Loss of contrast in the objective

[bromodomain]

I have a nice old Zeiss Standard and I noticed that when I point the 40/0.65 achromat lens towards a bright source and look at the aperture from its base there is certain internal reflection forming. The image it renders is sharp but lacks proper contrast, I have a few old 40/0.65's and they all appear to have the same problem.

I compared with a newer Zeiss 40/0.65 (black barrel) and a newer Olympus objective and they both produced an image of similar resolution yet much higher contrast.

Could anyone shed some light on this age-related phenomenon?

I am trying to get my hands on a 40/0.75 Neofluar to replace the old achromat. Is this particular objective a good choice?

[Pau]

There may be several causes, all related to lenses surfaces:

- dirt, scratches or damaged coating (specially at the front lens)
- delamination (element separation in optical doublet or triplet elements), a well known issue in Zeiss optics of this era
- fungus infection

(But it may be also the internal reflection issue you're talking about. Does the objective still have its black matte rear plate or has it been removed?)

You can see them examining the objective under a stereomicroscope or high magification loupe playing with the light source or mounted in the Standard with a "phase telescope" eyepiece.

Neofluars are very good (both shrapness, contrast and CA) and now can be found at good prices. They aren't fully plan corrected, but have more flat field than simple achromats.

[bromodomain]

Yes, there is a rear plate. What I see is more like a glare but at the same time its a multitude of images of different sizes of the light source (regular light bulb).