Lens hood for Microscope Objective

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BugEZ
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Lens hood for Microscope Objective

Post by BugEZ »

A week or so ago I performed a comparison of two reversed 50mm lenses and was reminded how important a lens hood can be. That got me thinking about the absence of hoods on microscope objectives, particularly when we illuminate the subject with diffuse omni-directional lighting.

I have constructed several ring lights that I use with my various objectives. I have made it my practice to keep the lights on the camera side of the objective lens opening. This has helped me avoid "fogging" of the images from having unwanted light bouncing around inside the optics. The down side of this is that the subject usually appears front lit and not uniformly illuminated.

Recently I was discussing this with canonian regarding his "octo-lite" LED ring light (discussed here: http://www.photomacrography.net/forum/v ... hp?t=15612 )

I decided to do a test using an older "tube light" that I constructed several years ago that would allow me to see if a lens hood on a microscope objective lens provided benefit.

The figures below document the test. In summary I found no benefit to the hood I constructed.

Please note that I did not spend much time post-processing the images. They are pretty much as they came out of Zarene (PMAX) and I did not go in and work on any of the ghosting. Also one stack did not quite go all the way up to bring the center of the eye into focus. The images do illustrate that there was little benefit to the hood.

Keith

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ChrisRaper
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Post by ChrisRaper »

Superb test - love it - as you say, not a lot of difference with/without hood ... and the tube light is ingenious - what sort of shutter speed were you getting with that?

BugEZ
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Post by BugEZ »

ChrisRaper asked:
What sort of shutter speed were you getting with that?
These are 1.5s exposures. The objective lens is .3NA. My ISO setting is 200 as that is the lowest the Pentax *ist DS has available.

This particular ring light is driven by a simple 9V DC adapter (vampire) that is not regulated. When the freezer in the basement starts, the lights momentarily dim etc. The 48 LEDs are arranged in 16 banks of three and are driven well below their rated amperage and brightness. I suspect it could easily be driven 2-3X brighter if a regulated power supply were used. I built it in May, 2009 and shifted to different illumination schemes in 2010. It is quite akward with large subjects such as a praying mantis. Here is a better description.

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Keith

canonian
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Post by canonian »

You have saved me the trouble of testing this with my own microscope objectives, thanks Keith. And very well documented !
It always amazes me that you get such wonderful shots out of a rig build with polystyrene, wood and tape. What brand and type of objective do you use?

Off topic: You also pulled a trick Rik Littlefield once did. Casualy using a book in one of your photo's.
Rik once used a book as paperweight in one of his posts "Light– Science & Magic, An Introduction to Photographic Lighting" (4th picture) which attracted my attention.
Very theoretical but it got me better aquainted with the behaviour of light (Family of Angles)
Now you got me all interested in "For Love of Insects".:wink: Is is any good?

BugEZ
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Post by BugEZ »

Canonian asked:
What brand and type of objective do you use?
I own 3 objectives all Olympus. My favorite is my UMPlanFL 10X .3 NA. It is infinity corrected and works well with my 200mm telephoto. The color correction is quite good. I splurged and purchased it new. Happily it was heavily discounted. My other 10X lens really needs a special tube lens to work well which I do not own. Thus it was not a good buy. The color aberration at the sides is quite pronounced. Happily, it was much less expensive. The 20X NEO would also benefit from a proper tube lens. It has the same chromatic aberration at the sides, but does allow me to zoom in a bit more than the 10X lenses. The working distance of the 20X lens is quite small. So when I send my list to Santa next year an infinity corrected 20X lens with long or extra long working distance will be on the list. Perhaps if Santa wins the lotto....

Canonian asked:
...Casually using a book in one of your photo's. ...Now you got me all interested in "For Love of Insects". Is is any good?
Well, I think it is excellent. I started it over the Christmas Holiday and I am enjoying it very much. It was on the kitchen table and I chose to leave it in the photo. It describes the various insect behaviors and characteristics that Dr Eisner (professor of Entomology at Cornell, now deceased) explored during his career and is an excellent read. I am a mechanical engineer but a entomologist want-to-be. I recommend it. It should be available in most public libraries, or you may choose to buy it (~$18 at Amazon), read it and donate it to the library...

Keith

Harold Gough
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Post by Harold Gough »

Looking at the side view of the head, there are small but significant improvements in the fine detail with the hood. If it were my setup I would definitely want to retain the hood.

Harold
My images are a medium for sharing some of my experiences: they are not me.

Craig Gerard
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Post by Craig Gerard »

Keith,

Does Dr Eisner's book have information related to an observation he made regarding moth scales and spider webs? I recall reading a digression in a report he wrote for Wings magazine at www.xerces.org. The old links I had to that particular article are no longer available online. I do have a hardcopy in my files. It was one of Thomas Eisner's articles that originally led me, indirectly, in a roundabout way, to this forum.


Craig
To use a classic quote from 'Antz' - "I almost know exactly what I'm doing!"

Harold Gough
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Post by Harold Gough »

A possible alternative to the tube/hood construction might be a wide, black annulus across the end of the tube light nearest the lens and wide enough to prevent any stray light from inside or outside the tube light from entering the lens.

Harold
My images are a medium for sharing some of my experiences: they are not me.

BugEZ
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Post by BugEZ »

Harold Gough wrote:
There are small but significant improvements in the fine detail with the hood
I am not certain what portion of the images you found that shows improvement. The detail area near the eye does not show much improvement (see below).

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Perhaps you refer to the lower profile area near the bottom of the images? The difference there is due to the stack depth differences in the images. The hooded image went a bit deeper and made this area appear crisper. Individual images at the same depth in the stacks are quite similar.

Harold Gough also wrote:
A possible alternative to the tube/hood construction might be a wide, black annulus across the end of the tube light nearest the lens and wide enough to prevent any stray light from inside or outside the tube light from entering the lens.
This is a good idea. Conceptually, this is what the small lens hood I tested is supposed to do. I like the portability of the lens hood as it moves with the lens, and allows me to position the lens and black annulus simultaneously.

Craig Gerard wrote:
Does Dr Eisner's book have information related to an observation he made regarding moth scales and spider webs?
I am about 40% through the book. I scanned ahead and found this the primary subject discussed in Chapter 10. The table of contents may be viewed on Amazon's web site... here... http://www.amazon.com/Love-Insects-Tho ... 674018273 The chapter titles are a bit cryptic and the TOC may not help much. As I read the book I can't help but to think how much better the illustrations might be if the stacking techniques and computer/digital photography technology we employ had been available to the author.

Eisner wrote several other general interest books on insect behavior and chemical defense which can be located with a internet search. I will put these on my wish list for next Christmas as they are closer to Santa's price point than a 20X APO ELWD infinite objective...

Keith

DQE
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Post by DQE »

I can't help much with such a complex project, but one thing came to mind from my now long ago lab experience doing quantitative photographic (film-based) measurements.

Specifically, it is harder than one might think to obtain excellent anti-reflection properties from many or perhaps most black materials. Most commonly, specular reflections can be significant even with what seem to be very dark/black materials such as black electrical tape. From now distant memories I believe that black "velvet-like" flocking material worked better than most things, significantly better than "Aqua-Dag" (carbon in an evaporating liquid, to be painted on the surface) and better than dull or satin-finish black paint. Flocking material traps light and just won't let it back out of its thick surface, pretty much independent of the angle of the incident light. Of course, flocking material can create debris if it sheds fibers. I think that one can buy flocking material that can be painted onto surfaces as well as fabric.

Especially in dark areas of a subject or scene, flare light from inadequate reflection control often degrades image quality significantly. It isn't just a matter of adjusting the image in Photoshop to offset the flare since the flare degrades the signal-to-noise ratio in the photograph in what is usually an irreversible manner.
-Phil

"Diffraction never sleeps"

BugEZ
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Anti Reflective Coatings

Post by BugEZ »

DQE wrote:
It is harder than one might think to obtain excellent anti-reflection properties
Yes, I agree, DQE has a very valid point. Black construction paper is not very good, but is better than a shiny black surface. A less reflective surface can be easily checked and I will put that on my list of things to test. I have found by experiment that black felt makes a good flocking material. I purchased several small flat sheets several years ago and use it where I need to reduce reflection. The down side is that some fibers may migrate to locations inside my optical path (mirror, lens, sensor, shutter etc.) and create mischief. I don’t see any problem with a washer shaped piece of felt on the lens side of my hood. It will definitely kill any reflection.

In my early photos, I glued my bugs to a wooden toothpick. Often, reflection from the toothpick would flare and spoil my photos. I still use a wooden toothpick, but first I blacken it with a permanent marker. This creates a very dark and dull surface and a dark bug is not hidden by glare from the mounting.

I had the occasion to blacken the inside surface of an optical “smoke detector” that looked for oil fog in the air vented from the gear and bearing cavities inside an aircraft engine. I used Krylon SP102 flat black paint and was pleased with the results. I have never tried a flocking paint.

In the local astronomy club I have friends that work at a high end telescope company. They guard their anti-reflective coatings and techniques carefully.

Keith

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Post by rjlittlefield »

I think that for this application the anti-reflection qualities of the hood are not so important. The inner surface of the hood will be shielded from most direct illumination anyway, and what light does hit the interior of the hood will be at a glancing angle where the strongest reflections won't enter the lens either. This is a very different situation from an extension tube or the tube of a telescope, where the sides of the tube are directly exposed to bright areas around the FOV, and glancing reflections go straight to the sensor.

--Rik

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Post by rjlittlefield »

Here's an article you may find interesting: "Accurate Contrast Ratio Measurements Using a Cone Mask". It's a publication by the National Institute of Standards
and Technology, describing the use of a shiny black cone to shield an imaging device from stray light.

--Rik

Excerpting from the article:
Figure 2, extraneous elements removed.
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Figure 3.
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BugEZ
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Post by BugEZ »

Rik wrote:
Here's an article you may find interesting
I read the article and found it quite interesting. Lots to think about!

The black shiny surface recommended in the article for the cone interior is not what I would want in a lens hood. We usually have an enclosed cavity between the hood opening and the lens. I think we want absorption in the interior walls of the cavity. The sensor-hood configuration used in the article allows the light reflected off a shiny cone interior to escape to infinity. That is not what I have in my camera.

Thanks for sharing!

Keith

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Post by rjlittlefield »

BugEZ wrote:The black shiny surface recommended in the article for the cone interior is not what I would want in a lens hood. We usually have an enclosed cavity between the hood opening and the lens. I think we want absorption in the interior walls of the cavity. The sensor-hood configuration used in the article allows the light reflected off a shiny cone interior to escape to infinity.
I agree about wanting absorption, but remember that absorption will happen even when the material is shiny, as long as the configuration is such that light reflected from the hood never gets into the lens.

Consider the following modification of the article's Figure 2. With the original cone, light can escape out the back of the cone as you describe. Likewise if the cone is simply extended. But now suppose the cone is extended and then capped with a back plate made of the same shiny black material. Then the light can no longer escape, but still it cannot get redirected to enter the lens. It just bounces between the cone and the back plate, getting absorbed at say 90% per reflection until it's all gone.

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The 90° configuration is kind of magical. Unlike the narrower cone shown in one of my early lens tests, in the 90° configuration there's little or no opportunity for light to reflect off the cone and enter the lens even when the cone is shiny. With a cone angle of less than 90°, like my earlier one, there are paths where a single direct reflection will take light from the outside world into the lens. Shiny material is not good in that case. A cone angle greater than 90° might be equally good from the standpoint of eliminating direct reflections, but would reduce the angle available for illuminating the subject. Dead on 90° seems perfect.

BTW, most of the time I make my hoods of matte black material too. It's simpler, and it works well in more configurations. But shiny black has its uses. Making really dark holes is one of them.

--Rik

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