Photonic Devices' precision Galilean beam expanders have been sold globally for over 20 years and include an extensive number of standard models. These multi-element beam expanders have been computer-designed to provide excellent wavefront quality with minimum beam deformation. All lens elements are air spaced and the majority of models have variable air spacing for collimation adjustment. All lens surfaces have multilayer antireflection coatings, with power handling capabilities over 1 kW (> 200 W for beam expanders operating at 10.6 microns).

Reflectivebeam expander

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This question is predicated on a misconception. The flange distance is included in the focal length in lens labels — it is the distance from the optical center of the lens focused at infinity to the imaging medium. This includes the flange distance. (See What is the reference point that the focal length of a lens is calculated from?, and What exactly is focal length when there is also flange focal distance?)

This aberration is familiar to anyone who has used a camera, videocamera, microscope, telescope, or binoculars. Optically, defocus refers to a translation of ...

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Optogama designs and manufactures laser beam expanders, beam reducers and divergence compensators which are used to increase or decrease laser beam diameter and control beam divergence:

The focal length marked on the lens is when focused at infinity. Some imagine that is the definition of focal length, but it is merely one setting for it. Zoom lenses also vary focal length.

1/F = 1/S1 + 1/S2, Where F is the focal length of the lens, ans S1 and S2 are the subject and image distance respectively. https://en.wikipedia.org/wiki/Angle_of_view

My question was: given that both parameters affect the FOV, why is it then that we attribute only the focal length to the FOV of the lens?

Unlike the angle of coverage, the field of view, or angle of view are not lens properties. They depend on the focal length of the lens, the size of the sensor and the distance between the rear focal point and the sensor. When a lens is focused at infinity, the distance between the rear focal point and the sensor is zero. When the lens is focused on something closer, the sensor is placed behind the focal point. The location can be determined with the thin lens formula:

Beam expanders are generally not designed for use with divergent beams, but only for collimated beams, and only within a certain range of beam radii. Otherwise, one may obtain clipping effects and/or not get a collimated beam out. Obviously, a beam can be collimated over a certain length only if its beam waist is large enough. As an example, Figure 3 shows the evolution of beam radius in the same beam expander is considered in Figure 1, but with a five times smaller initial beam radius. Here, the beams can no longer be considered as collimated beams.

There are variable beam expanders (zoom expanders), i.e., devices where the magnification can be adjusted in a certain range (e.g. from 2× to 5× or from 5× to 10×). Those contain at least three lenses and some fine mechanics to adjust the position of at least one of them.

However, in optics, it's well known that for a fixed sensor size, the distance from the lens to the image plane (which we can equate with the flange distance) also affects your field of view. Everyone who mounted his\her lens on a macro adapter had seen that increasing the flange distance will narrow down your field of view, and they will end up with a macro lens.

The lens focal length does provide some field of view, which the sensor size crops to capture possibly a lesser amount of it. Cameras with tiny sensors only capture a small field, so they have to use a much shorter focal length lens to compare to an expected "regular picture" view seen by other cameras with larger sensors. Crop Factor compares that sensor size view to the historical 35 mm film frame size view that so many of us are very familiar with.

Keplerianvs Galileanbeam expander

I realize that it could be the case that it was just an arbitrary historical decision, but I'm curious whether there's justification for that which I'm missing.

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Lenses are characterized by their optical and physical properties. Field of view is not an optical property, but a consequence of the camera system to which the lens is attached. The same 50mm lens has different fields of view when paired with differently sized sensors, such as crop frame vs full frame.

Edmund Optics has a variety of beam expanders, including devices for different wavelength regions (e.g. Nd:YAG and CO2), rotating or non-rotating optics for divergence adjustment and research-grade devices.

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If we expand the scanning beam, will the scanning angle decrease? Is there any way to expand the beam after the scanning optics?

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The very good reason for cameras is that field of view also strongly depends on the sensor size. A small sensor captures a more narrow view than a larger sensor can.

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Focal length and lens FOV are inherently the same thing. The variable is how much of the lens' FOV is utilized (i.e. extension tubes/TC's/crop sensor/etc). Why would you specify a lens' characteristic by something that may be variable?

Using cylindrical lenses, one can realize beam expanders which work in one transverse direction only. For that purpose, one may also use anamorphic prism pairs.

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Available in singlet, doublet (positive and negative focal lengths) and triplet varieties, Knight Optical can supply customised beam expanders for a range of laser-based, cutting-edge projects. Whether your application works in the ultraviolet (UV), visible, near-infrared (NIR), mid-infrared (MIR) or far-infrared (FIR) spectrum, opting for our custom-made components allows you to specify suitable substrates for your required wavelength.

This means that for a given sensor size, lenses made for different mounts are still comparable — a 24mm lens gives (approximately) the same field of view regardless of the mount distance of any given system. So, focal length does correspond to field of view.

In most cases, a beam expander is realized as an optical telescope consisting of two lenses (or in some cases of two curved mirrors). Two different configurations are common:

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Focal length is NOT at all about the mounting flange distance. The internal focus node can be moved by design. A "telephoto" lens means that node is slightly in front of the front lens element (lens is shorter than the focal length). A retro-focus (wide angle) lens places that focus node behind the rear element, to create space behind the lens. But focal length is to that focal node.

In the link above it can be seen that the angle of view depends on S2. If S1 is large, F and S2 are approximately the same, and one can say that the field of view is related to the focal length. In macro photography the subject distance S2 is not very large, therefore it has a significant influence, and the field of view is no longer related to the focal length of the lens. Because we are used to relating the field of view to the focal length, we define the effective focal length f, as the image distance. In that case the field of view is related to the effective focal length, which depends on the subject distance.

Beam expanders are optical devices which increases the diameter of the input beam to produce a larger output. Used with collimated light, they have important applications in remote sensing, interferometry, and laser scanning.

Keplerian beam expanderpdf

To further clarify by an example: you can have a 50mm and a 40mm lenses both with the same distance between the principal plane and the flange of the lens. In this case, if you place the 40mm lens sufficiently further from the sensor (compared to the 50mm lens) you will have the same effective FOV.

You note that using a macro adapter affects field of view. In fact, they also increase the focal length, although since macro extension tubes remove the ability to actually focus at infinity, we leave practical reality and get into the realm of theory.

For very high laser powers, purely reflective beam expanders (with mirrors instead of lenses) are used. This is because thermal effects such as thermal lensing are weaker on mirrors. Also, that way one can avoid any parasitic reflections. A disadvantage, however, is that some amount of astigmatism is generally introduced by the mirrors.

Shanghai Optics Inc's custom beam expanders are used in many applications such as laser ranging, laser illumination, interferometry, etc. In high-power laser systems, beam expanders are used increase the beam area without significantly affecting the total laser energy. This results in a reduction of the laser power density which reduce the risk of damaging the coatings and optical materials of optical components. In a laser ranging system, a beam expander is used to minimize the laser divergence, resulting in a smaller collimated beam at a long distance.

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In laser technology and general optics, one often works with collimated beams, by definition having a roughly constant beam radius over some length. Sometimes, it is necessary to substantially modify a beam radius, for example in order to achieve a reduced beam divergence for transmitting the beam over a larger distance. For that purpose, beam expanders can be built and are also available as fixed optical components.

These two properties are completely independent of each other, but changing either of them will change the effective FOV - this is a fact of geometrical optics.

Laserbeam expander

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A good familiarity with Gaussian beams is a good basis for understanding the operation of beam expanders and similar devices.

For application with pulsed lasers, the used lens coatings should also have a sufficiently high optical damage threshold. Further, one should avoid operation with misaligned high-power beams, which could lead to overheating of some parts.

Please try to answer my original question instead of trying to educate me about optics - that part of the conversation I got down pretty good :)

Standard off-the-shelf Galilean type zoom beam expanders and compact beam expanders. Our engineers can design a custom beam expander solution to fit your application.

When focused closer than infinity, the lens is extended forward (the frontal elements are, or possibly only internal elements) so that the internal focus node in the lens is further from the sensor plane. If we instead assume focal length is the distance from this internal node to the sensor plane, then of course the focal length is a bit longer when focused closer. That longer focal length changes things, like f-number, which can affect exposure, so regular lenses don't allow distances shorter than some nominal close distance, typically at around 0.1x magnification.

From the physics POV of it all, what determines the effective FOV for a given sensor is the effective focal length of the lens and the distance between the back principal plane of the lens to the sensor, which is determined unambiguously by the distance between the principal plane and the flange of the lens plus the distance between the flange of the camera and the sensor.

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Yes, beam expansion necessarily reduces the scanning angles – just as it also reduces the beam divergence angle. This cannot be avoided simply with additional (fixed) optics.

For achieving a given magnifying power (expansion ratio, ratio of beam radii), one may use different values of focal length. Most compact solutions are possible with small focal lengths, but there are limitations. In particular, one may then require lenses with very high numerical aperture, if at the same time a large output beam radius is required. Therefore, beam expanders for operation with large beams are tentatively longer.

For minimum losses of optical power, the lenses are usually equipped with anti-reflection coatings. These, however, work only within a limited wavelength range.

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When modifying the beam radius, one also modifies the strength of beam pointing deviations. For example, doubling the beam radius implies that angular changes of the output beam are only half as strong as those of the input beam.

In photography, it's the accepted norm to say that if you know the sensor size of your camera and the focal length of your lens, then you know the field of view of your system.