# Focusing process (Optics)

1. Oct 24, 2007

### kattie

It is actually not a homework, but rather a paper I need to present, talking about the advance attempts to raise microscopy axial resolution.
It is said the axial resolution is usually 3 times lower than lateral. The explanation is that the lens from 1 side of focus plane can only produce a segment of spherical wavefront, and thus, making the focal spot longer than wide.
(If there were a whole spherical wavefront, the focal spot would be spherical too.)
People also have tried to get close to this spherical wavefront by introducing another lens at the opposite side of the focus plan, thus reduce the length of the focal spot. (4Pi Fluorescence microscopy - invented by S. Hell, Max Planck Institute Germany)

However, I don´t get the idea here. Why could a segment of spherical wavefront be responsible for the longer-than-wide focal spot?

By the way, my background is not in Physics so please make it as simple as possible. And sorry if there is a similar question already (I can´t find the search tab to look for it)

Thanks a lot.

2. Oct 24, 2007

### Mindscrape

I don't really know how you get a solid angle of 4π, but as far as your question goes, lenses are not perfect, and this is just something that happens.

So there are a lot of things that contribute to a non-perfect lens, such as spherical aberration (what you are talking about). Most of the effects, comas, airy effects, etc. can be reduced by using aspheric lenses and making everything as paraxial as possible, but at a certain point there is not much else you can do.

Phase contrast imaging is pretty good, and the resolution can get pretty close to the wavelength limit. For imaging at sub-wavelength limits, I have no idea how it works. What is your background in?

3. Oct 25, 2007

### kattie

I major in Biology, and that´s why we have to study about microscopy.
So now I should learn about "spherical abberation"? :D

4. Oct 25, 2007

### mgb_phys

The axial and lateral resolution are different properties.

Focusing brings a bunch of rays together into an approximate spot - called the 'circle of least confusion' in optics. Objects slightly in front of and slightly behind the best focus are also almost in focus. This is the depth of field in photography - otherwise when you took a photograph only an infinitely thin layer would be in focus.
You try and design microscope lenses to have as small a depth of field as possible (a side effect is that this is also the design that collects the most light) but because the sample is slightly transparent an area infront of and behind will also be in focus.
The way to reduce this is confocal microscopy where you also use a lens to focus the illumination in a thin in-focus layer.

Lateral resolution is where two points on the image are just far enough apart not to be a single blur. Again this is due to optical aberations not bringing the light from slightly different directions into the same place = the 'circle of confusion' not being a perfect point.
This is reduced by reducing sphericla abberation and other geometric optical effects.