Question on the EF vs EF-s lens

[Blackey Cole]

I understand the designation of the EF-S lens are for the 1.6 crop factor lens from Canon only and not usable on the full frame bodies. I always thought that the xx-xxmm naming was adjusted for the smaller crop frame sensors. So that a 17-55 was a 17-55mm on the 1.6 sensor and a EF 17-55mm was actually a 27.2mm-88mm lens. I am reading that the EF-S 17mm-55mm would be the focal size of an EF-17-55MM lens and both would actually be a 27-88mm lens on the 1.6 cropped sensor bodies.

This doesn't make sense unless Canon had multiple sub-full frame sensor bodies that accept the EF-S lenses but I am not aware of any. Are they and is that why the lens are all marked relative to a FF 35mm size sensor.

[Jim G]

What?

17mm is 17mm. You just see the center part of it on a cropped sensor, missing the edges. There is no changing of the system for EF-S lenses, 17mm is still recorded as 17mm.

The field of view will approximate what a longer focal length looks like on full-frame. This doesn't mean that the lens takes on any characteristics of the longer lens. Depth of field etc. remain the same as whatever mm the lens is.

The field of view of a 17-55 on an EF-S body looks like a 27-88mm would on a full frame body.

There are multiple sub-full-frame sensors - APS-C and APS-H, 1.6x and 1.3x respectively.

[toxic]

This doesn't make sense unless Canon had multiple sub-full frame sensor bodies that accept the EF-S lenses but I am not aware of any. Are they and is that why the lens are all marked relative to a FF 35mm size sensor.

Uh...300-450D, 1000D, and 20-50D? The lenses are 17-whatever mm. I don't know what you mean by "marked relative to...", if you've never shot film and don't plan on shooting with a 35mm sensor, forget about the "equivalents." 17mm is wide, and 55 is short telephoto.

The purpose of the EF-S lenses is just to make cheaper and lighter lenses since a whole bunch of glass is wasted when putting an EF lens on an APS-C camera.

[ahendarman]

Read the FAQ, http://photography-on-the.net/forum/showthread.php?p=2271693#post2271693

[mrkgoo]

there's nothing to 'account' for because focal length never changes, regardless if what sensor you put behind it. Field of view changes, and it is related to focal length but it is not focal length.

I don't know why I come into these threads. I think I must enjoy it! But yes this question is asked by all new visitors. Probably around once every couple of days.

[mpistone]

I'm sure the FAQ explains it, but here's my take.
Suppose there was an 18-55mm EF lens (NOT EF-S). It would act just the same as an 18-55mm EF-S lens on a crop-body. But any regular (not EF-S) lens being used on a crop-body is under-utilized, because it's capable of imaging a 35mm-wide image onto the sensor, but your cropped sensor is too small so the edges of the scene are lost.
EF-S lenses are still the same focal length, but they anticipate a smaller sensor so they aren't built to cover a full-frame sensor with the image.

[mrkgoo]

I'm sure the FAQ explains it, but here's my take.
Suppose there was an 18-55mm EF lens (NOT EF-S). It would act just the same as an 18-55mm EF-S lens on a crop-body. But any regular (not EF-S) lens being used on a crop-body is under-utilized, because it's capable of imaging a 35mm-wide image onto the sensor, but your cropped sensor is too small so the edges of the scene are lost.
EF-S lenses are still the same focal length, but they anticipate a smaller sensor so they aren't built to cover a full-frame sensor with the image.

I wouldn't say under utilized exactly. Typically the centre portion of an image circle has the highest quality. A crop camera will use this highest quality portion to good effect because typically they can have higher pixel density. Corner sharpness would be higher than a full frame maybe even with an equivalent fog, there would be less aberrations and less vignetting.

[SkipD]

The "crop factor" is a reference number that relates to the difference in sensor size between cameras like the Rebel XTi and a 35mm film (or a so-called "full-frame" digital) camera. Let me list the facts:

35mm film cameras have a film frame size of approximately 24x36mm, while the Rebel XTi has an APS-C sized sensor, measuring approximately 15x22.5mm.

When camera manufacturers started designing digital SLR's, they decided that they should be about the same physical size of their 35mm SLR's. For that reason, they concluded that they could use the line of lenses they already had for their 35mm SLR's on the new digital SLR's.

All lenses designed for 35mm cameras project an image circle onto the film that covers the 24x36mm rectangle. The 35mm camera records the portion of that image circle that is defined by the opening behind the shutter for the film (24x36mm in size). A digital SLR with an APS-C sized sensor only records the smaller area (approximately 15x22mm) of the image circle projected by the lens.

When you put a 100mm lens on a 35mm camera and take a photograph, then put the same lens on a DSLR such as the Rebel XTi and take the same photograph - same subject, same position for the camera - with the same lens, and then enlarge both photographs to the same size print (4x6 inches, for example), it will appear as though the photo from the Rebel XTi was taken with a longer lens. That is because the image recorded by the Rebel XTi was of a SMALLER PORTION of the image circle projected by the lens - cropped, if you will - compared to the image recorded by the 35mm camera.

The special lenses for the Rebel XTi (and other Canon APS-C cameras) are called the EF-S series. These project a smaller image circle, making the lenses less expensive to design and produce in wide-angle and extreme wide-angle formats. However, an EF-S lens set to 20mm will produce the exact same image as an EF lens set to 20mm. Focal length is focal length, period.

Now to the primary point that I want to make: A lens NEVER CHANGES ANYTHING when you put it on different format cameras. Focal length never changes. Aperture range never changes. The only thing that would change is the apparent field of view, and that change is not a function of the lens but it is a function of the size of the sensor or film that will record the image.

The "crop factor" has only one valid use. Here's an example: Joe took a photo of Mount Rushmore with a 35mm camera from a particular place using a 200mm lens. You want to replicate that photo with your Rebel XTi. What focal length do you need to do that from the same location that he took his photo? Divide the 200mm by 1.6 and you get the answer - 125mm. The "crop factor" is a REFERENCE between the two formats that lets you compare the field of view of particular focal lengths between the two formats.

The "crop factor" (related to using lenses essentially designed for 35mm SLR cameras) is always given assuming that the 35mm format (24x36mm) is the reference master. Something to realize, though, is that the 35mm film format is not, never has been, and never will be the "master" format against which all other camera formats are referenced. It is simply the format of the cameras that have also evolved into today's commonly used digital SLRs.

[SkipD]

The APS-C format cameras such as Canon’s Digital Rebel series and their 20D through 50D series have a smaller sensor than a 35mm film frame. If you limit the lens selection to those lenses designed to fill a 35mm frame (such as Canon’s EF series lenses), you will find that there are no ultra-wide-angle lenses for the APS-C camera.

The shortest zoom lens focal length in the EF lens family is 16mm. There are two primes that are a bit shorter, and one of those is a "fisheye" lens. NONE of these lenses are what the average photographer would call "affordable".

To design an ultra-super-wide-angle lens such as a 10mm (non-fisheye) lens for a 35mm film camera is a VERY expensive proposition, which is why there are none.

By making some changes to the design criteria - reducing the "film" area to be covered by the lens, and allowing the lens to project deeper into the mirror box (moving the rear element of the lens closer to the "film"), it becomes much more economically possible to design lenses for the task. Thus, the EF-S family of lenses was born, the “S” standing for Short back focus.

The EF-S lens mount is purposely designed to be different from the standard EF lens mount so that you cannot mount the EF-S lenses on cameras that were not specifically designed for them. If you modified the mount of an EF-S lens to be able to put it on a 35mm film camera, there would be a high probability that the mirror would crash into the rear element of the lens at certain focal lengths (the mirrors in the APS-C cameras are significantly smaller than those in 35mm cameras and “full-frame” DSLR’s). Also, the 35mm film frame would have a dark circle around the edges and the image would be inside the circle (known as vignetting).

Now that you have some understanding of what the EF-S lenses are all about, you need to understand that focal length is focal length is focal length. An EF 50mm prime lens designed for a 35mm camera, when used on an APS-C camera, will provide you with EXACTLY the same image size as an EF-S 18-55 lens set to 50mm. If you could cobble together a mount for a Hasselblad 50mm lens and use it on a 20D, you would again have the same size image as a result.

NO CHARACTERISTIC of any lens changes when you mount it on different format cameras. Focal length (or focal length range for zooms) never changes. Aperture range never changes. The only thing that would change is the apparent field of view, and that change is not a function of the lens but it is a function of the size of the sensor or film that will record the image. There are some changes to the depth of field, but that is also a function of the size of the sensor or film that records the image.

EF-S lenses, by the way, will only fit on the Digital Rebel series cameras (300D, 350D, 400D, 450D, and 1000D), the 20D, 30D, 40D, and the 50D as of this writing.