Reading an objective's specs

Here are summaries and links to discussions for Frequently Asked Questions.

Moderators: Pau, rjlittlefield, ChrisR, Chris S.

Posts: 224
Joined: Tue Apr 03, 2012 6:18 am
Location: Bangalore, India

Reading an objective's specs

Post by hayath »

Any help on reading the specs on an objective?

E.g. The cnscope 4x mentioned here ... 2291wt_952

1) what does 0.10 indicate?
2) what does 017 indicate?

And what should one look for when making a decision on an objective :)



Site Admin
Posts: 5315
Joined: Wed Jan 20, 2010 8:57 am
Location: Valencia, Spain

Post by Pau » ... hp?t=12147

0.10 is the numerical aperture

160 the tube lengh (it can be 160, 170, 210 or infinite symbol in most cases)

0.17 is the cover glass thickness, but for NA under 0.30 it doesn't matter.

Posts: 224
Joined: Tue Apr 03, 2012 6:18 am
Location: Bangalore, India

Post by hayath »

Thanks Pau for the link and answers.

But most of it did fly over my head :(
NA ?

Site Admin
Posts: 8589
Joined: Sat Mar 14, 2009 3:58 am
Location: Near London, UK

Post by ChrisR »

NA ? a measure of the size of the aperture, which in microscopy leads to resolution, all other things being equal.
It's worth googling these terms, you'll find some very good explanations, especially at Wikipedia and the manufacturers' websites, eg Nikon and Olympus. Do expect to search a while to find one which has the right level though, some are much harder going than others.

Posts: 674
Joined: Sat May 01, 2010 6:12 am
Location: Nice, France (I'm British)

Post by ChrisLilley »

You are probably familiar with reading specs on camera lenses, so let us start with that, they see how microscope objectives are similar or different.

Camera lenses

The two most common specifications on a camera lens are the focal length and the maximum aperture. For example, 50mm f/1.4 means that the focal length is 50mm (broadly speaking, there is 50mm between the sensor in the camera and the aperture in the lens) and the diameter of the widest aperture is the focal length divided by 1.4 (50/1.4 = 35.7mm). Because of this division, smaller 'aperture numbers' correspond to larger aperture sizes.

Camera lenses typically have a variable aperture. Wide open, the lens gathers a lot of light but has a shallow depth of field and typically has lower contrast and lower resolution. Set to a smaller aperture, the quality improves up to some optimum aperture, then degrades again as diffraction becomes the dominant factor (but the depth of field continues to increase).

Camera lenses also typically have variable focus, from infinity to some closest focus distance. The quality may well be different along that range; some lenses are best at infinity while others are optimised for close focus (macro lenses).

A third number that is sometimes quoted is the magnification, usually at the minimum focus distance. For example, that 50mm f/1.4 might have a maximum reproduction ratio of 0.15x at it's closest focusing distance of 0.45 m. That means that the image of an object 100mm high would be 100 x 0.15 = 15mm high on the sensor. (the further magnification that would occur when the image is printed or displayed on a screen is not considered here.)

Less commonly quoted (because the effect at low magnifications is slight) is the effective aperture at close focusing distances. For example, that 50mm lens with a nominal aperture of f/1.4 would have an actual aperture of f/(1 + 0.15)*1.4 = f/1.61

For macro lenses, that effect becomes significant. An f/2.8 macro lens that can reach a magnification of 1x will be at an effective aperture of f(1 +1)*2.8 = f/5.6 wide open, and if it's sharpest nominal aperture is f/5.6 that becomes f/11.2 at a magnification of 1x.

The working distance is rarely quoted, but corresponds to the minimum focus distance (measured from the camera sensor) minus the space taken up by the camera and lens. In other words, how much space there is between the end of the lens and the subject being photographed.

Lastly, you may be aware that the magnification can be increased by using extension rings or bellows between the lens and the camera. This decreases the minimum focus distance (and also decreases the maximum focus distance, so the lens can no longer focus on distant subjects).

Microscope objectives

Most microscope objectives differ from a camera lens in a number of ways:
  • - No variable focus. The objective is like a lens set to it's mimimum focus distance, which gives the highest magnification. This also means that the working distance is fixed.
    - No variable aperture. Because of the higher magnification, the objective is firmly in the 'limited by diffraction' area. It gives its best quality 'wide open' and if it were possible to close the aperture, this would only result in more blurry photos (although with a greater depth of field). This also means that the effective aperture (at the fixed magnification) is important and the nominal aperture is typically not quoted.
    - Physical size constraint. Camera lenses come in all sizes from small to large, and there is no expectation that, after changing lenses, the subject is still in sharp focus. On a microscope, several objectives of different magnification are typically grouped in a rotating turret. The expectation is that a different objective can be swung into place without it crashing into the slide and with the image requiring only minor adjustment to still be in focus. Thus, the sum of the working distance plus the physical length of the objective (from the end of the objective to the part where it screws into the microscope, ignoring the length of the actual threaded part) is held constant. Typical values are 45mm (DIN objectives), 60mm (Nikon CFI), and 90mm (Mitutoyo).
So, when looking at a microscope objective, the first specification quoted is usually the magnification for example 4x, 10x, 20x.

The next specification is the effective aperture. Rather than being quoted as the focal length divided by some number, it is given as the numerical aperture, abbreviated NA. Larger values mean a better quality lens (but with a shallower depth of field). Typical values depend on the magnification of the objective. For example a 4x might have an NA of 0.1 or 0.16 while a 20x might have an NA of 0.3 or 0.4

The focal length is not typically quoted directly, but that 'how far does it need to be from the sensor to get an image in focus' aspect is given by the tube length - the distance from where the objective screws in, to the top of the tube into which the eyepieces go. Typical lengths are 160mm (for biological microscopes) and 210mm (for metallurgical ones). Because the image in the eyepiece is actually formed around 10mm down from the top of the eyepiece, we need to subtract that 10mm to get the distance between the objective shoulder and the camera sensor. The units (millimetres) are assumed and not directly quoted.

For biological microscopes, subjects are typically mounted on glass slides and often protected by a thin glass cover-slip. The cover slip, being between the subject and the objective, functions like a 'flat lens' and does affect the optical properties. So the thickness of the cover slip is often quoted. It becomes increasingly important with high NA objectives but has little to no effect when the NA is low. If the objective is designed for a specific cover glass thickness it will give that value (in millimetres, the units being omitted). A typical value is 0.17. If the objective was designed to work without a cover slip, or is not much affected by whether a cover slip is present or absent, this is indicated with a dash "-". (There are some objectives that have a special collar where the cover slip adjustment can be varied.)

Putting all that together, a microscope objective might be described as


That means a magnification of 10x, a numerical aperture of 0.3, a tube length of 160mm and no cover slip is needed.

Those values will typically be painted or engraved on the objective itself along with a coloured ring which also indicates the magnification (e.g. yellow for 10x). The working distance may also be stated:

WD 8mm

For biological microscopes where the subject is typically very thin, flat and lit from behind, the working distance does not much matter as long as it clears the slide. For photomacrography, where the subject is opaque, three dimensional, and lit from the front and sides, the working distance matters because there needs to be enough room for lighting and for focusing on different parts of the subject.

Another difference between camera lenses and microscope objectives is the medium = what is between the subject and the objective or lens. With the exception of underwater cameras, this is generally 'air' for a camera lens. For a biological objective it is typically air but can be water or immersion oil. Using an objective with the wrong medium degrades quality (a lot, to the point of not getting an image at all if you use a high NA oil objective in air). For photomacrography, unless you have specialised needs, just avoid water or oil immersion lenses.

So far, finite objectives which have a fixed tube length have been described. Just like fixed focus camera lenses, the image at the rated magnification is formed at a certain distance from the objective to the sensor. And just like fixed focus camera lenses, the magnification can be altered somewhat by increasing (or for microscope objectives, also decreasing) the distance between the lens and the camera sensor, for example using bellows. And just like a camera lens or bellows, the more you change the magnification away from the value it was designed for, the more the quality degrades.

Microscope makers found the fixed tube length to be inconvenient, because they wanted to put various things (filters, polarisers, mirrors, etc) between the objective and the eyepiece. There was not much room, and inserting them in the optical path required correcting for their optical effects. So a new class of infinite objectives was introduced. In effect the optical tasks performed by the objective in a finite system were split into two optical parts.

The objective in an infinite system captures the light from the subject but does not form a focused image. The 'tube lens' takes that light and forms a focused image at a particular distance given by the focal length of the tube lens. A typical value is 200mm. The benefit for microscope makers was that filters, polarisers etc could be inserted in between an infinite objective and the tube lens with much less impact on the optical quality.

Infinite objectives can be spotted because they use an infinity symbol, which looks like a figure 8 on it's side, instead of a tube length. For example


For photomacrography, infinity lenses differ from finite ones in two ways. First, you need a tube lens as well as the objective. Second, the magnification is varied not by increasing or decreasing the extension but by changing the focal length of the tube lens. For example, a 10x objective designed for a 200mm tube lens would give a magnification of (10 * 150/200 = 7.5x) when used on a 150mm tube lens.

Posts: 224
Joined: Tue Apr 03, 2012 6:18 am
Location: Bangalore, India

Post by hayath »

Oh wow ChrisLilley! Thanks a ton for that very elaborate and kind explanation - certainly was reading it like a little kid trying to soak in the max information possible. bookmarked and I'm sure I will revisit many times over :)

Thank you ChrisR! :)


Posts: 89
Joined: Wed Nov 28, 2007 5:35 pm
Location: Cairns, FNQ, Australia

Post by cadman342001 »

Great post Chris, cleared up a few things I wasn't sure of.

What I usually struggle with is that the WD isn't marked on the objective, and I'm not sure I trust the seller to know what it is. eg often they quote the 0.17 as the WD but I now know that the 0.17 is the cover glass thickness.

So, for an infinity 10X objective, is there a usual WD ? I see the nikon e-plan 10X has a 7mm WD but others have say 16mm. I'm presuming that 16mm is better for us lighting wise (more room) but I'm also assuming it wil be more expensive ?


Chris S.
Site Admin
Posts: 3606
Joined: Sun Apr 05, 2009 9:55 pm
Location: Ohio, USA

Post by Chris S. »

cadman342001 wrote:So, for an infinity 10X objective, is there a usual WD?
No, working distances vary greatly at 10x, as well as other magnifications.

There are a couple of ways to determine working distance. One is to obtain the exact model of the objective, and look up working distance in the manufacturer's specifications.

The other is to learn the "parfocal distance" for the objective (the distance from the shoulder of the objective to a subject in focus), then subtract the length of the objective, measured from the shoulder to the end closest to the subject. The shoulder is the broad, flat portion of the objective where the threaded portion joins the barrel.
cadman342001 wrote:I see the nikon e-plan 10X has a 7mm WD but others have say 16mm. I'm presuming that 16mm is better for us lighting wise (more room) but I'm also assuming it wil be more expensive?
At 10x, 7mm of working distance is doable, but a bit difficult. Much easier is 16mm. And yes greater working distance usually comes with a cost--either in dollars, decreased numerical aperture, or both.

--Chris S.

Posts: 89
Joined: Wed Nov 28, 2007 5:35 pm
Location: Cairns, FNQ, Australia

Post by cadman342001 »

OK thanks.

:roll: I was all set to get the nikon 10x MRP70100 that has a WD of 7mm direct from Nikon for approx. 250 aussie dollars but now I dunno

what can I get for similar money?

Post Reply Previous topicNext topic