I have the same microscope and essentially the same arrangement.
Instead of the PLI 2.5X I have the PLI 2X, Instead of the "F-Mount" tube shown in your diagram I made up my own tube that is adjustable in length. So except for the magnification difference, the same arrangement. (Naturally you need to be sure that the camera is parfocal with the eyepieces if you are focusing through the eyepieces, or you need to focus very carefully with the camera).
The "darkness" might be due to the Canon body used. The lower end Canon bodies (T2i, T3i) can only do live view in what is called "exposure emulation" mode. On a microscope, at very low light levels there seems to be a limit on how far the view will "gain up". To see if this is the problem try flipping up the flash on the camera. This has been discussed in a few other threads in the forum:
http://www.photomacrography.net/forum/v ... hp?t=13938
The primary one I am thinking about is the live view implementation. On the 50D you can select to have it "emulate exposure" or have the camera ramp the gain up/down automatically in order to provide a nicely "exposed" viewable image on the screen. With the 600D it is always in the emulate exposure mode (at least I have not found a way to switch it). I'll echo one of Chris' complaints here. If I am using electronic flash on the microscope I usually want to have the shutter speed set to 1/200 second. With the 600D that can result in a very dark image on the screen, since a proper "ambient/continuous light" exposure might require a 1/25 second exposure... and the camera wants to "tell me" that the set shutter speed is far underexposed. With the 50D (and "emulate exposure" turned off) I see a nice well "exposed" image on the screen regardless of the set shutter speed. (This is really only an issue when flash is used as the photo light, and the microscope viewing illumination is low). BTW... you can "fake out" the 600D by popping open the built in flash. Apparently the camera then expects that the correct amount of light for a good exposure will be provided by flash, so it makes the screen image appear properly exposed. Realistically, with the 600D, if you want fairly good overall external/non-dedicated flash control, and the ability to "overcome" this dark live-view issue you will want to have a dedicated Canon flash that can be set to manual, low power settings and second curtain sync.
http://www.photomacrography.net/forum/v ... hp?t=14701
This may be what you are referring to. The 600D is always in the "emulate exposure" mode, unlike higher end Canon bodies where this can be set to either on/off. If emulate is set to "off", for example, with a 50D body, the camera will automatically up the gain to try and provide a good live view image regardless of the exposure settings. The only way I am aware of that this can be accomplished with a 600D is to have a Canon flash attached (or the pop-up flash opened). The camera then "assumes" the flash will provide the needed light for proper exposure and increase the gain in live view to provide a viewable image.
This may (or may not
) be the "darkness" problem.
The bigger issue is whether or not this microscope, or any stereo microscope, can provide the image quality wanted at the magnifications you want. Stereo microscopes typically have very modest numerical apertures. While this gives great depth-of-field for viewing, if you are after really "sharp" photographs you need to keep your "in camera" magnifications in line with what is is reasonable given the numerical aperture. It is very easy, and very common to set up these stereo microscopes for photography at magnifications (on camera sensor) that virtually guarantee the images will not be very sharp due to diffraction. My SMZ-800 has the 1X objectives with a maximum NA of 0.09. But that NA is only obtained at the full zoom... 6.3X. This is actually lower than a typical "basic" compound microscope 4X objective. A basic 4X has an NA of 0.10, a "better" quality is about 4/0.13, and the highest quality is 4/0.16 to 4/0.20. There have been a few threads in which this topic has come up:
http://www.photomacrography.net/forum/v ... hp?t=16741
http://www.photomacrography.net/forum/v ... hp?t=15550
Think about it this way. With the 1X, NA 0.09 set to 6.3X, and a 2.5X photoeyepiece you will be getting 15.75X on camera sensor. The smallest subject detail that can be resolved is 3.73 microns in size. If you used a "compound microscope" with a 10/0.30 objective and a 1.57X magnification to camera you would also have a magnification of 15.57X on camera sensor, but the smallest subject detail that is resolve would be 1.12 micron.... more than 3 times (333%) greater resolution!
As a result I rarely use this microscope for "serious" photography. And when I do, it is at very low in camera magnifications. Here are a few examples:
http://www.photomacrography.net/forum/v ... hp?t=16053
http://www.photomacrography.net/forum/v ... php?t=6078
http://www.photomacrography.net/forum/v ... php?t=5964
One last thought. With this set-up you really need to pay very close attention to potential vibration problems (this is true of the physical arrangement of many stereo microscopes where the camera is greatly extended on various tubes, and the base microscope stands are less stable than typical "compound microscopes"). If you have a Canon body with EFSC be sure to use that feature, and be very wary of other external vibration sources.
