Circular Oblique Lighting (COL) on microscopic images of bacteria and algae

Using the slide from my previous post, I tried out Circular Oblique Lighting on the Zeiss IM today using appropriate COL annuli.

All of the following use Zeiss condenser NA 1.4 and low NA Leitz 25x objective (NA 0.22).


The following photo is a BRIGHT FIELD image of part of the slide – the bacteria are just visible as the blue/black mottling top right/and at bottom left:

The following picture is of the same field of field of view with same condenser and objective but adding in COL annulus – I don’t think there is a deep sky astronomy filter that works quite as well as this!

The following photo uses a COL filter with smaller hole for light to pass through – do you think there is a meaningful difference between the two?

I think there a difference – the bacteria stand out a bit more BUT so do the larger hill-like mounds in the background – these are due to dust on the slide or optics.

The following is a dark field photo (changed to greyscale in GIMP2) – again the contaminants stand out as well as the bacteria which is a real nuisance, although the bacteria are easily seen – same condenser and objective as above:


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