Even more off topic, in looking at all the continuous lighting options I see many microscope ring leds, lots of different types and prices, yet in looking at the setups that use objectives I don’t recall seeing any?
I have both flash and continuous-illumination ring lights, but rarely use them. I recommend against buying them for most uses.
A ring-light produces very flat light without modeling. We photographers most often want to create modeling—controlled use of shadows—to provide visual information about the subject’s shape and texture. Attractive and useful modeling is difficult to create with a ring light. Also, ring lights can create annoying, ring-shaped specular highlights.
I do occasionally use a ring light for quick documentary purposes. In these situations, I want to record visual information in a hurry, and don’t care if the image is appealing to the eye. As an example, sometimes I want a close-up photo of a part that I want to machine an adapter for. A ring light will give me a well-lit, shadow-less view that I can blow up on my monitor for study. As another example, ring lights have often been used by dentists to take intra-oral close-up photos of patient’s mouths for their records.
I think this is why one sees ring lights advertised for industrial or laboratory use, but not in common use by photomacrographers. In industry, speed and information capture are most important. But in the photomacrography community, we far more often value images that convey the shape and texture of the subject, and please the eye. For this sort of work, we are better off using a number of discrete light sources whose size, shape, distance, and diffusion we can control.
You also commented about harsh lighting you experienced with cross-polarization, and asked about the apparent size of the diffuse light source when cross-polarized.
I don’t have experience that addresses directly the situations you’ve described. But I have used cross-polarized light in ways that might offer potentially useful, if off-topic, insight. Cross-pol on-camera lighting seems fraught with issues. Alternatively, cross-pol lighting placed well off-axis, oblique to the subject, can be very revealing of certain aspects of a subject (such as cracks, in one important subject I documented).
And—of course—if you diffuse your light with cross-pol, you will want to place your polarization sheet on the subject side of the diffuser. (No doubt you know this well!)
Cheers,
--Chris S.
