Fresnel lens discovery

GaryB · Post by **GaryB** » Sun Mar 05, 2017 7:50 pm

I was playing around with one of those Fresnel pocket magnifiers (like the kind they give away as bookmarkers) and made a nice discovery for those with low end gear like mine, mirror, Abbe condenser, lamp etc. I found when setting up Köhler it was a pain aligning things because my condenser isn't centerable so it all had to be done via all the other elements, and if the field iris was too close the condenser due to lack of space, it was usually lowered quite a bit to get focus, thereby effectively lowering its NA.

Example Fresnel lens

Anyhow, I placed the Fresnel lens on the condenser filter tray and this apparently gave two benefits. One, the condenser now focused on the field iris when right below the slide. Secondly, because of the circular nature of Fresnel lenses, by adjusting it's position you could 'virtually' center the condenser by moving the Fresnel lens until the iris was perfectly centered. This all assumes it was all pretty close to begin with.

It all sounds odd and I'm not sure what real world benefit there is as yet but the good news is that it doesn't introduce any extra aberrations that I can see. It's mostly to aid me in getting my sub-optimal lighting optimized. Once again I'll happily call out my very suspect knowledge of optical intricacies, this is basically an observation that may warrant further investigation for those of us at the lower end of the gear spectrum.

ChrisR · Post by **ChrisR** » Tue Mar 07, 2017 4:36 am

That's a neat idea.

For some reason I have yet to work out, people like apochromatic condensers.
Yours will be the other end of the er, spectrum.
You might get poor-man's Hoffman Modulation

GaryB · Post by **GaryB** » Tue Mar 07, 2017 1:05 pm

Heh, now there's a thought.. *rubs chin*

Ichthyophthirius · Post by **Ichthyophthirius** » Wed Mar 08, 2017 4:42 pm

Hi Gary,

This reminds me a bit of the auxillary lens on Zeiss Standard microscopes (the one below the condenser; there are some with a centering mechanism as well). Definitely worth experimenting with.

Regards, Ichty

GaryB · Post by **GaryB** » Wed Mar 08, 2017 5:38 pm

Thanks, Ichthy.

It's fun playing with everyday things to see what they'll do.

Pau · Post by **Pau** » Thu Mar 09, 2017 1:30 am

Ichthyophthirius wrote:... the auxiliary lens on Zeiss Standard microscopes (the one below the condenser; there are some with a centering mechanism as well). Definitely worth experimenting with.

True! I use it most of the time, better illumination and the only way with my DIC above 25X, the centerable model is also much more convenient (at least in my case)

GaryB · Post by **GaryB** » Thu Mar 09, 2017 9:24 am

Cool.
Well hopefully it'll help someone else in a similar situation to mine. Funnily enough, for the last day or more I've been using a DIY Lucigen condenser. Basically a bulb and an 9mm opalescent top diffuser that works with any objective from Oil immersion down to a 2.5x by moving the thing up and down.

Instead of a bulb and opalescent glass, I used an LED and a piece of ping pong ball and my condensers top iris to limit the size to 9mm. and blow me sideways if it doesn't work every bit as well as my condenser. It's neither Köhler, nor critical, something in-between and it really, really works! Apparently it never took off, the reality is that it's so simple that any buffoon with a ping pong ball and some black paper can make it!