Understanding Camera Lens Specifications: A Beginner's Guide

This guide is for new camera users looking to increase their understanding of lens definitions and technical specifications, and how this applies to taking pictures. I do not make lens recommendations as each user may have different needs, but the info in this guide can help you make your decisions. It is primarily for 35mm SLR lenses, but does include info about Digital SLRs and Medium Format lenses. This guide is very basic and if you are experienced

Italized sections show important points.

Definitions:

Focal Length: The distance between the optical centre of the lens (the diaphragm) and the film.  The distance is usually given in millimetres (mm), but on some older lenses it will be in centimetres (cm). The longer the focal length, the larger your subject will appear, and the shorter the focal length, the smaller your subject will appear. Large focal length lenses have small angles of view, and small focal length lenses have large angles of view.

Aperture: A ratio defining the width of the lens glass with respect to the focal length. In simplest terms, it is a number that tells you the width of the glass inside the lens at the narrowest point. The smaller the number, the wider and faster the lens. These numbers can range from about 1:1 to 1:8 (f/1 to f/8), but most lenses fall between 1:1.4 to 1:4.5 (f/1.4 to f/4.5). A wide (or large or fast) aperture allows more light to reach the film or digital sensor, which allows you to take pictures with a faster shutter speed.

• Most 35mm & Digital lenses have a widest opening of about 1:2 to 1:3.5 and a smallest opening of around 1:22
• Most Medium Format lenses have a widest opening of 1:2.8 or 1:4, and a smallest opening of a 1:32

The wider the aperture, the smaller the depth of field, and the smaller the aperture, the larger the depth of field. Portrait photographs usually use a very wide aperture while focusing on a person's eyes & face while the rest of the image is out of focus, including a totally out of focus background. Landscape photographs usually use a small aperture with a wide depth of field so that the foreground and background are in focus.

The sharpness of lenses is always best in a middle aperture: typically around f/8 for 35mm and f/11 for Medium Format. This doesn't mean you should always shoot using a medium aperture, because varying depth of fields are useful for different looks.

A 1:1.4 lens for 35mm will have the following apertures labeled on the lens: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22. As the numbers increase, the opening shrinks, decreasing the amount of light allowed through. Each f-stop change (from f/5.6 to f/8 for example) halves the amount of light let through, so the shutter speed must double to accommodate.

The faster a lens, the more expensive it gets. Most 50mm 1:2 lenses are quite cheap, but 50mm 1:1.2 lenses are very expensive. Faster lenses are generally of higher optical quality than slower lenses, but this isn't always true.

For the mathematically inclined, the lens ratio 1:2 gives the ratio of focal length to the aperture width. A 1:2 50mm lens has a 50mm focal length and a 25mm widest aperture width. A 100mm 1:2.5 lens has a 100mm focal length and a 40mm widest aperture width. Each f-stop number increases by a multiple of the square root of 2. If you take a picture with a 100mm 1:2.5 at f/4, the aperture blades (diaphragm) will close down to being 25mm wide.

'Auto' Setting: Auto, Automatic, or 'A,' does not mean the lens is auto focus. Auto means that the lens will communicate with the camera and your aperture setting can be automatically determined. Auto focus lenses will often be labeled with 'Autofocus,' F, AF, FA, usually EF (but not always), and possibly more. If you aren't sure, ASK!

Filter Thread: The diameter at the end of your lens where filters can be screwed in. This is NOT the focal length of your lens, but both are given in millimetres (mm). Several 50mm lenses have 49mm filter threads, and occasionally inexperienced sellers will mistakenly write the filter thread instead of the focal length when describing their lens.

Diaphragm: The mechanism on the inside of the lens that closes down the aperture using aperture blades.

Screw Mount: A lens that screws on and off of the the camera body. These are slow to change, but if you are a patient photographer who doesn't shoot action very often this isn't a problem. Screw mount lenses are always manual focus.

Bayonet Mount: A lens that clicks into place on a camera with a small twist. They are released by pressing a lever or button and twisting the lens the other direction. They are very quick to change and are much more common that screw mount lenses. Most camera manufacturers have their own proprietary bayonet mount.

Leaf Shutter: A lens that contains its own shutter system. These are very useful for studio photographers or any photographer that uses a flash a lot. Using a leaf shutter lens allows photographers to use a flash with a much shorter (higher number) shutter speed. Most cameras cannot use flash with any shutter speed faster than 1/60 or 1/125 (some are faster), but a leaf shutter lens alleviates this restriction to allow faster shutter speed while using flash. If you want to know the details about why, read the following two paragraphs.

Most cameras have a focal plane shutter, which means that in front of the film (or digital sensor), there are two shutter curtains. One of them starts covering the film, and the other one starts not covering the film. When you take a picture at 1/60, one of the shutters moves out of the way, and 1/60 of a second later, the other one moves to cover up the film. A camera's flash sync is the fastest shutter speed in which the first curtain totally uncovers the film before the second curtain starts to move. For faster shutter speeds, the film is never totally uncovered because the second shutter curtain starts moving before the first one has cleared the film (or sensor).

When you take a picture using a flash with a shutter speed that is too fast, the flash only shines onto part of the film (or digital sensor) because the shutter curtains are blocking part of the film. By using a leaf shutter (and a very slow shutter speed on your camera), the first shutter curtain gets out of the way, THEN the lens' leaf shutter system opens up, the flash fires, the leaf shutter closes down, then the second shutter curtain moves. Because the leaf shutter opens up from the middle and not from side to side, this allows a faster shutter speed.

Types of Lenses:

Prime Lens: A lens with a fixed focal length. The optical quality of prime lenses almost always exceeds the optical quality of zooms lenses, but it is cumbersome to carry several prime lenses when a single zoom lens can do the same. That said, a cheap prime lens from a 3rd Party Manufacturer will usually not be as good as an expensive zoom lens from a big name camera maker.

Zoom Lens: A lens with a variable focal length. A common zoom is the 80-200mm, and can be set to every focal length from 80mm to 200mm. Zoom lenses are usually slower (have a smaller aperture) and are of lower optical quality than prime lenses. In general, the more of a range a Zoom has, the lower its optical quality will be. For example, an 80-200mm zoom will usually produce better pictures than a 28-200mm.

Zoom lenses usually have a variable aperture. For example, a 28-80mm 1:3.5-4.5 zoom lens has a widest aperture at 28mm of 1:3.5, but at 80mm the widest aperture is only 1:4.5. If a zoom lens is described as 24-50mm 1:4, then it has a maximum aperture of 1:4 at both 24mm and 50mm.

Wide Angle Lens: A lens with a short focal length that has a wide angle of view. There are also ultra wide angle lenses, and these are lenses with an extremely short focal length.

• For most digital SLRs (non full-frame), this is typically between 14mm and 25mm
• For 35mm cameras, this is typically between 20mm and 35mm
• For Medium Format cameras, this is typically between 40mm and 60mm

Normal Lens: A lens that has a focal length roughly equal to the diagonal distance of the film negative of digital sensor. The apparent perspective is similar to what a human eye sees.

• For most digital SLRs (non full-frame), this is around 30mm
• For 35mm cameras this is around 50mm
• For Medium Format cameras, this is around 75mm or 80mm

Portrait Lens: A lens that has a focal length that offers a pleasing perspective of people, and usually a wide maximum aperture.

• For most digital SLRs (non full-frame), this is around 50mm to 60mm
• For 35mm cameras this is around 85mm
• For Medium Format cameras, this is around 120mm to 140mm

Telephoto Lens: A lens with a long focal length to greatly magnify the subject size. Portrait lenses are considered short telephoto lenses, and anything longer than a portrait lens is considered a telephoto lens.

Summary:

While reading auctions, you should now understand the majority of lens specifications. If you encounter things like IS, ED, AS, try your favourite search engine to see what they mean (lenses containing these letters will cost significantly more than most). A Pentax-M 50/2 means it has a 50mm focal length and maximum aperture of 1:2. For info about what letter abbriviations like M means, try your favourite search engine again. Good luck and I hope this was helpful.