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canon ts-e 24mm


I've used the Canon TS-E 24mm L for a while and I came across a couple of things which google suggests others may not be aware of. Here they are..

1 chromatic aberration and lens shift

The 24mm TS-E is an "L" lens and it's not bad at all. It does have some CA however, like all the other wide Canon lenses I've ever seen. At least it does if you're using a decent full-frame digital sensor and you look closely.

The problem

If you try to fix TS-E CA in the standard manner, which is with the raw convertor, you'll rapidly realize that whilst this works for un-shifted images, for it doesn't for shifted ones. Specifically you can correct the CA in any one place quite effectively, but because the lens is shifted, the assumption that the CA varies radially doesn't hold, so you have a variation across the image which Photoshop's raw converter doesn't know about and hence can't handle.

the solution

You can't shift the centre-point of the CA algorithm in Photoshop's raw convertor, or in "Filter->Distort->Lens Correction..." (although you can set the midpoint for vignetting). The panorama tools plug-ins provide pretty comprehensive facilities for tweaking images, but I can't find a way to set a centre point for the "correct" functions.

So the easy answer is to live with the fixed centre point. Just increase the size of the file so that the centre of the file is right where the centre of the [shifted] lens was. Unfortunately you don't generally know how much the lens was shifted by, unless you use the approach suggested below or otherwise takes steps to make sure you know. Assuming you know or can guess the shift, it's pretty simple to calculate the size of the "canvas" that would equate to in Photoshop. Then just increase the canvas size to that and the lens correction tools work fine again.

In Steps:

  1. Open the image using the raw convertor. Don't change the size of it or crop it.
  2. Figure out which way you shifted the lens, and by how much (typically 7mm and "up" for me).
  3. Compute what fraction of the image width or height that shift equates to (remember a 35mm frame is 36mm by 24mm)
  4. Multiply the vertical or horizontal dimension by the above fraction, which tells you how many pixels you need to add to the bottom/ side of the image
  5. Select "Image->Canvas Size..." and adjust the appropriate dimension to the new (larger) value. Make sure that the extra bit is added in the right place (eg below the image if you shifted the lens up). It doesn't matter what you fill the new canvas with as you'll throw it away later.
  6. Use "Filter->Distort->Lens Correction..." to zap the CA. If you process all your images this way you can save the setting as it'll work for all this lens' images, assuming you centre them all correctly as above.
  7. Finally crop away the new bit of canvas you created.

As far as I can tell the "mm" of the shift scale on the lens do correspond fairly closely to the mm of the sensor. That could be because they're quite close in the optical system, or perhaps cannon calibrated it that way. I'm sure there will be an easy way to check this, but it doesn't seem to matter much: the above works within the tolerances of my systems.

2 quick and dirty - using the TS-E without a tripod for architecture

You're supposed to use a tripod. You level the tripod up (bubble level on tripod), then level the camera up (bubble level in hot shoe), then check that the tilt and shift controls are at zero. Then you're allowed to look through the viewfinder to frame the building. You squint through the finder as you tweak the shift control to bring the verticals parallel. Well nearly parallel - a little convergence should be left for the best effect I think. Then you focus, expose, check the exposure, compensate, then shoot. It's not as bad as it sounds, but the tripod thing is a bit of a pain.

the problem

If you don't like carrying a tripod, or if you can't, then you need a different approach. You can't in general be sure that the front of the lens is parallel to your subject, and even if you could check that with your hotshoe-mounted bubble-level, by the time your eye is back on the viewfinder you already moved the thing. Until Canon put a bubble level in the viewfinder window, you're knackered.

the solution

It's another "turn the problem around" solution.... The requirement isn't really to get the lens parallel to the building, it's to minimise the vertical convergence in the building. One way to do that may be with the lens parallel to the building and a shift of [say] 2mm, but there are many other combinations of lens angle and shift which would also achieve the same thing - say a shift of 4mm with a lens angle of a few degrees.

So set the shift lens to a fixed value, let's say the maximum shift which doesn't cause distortion issues - 7mm, marked on the lens barrel. Then you lock it and you're ready to go. Just point the lens at your building, and tilt it until the verticals appear as you want them. It doesn't matter to the image if the front of the lens is vertical, pointing up or down or anything else.

In Steps:

  1. Set lens to maximum "high quality" shift
  2. Look through viewfinder, tilt camera to achieve desired effect
  3. Focus, expose, check, repeat until done

Perhaps it's so obvious that everyone is doing it already and they just didn't bother to mention it. Well there it is anyway: tripod-free architectural photography.

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