A microscope objective lens is a fundamental component of a microscope responsible for gathering and magnifying the image of a specimen. Positioned in close proximity to the specimen, it plays a crucial role in determining the quality and clarity of the final magnified image.

So at one end of the scale, you’ll find the infinity symbol, and at the other end, you’ll find the lens’s minimum focusing distance (i.e., the closest the lens can focus).

First, focus your lens and set your aperture. Then look at the hyperfocal distance scale and find your chosen aperture on either side of the red line. Finally, look at the focusing distances that correspond to the apertures – these will be your near and far depth of field limit.

High power objective lenses in microscopes typically have magnifications ranging from 40x to 100x. These lenses are used for detailed examination of specimens and provide high levels of magnification, allowing for close-up views of fine structures and cellular details. Keep in mind that the actual magnification will also depend on the eyepiece being used in conjunction with the high-power objective. For example, if you're using a 10x eyepiece with a 40x objective, the total magnification will be 400x.

If you are using a prime or fixed lens, you won’t have a zoom ring. Your lens will simply indicate the focal length on its barrel, as you can see on my 85mm lens:

3. Working Distance: This is the distance required between the objective lens and the specimen to create an in-focus image. It's an important consideration, especially in applications where space is limited, or when using specialized accessories.

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So while newer lenses rarely include aperture rings, you’ll find them on plenty of older lenses. An aperture ring displays different aperture settings, like this:

What does this mean? It’s simple: the maximum aperture on the 85mm lens is f/1.8, and on the Tamron zoom, the maximum aperture changes from f/2.8 to f/4 as you zoom the lens. (At the lens’s widest, 17mm, I can open the aperture to f/2.8. But if I zoom all the way to 35mm, my maximum aperture becomes f/4.)

Now that you’re familiar with all the common camera lens numbers, let’s take a look at some of the less common markings. These numbers are pretty rare on lenses designed for digital cameras, but you may come across them if you purchase older, manual focus glass.

5. Correction Type: Depending on the type of microscope and the specific application, one may choose from achromatic, plan, or specialized objectives. Each type is optimized for different imaging conditions.

On some – but not all! – lenses, you will see a range of distances, usually marked in two scales, feet and meters. These lens numbers indicate the distance at which your lens is currently focused.

6. Long Working Distance Objectives: These objectives have a greater distance between the lens and the specimen, making them suitable for applications where space is limited or for examining larger specimens.

(Note that some zoom lenses have a variable maximum aperture, where the maximum aperture will change depending on the focal length; this is represented as a range of numbers, such as f/3.5-6.3.)

4. High-Resolution Objectives: These offer exceptionally high levels of detail and are ideal for applications requiring precise imaging, such as in medical or scientific research.

In the photo below, you can see two different lenses: my Tamron 17-35mm and my Canon 85mm. On the Tamron, you should see “1:2.8-4,” and on the 85mm, you should see “1:1.8.”

And this is by design. The hyperfocal distance scale uses the distance scale to display the expected depth of field. Here’s how it works:

5. Oil Immersion Objectives: These objectives are designed to work with a layer of immersion oil between the lens and the specimen. This reduces refraction and increases numerical aperture, allowing for higher resolution images.

Depending on the age of your lens, you’ll run into different markings. In this section, I’ll discuss numbers frequently found on newer lenses (though note that many will apply to old lenses, as well!).

8. Darkfield Objectives: These objectives are optimized for darkfield microscopy, a technique used to observe specimens against a dark background, enhancing contrast for certain types of samples.

7. Phase Contrast Objectives: These are specially designed for phase contrast microscopy, a technique used to visualize transparent or low-contrast specimens without the need for staining.

13. Near-Ultraviolet Radiation (NUV) Objectives: Objective lenses designed for the near-ultraviolet (NUV) radiation range are tailored for brightfield microscopy, making them well-suited for laser applications. These NUV objectives possess characteristics such as Plan Apochromat design, long working distance, and infinity correction. Specifically optimized for wavelengths from near-ultraviolet (355nm) to visible light, they offer high-performance imaging in this spectral range. Additionally, high-resolution NUV objectives are also available for specialized applications.

2. Plan Objectives: Designed for flat-field microscopy, these objectives ensure a sharp focus across the entire visual field, reducing distortion at the edges.

1. Achromatic Objectives: These are standard objectives that correct for chromatic aberration, enhancing color accuracy in the image.

2. Numerical Aperture (NA): A higher NA indicates better resolution and light-gathering capabilities, which can result in clearer and more detailed images.

11. Stereo Microscope Auxiliary Objectives: Stereo microscope auxiliary objectives are supplementary lenses used to alter the magnification and working distance of a stereo microscope. They are designed to be easily interchangeable, allowing users to adapt the microscope's capabilities for different tasks. These objectives can provide additional magnification for closer inspection of specimens or extend the working distance to accommodate larger objects. They play a crucial role in enhancing the versatility and functionality of stereo microscopes in various fields such as biology, electronics, and precision manufacturing.

Note that some modern lenses do include aperture rings; Fujifilm is known for this, as are other brands that offer manual focus lenses (e.g., Samyang).

Most zoom lenses don’t offer hyperfocal distance scales (because depth of field varies with focal length). But if you have a prime lens – especially an older model – you may see an extra ring of numbers on the barrel, such as in the image below:

Choosing the right type of objective lens depends on the specific requirements of the microscopy application, including the type of specimen, desired level of detail, and any specialized techniques being employed.

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1. Magnification Range: This determines the range of magnification levels the lens can achieve, allowing for versatility in examining different types of specimens.

So for the lens pictured above, the diameter is 77mm. And if I wanted to use a polarizing filter or a clear filter, I’d need to grab one with an equivalent diameter.

12. Near-Infrared Radiation (NIR) Objectives: Near-infrared (NIR) range objectives in brightfield microscopy are tailored for laser applications, enabling precise observation within the near-infrared radiation spectrum. These NIR objectives are characterized by their Plan Apochromat design, infinity correction, and long working distance, making them ideal for applications involving this specific wavelength range.

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That’s why, in this article, I’m going to run through all the important camera lens numbers you’ll encounter. I’ll explain what the numbers actually mean, and I’ll also explain why they matter for your photography.

Well, that’s it for lens numbers! Hopefully, you now feel much more confident (and much less confused) when looking at your lens.

Most newer lenses set and control the aperture through the camera. But back in the days of film, you would set the shutter speed on your camera and the aperture on the lens (via an aperture ring).

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Now, pretty much every lens has the maximum aperture written somewhere on its body. You can usually find this information in one of two places (or perhaps even in both):

Larger apertures like f/2.8 or even f/1.8 are highly desirable because they allow you to shoot in low-light conditions while maintaining a fast shutter speed. So the best lenses – and the most expensive lenses – tend to offer a very wide maximum aperture.

The maximum aperture is the largest aperture opening your lens is capable of achieving. Note that the larger the aperture opening, the smaller the f-number (so f/2.8 corresponds to a very wide aperture, while f/22 corresponds to a very small aperture).

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A scanning objective in microscopy refers to a specific type of objective lens used to observe specimens. It is characterized by a low magnification level, typically between 2x and 4x, and is designed to provide a wide field of view. Scanning objectives are commonly used for initial specimen location and overview. They allow for quick scanning of a large area to locate regions of interest before switching to higher magnification objectives for detailed examination. This type of objective is particularly useful in tasks where a broad perspective is needed before zooming in for more precise observation.

Objective lenses are typically labeled with two numbers, such as 10x/0.25. The first number represents the magnification, indicating that the lens magnifies the specimen by a factor of 10. The second number is the numerical aperture (NA), which signifies the lens's ability to gather light. This combination of magnification and numerical aperture is crucial in determining the level of detail and clarity that can be achieved when examining specimens under a microscope.

By the time you’re finished, you’ll be a lens number expert, and you’ll never find yourself confused by your lens markings again.

10. Strain Free Polarizing Microscope Objectives: Strain-free polarizing microscope objectives are specialized lenses designed for use in polarizing microscopes. They are crafted to minimize birefringence, which is the distortion of light passing through a material under stress. These objectives allow for precise examination of transparent specimens, particularly those with mineral compositions, by reducing optical distortions related to internal strains. This makes them invaluable in geology, petrology, and materials science for studying crystalline structures and material properties.

3. Fluorescence Objectives: These specialized lenses are optimized for fluorescent microscopy. They enable the observation of specimens labeled with fluorescent markers, which emit light of a specific wavelength when illuminated.

And if there are any lens numbers I missed, don’t worry – just share pictures in the comments below, and I’ll see what I can do to help out!

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14. Ultraviolet (UV) Radiation Objectives: Objective lenses tailored for the ultraviolet (UV) radiation range are crafted for brightfield microscopy and find applicability in laser-based tasks. These UV objectives boast features like infinity correction, long working distance, and a Plan Apochromat design. They excel in performance specifically within the ultraviolet spectrum (266nm) and also maintain high-quality imaging for visible wavelengths.

Check out the two lenses below. The distance scale on the 70-200mm (left) is under a cover, and you can see that the lens is focused somewhere between 10 meters and infinity. The distance scale on the 17-35mm (right) is on the lens’s focus ring, and you can see that the lens is focused quite close, at around 0.5 meters.

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Next to this ring, you’ll generally find focal length numbers. For example, if your lens is a 70-200mm zoom like mine (below), you’ll see markings that span from 70mm to 200mm. I’m currently at around 100mm:

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These variable maximum apertures are pretty common with kit lenses, and especially kit lenses with a large focal length range such as 28-300mm or 18-200mm.

The primary function of objective lenses in microscopy is to capture light emitted or reflected by the specimen. They then focus this light to form an enlarged image, which can be further magnified by the eyepiece lens. By adjusting the objective lens, users can achieve different levels of magnification, allowing for detailed examination of minute structures.

By understanding these specifications, users can make informed decisions about which objective lens is best suited for their specific microscopy needs. This ensures that they can achieve the highest level of detail and clarity in their observations.

A hyperfocal distance scale helps you determine the depth of field for a scene, given a particular focal length, point of focus, and aperture.

9. Metallurgical Objectives: Unlike standard microscope lenses, it has a longer working distance, is designed for an inverted microscope configuration, and often boasts a high numerical aperture for better resolution. These features make it particularly effective for analyzing opaque, irregularly shaped metal specimens in fields like materials science and metallurgy.

Every lens has a diameter, the distance across the center of the lens. This diameter also corresponds to the filter size (if the filter’s diameter doesn’t match the lens diameter, it won’t properly screw onto the front of the lens).

4. Field of View: This specification indicates the area of the specimen that can be observed at a specific level of magnification. It's crucial for understanding the scope of what can be viewed.

You’ll find the lens diameter written on the end of your lens (often on the edge of the barrel), preceded by a symbol that looks like a zero with a strike through it:

A lens will never display every focal length but will instead offer a few useful intervals, as you can see in the image above.