#14
Post
by apochronaut » Tue Jul 28, 2015 5:01 pm
There are a couple of things here, that make this test more an interpretive assessment rather than a critical assessment.
Even though the test subject is uniform, such stained commercially prepared slides tend to obscure some microscope characteristics and enhance others and these can vary from optical system to optical system. Traditionally, the best test subjects with which to determine optical performance are unstained samples . Diatoms have been used for years and are still one of the subjects of choice.
Older microscopes, tend not to have as defined a contrast level with average adjustment because they have been made through mathematical computation and empirical testing, rather than mathematical computation( or reverse engineering), computer ray tracing and empirical testing. Modern microscopes therefore maintain better image quality over a broader range of maladjustment. For this reason , older optical systems need to be very carefully adjusted to optimize their performance. This is the prime reason that novices find new microscopes better for them...it is simply because they don't know how to properly adjust an older one, so they appear to provide poorer quality, when in fact , properly used, they usually provide better quality. Additionally, although the lenses may be clean and clear, often the condition of the prisms in the head get neglected. It doesn't take very much in the way of airborn fine particulates over a few years, to leave a film on prisms and lower the contrast. All older microscopes , should have the head cleaned.
When the Bausch & Lomb model tested here was made,there were standard abbe condensers and wide field condensers. The standard practice in order to fill the field for low power objectives with a conventional abbe 1.25 N.A. condenser was to match the N.A. as closely as possible between the condenser and objective, by unthreading the top condenser lens. Using the lower lens only set at a low position, this reduces the N.A. of the condenser sufficiently and increases the breadth of the illumination field, so that the low power objectives achieve a good uniform background illumination and maximum contrast unobscured by scattered rays. The type of illumination is important here too. Not all illumination systems are created equal, and this is another factor that skews novice user choices towards microscopes with factory installed illuminators. Obviously it is hard to mess with those, whereas a microscope made to be used with a remote or understage illuminator, could have all sorts of imperfections in the illumination beam, depending on a large array of factors. This is a critical issue , when determining an optical systems performance.
Diffusing filters , are kind of a bandaid kind of compromise always. While they can sort out some problems of evenness, it is always at the cost of illumination intensity and contrast. Generally, if a diffusion filter is absolutely necessary , then it is an indication that something is wrong with the illuminator/ condenser set up and it is best to tackle that problem primarily.
. By the way, I've found that different "white" plastic diffusers (or "gekkos" if you like) can have different effects on the color temperature of the light, so one should set custom white balance with the diffuser in place if one is being used.