What is the standard method of comparing magnification?

Guercini · Post by **Guercini** » Fri Nov 08, 2019 9:23 am

I am trying to understand what the "best image size" seen on a camera LCD screen (attached to a microscope) compared to the size of the view seen through the eyepieces. Let's say the view through the eyepieces is a circle covering 3mm on the microscope slide. What is the most desirable rectangular image coverage seen by the camera?

Scarodactyl · Post by **Scarodactyl** » Fri Nov 08, 2019 9:57 am

Typically a rectangle that captires as much of the FoV without vignetting. This could vary though. On my Wild M400 the camera actually gets a wider FoV than the eyepieces see. On some other systems you might not want the edges since they might not be very good. But broadly speaking wider FoVs are more expensive and desirable so it is good to be able to capture as much of them as possible.

Guercini · Post by **Guercini** » Fri Nov 08, 2019 11:10 am

Scarodactyl wrote:Typically a rectangle that captires as much of the FoV without vignetting. This could vary though. On my Wild M400 the camera actually gets a wider FoV than the eyepieces see. On some other systems you might not want the edges since they might not be very good. But broadly speaking wider FoVs are more expensive and desirable so it is good to be able to capture as much of them as possible.

Thanks for the comment. My preference is the camera view that includes full circle with the short side of the rectangle matching the diameter of the circle. What do you think? I tried to attache a picture showing this and two other possibilities defined by purple, green and blued colors, but I didn't know how to include the image in my response. Here is the link to the picture:

https://www.dealcorner.com/MossViews.jpg

I also like the view defined by the green rectangle, but not the blue one.

Scarodactyl · Post by **Scarodactyl** » Fri Nov 08, 2019 12:53 pm

That does allow you to control final framing of the image completely, but it wastes some of your camera resolution on black space. It is a tradeoff but I don't think there is anything inherently wrong with it.

Ichthyophthirius · Post by **Ichthyophthirius** » Sat Nov 09, 2019 5:31 pm

Hi,

If you are OK with cutting away the vignetted sensor area that's fine.

However, make sure that you don't open the field aperture so far as to illuminate the entire purple area even if your camera adapter allows it.

The field aperture must be closed so far that it matches the red circle (for most microscopes that's FN18-20, for super widefield microscopes a bit more (matching the maximum eyepiece FN of the microscope)). Opening the field aperture further than it is designed for causes stray light that can severely degrade the image.

Regards, Ichty

chanik · Post by **chanik** » Sun Nov 10, 2019 5:00 pm

There is one more important calculation: sampling size. You want to be ~3x over-sampled so as not to lose image resolution. Take a 40x 0.9NA objective. The field will be about 0.5mm and the resolution L/2*NA so about .55um/1.8. If you expand that to a 1/2" (12mm) image through your photo-tube then you have .3um resolution blown up to 7um in your image disk. You want your camera pixels to be ~2.5um. If they are much smaller then you just have a blurry full size image so you might as well have projected a bigger image disk. Less and you are losing resolution to undersampling. An IMX294 sensor has 4.6um pixels so you would want to project to more like a 20mm diameter for instance