Doug Kerr
Well-known member
In an interchangeable lens camera system, the term flange focal distance (often also called the back flange distance, register distance, and several other terms) refers to the distance between the reference contact surface of the mount and the focal plane (where the sensor resides).
In the Canon EOS R system, that distance is 20 mm, significantly less than in the familiar EOS (EF) mount, where it is 44 mm. This is of course made possible by the fact that in the non-SLR formulation of the EOS R there is no mirror between the lens and the sensor. Thus, we can allow the mechanical aft end of the lens to reside closer to the sensor, and a closer mount is an obvious corollary.
One obvious advantage of this is that, "all other things being equal", it allows for the body to be made thinner, perhaps by 24 mm (almost an inch) or so.
The Canon white paper on the EOS R system discusses at some length other advantages of this change in flange focal distance with respect to lens design. The description nicely avoids mathematics or discussion of actual optical theory principles, and while it sounds facile, I have not yet been had time to wrap my engineer's head around it. Nevertheless, I will tell the story essentially as it is presented in the white paper.
The author points out that the various schemes used in lens designs to minimize certain aberrations can be made "easier" if we:
• Move the last element of the lens closer to the sensor, but this would result in some of the rays traveling toward the outermost parts of the frame striking the sensor at a more oblique angle, which is not desirable. So to overcome that, we
• Make the last element larger. Then, many of the rays destined for the outermost parts of the frame come thorough places in the outer regions of the lens, and thus striking the sensor at a less oblique angle.
But does this mean that the lens is now heavier and more costly because of the increase in size of the last element? Evidently not, since (according to the example sketches in the white paper) we can now make the first element smaller (simplistically, having turned the original lens recipe around).
Interestingly enough, as this story is told (second- or third hand) in some comments about the new camera system, "The smaller flange focal distance allows us to make the last element of the lens larger, which is advantageous."
As well as I now understand all this (not too well, I'm afraid), that is sort of backward. We are now able to make the last element closer to the lens, but to benefit from that we need to make it larger.
Over the next day or so I'll try and do some bush-league ray tracing which will then (hopefully) prepare me to construct a more fluent explanation of this matter.
Best regards,
Doug
In the Canon EOS R system, that distance is 20 mm, significantly less than in the familiar EOS (EF) mount, where it is 44 mm. This is of course made possible by the fact that in the non-SLR formulation of the EOS R there is no mirror between the lens and the sensor. Thus, we can allow the mechanical aft end of the lens to reside closer to the sensor, and a closer mount is an obvious corollary.
One obvious advantage of this is that, "all other things being equal", it allows for the body to be made thinner, perhaps by 24 mm (almost an inch) or so.
It is worthwhile noting here, so as perhaps to avoid anybody later barking up a wrong tree, that the throat diameter of the new mount is identical to that of the EF mount.
The Canon white paper on the EOS R system discusses at some length other advantages of this change in flange focal distance with respect to lens design. The description nicely avoids mathematics or discussion of actual optical theory principles, and while it sounds facile, I have not yet been had time to wrap my engineer's head around it. Nevertheless, I will tell the story essentially as it is presented in the white paper.
The author points out that the various schemes used in lens designs to minimize certain aberrations can be made "easier" if we:
• Move the last element of the lens closer to the sensor, but this would result in some of the rays traveling toward the outermost parts of the frame striking the sensor at a more oblique angle, which is not desirable. So to overcome that, we
• Make the last element larger. Then, many of the rays destined for the outermost parts of the frame come thorough places in the outer regions of the lens, and thus striking the sensor at a less oblique angle.
But does this mean that the lens is now heavier and more costly because of the increase in size of the last element? Evidently not, since (according to the example sketches in the white paper) we can now make the first element smaller (simplistically, having turned the original lens recipe around).
Interestingly enough, as this story is told (second- or third hand) in some comments about the new camera system, "The smaller flange focal distance allows us to make the last element of the lens larger, which is advantageous."
As well as I now understand all this (not too well, I'm afraid), that is sort of backward. We are now able to make the last element closer to the lens, but to benefit from that we need to make it larger.
Over the next day or so I'll try and do some bush-league ray tracing which will then (hopefully) prepare me to construct a more fluent explanation of this matter.
Best regards,
Doug