Fresnel lens - fresnel lens vs pancake lens

 2024-11-27 04:57:07

To help to understand NA, it is useful to also have some understanding of refraction. In microscopy and optics, refraction refers to the change in direction of light waves which results from a change in the medium though which light passes (for example, glass, air, oil or water).

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Numerical Aperturecalculator

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Confocal microscopy provides optical sectioning, the ability to observe discrete planes in 3D samples, by using one or more apertures to block out-of-focus ...

A zoom lens could "zoom" from a short (wide-angle) to long ("telephoto") focal length, making things look bigger and closer as you zoom in. Or it could zoom from an extreme wide-angle to a moderate wide-angle, never coming close to a "telephoto" focal length. Or any other range of focal lengths.

'Zoom' just means that it has a range of focal lengths. That range could be telephoto, it could be below telephoto, or it could range from below telephoto to above telephoto.

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Oct 27, 2024 — Eyepiece – The eyepiece (ocular Lens) is closest to the viewer's eye. They are located at the top of the microscope. This part is used to look ...

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The maximum extension for this camera is 315mm. You can't move the front of the camera more than that distance out. But what if you want to use a longer lens? The Schneider 400mm f/5.6 Apo-Tele-Xenar (that's a mouthful - the 'apo' means it is an apochromat lens and then there's that 'tele' there...) has a focal length of 400mm... but its flange focal distance is 285.1mm. It could fit on that camera (well, in theory - it also has a #3 shutter and that lens board can only fit #1 and #0 shutters... but other than that).

Numerical aperture ofmicroscope

A 300mm lens is a telephoto but is not a zoom because 300mm is high mm (in other words, 'long focal length' or 'zoomed in') but it does not cover a range of focal lengths. (You can only use that lens at 300mm, not 299mm or 472674mm) Insead, we call these lenses prime lenses. A prime lens does not cover a range of focal lengths, just one.

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The NA of an objective is an important aspect as it relates to the final image formation seen when looking down through the eyepieces (which will be covered in full in a forthcoming blog article). Briefly, resolution relates to the amount of detail which can be seen in the final formation of an image. An objective with a high magnification would be unable to resolve detail in your sample without a similarly high NA.

Please talk to me like I'm stupid. :) I am hoping for a completely lay explanation. In particular, I'd like to understand in what situations I'd use a telephoto lens, and in what (other) situations I'd use a zoom lens.

Most of the time people say 'Telephoto' they just mean 'zoomed in' or in other words 'high mm' or 'long focal length' As has been mentioned, Nikon seems to say that 85mm is the shortest focal length that they will call telephoto. I have seen people call anything above 50mm telephoto.

The compound microscope has two systems of lenses for greater magnification, 1) the ocular, or eyepiece lens that one looks into and 2) the objective lens, or ...

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The focal length is normally measured in millimeters (mm). A zoom lens will have two measurements, for example "18-200 mm" (a wide-angle to telephoto zoom). It zooms from a short focal length of 18 mm to a long focal length of 200 mm. A non-zoom lens, also called a "prime" lens, will have a single focal length, for example "135 mm" (a moderate telephoto).

Make it simple: zoom is a lens of multiple focal lengths of all focal ranges; telephoto is a prime from 85mm-300mm range and super telephoto is over 300mm+. (Info from Greengo).

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Where ‘n’ is the refractive index of the medium between the cover glass and the objective front lens (e.g. air, water or oil).

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Snell’s Law describes the relationship between the angles of incidence and refraction of light as it travels through the boundary of two different medium (e.g. from air to glass). The Law states that the ratio of the sines of the angles of incident and refracted light are equivalent to the reciprocal of the ratio of the refractive indices through which the light passes.

A zoom lens means the lens can change focal length via zooming, i.e. it is not a prime lens. A telephoto lens has a long focal length (I do not know if there is an official threshold to call a lens telephoto, but Nikon seems to start it at 85mm.

Why bother with specific telephoto lenses? Because as opposed to generic zoom lenses with wide and long reach, they will have fewer image defects in their core range and will usually be considerably faster in that range. Their default long reach without too much detrimental optical trickery will also render the telephoto lenses the longest lenses to lug around, and being fast comes at the cost of a large entrance pupil and large front lens diameter. They are usually the largest and heaviest of all lenses. If you have an f=300mm lens with an aperture of f:1.4, its entrance pupil has a diameter of 214mm, and if it is not to suffer from significant vignetting, its front lens should be larger than that. Of course, you don't get them quite that fast. But there is little point in making them into a zoom lens also good at wide angle when a separate fast wide angle lens with good quality will add less weight and cost into your gear box.

Numerical aperture is proportional to refractive index. For example, air has a refractive index of 1.00, water has a refractive index of 1.33, whereas many of the immersion oils have refractive indexes around 1.52.

An 18mm lens is not a telephoto lens and it is also not a zoom lens because it has low mm and there is only one focal length. You'd call it a wide angle, prime lens.

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Telephoto, roughly, means that the lens has a relatively narrow field of view, thus it can be used to look at things further away.

When you go to other formats, the distinction between a telephoto lens and a non-telephoto lens becomes important. Consider a nice 4x5 field camera:

The focal length of a lens determines its field of view on your camera. If it has a long focal length, it has a narrow field of view, making the things in front of you appear large in the photograph. If it has a short focal length, it has a large field of view--it's a "wide angle" lens that takes in a large area, making objects appear small.

Unless you are dealing with a large format system and looking at the rail on your camera, you aren't going to care about how the lens is designed and if the actual focal length is the distance between the lens and the focal plane or not. You've got a lens, you use it.

There's another flip to that which is what you see in SLR photography a lot - the retrofocus design. With a Nikon F mount, the flange distance is 46.5mm. This allows the mirror to clear the back of the lens when it flips up (this is a major issue in SLR design). So the closest you can put a lens would be about 47mm away from the focal plane. But yet, there are lenses such as a 24mm lens which have a focal length that is shorter than this distance. (Note: this is part of why interchangeable lenses on rangefinders and mirrorless systems can made more cheaply - they can use simpler designs for their shorter flange distances).

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The simple definition of a telephoto lens is a lens that has a focal length longer than the physical lens. Many lenses that are 'long' (as opposed to 'normal' or 'wide') are telephoto in design. This is because it sometimes is impractical to put that much of a barrel on the lens. In the SLR world, one often uses 'long lens' and 'telephoto lens' synonymously.

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Technically 'Telephoto' means that the focal length [the mm] is longer than the lens is. In my experience, people in the photography world usually don't talk about it under that definition.

A zoom lens is useful when you are unable to move to get the crop of the scene that you want. Sometimes you can't step back further to get a wider view. Other times you can't go half way to the middle of the river to photograph the other side of the shore. You could pack a lot of lenses and select the one you want for the situation you want, or you could carry a zoom lens.

A 10-20mm lens is not a telephoto lens but is a zoom lens. It is not zoomed in at all. It has a short focal length, low mm. It's called wide angle. If you felt like it, you could shoot at 15mm when you feel jumpy and 16mm when you feel bumpy. You could not, however shoot at a high focal length as you could with the 300mm lens.

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There is an inverse relationship between the angular aperture and the working distance of an objective. I have covered working distance in my article entitled ‘Looking Down and Through: Microscope Optics 3: Oil Immersion Objectives’, but to briefly recap, the working distance is the actual distance (in millimetres or microns) between the objective front lens and the surface of the cover slip when the object is in sharp focus. Objectives with short working distances will consequently have a greater ability to gather more oblique light rays from a specimen compared to longer working distance objectives. Angular aperture is usually determined by the optics within the objective and each objective lens will have an optimal focal length and working distance- it can’t simply be increased by moving the objective closer to the slide!

So, if you want the sharpest images possible, you are looking at using a prime (not a zoom) lens. If it is longer than 85mm or so, it is likely a telephoto lens of some design. Glance at Canon's Forgotten 400 which shows a comparison between a 400mm f/5.6 telephoto prime and a 100-400mm f/3.5-5.6 zoom lens.... though do realize that with real life one typically isn't looking at brick walls at the edge of the frame.

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What is the difference between the two? Why would I want one over the other? I wiki'd telephoto lens but remain confused about this distinction.

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Telephoto and zoom are two completely different things. Zoom simply means that the focal length (apparent magnification) of the lens can be changed, ie, it looks like it can look at things either closer up or further away by adjusting it.

The theoretical maximum angular aperture of light entering the front lens of an objective is 1800. This would give a θ value of 900 (half of the angle of the light cone). As a result, the theoretical maximum NA of an objective would be one (which is equal to the sine of 90). The refractive index of air is also one, therefore the maximum (theoretical) NA of an objective with an air gap between the front lens and the specimen would only equal one.

A telephoto lens is optimized for long reach (good magnification at long distances). A zoom lens is one with variable focal length, usually ranging from wide to long. While a telephoto lens may have either fixed reach or variable reach, its focusing distance will contain ∞ and will usually start at a distance of more than a few meters (macro lenses also have good magnification but focus on near distances).

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Telephoto lenses can be either zoom or prime. Zoom means that they can change how far they are looking at or prime means they have a fixed amount of magnification and can't be altered. (Like an old instant camera with no zoom, but with much better picture quality.) The reason for using a prime is that they are a) cheaper and b) produce much, much higher quality images for the price.

Therefore, the refractive index of air is a limiting factor in achieving the highest possible NA of an objective. As a result, objectives with NA values greater than one are the immersion objectives where the air gap is replaced by a medium such as water or oil. An angular aperture of 1800 is physically unachievable- the widest angle of light which can be collected by an objective is around 1440. Consequently, the maximum achievable NA of a non-immersion objective is approximately 0.95 (which is equal to the sine of 72).

In summary, without a correspondingly high NA, a high magnification objective will have low resolution. Most microscope companies offer objectives which have high NA values for use with immersion medium. If you are in the lucky position of buying a custom microscope, or buying new objectives for your existing instrument, you should always consider buying objectives which offer the highest NA value which you can afford.

Other answers have stated the same thing in the same technical terms which might be meaningless in this instance. There is also a practical approach.

So "zoom" = focal length you can change, and "telephoto" = long focal length. A lens can be one, or the other, or neither, or both.

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The light from the microscope source passes through the specimen/slide and continues through the air (or an immersion medium) as a cone of light between the cover glass and the objective front lens. The ‘angular aperture’ refers to the maximum angle of the edges of this image-forming cone of light which can be collected by the objective front lens when the specimen is in focus. In addition to an increasing NA, image brightness and image detail (resolution) are also related to the angular aperture.

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On the other hand, recall that bit about the complex lens design? The wider the range of the zoom, the more compromises in the design to give you that range. It comes at the expense of aberrations, less light getting through the lens (requiring longer exposures), and other distortions. The 'super zooms' of 20-200 or 15-300 have much more compromises to the design than one that has a narrower range (the classic 100-300). Some photographers try to avoid zoom ranges of more than 3x or 5x except when necessary (glass weighs a lot - if you go hiking, it might be easier to carry a zoom than 50 lbs of glass). While the zoom lenses of today are better than those from a decade or two ago, there is still some truth to that.

Metaphorically, think of it this way: if you are standing in front of a door with a key hole which leads into another room, then when you are at a distance, you will only be able to see a little of the light and objects within the room. If you press your eye against the key hole, you will then see more of the detail and light in the room as you have, in theory, increased the angular aperture of your eye.

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Refraction is described in a formula known as ‘Snell’s Law’. Refraction was first described in the year 984 by a Persian physicist and mathematician called Ibn Sahl. In 984, he presented a manuscript in which he described how mirrors and curved lenses focused and bent light. Snell’s Law is actually named after a Dutch mathematician and astronomer called Willebrord Snellius (1580-1626). Although he was credited for mathematically describing refraction, it is more accurate to say that he ‘rediscovered’ diffraction after the work of Ibn Sahl.

An 18-200mm lens is a strange beast. It is a zoom lens but it can be considered both a wide angle (at 18mm) and a telephoto lens (at 200mm) because it's zoom range is so huge. You could shoot at 200mm and it's telephoto or you could shoot at 18mm and it's wide angle. In my experience if it is capable of telephoto, then you call it a telephoto. You'd then call it a telephoto zoom

Numerical apertureand resolution

Numerical aperture ofoptical fiber

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The term "telephoto lens" has a particular technical meaning in terms of lens design, but in common usage it refers to a lens with a long focal length.

Similarly, a zoom lens can be in any part of the focal length spectrum. You can have a 17-40mm zoom, which is basically wide angle (and not at all telephoto) or you can have a 100-400 zoom, which is entirely telephoto. You can also have any variety of different combinations including things like 24-250mm lenses that span from wide angle all the way to telephoto in one lens.

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I've heard it said that 50mm is the standard as far as lens focal length goes, meaning that at 50mm your camera will take a photo as close to how the human eye sees it, i.e. a similar zoom and angle of view. So, anything over 50mm is considered a telephoto (zoomed in from how you see it) and anything under is considered wide angle (zoomed out from how you see it). A fixed focal length lens will also be called a Prime lens, a variable focal length lens is a zoom. So to sum up, a lens with- 50mm is considered standard. 24mm would be a wide angle prime (only one focal length). 18-35mm would be a wide angle zoom. 200mm would be a telephoto prime. 70-200mm would be a telephoto zoom. 18- 200mm would be both a wide angle (below 50mm) and telephoto (above 50mm) zoom. Basically if you're focal length is below 50mm you're in wide angle mode and if you're above 50mm you're in telephoto mode. Hope this helps.

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Etched onto the barrel of each objective on a microscope, you will find a variety of information. In addition to the magnification and the optical correction (see my article published entitled ‘Looking Down and Looking Through: The Optics of a Microscope 2: The Objectives’ for more information on aberrations and corrections), you will find a number without units. This is the Numerical Aperture (or ‘NA’) of the objective.

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A "zoom lens" is a lens whose focal length can change. You twist the barrel, or push a switch on the camera, and it takes in a narrower or wider field of view, making objects appear bigger or smaller.

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Where ‘θ’ is half of the angle of the cone of light which is collected by the front lens lens (i.e., the angular aperture).

Numerical apertureformula with refractive index

Numerical aperture ofobjectivelens

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The NA of an objective is the simply the ability of the lenses to collect light at a fixed distance from the sample which you are viewing. When light passes through and leaves a specimen, it enters the front lens of an objective as an inverted cone. However, a percentage of this image-forming light is refracted and reflected. Objectives which have a high NA allow for increasingly oblique light waves to be collected by the front lens which will in turn form a final image which is not only relatively brighter, but contains more information and detail and is highly resolved.

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To simplify this further, as light travels through one medium to another, it changes speed (e.g. when passing from air to water, light slows down). When light passes across the boundary of two different medium at an angle other than 900, this results in a change of direction. Although the frequency of light doesn’t change, the resultant wavelength will be determined by nature of the medium.

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Now to zoom lenses... and the reason I mentioned that bit about retrofocus design. Typically a zoom lens is made of a prime lens group in the rear, a middle group, and then a retrofocus group in the front. That's the 'ideal' design, though often they are more complicated to deal with aberrations and distortions that inevitably come with more complex lens designs. You can get a hint of how this works in the What Is Inside a Zoom Lens? article from Tamron. Though, it is probably more accurate to say that zoom lenses take design elements from telephoto design, retrofocus design, prime lens design, and a bit of other to make a very complex system.

So, instead of making a lens that has a focal length that is longer than the distance it is at, you make a lens that has a focal length that is shorter than the distance it is focused at. Retrofocus lenses often have big front elements.

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