Microscope Objective Degradation Limits for Photomicrography

[Craig Gerard]

Microscope Objective Degradation Limits for Photomicrography.

Are there any guidelines or 'rules-of-thumb' regarding an acceptable degree of optical degradation (damage, physical or otherwise) of microscope objectives that are to be used primarily for photomicrography?

As an example; I have one objective that has visible 'pits' on the front element. These 'pits' are clearly visible to the naked eye from a distance of one foot; so I expect I can most certainly expect this objective to 'shadow' (if that is the correct term)?

Closer inspection of other objectives using a macro lense as a loupe and a good desklamp, I have observed various levels of degradation that may or may not impact on imaging quality.

Is there a point where an objective becomes suspect? What should I look for and is there some appropriate imaging test recommended that could help determine an objective's suitability for photomicrography?

What are the more common problems and how can they be identified?


Craig

[rjlittlefield]

Craig,

I'm afraid the best test is "try it and see", using a subject representative of what you eventually want to shoot. I've never seen a "grading guide" for objective damage.

The front element of the lens is so out-of-focus that very likely it will not cast a visible shadow.

The main issue will be that light scattered from the pit will spread evenly over the whole image. This cuts contrast, especially in darker regions. If you're interested mainly in overall appearance, the contrast loss may be compensated with a curves adjustment. But loss of contrast also implies some loss of resolution. You won't be able to back out all of the degradation, and you will lose the finest details that would have been captured by the same lens when it was intact.

If the damage is isolated, you may be able to fill the pits with black ink to stop the scattering. And a really good lens even with a non-filled pit may still beat a lesser lens in pristine condition.

Not much help, eh?

--Rik

[Craig Gerard]

Rik,

Thanks for the reply.

I made the question rather general, without mentioning specific 'brands' of objectives.

Photomicrography is new territory for me. I received a BHT and have spent some time carefully cleaning and checking all the areas that I feel comfortable accessing; basically a more intensive version of routine maintainence. During this exercise I became concerned with some of the objectives that appeared to be okay; but on closer inspection exhibited some level of degradation.

In addition to the notes above, I also went 'shadow chasing' - looking for dust and other undesirables in the eyepieces and the trinocular head. The eyepieces are easy, you just rotate them, if specific dust spots rotate at the same time, then you have found their location.

In regard to the objectives, as you've suggested, "try it and see" is one approach; but if you were presented with a number of objectives and needed to make a purchase decision, at what point would an objective become unacceptable? Is there an imaging test that would be applicable?

Craig

P.S. This is not simply a case of constructive procrastination

[Retro]

Craig:
Some time ago I bookmarked a fascinating article called "Why the Bubbles in the Lens Glass?" in a feature about Contax Lenses (half-way down the page) that I've been anxious to share with this forum... and you stepped right in it :-)

I hope you find it enlightening.
Jim

[Craig Gerard]

Thanks Jim,

I'm always stepping into things




Craig

[PauloM]

In addition to the notes above, I also went 'shadow chasing' - looking for dust and other undesirables in the eyepieces and the trinocular head. The eyepieces are easy, you just rotate them, if specific dust spots rotate at the same time, then you have found their location.

Craig, with a phase telescope you can gradually move its focus through each element of the optical train. This allows you to locate each "problem area" with some accuracy, even inside the objectives. It allows you to recognize problems like delamination and oil seepage that would otherwise be very hard to identify.

[Blame]

There is one thing you can do with pits. You can ink them in. They will still not function in gathering light but it should stop them scattering light and reducing contrast.... or so I have been told.

[Craig Gerard]

Thanks for the responses.

Pau, the phase telescope sounds like an appropriate tool for objective analysis. What is a phase telescope?

Blame, the ink approach is something that would not have ocurred to me before starting this thread. Rik also mentioned a similar approach.

I have a replacement for the very badly pitted Olympus SPlan FL2; but will experiment with the damaged copy.

Delamination, fogging, haze, and fungus are faults that I am able to recognize visually.

I do not have the appropriate tools or experience to consider dismantling objectives to remove internal dust.

If I was to consider buying very expensive Apo objectives, then I think the phase telescope Pau mentioned would be something to have in my pocket, else purchase objectives from, or via a very reliable source.

*This exercise has been beneficial. I have been able to compare a number of objectives, some with known faults and other identical objectives without faults. I have also been able to determine which type of problems may have an impact on imaging and which ones can be comfortably ignored.

Next, play with the badly pitted FL2 and see if the suggested approaches minimise or significantly reduce any shadows that are visible when looking through the eyepieces (if no success, it goes into the 'parts' box).

*later note: the inking didn't work. I can still see faint, but distinct shadow clouds. The rest of the objective is okay; but the front element looks like it was used in a MythBusters project.


Craig