Thoughts concerning cleaning and scratches
By Paul James
It never fails to amaze me that many lenses I have seen over the years, are to a greater or lesser degree marked in some way. The coatings may be abraded, or even the glass surface below has been scored, and I have seen several lenses which sport a generous helping of fine scratches over their surfaces. Fortunately the effects on the imagery they produced vary from slight to nothing, but this reflects the attitudes of owners and subsequent sales value.
Ideally, the external lens surfaces should never be touched, but in reality deposits of one kind or other find their way onto these delicate surfaces. No matter how careful we are it is quite likely that we will need to clean the lens surface occasionally. The traditional 'cleanliness culture' were 'elbow grease' is applied with vigour has no place in the realms of optics. A lens is quite special, being manufactured in most cases to very high tolerances, and makers go to great lengths also to see that they are mounted with great precision, like those found in high power objectives.
The following notes apply in general terms, but more specific thoughts on this subject are contained in the last table at the end of this article.
Before considering any method of cleaning lenses, it might be prudent to try to understand why scratches and abrasions occur in the first place. Generally speaking most cloths and tissues used to remove deposits on the lens surface are not capable of scratching the glass, or coating. They are too soft to do this, unless undue pressure is used of course. The cause of the marking lay in the inherent abrasive qualities of the 'dirt' already present on the lens, which can easily form an instant partnership with the cleaning tissue to become a fine 'emery cloth'.
The problem is how to remove the potentially abrasive dust in the first instance. Fortunately a fine brush with soft bristles such as camel hair will brush most of the free dust off, and those which can blow a jet of air as well, seem to be ideal, though I have some reservations about this. Even an ordinary artist's paint brush is far better to use gently at first, than wiping the lens surface boldly with a tissue.
Once the majority of dust is removed this way, a tissue can be used gently to sweep off the finer debris.
A greasy lens brush is worse than useless !
Smears are more difficult to remove, depending on their chemical makeup. Some can be removed by gentle wiping with a dry tissue, others need solvents. If need be then an appropriate solvent such as isopropanol should be applied to the tissue in a very small quantity, and never to the lens in case some finds its way into the interior structure of the lens.
The common method of 'breathing' on the glass surface and then wiping off the condensate and debris after using a brush works quite well, though it appears to be a crude method, it is highly effective in actuality.
Supporting an inverted lens over a small container of the solvent, can be effective, because the smear will absorb some of the volatile solvent, and has the advantage of never allowing any liquid to creep into the internal elements.
Several proprietary solutions have been available over the years which claim to aid the cleaning process. They tend to be of the multi solvent variety, and appear to be highly effective, but I have reservations about those in spray form. If used to excess they can lead to solvent creeping inside lens housing more easily than imagined. I prefer 'brush, OR brush & tissue' in all but the most difficult cases.
Special considerations and effects of abrasions on functionality of lenses
Some lenses need a little more thought and care when cleaning. The front element of a high power objective can easily be scored or abraded, and since this lens is very small, the scoring could have an observable effect on performance, since the proportion of damage etc., may be quite high in relation to the total area of the lens.
Eyepieces are notoriously susceptible to dust and markings, depending much on their design. Field lenses especially can become dusty, and when the iris is closed down, it becomes more apparent.
Marks and scratches disperse light in a random way, and usually cause a lowering of contrast in most optic systems. Apart from the obvious cosmetic appearance, small or light scratching will have little or no observable effect on imagery (objectives excepted). Only when the area of marking is significant will any effects become apparent. Uncoated lenses scatter light by their very nature, and an identical lens which is coated will produce a brighter and more contrasty image even if moderately scratched .
Smears of a 'greasy' nature on the other hand can ruin the imagery of a perfectly fine lens, simply because the 'greasy' film becomes effectively the lens surface, causing a significant amount of light to divert from its intended path. The result is lower contrast in the image with lower resolution too. If left, a grease smear such as a finger print, can become more permanent, and more difficult to remove.
possible lenses should be left, and only cleaned if really necessary.
I'm certain that more lenses are damaged by ill conceived cleaning,
than by any other cause.
Dust and dirt on the lens, together with a wiping tissue is a recipe for
scratches. Brush first, then wipe after if necessary.
Smears are infinitely worse than a few small scratches.
Markings on the front element of an objective, especially those of high power,
cause far more image deterioration than similar marks on the eyepiece.
Do not take apart a complex lens assembly such as a condenser just because
there are a few 'specks' in its interior, as the chances of damage from
accidental causes are fairly high, so it is is not worth taking the risk,
and its performance will be unaffected anyway.
Never disassemble a high power objective.
The internal or rear element of an objective, especially those of
x 20, x 40 and x 100 can be a major problem if very dusty. Cleaning these deeply inset lenses
is not a job for the 'cotton bud'. Ideally, this task should be undertaken by
the maker, or competent technician.
Finally :- If you are buying a secondhand instrument, the quickest way to spot defects such
as scratching etc, is to view at about x 200-400 brightfield , focus on a specimen then remove it
and close down the iris significantly. The eyepiece dust and scratches if present will be glaringly
apparent, and this can be verified by turning each independently. Any 'shadowy' blurred marks
that do not revolve are very likely to be defects, markings or dust on prisms further below.
A focusing telescope will reveal any defects too through the optical train, especially in
the internal glass structure of the objectives. The front element of an objective can be viewed
through a loupe using incident light, and any scratches and defects will be easily observed.
|All comments to the author Comments to the author sent via our contacts page quoting page url plus : ('pjames','')">Paul James are welcomed.|
Please report any Web problems
or offer general comments to the
via the contact on current Micscape Index.
Micscape is the on-line monthly
magazine of the Microscopy UK web
site at Microscopy-UK