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Specific image quality issues

As always, our studio tests are backed up by taking hundreds of photographs with the lens across a range of subjects, and examining them in detail. This allows us to confirm our studio observations, and identify any other issues which don't show up in the tests.

Flare

Control of flare is a critical feature of a wideangle lens; with such a broad view of the world, bright light sources will find themselves in the frame on a regular basis. This is especially the case with a shift lens, which has a wider angle of view than a conventional lens with the same focal length (in effect the TS-E 24mm has the same angle of view as a 15mm lens, and a full frame sensor effectively imposes a 1.6x crop on the overall image circle). Fortunately the lens does pretty well dealing with flare - even shooting outdoors in bright summer sun we've encountered few problems - but it's certainly possible to pick up some nasty artefacts when shooting into the light and torture-testing the lens.

With the sun placed in the corner of the frame we see the expected multicolored flare patterns - these are most pronounced at the extremes of the aperture range, and tend to be least objectionable at around F5.6 - F8. With the sun placed just outside the frame, we were able to induce a large patch of red veiling flare, most visible at smaller apertures of F11 and smaller. In this scenario it's worth bearing in mind that the (necessarily) very shallow lens hood doesn't do very much, and providing additional shading makes a great deal of sense whenever possible. So it's worth heeding the advice Canon gives in the manual, and carrying around a piece of cardboard to use as a 'countermeasure for harmful rays'.

F3.5, Canon EOS-1Ds Mark III F11, Canon EOS-1Ds Mark III

Chromatic aberration

Canon's original TS-E 24mm F3.5 L was somewhat notorious for lateral chromatic aberration, with a distinct tendency to display green/magenta fringing around high contrast transitions towards the edge of the frame, especially when shifted. Our studio tests suggest that the new lens addresses this problem almost completely - and after sifting through several hundred real-world shots too, we're happy to report that CA is rarely a problem at all.

The example below illustrates this. Specifically set up as a CA 'torture test', with the lens fully shifted and light/dark transitions placed in the corner of the frame, we see only a little fringing even in the extreme corner, and it's of the relatively unobtrusive red/cyan kind (as opposed to much uglier green/magenta). Most of the frame is free of fringing in practical terms - which given this lens's huge image circle and angle of view is very impressive indeed.

F11, EOS-1Ds Mark III, 12mm shift 100% crop, top left corner

Corner softness and falloff when shifted

Our studio tests indicate that corner softness and falloff are potentially problematic with this lens when it's shifted, especially in the (rather unlikely) event you want or need to shoot at larger apertures. It's important to appreciate that the asymmetric nature of the falloff (which darkens just one edge of the image) makes it visually much more objectionable than the radially symmetric type that's normally seen. The example below shows how falloff changes with aperture - it's very visible across the top of the frame at F3.5, but much less obvious at F8, beyond which it scarcely changes.

F3.5
F5.6
F8
Crops

To demonstrate the edge and corner softness, we've taken crops at the positions indicated above. The lower one is very close to the optical centre of the lens; the top edge one roughly corresponds to the corner of the frame when the lens is fully shifted vertically in landscape format. This confirms that the centre is very sharp even wide open, and softens progressively due to diffraction at apertures smaller than F8. The edge crop is at its best at F11, and the extreme corner at F16; but neither ever achieve the heights of sharpness seen in the centre.

Center
Edge
Corner

Of course this all has to be placed in context - these are 100% crops from a 21Mp EOS-1Ds Mark III frame, after all, and a print made at similar magnification would be about 5 ft (1.5m) wide. The results at larger apertures are largely irrelevant - in practical use the lens is most likely to be stopped down to at least F8, simply for depth of field. It's also important to appreciate that full shift isn't necessary all of the time, and even when it is, there may not be a need for absolutely critical detail in the extreme corners of the frame. Also sharpness isn't the sole determinant of image quality, and the lens's extremely low distortion and almost nonexistent chromatic aberration are equally important in the overall equation. To get a better feel for how problematic corner softness really is in normal use we'd recommend (even more than usual) downloading and printing the full size images available in the sample gallery.

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Comments

BobFoster

One of my very favorite "core" lenses. I'm the perfect demographic for this lens: I like outdoors and buildings, and I insist on excellent detail and image control. Coming from 4x5 shooting, this is a natural fit, but it is easy enough for beginners to pick up. Great images can be had without any adjustments -- it is one of Canon's premier lenses within all the "L" lenses.

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