Canon TS-E 24mm 1:3.5 L II review
Applications and Examples
Tilt and shift lenses certainly aren't the kind of equipment that most photographers will be able to afford or justify, indeed many may well question their continued existence given that their capabilities can to some extent be imitated using software. But the key here is that word 'imitated'; for critical applications which require the highest quality, the use of lens movements gives superior results. This is nothing new - indeed it's long been a staple technique for photographers using large-format film cameras - but in the context of modern digital capture it becomes important when trying to extract the maximum possible image quality from the latest round of high-resolution DSLRs.
The purpose of such lenses is two-fold. Firstly, shift movements are used to help control perspective, and most famously avoid the 'converging verticals' effect commonly seen when shooting architecture with wide angle lenses. This effect can of course be corrected using software (e.g. Photoshop's 'Lens Correction' filter), but such methods work by stretching parts of the image, and lose resolution in the process; in contrast the use of shift movements maintains detail from corner to corner. Software correction will almost inevitably give the wrong proportions in the final image, which will also have to be cropped afterwards to a narrower angle of view.
Secondly, tilt movements are used as a means of controlling depth of field. Most notably, this allows the user to achieve extended depth of field without having to stop down to small apertures, and therefore avoids the associated penalty of image softening due to diffraction. However tilt movements can also be used to manipulate the direction of the plane of focus across an image for creative effect; perhaps the currently most fashionable application is to use shallow depth of field to produce 'fake miniatures'.
Shooting with a perspective control lens requires patience and technique. Visualizing the effect of shift on geometry is relatively simple, and it's quite possible to shoot hand-held for this kind of work. Ideally you need to be using a grid-type focusing screen - we used Canon's Ec-D screen in the EOS-1Ds Mark III - and be prepared to make minor corrections in post-processing. However using tilt to manipulate the plane of focus is more complicated. For this you really need to be working with the camera on a tripod equipped with a good 3-way (and preferably geared) head, and ideally be using a camera with a live view system that allows you to check fine focus critically across the frame (fortunately all of Canon's latest DSLRs are very good at this).
Metering and exposure
Setting the exposure when using a tilt lens isn't entirely straightforward, and requires a bit of thinking. The problem is that the camera's conventional TTL meter works by measuring light scattered off the focusing screen. Tilting or shifting the lens will change the scattering behavior and confuse the camera's meter, resulting in under or over-exposure in a fashion that's not easily predictable. This means it's normally best to work in manual exposure mode, metering and setting the exposure before applying lens movements. As always it also makes sense to check your histogram regularly, and shoot RAW for maximum leeway.
However it's also worth appreciating that when you're working in live view, the imaging sensor is used directly for metering, and the recommended exposure is generally correct. If you set the live view mode to 'exposure simulation' and enable the live histogram, everything suddenly becomes very much easier. So when working on a tripod this is very much the best option (especially as it also effectively provides mirror lock-up for free).
Perspective correction using shift movements
Probably the first application most people will think of in connection with a tilt and shift lens is the correction of converging verticals (most obviously when shooting architecture), and this is a major application of the TS-E 24mm. The 12mm maximum shift corresponds to half the frame height when shooting in landscape format, which effectively means you can place the horizon right at the bottom edge of the frame while still keeping the camera back vertical and therefore maintaining correct geometry. In portrait format, you can place the horizon one-sixth of the way up the frame and still avoid distortion.
The example below illustrates the affect of using shift. Normally, when confronted with shooting a tall building from ground level we have to tilt the camera upwards, but this results in the infamous 'falling over backwards' look. However keeping the camera back upright and applying maximum shift allows the verticals to be rendered correctly.
It's possible to take the distorted image and correct it in software - we've used Photoshop's lens correction filter turned up to maximum (-100 on the 'Vertical Perspective' slider), followed by substantial cropping to remove the blank areas of the frame that result (down to about 16Mp; a 14% loss of linear resolution). However this doesn't get the proportions quite right - the building is rendered too wide and not tall enough (although in isolation this probably wouldn't be obvious). More problematic, though, is the fact that the bottom corners are missing, and the image would have to be cropped further for printing. This is an inevitable consequence of tilting the camera back, and something which you might not necessarily appreciate, or be able to take steps to correct, at the time of taking the image.
All of these manipulations also affect the resolution and detail in the corrected version; to give an idea of just how much, we've resampled it back to 21 Mp (using Photoshop's bicubic method) and taken a series of 100% crops from similar regions of the image.
Software corrected, F8
At the bottom of the frame, there's little difference in sharpness between the two images. But as we move upwards into regions where the software-corrected version has been radically stretched, the difference becomes stark, and by the time we hit the top of the building the corrected file is visibly breaking up at the pixel level from all that resampling (offsetting the lens's softness in this region). The bottom line is that using shift gives better results.
Depth of field control using tilt movements
The second major application for a perspective control lens is the use of tilt to manipulate depth of field. When using a wideangle, this is most widely used for extreme near-far compositions (common in landscape photography) which require the depth of field to extend from a few inches in front of the lens to the far distance. Use of downward tilt can achieve such depth while still shooting at the lens's optimum aperture.
The samples below illustrate this. On the left is conventional shot with the lens set straight at F8 - the foreground is in focus, but the background isn't. Apply downward tilt, and both the foreground and background can be brought into sharp focus simultaneously. In this particular shot, we've also used considerable downwards shift to maintain the verticals of the church in the background.
Canon EOS-1Ds Mark III, F8
Of course it's also possible to achieve extended depth of field simply by stopping down, especially with a 24mm wideangle. But below we can see what happens when the lens is stopped down to F22 - the church spire is now within depth of field, but diffraction has kicked in and softened everything noticeably. For the most critical uses, it's clear that the F8 tilted version is better.
No tilt, Canon EOS-1Ds Mark III, F22
In practice, visualizing the effect of tilt on the final image is far from easy. It's more or less impossible to judge the position of the plane of focus sufficiently accurately using the optical viewfinder of an SLR, and this is therefore one application for which Live View is ideal, especially given Canon's excellent implementation of magnification for checking focus. Of course it's still advisable to check the final shots on a computer whenever possible before calling it a day on the shoot.
Using tilt for selective focus; 'fake miniatures'
Tilting the lens can also be used to focus selectively on a specific element in an image. In the sample below, full 8 degrees tilt to the left places the plane of focus selectively through the tree and its shadow, throwing the left and right hand side out of the frame in a fashion which is normally impossible with a 24mm lens. (In this shot we've also thrown in significant upwards shift to prevent converging verticals.) This gives the scene the appearance of being a miniature model; a result of the angled focus plane giving unexpected transitions between in-focus and out-of-focus regions, which resemble the shallow depth of field effects of close-up photography.
Canon EOS-1Ds Mark III, F4, 8 degrees tilt left
Panoramas and composites
One less-appreciated application for shift lenses is that of panorama shooting. The method is simplicity itself; mount the camera on a tripod, and make two exposures, one with the lens fully shifted to the left, the other with the lens fully shifted to the right, and layer the two in Photoshop to create the final panorama. The example below illustrates this in practice; the two images combine to give a 35Mp file, with a horizontal angle of view approximately equivalent to that of a 17mm lens.
However while this appears almost seamless in the reduced-size version, look a little closer and it's clear that the overlay is far from perfect - shifting the lens has caused a perspective change between the two shots that makes masking a complicated task. Ideally the position of the lens's entrance pupil, rather than that of the camera, should be kept constant; this can be achieved by mounting the camera on a slider and moving it sideways to compensate for the lens shift.
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