MicrobeHunter.com Microscopy Forum

I am confused (which at my age I am beginning to get comfortable with).

I thought that the idea of an infinity objective was that the rays exited the objective parallel and were then put in focus by the "tube lens".

I thought that this was so that you could add accessories (distance) between the objective and the eyepiece without upsetting the focus.

Most of my infinity objectives have a -19mm BFD, recently i purchased an odd ball that has a BFD of -25. I was told that to get optimum results for DIC (from the -25mm) I would have to purchase a Nomarski prism slider that had a "shift" function which shifted the prism upwards.

Others with the shift function slider said that they couldn't see a difference in the positions.

This seemed reasonable to me as if the rays are parallel until they get to the tube lens and if putting (for example) an EPI light housing in the light path didn't make a difference I couldn't see what difference 6mm would make for the Nomarski prism.

I tried it and DIC with the standard prism slider seemed to work but I was told it wasn't optimal.

So I reasoned I can pay about $1000 to buy a slider and move the prism up or for about $10 on eBay, I can by a spacer and move the objective down.

I went for the $10 fix after a great deal of gnashing of teeth.

I couldn't see a difference (exactly like the person with the slider prism said.)

However all of this means that I don't understand what a ray tracing from an infinity objective to the tube lens looks like. Surely if it is an infinity lens it doesn't have a BFD?

Can anyone explain or point me to a Youtube video?

Thanks
Neal

You have the sign wrong on the spacing. It would need to be 6mm closer, not further. This is the back focal plane and it is inside the objective. The nomarski prism accounts for that in its shear.

I did some more experiments:

With a 40x with a BFD of -19 I set up Kohlar illumination and stopped down the field iris and looked at the image circle on a frosted slide I substituted for the Nomariski prism till I got a small dot about 1/5th the FOV. If i moved the frosted slide from the bottom to the top of the opening it stayed the same size.

With the 40X water and a BFD of -25 the circle of illumination on the frosted slide was about 5x as big and if I moved the frosted slide from the bottom of the slot to the top it got bigger.

If I spaced the -25 objective down 6 mm i got similar results to the normal -19 objective.

I think I figured it out thanks to the Olympus link below:

https://www.olympus-lifescience.com/en/ ... nityintro/

A curse on those who "simplify" things to make them understandable.

I thought that a normal objective had ray paths that exited and formed a cone of light and that the rays that exited an infinity objective where parallel (to the axis of the microscope tube).

The reality is that there are bundles of parallel rays that are parallel to each other but at an angle to the axis of the microscope tube. (See figure two in the link).

That means that they form a cone and that cone has a "back focus distance".

In my case I have two 40x infinity corrected lenses. One with an NA of .75 and one with an NA of 1.15.

That means that the cone of light that enters the 1.15 lens is dramatically larger than the one that enters the .75 NA lens. (If you look at the front of the objective this is VERY obvious.)

That implies that, all things being equal, the exit cone will be bigger. So much bigger that if nothing was done, some of the rays exiting the objective would completely miss the tube lens.

Actually some probably do because the 1.15 lens although being much more expensive has a much smaller field of view than the .75 lens.

What I believe now, is that the reason they changed the back focal distance is that this effectively makes the objective to tube lens distance shorter and gets more of the image onto the tube lens.

Corrections and or expansions appreciated.

Neal