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Useful magnification of the microscope

 
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Marek Mis



Joined: 10 Jul 2010
Posts: 2560
Location: Suwalki, Poland

PostPosted: Tue Mar 25, 2014 3:50 am    Post subject: Useful magnification of the microscope Reply with quote

I have thye question concerning the useful magnification.
We all know that the useful magnification of the microscope is described by:

useful magnification = (500 - 1000) x NA

If I use for example objective 10X/040 this range is 200X - 400X.
So I should use the eyepieces in range of magnification from 20X to 40X, shouldn't I ? The most often we have in our microscopes the "standard" 10X eyepieces which we use for most of the objectives.
Could anybody explain me, why ? We should use the eyepiece at least 20X.

Marek
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Pau
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Joined: 20 Jan 2010
Posts: 4805
Location: Valencia, Spain

PostPosted: Tue Mar 25, 2014 6:58 am    Post subject: Reply with quote

With low power objectives you have room to use high mag eyepieces without falling into the empty magnification territory, but with high power objectives lilke the 100X you fall very easily. Of course with a low power high NA objective like your 10/0.4 you have lots of room to play with secondary magnification, you could perfectly use 20X or even 25X eyepieces.
Another important point is that high power eyepieces usually have smaller field of view (field number).
When applied to the camera sensor, you can magnify the image as much as you want with relay magnification, but if pixel peeping you usually don't want to do it

If your vision is not limitating, in general is better to stay with 10X or 15X eyepieces.
At work I have 10X and 16X eyepieces and I allways prefer the 10X, while my pupils not used to find small details and fine focusing usually prefer the 16X (but we use achromats, not plan apos!)
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Pau
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g4lab



Joined: 23 May 2008
Posts: 1434

PostPosted: Tue Mar 25, 2014 11:29 am    Post subject: Reply with quote

To expand a small bit on Pau's correct remarks, 10x oculars are always easier to use because they have longer eye relief, and larger diameter exit pupils than higher magnification oculars. So your eye and brain can compensate better for slight maladjustments with less eyestrain.

When you use even 20x oculars you really have to have the interpupillary distance adjusted perfectly and also since the exit pupil has such an angular cone, you also need to adjust for differences in the strength of your eyes. This is more trouble than many people want to go to. (but probably does not apply to most listmembers here Very Happy ) Your head position becomes more critical.

On my beloved Wild Makroscopes , Wild who are very concerned with optical quality, sold these with 8x, 10x(Fields of View available from 21mm up to 26mm from other vendors) , 15x(usually FN 17) 16x, 20x(FN 13) , 25x, 32x and 40x oculars.
Although I have collected large amounts of gear for this system The highest mag oculars I have bothered to collected are 20x. The NA of the system with an ApoZoom and a 2x auxiliary lens is 0.230. (and 12x initial mag so anything over a 20x is going to be empty mag)

I do have a Zeiss stereo that came to me with a 25x pair of oculars. They do need to be carefully adjusted. I stick with 10 or 15x.

Of course , compound scopes operate at much higher NAs but the ocular restrictions still apply as far as IPD and dioptric adjustment are concerned.
You still have to get the light onto your retina with your ocular muscles as relaxed as possible and lower mag oculars make this easier.
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Marek Mis



Joined: 10 Jul 2010
Posts: 2560
Location: Suwalki, Poland

PostPosted: Fri Mar 28, 2014 5:30 am    Post subject: Reply with quote

Thank you for your explanations.

I was thinking about it and I assume, that:

I think the 10X eyepiece is good to achieve the magnification in the range of the useful magnification because:

- the full NA objective (0,40) usually is not used and it is decreased by condenser aperture diaphragm to 2/3 to obtain the better contrast and depth of field (in Kohler illumination)

- so, the new, effective aperture of the objective 10X/0,40 is not 0,40 but about 0,27 and then 500 x 0,27 = 135X. So the 10X eyepiece is very close to optimal 13,5X power of the theoretical eyepiece.

That is my idea why it is good enough using the 10X eyepiece with my 10X/0,40 objective instead 20X one.

I am not sure if about my way of thinking, so I will be very grateful to get the new explanation if I am wrong.

Marek
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