Useful magnification - how to understand the _lower_ limit?

uimike · Post by **uimike** » Mon Apr 22, 2013 1:26 pm

Hi all,
I understand empty magnification, but I am having a bit of trouble understanding the implications of the lower limit.

I just bought a Zeiss 10x planapo - delaminated but still very nice -and on reading about useful magnification, I realize it falls _below_ the lower limit, when used with my 10x KPL (microscope is a Standard 16, 0.9 z condenser).

What does that mean, that I am actually not seeing all (detail) I could see from that objective?

If so, should I get a pair of 16 or 20x KPL, or even an Optovar?

thanks for any help!

mike

rjlittlefield · Post by **rjlittlefield** » Mon Apr 22, 2013 1:54 pm

I don't recall seeing that exact phrase before, "lower limit" of magnification. Can you give a pointer to what you've been reading?

What does that mean, that I am actually not seeing all (detail) I could see from that objective?

That sounds correct. A wide aperture 10X objective will resolve finer detail than most eyes can see through 10X eyepieces. In that case having higher power eyepieces will give you the option to see more detail within a smaller field. This is very much like what you would usually get by switching objectives. For example a 10X NA 0.40 plan apo objective combined with 20X eyepieces will have very similar resolution to a 20X NA 0.40 plan achromat with 10X eyepieces. If you have a 10X NA 0.45 objective, it will be a little sharper. With 16X eyepieces you would have an intermediate magnification of 160X total, still at NA 0.40 (or 0.45, whatever the objective is).

--Rik

uimike · Post by **uimike** » Mon Apr 22, 2013 2:13 pm

Hi Rik, thanks for clarifying! That means I can get some extra mileage from my coming Lomo Apo 20/0.65 as well :-)

I was calling the lower limit the value of 500x cited in the literature (the upper limit being 1000x, beyond which magnification is empty) - I hope I got it right.

So for the Lomo Apo, the useful magnification range is 325 - 650, which means using my KPL 10x, the magnification is 200x, well below what the objective can resolve, right?

And that would mean if I want to go for magnification instead of field of view, I could use a 16x or even a 20x ocular, right?

How useful is an Optovar here, as opposed to having a second set of oculars/

thanks!

rjlittlefield · Post by **rjlittlefield** » Mon Apr 22, 2013 3:08 pm

That all sounds correct.

Sorry I can't comment about the Optovar -- no experience with that.

--Rik

Free2Fish · Post by **Free2Fish** » Mon Apr 22, 2013 5:08 pm

I really like the optovar, especially with phase contrast. The Bertrand lens allows you to align the phase plates and anulli and the extra magnification options are often nice. It's also useful for checking the condition of objectives as you focus through the glass.

I just read about minimum magnification to achieve the resolution of the objective NA in a book called "Practical Microscopy" by C. L. Duddington. He states you need a magnification of at least 500X NA.

I had a situation recently where it was suggested that I didn't have enough magnification to resolve some diatom details. Using 2x instead of 1.25X seemed to help quite a bit.

Harry

rjlittlefield · Post by **rjlittlefield** » Mon Apr 22, 2013 6:57 pm

When I'm testing 10X NA 0.25 objectives, it's very common that I see more detail in pixel-peeping the digital captures than I can see live through a 10X eyepiece.

That's at 100X = 400 times NA, so the observation is consistent with the idea that total magnification should be at least 500 times NA if you want to see all the detail that the objective is resolving. Even more magnification, like 1000 times NA, will make it easier to see that finest detail.

--Rik