From essentially the introduction of the EOS camera system in 1987 through to 2003 Canon standardized on a single lens mount system for all of their SLR cameras - the EF (electrofocus) lens mount. So throughout this time there was no possible source of confusion, since all EF lenses made by Canon and other lensmakers will physically fit all Canon EOS cameras.

However, in 2003 Canon introduced a new digital camera, the consumer-oriented EOS 300D/Digital Rebel/Kiss Digital camera, which sported a new lens mount design dubbed EF-S. All consumer to midrange digital EOS cameras released since have been both EF and EF-S compatible. For reasons explained in a moment, no film camera has ever been EF-S compatible.

So it’s important to remember that digital camera bodies with EF-S lens mounts are totally compatible with all regular EF lenses. However an EF-S lens can fit only EF-S compatible cameras and no others. (unless the lens is altered - see the section on hacking below).

EF-S bodies have small mirror boxes - roughly 2/3 the size of a regular EOS camera (also known as a 1.6x cropping factor) - because they use image sensors which are smaller in area than 35mm film. They, and APS cameras which similarly used small imaging areas, are thus often called subframe cameras. Cameras which use 35mm film or which use large sensors that are the same size as a frame of 35mm film are commonly called full frame cameras these days.

EF-S cameras thus support lenses with a shorter back focus distance than EF lenses, because the mirror swings further back. This is where the “S” comes from - EF-S lenses have shorter back focus distances. (ie: the back part of the lens can get physically closer to the image sensor since the mirror is smaller) Having a shorter back focus distance allows Canon to produce cheaper wide-angle lenses that work with the smaller image format of a subframe digital SLR, since it’s optically very challenging to create a wide angle lens with a long back focus distance.

Canon have a small but growing series of EF-S lenses available, ranging from inexpensive kit lenses to very good high-quality lenses with image stabilization. There’s even a very interesting 60mm macro lens with an EF-S mount. The super wide angle EF-S 10-22mm 3.5-4.5 USM (roughly 16-35mm coverage if it were full frame) is particularly well regarded, as is the EF-S 17-55 2.8 IS USM, which is an L lens in all but build quality and name.

The main issue to be concerned about with EF-S is the future value of the lenses. Right now full-frame image sensors are extremely expensive to make, which is why nearly all digital SLRs out there have image sensors smaller than that of a frame of 35mm film. But in the future it’s likely that prices on such sensors will drop, at which time full-frame digital SLRs will become more affordable and thus EF-S lenses will no longer be of use except on pre-existing cameras. The two questions are - how long will this take and will you be able to get good use of your investment in EF-S lenses before this occurs? The first nobody knows the answer to, and the second can only be answered by you. For the time being it seems likely that it’ll be some years before affordable full-frame sensors are ubiquitous, so EF-S lenses aren’t necessarily a bad idea, assuming you aren’t planning on upgrading to full-frame as soon as you can.

Can my camera take EF-S lenses?

Any Canon EOS camera with a red dot on the lens mount can take EF lenses. Any Canon EOS camera with both a red dot and a white square on the lens mount can take both EF lenses and EF-S lenses.

What you see is what you get

This whole discussion about focal lengths and EF versus EF-S can be very confusing if you’re a beginner. The key thing to remember if you’re just starting out is what you see is what you get. When you look through the viewfinder you’re going to see pretty well exactly what will be in the photograph - minus a tiny little strip along each edge on consumer cameras. So there’s no need to worry about mathematical calculations and whatnot to figure out what will be in your picture and what won’t.

The only time the whole cropping factor is really an issue is when you’re comparing how much of a scene (coverage angle) is taken in by an EF lens on a full-frame camera, compared to an EF lens on a cropped body, or compared to an EF-S lens on an EF-S compatible camera. For guidance on this, take a look at my Complete EOS Lookup Page. It allows you to compare multiple lenses to see how much of a scene each lens can take in when used on cropped or full frame bodies.

Distinguishing EF from EF-S

Regular EF bodies and lenses use raised red dots for aligning lens and body. EF-S bodies have a white square as well, which must be lined up to the white square on EF-S lenses. Interestingly enough, EF-S lenses also have rubber rings around the end which press up against the interior of the camera body. This is not as sophisticated as the weather seals used in L series lenses, but presumably helps reduce dust levels inside the camera slightly. Also, according to Canon, this ring minimises damage if you were to try and mount an EF-S lens on a EF-only camera.

Full frame equivalent

You’ll sometimes see phrases such as “full frame equivalent” or “35mm equivalent” in conjunction with lenses designed for subframe sensors, such as EF-S lenses. Properly speaking these are not equivalent values for a variety of technical reasons, but they are nonetheless useful points of reference. For instance, a 60mm EF-S lens has the same field of view as a 96mm lens on a 35mm film camera. So sometimes 60mm EF-S lenses are said to have a full-frame equivalent field of view of 96mm.

EF-S lenses and film cameras

No EOS 35mm film camera has or is ever going to use an imaging area smaller than 36x24mm, so no EOS 35mm film camera will ever be able to support EF-S lenses. Even if you were to attach one somehow the mirror would collide with the back of the lens when it flipped up, and the lens would vignette severely because EF-S lenses can’t image an image circle as large as 35mm film. In theory Canon might be able to build an APS camera with support for EF-S lenses, since APS film uses a surface area smaller than that of 35mm film, but since APS is a commercially moribund format that isn’t ever going to happen. So the only cameras you’ll ever see with EF-S lens mounts are going to be digital.

Hacking EF-S lenses

While EF-S lenses are not designed to fit EF-only cameras, it is possible to saw off or, in some cases, unscrew the back of an EF-S lens and attach the lens to any EOS camera. The problem is that if you put a modified EF-S lens on an EOS camera with a standard sized mirror box then the mirror will flip up and smash into the lens when you try to take a photo. So modified EF-S lenses will only work with older EOS digital cameras with cropping factors of 1.6x that predate the EF-S system: namely the EOS D30, D60 and 10D. Even then there are risks. For example, at its widest position the EF-S 10-22mm lens collides with the mirror in non-EF-S compatible cameras, even those with small mirror boxes. (for this reason it’s unwise to use a modified EF-S lens on a 1.3x digital camera such as the 1D) Messing around in this fashion is fun - I’ve done it myself and it works well - but obviously invalidates your Canon warranty and runs the risk of breaking something.