, first open it fully and then pull out an eyepiece and peer down the tube. Close down the condenser aperture until you see that it closes down and "blocks" about 25 percent of the full aperture as seen down the eyepiece tube. (Usually no more than 50 percent or your resolution takes a big hit)

Try this, and if you still have the "hotspot" problem we can continue to trouble-shoot.

[georgedingwall]

Hi Charles,

Thanks for the guidance.

I've gone through your instructions step by step, and it does seem to have a positive effect on the image. It seems more balanced across the frame, although there may still be a slight hot spot. That may be my inept attempts at doing the adjusments accuratley. I'm sure I'll improve with practice.

Here is an image taken after I applied your advice.



The balance of brightness across the frame is much better than the same 400x image in my original sample image. There is also a bit less vignetting at the bottom of the image, but still quite a bit there.

I'm not sure if this is caused by me during the setup or whether the camera position on the scope is throwing the image sensor outside of the image circle. When I look at the setup, I feel that the camera is a long way above the scope. If the later is the case, there's not much I can do about it as the camera adaptor has no height adjustment.

Do you think ther is anything I can do about this vignetting or is it simply a metter of cropping out the offending bits.

Many thanks for your help.

[Charles Krebs]

George,

It looks as if the photoeyepiece (the optic in the trinocular head) is not projecting an image circle large enough to "cover" your camera sensor. The sensor in the D200 is roughly 16x24mm in size.

I'm wondering if the photoeyepiece in the trinocular head is designed for video use, and thus designed to cover a smaller sensor. If this is not specified in the info that came with your microscope, you may have to question your dealer to see if he knows.

If this is the case, a different photoeyepiece could solve it (if there is a capability to change it). If not you will likely have to crop the image.

[georgedingwall]

Hi Charles,

As far as I can tell, there is no seperate photoeyepiece in the photo adapter that came with the scope. There are some optical components in the trinocular head in the same light path as the photo adapter.

I did insert a spare photoeyepiece from my stereo scope into the photo adapter, and this got rid of the vignetting. I'm not sure if this is caused by the optical quality of the eyepiece, or simply by causing the camera to be a little futher away from the subject, and allowing the image circle to get a bit larger when it is in focus on the sensor.

The arrangement with the extra eyepiece isn't very stable, as it is not possible to firmly clamp the setup with the existing adaptor. I might try to make a small extension tube to fit firmly into the adaptor to lift the camera up a bit and see if that corrects the problem.

I'm much nearer to a permanent solution now than I was. Many thanks for your help in getting me to understand the proble. I really appreciate it.

Bye for now.