Tilt shift 101

No, they’re not broken. But your wallet will be after buying a set.

One of the most frequent things I get asked about is the use of tilt shift lenses; it isn’t surprising given the apparent complexity of the hardware and lack of any clearly understandable documentation or literature. There are plenty of good technical explanations of movements, but often they leave the reader more confused than when they started especially if you do not have a background in optics! This article will therefore aim to address the whole question of camera movements in as straightforward a manner as possible – necessitating some simplifications. Read on if you’ve ever been bothered by insufficient (or too much) depth of field, or geometric conversion of verticals with a wide angle…

A monorail 4×5″ view camera: the Arca-Swiss F-Line Metric.

To understand movements, a little bit of history is required. Early cameras used long real focal lengths paired with large capture areas (formats) to achieve normal-ish angles of view; simple geometry dictates that the larger the format, the longer the focal length you need to maintain the same angle of view. Depth of field, however, remains a property of a) the real focal length; b) the aperture and c) the distance between camera and subject. Largish apertures were required to obtain sufficient light for viewing, and to keep exposure times short; however this was always at the expense of sufficient depth of field. Smaller formats would render poor image quality due to the film emulsions of the time. Camera makers had two cures for this: firstly, provision of a closable iris (the aperture) to allow for a large viewing aperture but a small capture aperture; secondly, the Scheimpflug principle.

Tilt, extending or reducing depth of field depending on direction, and employing the Scheimpflug principle.

The Scheimpflug principle is a simple geometric rule that relates the plane of focus to the orientation of the camera lens relative to the capture plane. In practical terms, imagine your focal plane like a book-shaped box superposed over your subject: it is effectively flat, and has some thickness. Anything that falls within the box will be in focus. You can increase the thickness of the box by stopping down, but that is only useful up to a point: you will run out of ability to stop down infinitely, and you will start to see degradation in resolution caused by diffraction*.

*Diffraction is a wave effect that causes rays from a point source to spread out inversely proportional to the size of the aperture. Couple this with shrinking photosite sizes, minimum apertures before diffraction is visible (i.e. light spill to adjacent photosites) become larger, limiting effective resolving power/ perceptual depth of field gains on stopping down.

What a tilt allows is the ability to place this focal plane at an angle that isn’t parallel to the imaging plane: you can lie it down, tilt it upwards, or to either side, or some angle in between. The focal plane is still a plane – you cannot make it curve or bend in a complex shape – but you might be able to find a plane that intersects all of your desired subjects and cover the rest by stopping down a little. It therefore makes it easy to get any planar subject entirely in focus without having to resort to very small apertures.

Tilt employed for focal plane to cover entire dial and front roller; this is not a stacked or merged image

The easiest way to think about the resulting movement in the focal plane when you tilt is to imagine it being parallel to the front element: if you tilt the lens down, then the focal plane also moves parallel to this and ‘lies down’. Thus more elements of the scene from near to far will be in focus, increasing apparent depth of field in the image plane. Doing the opposite will reduce depth of field. Note that you may also hear some other terminology mentioned: for the purposes of a DSLR, a swing is a horizontal tilt; however, view cameras usually have both tilt and shift on both the lens plane (front standard) and the image plane (rear standard), which allows for some even more complex manipulations of the focal plane. DSLRs only allow for limited movements in the lens plane, and worse still, tilt and shift axes are almost always coupled.

Tilt employed again to create focal plane that slices through the watch name and the foreground lug/strap

In practice, shorter focal lengths require less tilt to get the same depth of field extension compared to longer ones. I’ve almost never had to use any tilt on the 24PCE simply because there is so much depth of field already by f8. Even wide open, use of a degree or less will render a very clear change in the focal plane – which makes it all the more bizarre that you have eight degrees of tilt, and no real precision in controlling them. Conversely, with the 85PCE, the full eight degrees is often not enough to lie the focal plane down sufficiently – especially when working with close subjects that are mostly flat. There is no choice here but to resort to small apertures coupled with tilt.

In the example (somewhat simplified and also exaggerated) drawing above, you can see that the focal plane remains parallel to the image plane and slices straight through the rear tree when there is no tilt applied (centre). However, if we tilt upwards, the focal plane follows and we take a ‘slice’ diagonally through the rear tree only; if we tilt downwards, then the focal plane ‘lies down’ and transects both the front bush and the rear tree. The front tree will never be in focus in either example because you can see it always lies outside the focal plane. It will be more in focus in the lower (tilt down) situation than the other two.

Shift. Or more accurately in this case, rise and fall.

Shift, on the other hand, is a movement of the optical cell in a direction that is still parallel to the imaging plane. Effectively, the imaging plane is now looking at the outer portion of the lens’ imaging circle**. By doing so, we are therefore either looking above or below the effective camera position; you could alternatively imagine the lens as being much wider on a larger format and cropping a portion of the image that’s higher up or lower down, to match the sensor/capture size. Since we are effectively working with a much wider lens, we can keep the camera level – keeping verticals vertical – and still effectively be shooting from a ‘higher’ or ‘lower’ position.

**This is why tilt shift lenses have to be large, and usually relatively slow aperture: they have to maintain adequate optical performance over a much larger image circle than the format would normally require. It is also why lenses from larger formats can generally be adapted to smaller formats with the added possibility of movements.

Rise employed to ensure verticals remain vertical

This is the main reason to use a shift lens: it allows you to effectively shoot from a much higher or lower camera position without physically having to change the height of your vantage point. By doing so, geometric distortion caused by converging verticals from aiming the camera upwards is eliminated. Generally, the most common movement used is rise – the lens is raised and the camera is static. (Lowering the lens is of course ‘fall’). Once again, you can imagine the focal plane like a book: except now it stays parallel, but moves upwards or downwards as the lens does the same. If the book moves parallel to the long axis of the format, i.e. horizontally, then this is shift. There are few uses for shift other than flat panoramic stitching – since there is no change in camera position, there are no issues with nodal point, alignment etc. enabling for very easy merging of the left and right portions of the image afterwards. Effectively, you can turn your 24×36 sensor into either something very wide – 24×60 or thereabouts – or stocky – 36×48 or thereabouts. (This of course depends on the amount of shift built into the lens.)

Left column: rise and fall. Centre column: tilt. Right column: compound movements including tilt/rise and tilt/fall/rotate. Centre row: zero position for comparison.

Tilt and shift can of course be combined to effectively say eliminate converging verticals in a building, and still have everything in focus from the foreground gardens to the very back of the roof (combined rise and tilt) or lay the focal plane very carefully to match say an inclined watch face. Unfortunately, few lenses allow for the tilt and shift axes to move independently; all of the Nikon PCEs require factory modification (except the 24) and even then you have to choose whether to have the axes parallel or perpendicular – there are no intermediate orientations. I understand the newer Canon lenses allow for independent rotation of tilt and shift axes, however.

In practical implementation, it can be easy to get confused once you start moving things around – especially if you are trying to match a subject plane that’s inclined at an odd angle, and combining both tilt and shift. My suggestion is to start with rough focus, then decide your shift orientation first and shift the lens – you will have to move the camera quite a bit after this to compensate for the change in perspective. Next, tilt as necessary to match your subject plane. The amount of tilt required varies enormously – for a wide lens, almost none. For a short tele, sometimes more than the 8 degrees offered by the lens. Then only achieve focus and fine tune your composition. Check your intended near and far subjects for focus if tilting. If you get confused or can’t get what you want in focus, re-zero everything and start again. In general, I find that you need more shift/rise/fall when working wide since these are typically used for perspective corrected architecture; with a long lens, you tend to be focusing on details – e.g. product photography – and always use a lot of tilt, but rarely any shift/rise/fall. There are of course exceptions to this.

Shift stitch, no tilt, 24PCE. Merged from a left and right (camera vertical)

Remember to lock the tilt and shift knobs to avoid knocking things off; whether after adjustment or in transit. I make a habit of leaving my lenses zeroed to ensure a consistent starting point. You also want to be using live view with magnification if possible – the optical finder is simply not accurate for this kind of work, and your meter is probably also going to be rendered inaccurate because the movements may well result in shading of the metering cell. Though for small perspective corrections you can always straighten the edges using the distort tool in Photoshop, remember that this will also result in image degradation since you are effectively interpolating one edge. The proportions of the subject will also be off because you will have to compensate the non-stretched dimension, too – resulting in further degradation; you may well run into a point where no amount of stretching is going to fix the perspective. On top of that, there is no easy way to replicate tilt – even focus stacking isn’t the same, since you are effectively putting one ‘book of focus’ in front of the other; this doesn’t render the ‘above’ and ‘below’ areas the same way at all.

Rise and a small amount of tilt to keep entire image in focus from foreground to background. Not achievable with stacking – there’s no way I could have focus stacked a moving bus…

Beyond the obvious creative freedom, there is one additional reason tilt and shift lenses are going to become an increasingly important part of the photographer’s arsenal: resolution. As sensor density increases, the smallest aperture usable before visible diffraction at the pixel level sets in gets larger and larger. On full frame, with the 12MP cameras, f16 was fine, f22 slightly soft. Go to 24MP and f16 starts to get soft. 36MP, f8-11 is your limit. On the 50MP 5DSR, I’m seeing softening anything beyond f8. This of course defeats the point of having more resolving power; the only way around it is careful management of the focal plane using tilt. Given that you really have to use these cameras on a tripod anyway to maximise image quality, it doesn’t really present much of an additional hardship. Are movements complex? Somewhat. Intimidating? It doesn’t have to be. MT

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