Doug Kerr
Well-known member
Most here are familiar with the Canon EF-S lens series. But sometimes the entire story may not be grasped clearly, or correctly. I thought I would review the topic here.
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The Canon EF-S lens series is a subgroup of the EF series. EF-S lenses are only intended for use with, will only work on, and will only physically mount on Canon EOS digital cameras having the smallest of the three current sensor sizes, the one often identified as "APS-C" (or "1.6x").
The prominent differences in their specifications compared to EF-series lenses proper are:
• The minimum diameter of their image circle (the boundary of the image they form on the focal plane) is smaller.
• Their minimum back focus distance (the distance from the rearmost point on the lens to the focal plane, for focus at infinity) is shorter. (In fact, the "S" in EF-S is said to be evocative of "short back focus".)
We can easily realize the significance of the former, but what about the minimum back focus distance?
The smaller the back focus distance, the more the lens protrudes into the camera. Lenses with a short focal length, if designed in a "classical" way, will have a small back focus distance, and thus a significant protuberance.
In an SLR camera, we cannot have the lens protrude an arbitrary distance into the camera, as beyond a certain point it will interfere with the reflex mirror as it swings up (or maybe even with in in its normal position).
Thus, in the design of short focal length lenses, special design techniques (such as the so-called "retrofocus" design) must be employed to increase the back focus distance (and thus reduce the protuberance of the lens into the camera to an acceptable amount). There are penalties in so doing - a necessary compromise.
For a smaller format camera, the reflex mirror is typically smaller, and thus a greater protuberance of the lens can be accommodated; that is, a shorter back focus distance is allowed.
Thus, in a lens intended only for such use (such as an EF-S lens), the degree to which special techniques of design to increase the back focus distance are applied can be reduced, and their bad side effects minimized.
We might hope that both of these unique, "relaxed" specifications lead to lenses that, for given optical parameters and a given performance bogey, will be less expensive, lighter, and/or smaller than if they were suited for general application to the range of EOS cameras.
Is the mount different
Basically, no.
However, EF-S lenses are designed with a rear protuberance (partly comprising a rubber collar) that will interfere with a feature on the mirror box of cameras with the two larger format sizes ("APS-H"/"1.3x" and "full frame") so the lens cannot be mounted there. (The use of the rubber part makes the attempted but failed penetration benign.)
Can we cheat?
Suppose that for certain EF-S lenses, we could excise the rubber collar (and maybe also machine off some other material) so that the lens would now physically mount on a larger format camera. What then?
Well, in general:
• There would be the risk, if not the certainty, that the reflex mirror would strike the rear protuberance of the lens proper.
• The reduced image circle would mean that there would be substantial vignetting of the images.
Equivalent focal length
Under the equivalent focal length concept, we express the field of view of a certain lens, with a certain focal length, when used on a camera with a certain format size, by stating the focal length of lens that, on a full-frame 35-mm camera, would give that same field of view. This value is called (to give its full name) the "full-frame 35-mm equivalent focal length" of this lens when used on a camera with "this" format size.
It is not a focal length of the lens, under any situation. The focal length of a lens is a basic optical parameter. It does not depend on what type of camera (if any) the lens is mounted to. The terms in which it is expressed are not in any way dependent on any presumption as to what kind of camera it might be used on.
Ef-S lenses, like all EF-family lenses, are marked with their (nominal) focal length (what some people would call the "real focal length", but of course this is the only focal length a lens has).
Some people believe that, since EF-S lenses are only intended for, and suitable for, operation on cameras with a very small range of sensor sizes, smaller than full-frame 35-mm size, they should be marked with (or designated by) their "full-frame 35-mm equivalent focal length". Thus, these people say, we could most directly recognize the field of view the lens would give under the familiar metric of full-frame 35-mm equivalent focal length.
But doing so would cause at least these two problems:
• Many properties of a lens in use (such as depth of field) depend on the focal length, not any "equivalent focal length". If our lenses were marked with an equivalent focal length, predicated on a certain sensor size, we would have to convert that to the (real) focal length before we could use it in depth of field calculations and the like.
• Suppose we had an EF-S lens with a focal length of 50 mm, and (under the doctrine suggested by these commenters) it was marked "80 mm". We would get familiar with its behavior with respect to field of view, and when it would be an appropriate choice for a certain photographic task. Now we buy an EF lens with a focal length of 80 mm (and marked "80 mm"). It would have a much different field of view that the first lens, even though they bore the same marking. It would be as if some of the wrench sockets in our toolbox were marked with the size across the flats, and others with the size across the points.
Best regards,
Doug
***********
The Canon EF-S lens series is a subgroup of the EF series. EF-S lenses are only intended for use with, will only work on, and will only physically mount on Canon EOS digital cameras having the smallest of the three current sensor sizes, the one often identified as "APS-C" (or "1.6x").
The prominent differences in their specifications compared to EF-series lenses proper are:
• The minimum diameter of their image circle (the boundary of the image they form on the focal plane) is smaller.
• Their minimum back focus distance (the distance from the rearmost point on the lens to the focal plane, for focus at infinity) is shorter. (In fact, the "S" in EF-S is said to be evocative of "short back focus".)
We can easily realize the significance of the former, but what about the minimum back focus distance?
The smaller the back focus distance, the more the lens protrudes into the camera. Lenses with a short focal length, if designed in a "classical" way, will have a small back focus distance, and thus a significant protuberance.
In an SLR camera, we cannot have the lens protrude an arbitrary distance into the camera, as beyond a certain point it will interfere with the reflex mirror as it swings up (or maybe even with in in its normal position).
Thus, in the design of short focal length lenses, special design techniques (such as the so-called "retrofocus" design) must be employed to increase the back focus distance (and thus reduce the protuberance of the lens into the camera to an acceptable amount). There are penalties in so doing - a necessary compromise.
For a smaller format camera, the reflex mirror is typically smaller, and thus a greater protuberance of the lens can be accommodated; that is, a shorter back focus distance is allowed.
Thus, in a lens intended only for such use (such as an EF-S lens), the degree to which special techniques of design to increase the back focus distance are applied can be reduced, and their bad side effects minimized.
We might hope that both of these unique, "relaxed" specifications lead to lenses that, for given optical parameters and a given performance bogey, will be less expensive, lighter, and/or smaller than if they were suited for general application to the range of EOS cameras.
Is the mount different
Basically, no.
However, EF-S lenses are designed with a rear protuberance (partly comprising a rubber collar) that will interfere with a feature on the mirror box of cameras with the two larger format sizes ("APS-H"/"1.3x" and "full frame") so the lens cannot be mounted there. (The use of the rubber part makes the attempted but failed penetration benign.)
Can we cheat?
Suppose that for certain EF-S lenses, we could excise the rubber collar (and maybe also machine off some other material) so that the lens would now physically mount on a larger format camera. What then?
Well, in general:
• There would be the risk, if not the certainty, that the reflex mirror would strike the rear protuberance of the lens proper.
• The reduced image circle would mean that there would be substantial vignetting of the images.
Equivalent focal length
Under the equivalent focal length concept, we express the field of view of a certain lens, with a certain focal length, when used on a camera with a certain format size, by stating the focal length of lens that, on a full-frame 35-mm camera, would give that same field of view. This value is called (to give its full name) the "full-frame 35-mm equivalent focal length" of this lens when used on a camera with "this" format size.
It is not a focal length of the lens, under any situation. The focal length of a lens is a basic optical parameter. It does not depend on what type of camera (if any) the lens is mounted to. The terms in which it is expressed are not in any way dependent on any presumption as to what kind of camera it might be used on.
Ef-S lenses, like all EF-family lenses, are marked with their (nominal) focal length (what some people would call the "real focal length", but of course this is the only focal length a lens has).
Some people believe that, since EF-S lenses are only intended for, and suitable for, operation on cameras with a very small range of sensor sizes, smaller than full-frame 35-mm size, they should be marked with (or designated by) their "full-frame 35-mm equivalent focal length". Thus, these people say, we could most directly recognize the field of view the lens would give under the familiar metric of full-frame 35-mm equivalent focal length.
But doing so would cause at least these two problems:
• Many properties of a lens in use (such as depth of field) depend on the focal length, not any "equivalent focal length". If our lenses were marked with an equivalent focal length, predicated on a certain sensor size, we would have to convert that to the (real) focal length before we could use it in depth of field calculations and the like.
• Suppose we had an EF-S lens with a focal length of 50 mm, and (under the doctrine suggested by these commenters) it was marked "80 mm". We would get familiar with its behavior with respect to field of view, and when it would be an appropriate choice for a certain photographic task. Now we buy an EF lens with a focal length of 80 mm (and marked "80 mm"). It would have a much different field of view that the first lens, even though they bore the same marking. It would be as if some of the wrench sockets in our toolbox were marked with the size across the flats, and others with the size across the points.
Best regards,
Doug