Help me understand something about tilt shift lenses

[Kit Lens Jockey]

Regarding the shift function of tilt shift lenses... Because of the way the lens can be shifted, does this mean that they actually project an image circle that's larger than all of the other, normal full frame lenses, and when you shift the lens, you're in effect capturing a different part of this one huge image circle? Or is something more complex going on, like the shift is happening somewhere between the groups of optics, so the image circle is only full frame sized, but when you shift, you're somehow shifting the direction that the lens pulls in light from?

Just curious.

 

[Zeidora]

Yes, large image circle. That's also why image quality goes down as you go towards the periphery of the oversized image circle. Same is true for LF lenses. That's also why the same focal length my not work on larger formats. A 4x5" lens with some movements may not even cover 8x10".

 

[Kit Lens Jockey]

Ah, so is this why the max aperture ratings of tilt shift lenses are pretty underwhelming? A 17mm f4 prime is pretty lame, but perhaps making this lens in a tilt shift at f2.8 would result in a gigantic lens due to the large image it has to project?

 

[Loswr]

Standard EF iamge circle is ~43mm. The image circle for the current L-series TS-E lenses is ~67mm (a bit larger than the older TS-E lenses).

More details from Canon here.

 

[Darkly]

Ah, so is this why the max aperture ratings of tilt shift lenses are pretty underwhelming? A 17mm f4 prime is pretty lame, but perhaps making this lens in a tilt shift at f2.8 would result in a gigantic lens due to the large image it has to project?

It's interesting that you think f4 is lame for a lens that's primarily used for architecture and landscape photography. Aside from the sheer bulk a faster aperture would necessitate (and the 17mm TS-E is bulky enough as it is), fast apertures aren't a high priority for lenses used for such work, especially given they're all MF designs and require patience and a considered approach (and usually a tripod too).