But there’s more. Depth of field determines what part of the image is in focus going from front to back. Lenses with longer focal length tend to have a shallower depth of field, meaning you can focus on and separate (or isolate) a particular object far away, whereas shorter lenses have a deeper depth of field, which means you can get more objects in focus throughout the image.

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Try: NIKKOR Z 20mm f/1.8 S, NIKKOR Z 14-30mm f/4 S, NIKKOR Z 17-28mm f/2.8, NIKKOR Z DX 12-28mm f/3.5-5.6 PZ VR, NIKKOR Z 24mm f/1.8 S

From 35mm and 70mm, we have the ‘standard’ focal length, which is pretty close to what the human eye sees. Photographers talk about the ‘nifty fifty’ – a large aperture 50mm prime lens such as the NIKKOR Z 50mm f/1.8 S – because a 50mm lens is an ideal day-long companion, suitable for so many different types of shooting, especially in low light situations or indoors. This type of lens also creates a shallow depth of field and great bokeh blur.

Linewidth

When interfering two beams split from a laser source, the fringes disappears if the delay $\tau$ between the two beams is too large. The delay $\tau = \tau_{c}$ beyond which the fringes are blurred corresponds to the typical phase stability timescale of the laser and is called the coherence time. The related distance $L_{c} = c \tau_{c}$ is the coherence length. Finally, the value $\Delta f = 1 / \tau_{c}$ is called the laser linewidth and it is used to characterize the coherence of a laser source.

A short-delay is placed in one arm of a Mach-Zehnder interferometer and an acousto-optic modulator (AOM) in the other one. After demodulation of the detection interferometer output, it is possible to track the phase of the laser. This provides full information on the laser phase, from which we can derive its linewidth. This method is very precise but it requires an AOM and the signal processing is quite involved.

相干长度

Note that if the laser under test has a significant low-frequency relative intensity noise (RIN), a self-heterodyne technique with long delay can be used placing an AOM in an interferometer arm. The laser line shape will then be observed around a carrier at the AOM modulation frequency.

Here we characterize the Koheron LD100 laser. We expect the laser to have a Lorentzian line shape and a linewidth of a few MHz. We use the following long delay self-homodyne interferometer:

He-Nelaser

Camera lenses are usually described by two main factors: one is the aperture or f-number (the maximum size of the hole where light gets through to the sensor, where the lower the number, the bigger the hole). The other is focal length, which is in millimetres. You will usually see lenses described by focal length first and then aperture, for example 85mm f/1.8.

If you had a simple lens made of a tube with one piece of optical glass at the front that focused the incoming light rays on a point at the back of the tube, the length of the tube would be the focal length of the lens. Modern lenses, however, use lots of different pieces of glass inside the casing to move light around before it hits the sensor and therefore focal length has nothing to do with the physical length of your lens. You can find two 85mm lenses that are completely different in size while, for example, the new NIKKOR Z 600mm f/6.3 VR S is less than 300mm in length.

As angle of view goes down, magnification goes up. For example, a 24mm lens has a wide angle of view (around 84 degrees) and low magnification – perfect for sprawling landscapes. A 600mm lens has a very narrow angle of view and large magnification – great for taking close up shots of faraway wildlife (especially useful when you don’t want to get too close to a lion!).

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Camera lenses are usually described by two main factors: one is the aperture or f-number (the maximum size of the hole where light gets through to the sensor, where the lower the number, the bigger the hole). The other is focal length, which is in millimetres. You will usually see lenses described by focal length first and then aperture, for example 85mm f/1.8.

For a laser with a fairly large linewidth, a Lorentzian profile and a low RIN, the long delay method is a good choice since it is easy to setup and the results are quite straightforward to interpret.

At 300mm and upwards, we have the super-telephoto range. This is most commonly used by sports and wildlife photographers, where it would be impossible to get close to the subject. At this range, and with such a narrow angle of view, it’s usually advisable to use a monopod or tripod to reduce camera movement (although Nikon’s in-camera vibration reduction (VR) and VR lenses can help with that), and also because lenses this size can be heavy to hold for long periods. Telephoto lenses can also be used with a teleconverter, which can double the focal length of the lens, giving you even more ‘reach’.

Aspheric lenses minimize spherical aberrations and offer a smaller footprint resulting in lighter, more compact optical systems.

A laser with a 1 MHz linewidth has a coherence length of 300 m so a 10 km delay is well above the expected laser coherence length. Though the LD100 laser integrates a 30 dB optical isolator, we add an isolation stage to ensure no disturbance. A Koheron PD100 photodetector is used at the interferometer output and the detection result feeds a spectrum analyzer.

Coherent light

Coherence length

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Angle of view is how much of the scene your lens will capture from side to side (holding the camera horizontally). A wide angle of view takes in a lot, a narrow angle of view less. Magnification is how ‘close’ you get to the subject of an image with the lens, like a telescope.

Try: NIKKOR Z 180-600mm f/5.6-6.3 VR, NIKKOR Z 100-400mm f/4.5-5.6 VR S, NIKKOR Z 400mm f/4.5 VR S, NIKKOR Z 600mm f/6.3 VR S

This method is easy to implement since no modulator is required. Also the laser phase-noise is directly visible on the spectrum analyzer.

The most important thing about focal length, however, is not what it is, but what it does. Focal length defines two main things about any lens: its magnification and its angle of view.

There are two types of lenses: primes, which have a fixed focal length, and zooms, where the focal length is variable. Zooms are super-handy as you often only have to carry one lens around that will be capable of shooting lots of different subjects, from landscapes to portraits, so it’s great for travel. Prime lenses, however, are usually lighter and are often available with larger apertures than zooms.

If you had a simple lens made of a tube with one piece of optical glass at the front that focused the incoming light rays on a point at the back of the tube, the length of the tube would be the focal length of the lens. Modern lenses, however, use lots of different pieces of glass inside the casing to move light around before it hits the sensor and therefore focal length has nothing to do with the physical length of your lens. You can find two 85mm lenses that are completely different in size while, for example, the new NIKKOR Z 600mm f/6.3 VR S is less than 300mm in length.

Coherent detection relies on interferences between a probe laser beam and a local oscillator. Phase instabilities in the laser beam are responsible for the blurring of the interferences fringes, thus limiting the performance of coherent measurement.

Superconductingcoherence length

Lens choice can often be difficult, especially with so many options in the Nikon Z mount range (and access to another 300 or so Nikon F mount lenses with the FTZ II converter). So, here’s a quick guide to the different focal lengths and what they often get used for to help you.

Try: NIKKOR Z 70-180mm f/2.8, NIKKOR Z 85mm f/1.8 S, NIKKOR Z 135mm f/1.8 S Plena, NIKKOR Z 70-200mm f/2.8 S, NIKKOR Z DX 50-250mm f/4.5-6.3 VR

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Check out these images of the same scene shot from the same position but with focal lengths from wide angle 24mm to telephoto 180mm and you can see how the angle of view gets narrower as the magnification gets larger.

Usually between 70mm and 200mm, a telephoto lens is a popular choice for portraiture and weddings (especially an 85mm focal length) and at the longer end is great for wildlife where you can’t get too close to the subject. At the longer end, a telephoto lens gives you the freedom to shoot as an observer where the subject is unaware of the camera, and so is also great for fly-on-the-wall, reportage-style shooting. This is also the focal length range for many macro lenses.

The focal length of a lens also affects the perspective of an image. With a long lens, perspective tends to be compressed, with objects in the background appearing closer to the subject in the foreground, whereas with a wide-angle lens the relative distance between two appears greater.

Ledcoherence

You should also bear in mind that focal length translates differently on a DX camera (such as the Nikon Z 30, Z 50 or Z fc) compared to an FX ‘full-frame’ camera because the sensor on a DX camera is 1.5 times smaller than the FX sensor. For example, if you put a 50mm lens on a DX camera, you will actually get the angle of view and magnification similar to an 75mm lens on an FX camera (because 50 x 1.5 =75).

[1]: Chen X. Ultra-Narrow Laser Linewidth Measurement. PhD Thesis. Faculty of the Virginia, Polytechnic Institute and State University. Virginia, 2007.

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Many phenomena alter the phase stability of the laser, for example the spontaneous emission which is not synchronized with stimulated emission, or noise in the DFB current driver.

From 24mm to 35mm, lenses are wide angle. These are beloved by landscape, interiors and architecture photographers, as well as being useful for street scenes and dramatic pictures of the night sky. Get close to your subject and you will accentuate the perspective in the scene. A wide angle is a great travel companion, allowing you to shoot landscapes, cityscapes, people and much more.

For narrow linewidth lasers the required delay might become very long (hundreds of kilometers) and absorption in the fiber becomes a limitation (especially for non telecom wavelengths). Moreover, narrow linewidth laser often have a complex line shape (such as a Voigt profile instead of a Lorentzian) and interpretation of the power spectrum becomes difficult [1]. In this case, using the short delay technique will probably be beneficial.

Coherencetime

Try: NIKKOR Z DX 24mm f/1.7, NIKKOR Z 26mm f/2.8, NIKKOR Z 28mm f/2.8, NIKKOR Z DX 16-50mm f/3.5-6.3 VR, NIKKOR Z 35mm f/1.8 S

We are actually observing the autocorrelation of the laser line shape with itself so that the measured signal has twice the laser linewidth. In other words, the linewidth defined as the full width at half maximum of the laser line shape corresponds to the half width at half maximum of the spectrum analyzer signal. Here, when driving the laser with 50 mA the linewidth is 2.4 MHz. We also observe that the laser linewidth decreases when the current increases.

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Linewidth measurement methods rely on a delay introduced between the arms of an interferometer. They can be classified into short and long delays (compared to the laser coherence time) setups.

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Any lens with a focal length of between 8mm and 24mm is usually described as an ultra-wide. You’ll be taking in a huge angle of view of what’s in front of the camera. These are lenses for getting in really close with your subject to create drama and are also used extensively for astrophotography. However, at really low focal lengths there will be significant distortion at the sides of the image where straight lines start to look curved. Ultra-wides are one the hardest lenses to master but, with effort, they can deliver incredible results.

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