Power Standarization questions

[cpsTN]

Why are microscopes sold with 10x EPs and 4x, 10x, 40x and 100x Objectives (if they have four)? Are there reasons for the standars being here? Also, why are people encouraged to add objectives rather than add eyepieces? Is there a problem with having intermediate powers?

[gekko]

(1) Using a higher power (larger NA) objective gives better (finer) resolution, hence more detail. Using a higher power eyepiece does not give better resolution (does not show more detail, but merely enlarges the image), and after some point, results in a blurry image ("empty magnification"). It is like using a high power magnifier to look at a picture in the newspaper.
(2) Even if you had 4x, 5x, 6x, 8x, 10x, 12x, etc. objectives, I very much doubt that you will be using all those fine steps, but would go directly from 4x to 8x or 10x, in other words, use steps that double the magnification unless you really have a specific purpose why you want to go, say, from 4x to 5x (I, for one, cannot think of a reason).

[cpsTN]

So, its like optical zoom compared to digital zoom. Increasing optical zoom increass the amount of incoming light, giving a cleaar image. Digital zoom just spreads out the available light, making things dimmer and less clear. Is that pretty much it?

[lorez]

Increasing optical zoom increass the amount of incoming light, giving a cleaar image

I don't think there is a connection between the optical zoom and a clearer image, but I'm not sure I understand what you mean. My experience with zoom lens systems is primarily with stereo microscopes and with the old B&L DynaZoom of the 1960s and the clarity of the image was always due to the quality of the lenses in the system. Numerical Aperture enters here, but the quality of the lens itself will always make the final difference.

As for more light at higher magnification; it is really the opposite. With higher magnification there is less light because the area being observed is smaller so there is light allowed into the objective lens group. Think of the diminishing field of view with the increasing magnification.

lorez

[gekko]

So, its like optical zoom compared to digital zoom. Increasing optical zoom increass the amount of incoming light, giving a cleaar image. Digital zoom just spreads out the available light, making things dimmer and less clear. Is that pretty much it?

Yes, I think this is a very good analogy. [Edit:] Both lorez and Oliver are, of course, correct regarding the spreading of light analogy, which as they indicated, is not quite valid. I am referring to your analogy in terms of the increase in resolution with optical zoom in a digital camera, which I think is what you are thinking about: optically zooming-in spreads a given part of the image over a larger number of pixels, hence increases resolution, whereas digitally zooming-in merely enlarges the image comprising the same number of pixels.[End of edit]
The resolution of the microscope is determined by its numerical aperture-- the numerical aperture (NA) of the objective and the actual NA of the condenser, and not by the eyepiece. The eyepiece is there to enlarge the image produced by the objective in order for the eye to see it clearly**. When eyepiece magnification is too low, the eye cannot perceive all the detail that the objective has resolved. If the eyepiece magnification too high, the eye sees a rather blurry image (empty magnification). The rule of thumb, as you know, is that the eyepiece magnification should lie between 500 X NA and 1000 X NA (where NA may be taken as the NA of the objective).
_______________
** In many microscopes, the eyepiece also provides some necessary optical corrections that complement those of the objective.

[Oliver]

Increasing optical zoom increass the amount of incoming light, giving a cleaar image

Well, actually increasing the zoom decreases the light, because you are looking at a smaller part of the specimen. The light of this "small part" is then magnified, hence spread out over a larger area. This reduces the overall brightness.

There are several factors that influence the subjective clarity and impression. Closing the condenser aperture diaphragm below the ideal settings lowers the resolution, but the image subjectively appears sharper because of the larger depth of field and higher contrast.

As said, resolution is managed by the objective and depends on the angle of the light that can be caught by the objective. Increasing the eyepiece magnification just makes the unclarity more visible. Having said that, there are stereomicroscopes that have zoom eyepieces, because they operate below the resolution limit.

Other reasons why there are no zoomable eyepieces might include
more lens elements needed, reducing contrast
possible shift in focus when zooming
technical difficulties making tolerance-free zoom
bad cost to benefit ratio: one can always crop the pictures and resize them for intermediate magnifications

Oliver

[cpsTN]

The light isn't decreased overall, it's just pread out so that the brightness per area decreases, making it darker. Splitting hairs, I know.

[gekko]

The light isn't decreased overall, it's just pread out so that the brightness per area decreases, making it darker. Splitting hairs, I know.

I don't think the brightness or dimness of the light has anything to do with the resolution analogy. I took your opticel-vs.-digital zoom analogy to be that objective magnification corresponds to optical zoom, and eyepiece magnification corresponds to digital zoom, in terms of the effects on resolution, and that I think is a good analogy. The spreading of light resulting in dimming is something else, not related to resolution as I see it, and I think is a red herring.

[lorez]

The light isn't decreased overall, it's just pread out so that the brightness per area decreases, making it darker. Splitting hairs, I know.

Speaking only of the microscope objectives, I think this is still backwards. The light is constant and is either reflected by or transmitted through the specimen. As the objective of the microscope increases in magnification the area of the specimen observed decreases and the amount of light passing through a smaller area is less.

lorez

[gekko]

The light isn't decreased overall, it's just pread out so that the brightness per area decreases, making it darker. Splitting hairs, I know.

Speaking only of the microscope objectives, I think this is still backwards. The light is constant and is either reflected by or transmitted through the specimen. As the objective of the microscope increases in magnification the area of the specimen observed decreases and the amount of light passing through a smaller area is less.
lorez

lorez, you are, as always, correct. But you have defined the conditions on which you are basing your statement. I suspect that Charles (cpsTN) and I were trying to address the issue without defining precisely what we were actually talking about. I think that Charles' statement is also correct if, as I surmise, he is talking about, for example, going from a 10x eyepiece to a 20x eyepiece; in this case the image would be more spread out with the 20x eyepiece, and it would be dimmer. The resolution per unit area of the image would also be lower, even as the overall resolution (i.e. visible detail) of the image remains unchanged.

[lorez]

lorez, you are, as always, correct.

Thanks for the vote of confidence, gekko, but if you were to visit with my lovely wife you would soon find out how incorrect you are.

lorez

[apochronaut]

To answer the original question.
In the past, most microscopes had objectives measured in inches, just like the situation that took place with telescopes for instance, where imperial measure still holds sway. So, in the case of microscope objectives and eyepieces, it was convenient to have 2 in, 1 in., 1/2 1n.,1/4 in. objectives etc. I think this was primarily because England was the primary seat of development of the microscope. There was a lot going on in Germany and France too but even makers in those countries fell in to line using imperial measure, for the most part.
Microscopists often changed objectives frequently during a session, in those days so there were many more objective options( 3" to about 1/30") but over time the round figures of 100 diameters, 400 diameters etc. must have had some appeal because certain objective and eyepiece focal lengths seem to be overly represented in the current second hand market.
The R.M.S., adopted a metric system in the late 19th century. This made it a defacto standard for the industry and makers going that route just transposed over from the popular inch sized focal lengths of the time to the closest metric equivalent.
Thus a 1 1/2" objective became 40mm, 1" became 25mm,, 2/3" became 16mm, 2/5" became 8mm etc. This likewise happened for eyepieces too.
One of the standards the R.M.S. adopted as well, was a standard for a microscope tube length; 160mm. Using a 25mm eyepiece and a 16mm objective gave 100 diameters on that length tube and shortly thereafter microscope makers almost universally adopted marking objectives with magnifications as well as focal lengths stamped on them, if they hadn't already done that.

Why, did 4,10,40,100 become the standard for a 4 objective microscope? Firstly, it should be pointed out that those options are usually the most common in kits but when purchasing higher end microscopes, any available option is possible. It's a kind of department store mentality that the bargain microscope sellers have, in order to lower costs through volume purchasing.

The first change came with the revolving nosepiece. Prior to it's widespread use, objective changing was routine. What one sees in some old catalogues are lots of objectives and relatively fewer eyepieces. When the nosepiece became ubiquitous, the number of objective options declined. The choice of kit magnifications has been close to those 4 selections for a long time , because it is historically based on a 25mm,16mm,4mm,1.5mm spread and with a 160mm tube and a 25mm( 1") eyepiece you get roughly 50X, 100X, 400X and 1000X , depending on the design of the lenses. The actual values could vary by as much as 10%( up or down). These magnifications, given the limitations of the nosepiece ,give a pretty good spread with the ever present possibility of using 5X or 15X eyepieces in order to make small alterations.
Two of the "gaps" seem large to some. The 10-40 gap and the 40-100 gap. With the purchase of higher end microscopes, no one is inhibited from buying a more flexible nosepiece or putting a 20X or 60X objective in there but the convention of 4,10,40,100 has become entrenched and does seem to cover the bases as well as possible.

[gekko]

lorez, you are, as always, correct.

Thanks for the vote of confidence, gekko, but if you were to visit with my lovely wife you would soon find out how incorrect you are.
lorez

lorez, I should have said "usually correct". You are right, only your wife (or my wife, etc.) is always correct .

[gekko]

apochronaut, thank you for, as always, a very interesting, enjoyable, and informative historical explanation. I think if one were to collect all your posts on this forum so far, one would already have an excellent reference source on microscopy and the history of microscopes and their makers. Have you thought about writing a book on the subject?

[lorez]

Apo...

Thanks for the explanation. I was unsure of the historical track of the evolution of the objectives. I still have a question.

Firstly, it should be pointed out that those options are usually the most common in kits

Why is it that these four objectives became the default objectives of the kit ?

I remember, back in the day, that AO, B&L and Olympus all had "kit configurations" in their lower line models.

lorez

[apochronaut]

Thanks, Gekko. I think there have been lots of books written by those, more capable than I.

Lorez. I've thought about that some. The low power objective can be anywhere from about 3X to close to 6X, so there is a degree of variation there. 4-5X seems to be favoured and provides adequate scanning capability. 10X seems to be a fixture and I really think it is because with a 160mm tube 10X10= 100, which is a nice round figure for low magnification. And of course, the more that combination was used, the more other companies had to follow suit, in order to be offering a "normal" kit. However, why jump to 40X and not 50X? I think that has to do with economics. A microscope with 4 objectives, or even 3 for that matter must have one high dry objective. It doesn't make sense to have a 35X objective for instance and then jump to 100X and although a 50 or 60 would be nice, the cost to make those is generally more than to make a 40 and the N.A. has to be pretty high in order to get good high dry performance. 4mm or 40X probably fits the bill for a good economy X performance ratio. That is also the one achromat objective, many companies also offers in various grades; a higher N.A. or even as an immersion version; so getting an intermediate magnification with an eyepiece change is practical and with an un-oiled condenser too.
Personally, I have always been disappointed in the performance of high dry objectives much above 40X. There may be some sort of optical limit there. I have tried all kinds of 50 to even 80X dry objectives and always feel that there is something lacking and that there must be a better way. When the magnification is lower, say 60X, unless the objective is an immersion type with a high N.A., the increase in magnification is seldom an advantage. When it is higher, at around 70 or 80x, accepting the loss in light levels and insufficient clarity doesn't make sense ; when one can go to 90X with water or oil immersion and get far better resolution.

[lorez]

Thanks Apo...,

I appreciate the insight. I was having some difficulty finding a coherent discussion among my usual haunts. I am glad you are old enough to wear the mantle of historian for the group.

lorez

[gekko]

Thanks, Gekko. I think there have been lots of books written by those, more capable than I.

My impression is that your wide and deep knowledge of the subject is derived from your having lived it, rather than researched it or read about it.

[apochronaut]

I have been around microscopes for some time and I have had the possibility to test and compare. So yes, I have an experience generated view of things , microscopic. This is often necessary because the companies can be very close mouthed about what they are doing. I recently came across a website that listed numerous manuals, from almost any notable maker of the past. All of the Zeiss entries were blocked because Zeiss didn't want anyone accessing them.

[gekko]

[quote="apochronaut"All of the Zeiss entries were blocked because Zeiss didn't want anyone accessing them.[/quote]I once downloaded from the web a Zeiss document explaining succinctly how to use the Michel-Levy chart, but later discovered that one page was missing. I tried in vain to find it: it just was not to be found. That explains why. Thank you.

[75RR]

I once downloaded from the web a Zeiss document explaining succinctly how to use the Michel-Levy chart, but later discovered that one page was missing.

As that particular brochure is available from the Zeiss Home site (http://www.zeiss.com/microscopy/en_de/d ... dge&page=2) I suspect that the missing page (#5) is an oversight.

The site apochronaut referred to may well be frankshospitalworkshop (http://www.frankshospitalworkshop.com/e ... nuals.html)

I suppose Zeiss wish repairs of their more modern microscopes to be in-house.

The impression I got when talking to a local Zeiss representative is that they have little interest in servicing non institutional customers.

Additional brochures: http://www.zeiss.com/microscopy/en_de/d ... loads.html

[apochronaut]

Offering brochures is a way of making money. Restricting access to manuals is a way of making money. Companies take this too far, even to the point of restricting parts access to those under the umbrella of their organization. Greedy, is not a word that should have to exist but it does and shareholder and corporate actions increasingly define that word much more than a well fed puppy growling over a bone.

[Dale]

Lorez, you know this discussion is leading me to a barrage of questions. Perhaps a tad of scotch
would enable me to improve my resolution, and to be more objective about magnification selection!
Dale.

[lorez]

Dale,

I told my lovely wife I was going to the store to get some scotch for Christmas... she thought I was talking about tape

I am glad you are resolved to be more objective about magnification.

lorez

[Dale]

I tried the Wild Turkey for Thanksgiving trick, no luck, I'd better find something or
the Nikon is at risk!
Dale

[Crater Eddie]

In times like these I often turn to my old friend Captain Morgan for advice.
(yeah, I know, that's as close as I could get)
CE

[lorez]

the Nikon is at risk!

Oh, NO ! Say it ain't so! I just ordered twelve sets of hex drivers.

my old friend Captain Morgan

If I remember correctly, from college, he was Captain of the Cutty Sark.

lorez

[gekko]

Offering brochures is a way of making money. Restricting access to manuals is a way of making money. Companies take this too far, even to the point of restricting parts access to those under the umbrella of their organization. Greedy, is not a word that should have to exist but it does and shareholder and corporate actions increasingly define that word much more than a well fed puppy growling over a bone.

My memory may be failing me, but I seem to recall that 8 or 10 years ago, many journals offered the contents of their old (10 or 20 years) issues on the web for free. Now, one has to pay $35 for a single paper no matter how long ago it was originally published.