Sigma 100-400mm f/5-6.3 DG DN OS | C review: specs and price

A metal mount is generally superior to a plastic mount as it is more durable.

The device is protected with extra seals to prevent failures caused by dust, raindrops, and water splashes.

We consider a lower weight better because lighter devices are more comfortable to carry. A lower weight is also an advantage for home appliances, as it makes transportation easier, and for many other types of products.

A longer maximum focal length allows you to focus in on a small part of a scene, and offers a narrower angle of view than shorter focal lengths.

A prime lens has a fixed focal length. In general, these are sharper, with a wider aperture.

A telephoto lens allows you to zoom in on far away objects. This is particularly useful when you need to photograph a subject from a distance, such as wildlife photography, or even in some cases street photography where you can’t get near the subject.

The front element doesn’t rotate. This is important if you use filters, as some such as polarising or gradient filters have to be orientated a certain way.

The lens hood can screw onto the lens in reverse so that you can keep it on your camera at all times, ready to use.

A macro lens is used to take very close-up photos, allowing you to capture flowers, insects etc. in great detail. A macro lens is any lens with 1:1 magnification.

Optical image stabilization uses gyroscopic sensors to detect the vibrations of the camera. The lens adjusts the optical path accordingly, ensuring that any type of motion blur is corrected before the sensor captures the image.

A true macro lens has a magnification of 1:1. This means that the image produced is a life-size representation of the subject being photographed.

At the shorter end of the lens you get the widest angle of view. This allows you to fit more of the scene into the photograph (based on APS-C format).

At the longest end of the lens you get the narrowest angle of view. This allows you to fit a small portion of the scene into the photograph, such as when you are zooming in on a subject (based on APS-C format).

The zoom range is the ratio between the longest and shortest focal lengths. A higher zoom range means that the lens is more versatile.

With a wider aperture the sensor can capture more light, helping to avoid blur by enabling a faster shutter speed. It also provides a shallow depth of field, allowing you to blur the background to focus attention on the subject.

With a wider aperture the sensor can capture more light, helping to avoid blur by enabling a faster shutter speed. It also provides a shallow depth of field, allowing you to blur the background to focus attention on the subject.

Similar to the number of aperture blades, rounded blades affect the way the light gets through to the sensor. Rounded blades, often only found on more expensive lenses, improve the appearance of the out-of-focus areas. This allows you to attain better, softer looking bokeh in your photos.

A smaller aperture reduces the amount of light reaching the sensor. This is important in bright conditions where a wider aperture could result in your image being overexposed. Another advantage is that with a smaller aperture you get a greater depth of field, and can keep all of the image in focus.

The aperture controls how much light gets through to the camera’s sensor. More blades is often an indicator of a better quality lens. It also allows you to achieve much nicer looking bokeh when blurring out your background, whereas a lens with less blades will often produce harsher, more polygonal bokeh.

A smaller aperture reduces the amount of light reaching the sensor. This is important in bright conditions where a wider aperture could result in your image being overexposed. Another advantage is that with a smaller aperture you get a greater depth of field, and can keep all of the image in focus.

Many lenses allow you to focus to infinity. This is essential when you wish to take photos including far off objects, such as when shooting landscapes, in order to make sure everything is sharp and in focus.

Lenses with a built-in focus motor can autofocus even if the camera does not have its own focus motor.

Lenses with built-in focus motor focus faster and more quietly than lenses without a focus motor which rely on the camera's body focus motor.

With full-time manual focus, you can move the focus ring whilst it is in AF (autofocus) mode. This means that you can make manual adjustments once the AF has finished, without changing to manual mode.

This is the closest distance that the lens can focus. A shorter minimum focus distance allows you to get closer to your subject, and is particularly important when doing macro photography.

The sharpness result from the DxOMark set of metrics. This result is based on the MTF (modulation transfer function) measurement, and gives an overall indication of the sharpness of images produced by the lens. Tested with Nikon D7000 or Canon 7D. Source: DxOMark.

DxOMark is a set of tests to measure the performance and quality of lenses and cameras. The DxOMark score is the overall score given to the lens. Tested with Nikon D7000 or Canon 7D. Source: DxOMark.

The transmission result from the DxOMark set of metrics. Transmission refers to the amount of light that reaches the sensor through all of the glass elements of a lens, with a lower TStop signifying more light. This is important as less light reaching the sensor can lead to the requirement of higher ISOs or slower shutter speeds. Tested with Nikon D7000 or Canon 7D. Source: DxOMark.

The distortion result from the DxOMark set of metrics. Distortion in the lens refers to the variation of magnification throughout the image. More distortion will result in straight lines being recorded incorrectly in the image. Tested with Nikon D7000 or Canon 7D. Source: DxOMark.

The vignetting result from the DxOMark set of metrics. Vignetting refers to when the brightness of an image changes from the center towards the edges, resulting in darkened corners. A result of 0 is perfect and the image will have no vignetting. Tested with Nikon D7000 or Canon 7D. Source: DxOMark.

The lateral chromatic aberration result from the DxOMark set of metrics. Chromatic aberration is a type of distortion which results in color fringing along edges within the image. Tested with Nikon D7000 or Canon 7D. Source: DxOMark.