Understanding the markings on lenses can be confusing for anyone especially as manufacturer's will often use different terms for the same feature. Also the increasing popularity of Digital SLR's now means that many more people are making the leap from compact cameras to SLR's where lens terminology also differs. So I thought I would produce this guide to lens markings. To begin with, lets take a look at two sets of markings you will find on all lenses.

70-300mm.
Compact camera's usually measure their power as, for example, 3x zoom. Although this tells you that, at maximum zoom, the lens would be able to make something appear 3x closer it doesn't tell you about what field of view the lens can see, in other words 3x what? SLR lenses mark their power as a focal length measured in mm. The human eye see's the equivalent field of view as that of a 50mm lens on a 35mm camera or full frame DSLR, therefore a number smaller than 50 indicates that the lens will see more, i.e. is wider, than what the human eye would see. On a 35mm camera or full frame DSLR a wide angle lens would be one that offers a focal length of around 24mm, however on a non full frame DSLR a wide angle lens would be one that offered a focal length of around 18mm. The reason for this is that the smaller sensor on these camera's has the effect of multiplying the focal length typically by a factor of either 1.5x or 1.6x. Therefore a 24mm lens on these cameras would give the field of view of a 38mm lens on a 35mm camera.

f/4 - 5.6.
This is the maximum aperture achievable with that lens. The smaller the number, the larger the aperture which allows for a shallower depth of field and also for the lens to work better in low light. If a lens show two numbers like f/4 - 5.6 than this means that it is a zoom lens (e.g. a 70-300mm lens) and that at the short end of the zoom (70mm) the maximum aperture is f/4 but at the long end of the zoom (300mm) the maximum aperture drops to f/5.6. If a lens only displays one number, e.g. f/2.8 then it means that it maintains that maximum aperture across its focal range. These lenses will always cost more and typically perform better than lenses that do not have a fixed aperture.

Now lets take a look at markings that are specific to certain manufacturer's.

 

 

 

IS IS Stands for Image Stabilization. Image stabilization works by having a correction lens inside the barrel of the lens that corrects for any camera shake. It's not a guarantee to avoid camera shake but typically it allows you to shoot hand held by up to 2 or 3 stops slower than you would normally be able to.	USM This is Canon's Ultra Sonic Motor and means that the lens is quiet and fast at auto focusing
L This is Canon's professional range of lenses which offer superior optical performance. These lenses are often white in colour but also feature a red band around the top of the lens.

 

APO lenses use a special type of glass that minimize chromatic aberrations called SLD or Special Low Dispersion. APO lenses also often use ELD or Extraordinary Low Dispersion glass which also combats chromatic aberrations and aids in the capturing of sharp images by ensuring all light entering the lens converges to the same point. APO Zoom Macros offer up to 1:2 magnification in the telephoto range with instant shifting from normal to macro shooting. APO Tele-Macros offer 1:3 image size close focus, long thought to be impossible for a telephoto lens — and are no larger than a fixed focal lens of the same class.	Excellence. These professional type lenses feature the distinctive EX finish and EX logo on the lens barrel.
Aspherical lenses. A spherical lens element can mean that additional elements are required which increases the weight and can decrease the performance. Aspherical lenses avoid this and can reduce problems like lens flare and edge distortion that can often found with wide angle lenses and zooms	HyperSonic Motor. Enables virtually silent, highly responsive auto and manual focusing, as well as manual focus only override. UC Ultra Compact. The smallest, lightest lens of its type available. DL Deluxe. DL lenses are full feature lenses in spite of their modest price. Like other Sigma lenses, they are supplied with a custom lens hood and feature half-stop increments on manual aperture settings, depth of field scale, distance scale, infrared correction mark. DF Dual Focus. It is easy to hold the lens, since the focusing ring does not rotate during auto-focusing and yet provides adequate focusing torque of the focusing ring during manual focusing of the lens. HF Helical focus. This system of the lens eliminates front lens rotation, thus allowing the use of a Perfect Hood and easy to use polarising filters.
Conv. (APO Teleconverter EX) This lens can be used with the APO Teleconverter EX. It can increase the focal length and will interface with the camera's AE (automatic exposure) function.	Internal Focusing. Typically when a lens focuses the entire barrel moves in and out, however some lenses are capable of internal focusing where elements inside the lens move rather than the barrel itself. It is sometimes thought that internal focusing lenses up less susceptible to dust than the 'normal' focusing counterparts.
DC Lens. These are special lenses designed so that are designed to only be used with non full frame Digital SLR's. If used on a full frame Digital SLR or 35mm camera vignetting will be visible in the corners of the frame.	Optical Stabilizer (OS). This function utilizes a built-in mechanism that compensates for camera shake. It dramatically expands photographic possibilities by alleviating camera movement when shooting by hand held camera.
DG Lens. These are large-aperture lenses with wide angles and short minimum focusing distances. With an abundance of peripheral illumination, they are ideal lenses for Digital SLR cameras.	Rear Focusing. This is used on super wide angle lenses where only the rear element of the lens is moved. This helps to keep lens sizes down and reduces the focusing time..

 

A lens with this mark uses AD (Anomalous Dispersion) element(s)	A lens with this mark uses LD (Low Dispersion) element(s)
A spherical lens element can mean that additional elements are required which increases the weight and can decrease the performance. Aspherical lenses avoid this and can reduce problems like lens flare and edge distortion that can often found with wide angle lenses and zooms	Tamron SP-series lenses feature high-performance specs.
Typically when a lens focuses the entire barrel moves in and out, however some lenses are capable of internal focusing where elements inside the lens move rather than the barrel itself. This allows for faster focusing and it is also sometimes thought that internal focusing lenses up less susceptible to dust than the 'normal' focusing counterparts, although this point is somewhat debatable.	A lens with this mark uses a zoom-lock mechanism. When a lens is pointed downwards e.g. for macro photography, or upwards, e.g. astro photography, gravity can cause the lens to extend or compress. Zoom lock prevents this from happening and is also useful for when carrying long lens around.
A lens with this mark uses LD-Hybrid aspherical element(s)

 

CRC Close Range Correction. CRC lenses have floating elements that move in relation to others to ensure that objects remain sharp and in focus even when close to the lens. This is useful for macro lenses and some wide angle lenses.	IF Typically when a lens focuses the entire barrel moves in and out, however some lenses are capable of internal focusing where elements inside the lens move rather than the barrel itself. This allows for faster focusing and it is also sometimes thought that internal focusing lenses up less susceptible to dust than the 'normal' focusing counterparts, although this point is somewhat debatable
DX These are special lenses designed so that are designed to only be used with non full frame Digital SLR's. If used on a full frame Digital SLR or 35mm camera vignetting will be visible in the corners of the frame.	RF Similar to IF except that only the rear element moves. This is used on super wide angle lenses where only the rear element of the lens is moved. This helps to keep lens sizes down and reduces focusing time.
ED Extra Low Dispersion. This helps reduce chromatic aberrations which are seen as green and purple fringes around the edges of the subject.	VR Vibration Reduction. This function utilizes a built-in mechanism that compensates for camera shake. It dramatically expands photographic possibilities by alleviating camera movement when shooting by hand held camera.