Why are there no long EF-S lenses?

[Elliotte Rusty Harold]

Consider an EF lens such as a Canon 400mm f/4 DO on an crop factor camera such as a Rebel or a 7D. My understanding is that the lens produces an image circle substantially larger than the sensor. (99% sure of that.) The extra light is effectively blocked somewhere in the camera. Not deliberately, but it just doesn't hit the sensor at all.

Is the light path such that one could place black tape around the edges of the lens aperture without effecting the final image? I.e. could we block off the light that won't be used anyway on the lens aperture? (and if not, why not?)

And if that is true, why does nobody make a long lens for EF-S cameras that produces the same light with a smaller size and aperture? I.e. why is there extra glass and weight we don't need? Is it simply that there's not a market for long lenses only for crop factor cameras? Or is there some part of the physics or lens design I'm misunderstanding?

There are EF-S lenses with smaller image circles in the under 100mm range. Why is there nothing in the 100mm+ range?

[Graeme Sheppard]

I'm pretty you got it right - market forces (or at least perceived market forces).
There's actually a lot of movement in lens design and production now as far as I can tell, so companies need to prioritise, I reckon.

[Roger Clark]

Consider an EF lens such as a Canon 400mm f/4 DO on an crop factor camera such as a Rebel or a 7D. My understanding is that the lens produces an image circle substantially larger than the sensor. (99% sure of that.) The extra light is effectively blocked somewhere in the camera. Not deliberately, but it just doesn't hit the sensor at all.

Is the light path such that one could place black tape around the edges of the lens aperture without effecting the final image? I.e. could we block off the light that won't be used anyway on the lens aperture? (and if not, why not?)

Hi Elliot,
You are correct that the EF lenses have a larger image circle. One can not block light near the lens because that is the out of focus light. It would be more like the effect of closing the aperture: you are blocking light from the edges of the lens so reducing light to the center of the field of view as well as the edge.



And if that is true, why does nobody make a long lens for EF-S cameras that produces the same light with a smaller size and aperture? I.e. why is there extra glass and weight we don't need? Is it simply that there's not a market for long lenses only for crop factor cameras? Or is there some part of the physics or lens design I'm misunderstanding?

There are EF-S lenses with smaller image circles in the under 100mm range. Why is there nothing in the 100mm+ range?

At longer focal lengths, it is easier to design lenses that cover the full 35 mm frame. Thus, it would actually cost more to produce two separate lines of lenses. And a cropped sensor uses the central part of the image circle where image quality is better. Producing an EFS lens to be better than that of the EF lens would be diminishing returns, especially to make it cheaper (which is the general idea of EFS lenses).

At shorter focal lengths it is more difficult to make a lens with a large image circle, so a simpler design can be used for EFS and the smaller image circle, reducing cost and weight.

Roger

[Andrew Merwin]

Wikipedia (http://en.wikipedia.org/wiki/Canon_EF-S_lens_mount) has a good discussion of EF-S lens.

[Don Lacy]

Hi Roger, Just curious isn't the aperture of the lens determine by the size of the front element the larger the element the faster the lens, in simple terms layman terms that is
RThanks, Don

[Roger Clark]

Hi Roger, Just curious isn't the aperture of the lens determine by the size of the front element the larger the element the faster the lens, in simple terms layman terms that is
RThanks, Don

Hi Don,

For a simple lens what you say is true, but camera lenses are anything but simple, especially wide angle lenses. The front element must field the light from many degrees off-axis, so must be much larger than the aperture. The iris diaphragm best defines the real aperture. More technically, the aperture is defined by the entrance pupil, which might be inside a piece of glass, or even outside the lens. In those cases, the iris diaphragm is placed at the next best location. For telephoto lenses, the front element is very close to describing the true aperture.

Roger