DSLR models can  have different sensor sizes. The FLM or focal-length-multiplier indicates the relative proportion of the sensor in relation to 35mm film. The larger the FLM, the smaller the sensor, and vice-versa. A full-frame sensor has the same size as a 35mm film and therefore its FLM is 1. The most common FLMs are 1.5, 1.6 and 2. Here are the important consequences of sensor size:

• Larger sensors produce images with less noise and higher dynamic range compared to smaller sensors of the same resolution.
• Larger sensor require larger lenses and are more sensitive to lens defects.
• Larger sensors show less depth of field at a fixed aperture.
• The angle-of-view of a lens is proportional to the sensor size. The effective focal-length of a lens is equal to the actual focal length multiplied by the FLM.
• Lenses are designed with a minimum FLM. So, a full-frame lens which is designed for a FLM of 1 can be used on smaller sensors, like those with a FLM of 1.5. The converse is not true.

A lens mount serves two purposes, it connects and enables communication between a camera body and its lens. Since physical connections have seldom changed, most lenses can be physically connected to a body with the right type of mount. As lenses were modernized, new features were added to newer lenses and mounts. The consequence of this is that while most older lenses can mount on cameras of the same manufacturer, not all such lenses can fully function on DSLR cameras.

 

The greatest differentiating factor among DSLR cameras is the lens mount because it gives access to a specific line-up of lenses from the camera manufacturer. Certain third-parties make lenses in multiple lens mounts which generally differ specificationwise and qualitywise from camera manufacturers' line-ups. Lenses made for small sensors are smaller, lighter and possibly cheaper than equivalent lenses for large sensors.

A lens which cannot be zoomed is called a prime lens. Lenses which can be zoomed are called zoom-lenses. Zoom lenses may be quite versatile for framing but prime lenses commonly have several advantages: lighter weight, wider aperture and smaller size. Their reputation for higher image quality is being disputed.

Lenses have a maximum aperture which frequently depends on its focal length. A wider maximum aperture means that less light is needed and depth-of-field is shallower. When a lens is labeled with two aperture values, one is the maximum aperture at the shortest focal length and the other is the maximum aperture at the longest focal length. When a lens is labeled with only one aperture value, it means that the maximum aperture is the same across all focal lengths. Most new lenses are auto-focus lenses, as opposed to manual-focus lenses which must be focused manually. A lens which can focus closer than a regular one is labeled as a macro-lens.

Like cameras, lenses come in different qualities that cannot be deduced from their specifications alone. Manufacturers each have designations for lenses of vaious qualities. Lens quality determines its potential sharpness, resolution, contrast and aberrations. Lens quality can strongly influences focus speeds when they include a focus-motor. Canon lenses having a quick and quiet focusing system are labeled USM, Pentax labels those SDM, Nikon calls those as AF-S and Sony calls them SSM.

Lenses must be primarily chosen by their angle-of-view on a particular DSLR. Commonly, the 35mm equivalent focal-length is used instead. In 35mm terms:

• A 50mm lens is considered normal because its shows perspective similar to the perception of a single human eye.
• 35 to 70mm lenses are considered standard. They are used for general purpose photography and snapshots.
• Lenses longer than 70mm are considered telephoto.
• Lenses longer than 250mm are considered ultra-telephoto.
• Lenses shorter than 35mm are considered wide-angle.
• Lenses shorter than 24mm are considered ultra-wide angle and show pronounced perspective distortion. Much shorter lenses are mostly fisheye lenses.
• Portraits are often taken with at 105mm, since it produces a pleasing perspective.

To decide what lenses are needed:

1. Select a range of useful focal-lengths and divide by the camera's focal-length-multiplier.
2. Divide the maximum by the minimum focal-length to determine the optical zoom.
3. If the optical zoom is higher than 15X, then more than one lens will be required.
4. If the optical zoom is higher than 6X, it may be possible to get a single lens but such a lens typically compromises on image quality.
5. A single zoom or several prime lens can be chosen when a short range is required.
6. Select one or more lenses to match closely the desired range. Additional requirements such as maximum aperture and minimum focus distance can be considered here to make a final decision.

Two mechanisms exist for reducing the possibly of camera shake due to long exposures or long focal lengths. One way is to incorporate stabilization into the camera body. This way was pioneered by Konica-Minolta and works by moving the sensor to compensate for camera movements. It was orignally called Anti-Shake. Sony, who purchased the technology, calls it Super-Steady-Shot. Pentax now has similar technology which it calls Shake-Reduction. Olympus calls this iS for image stabilization. Other manufacturers embed stabilization in selected lenses by moving an internal lens element to counteract movement, this is called Optical-Stabilization. Canon labels stabilized lenses with IS and Nikon labels them with VR.

Anti-Shake and Optical-Stabilization appear to be equally effective and allow 2 to 3 extra stops of hand-holding ability. In other words, shutter-speeds can be 4 to 8 times slower than without stabilization. The main differences between these two mechanisms are:

• Body-based stabilization applies to all lenses and its cost is incurred once per camera.
• Optical stabilization is only available on selected lenses and its cost is incurred once for each such lens