Tilt-shift photography: how it works
16th Jun 2012 | 11:00
Special lenses and clever post-processing
Tilts and swings
When I go on holiday, I always take pictures of buildings in the places I visit. It doesn't matter where I travel to, there's always some beautiful architecture just waiting to be appreciated and photographed.
The problem is, to get a whole building in a shot, you either have to move further away than you'd like, or point the camera upwards in an effort to get the top in as well as the bottom.
When you do the latter, the verticals in the photo converge upwards and the building appears narrower at the top than at the bottom.
One solution to this issue is to use a wide-angle lens, but this isn't without its own problems: this type of lens will tend to include more of the foreground, even as it provides a better shot of the building. The better answer is to use either a view camera or a tilt-shift lens on a DSLR.
Back in the very early days of photography (and even today with specialist cameras), the camera itself was a very simple affair comprising two standards or planes – one holding the film plate (and a ground glass viewfinder) and the other holding the lens. In between the two was a bellows that didn't let in light.
The standards were mounted on one or more rails so they could be moved closer to or further from each other to focus the image, and to provide other options like perspective and depth of field.
Figure 1 illustrates the basic setup.
Notice that the image is seen upside down in the viewfinder. DSLRs reverse the image so that the view you see through the viewfinder is the same way up as the real view.
Rise and fall
Since the bellows were flexible and the lens and film standards were separate, these view cameras could provide a whole set of effects that are hard to produce with modern point-and-shoots or DSLRs.
The first of these were shift movements. The lens could be raised or lowered (known as rise and fall), or moved from side to side (the traditional shift), all while keeping it in the same plane.
Modern tilt-shift lenses on DSLRs provide much of the same functionality, but they are specialised pieces of equipment and are generally very expensive compared to normal prime or zoom lenses.
The rise movement on a view camera is important for architectural photography.
To properly use the rise shift, the image circle of the lens must be larger than the rectangle covered by the film. The image circle is the area of the film plane formed by the cone of light that comes through the lens.
The majority of the image circle is masked off, leaving just a rectangle that exposes the sensor of the camera (or the frame of film). If the camera's lens is shifted, a different part of the light cone exposes the sensor or film frame.
If the image circle is small enough, the film frame will not be fully exposed to the light and you will get the effect known as vignetting, where the photo fades and darkens towards the edges and corners.
Figure 2 shows what happens with rise shift.
In the first image (I've flipped the view so we're not looking at it upside down), we see the normal state of affairs: the image circle encompassing the film frame with the top of the building cut off. The foreground is also prominent.
The second image shows the effect of the rise shift: the film frame is in the same place (we're moving the lens up and down, not the film standard), but the image circle has moved down. A different part of the view is captured by the film: the one that contains the top and bottom of the building.
I'm sure you can imagine the issues posed by vignetting if the image circle were smaller than the one shown.
Side-to-side shift movements are used when photographing reflective surfaces.
One way to photograph paintings that are behind glass is to deck yourself out in black and keep the camera behind a black curtain with only the lens poking through so that only black surfaces are reflected (just try that in the National Portrait Gallery).
Another is to photograph the painting from a slight angle, making sure that the incident light is behind you so that reflections go away from the camera, and then fix the perspective in your favourite photo editing program (Photoshop has such a feature).
Alternatively, you can use a left/right shift movement to remove the reflection of the camera using the same technique as for architectural photography.
Tilts and swings
The other main set of possible movements with a view camera are called tilts and swings, known collectively as tilt.
In normal use, the axis of the lens is perpendicular to the plane of the film or sensor. Even shift movements obey this simple rule.
The result of this is that the sharp focus of a photo is a plane parallel to the film. The wider your aperture, the more obvious this sharp focus plane is.
In Figure 3, I took a photo of the word 'tilt' in the Oxford Modern English Dictionary at an angle with the aperture wide open to illustrate the focus plane.
There are four planes here: the plane of the sensor, the plane of the lens, the focus plane (which is parallel to the first two), and the plane of the page in the book (at an extreme angle, almost perpendicular to the first three planes).
Notice how the focus tapers off quickly the further or closer the text is to the camera.
With a tilt movement, the lens is tilted up or down with respect to the film plane – they are no longer parallel. The plane of the lens now intersects the plane of the film – usually below the camera.
More importantly, from some simple geometric proofs known as the Scheimpflug principle, the focus plane – the part of the view in focus – also tilts (geometrically it intersects the previous two at the same position).
In fact, by varying the tilt, you can bring nearer and farther parts of the view into focus at the same time (the effect on Figure 3 would be to bring all of the text into focus).
Notice though that this is not the same as increasing the depth of field by reducing the size of the aperture; this is a totally different effect, although a photographer can balance the need for smaller apertures to increase the depth of field by using a tilt movement.
Focus on blurring
Tilt movements are used in two contexts: landscape photography where you would prefer having more of the view in focus, and diorama effects.
The diorama effect is a process that converts a photograph of an ordinary view into one that looks like it was taken of a scale model.
Figure 4 shows this effect on a line of beach huts, and you may also have noticed the diorama effect in the titles for the BBC TV series Sherlock.
The reason this effect works is that, through exposure to photos of miniature scenes and landscapes, we know how the depth of field operates for small scenes versus large scenes.
For miniatures, the depth of field is very shallow, even at small apertures. This means that, even with small distances closer to or further away from the camera, the depth of field falls off dramatically. Take a look at Figure 3 again to see the effect.
For landscape photos, even when the aperture is wide open, the depth of field is usually quite large; more of the view is seen as being in focus, even though the focus plane is still well defined and shallow.
A simple way to create the diorama effect, therefore, is to trick the eye by blurring the region of the image closer to and further away from the object of interest in the view.
This reduction of the depth of field can be achieved using a tilt movement, or it can be done through some post-processing of the image. Figure 4 was produced by post-processing – in this case through some Gaussian blurring in Adobe Photoshop.
A blur is a mathematical operation used in graphics software to smooth out detail and reduce image noise. A Gaussian blur gives the effect of viewing a photo though a ground glass screen.
The simplest blur is a box blur. In this type of blur the pixels in the original image are transformed into the pixels of the resulting image by averaging out the values of neighbouring pixels.
So if we take the nine-pixel box around a pixel (the pixel itself plus its eight neighbours), a simple box blur would be to take one ninth of each pixel value in the box and sum them (which, of course, is the same as averaging the nine pixels).
In reality you would average each of the three colour channels and recombine them to form the value for the new pixel. You could also average the pixels in a 5x5 box or a 7x7 box to emphasise the blurring even more.
Another technique would be to vary the weighting of values of the pixels in the box so that, for example, the pixels that are closer to the reference pixel have a higher weighting than those further away (and thereby affect the final value of the resulting pixel more than those further away).
These types of box blurs are more properly known as circular blurs, since it's the distance from the central pixel that's of importance, and that can be viewed as a series of concentric circles.
One of the best known examples of circular blur is the Gaussian blur.
Here the weighting of the pixels in the box follow a Gaussian or normal curve (the familiar bell-shaped curve) so that pixels close by have a large weighting. This falls off rapidly the further away you get from the mid-point of the bell until you reach the long tail.
The problem with the Gaussian blur is the sheer number of calculations that are required in order to calculate the new value of a pixel. In essence, each neighbouring pixel value is multiplied by its weighting, the total of these products is accumulated, and the final total is divided by the total weighting.
For a reasonably sized box, the number of multiplications grows quite quickly (it varies as the square of the side of the box).
Luckily, there are some shortcuts that you can use to calculate a Gaussian blur.
The first one lets you apply a box blur in two steps instead of one: you can apply the blur horizontally (it's essentially then a one-dimensional 'line' blur), and then apply it vertically. The result is the same as applying the standard box blur.
With a box blur that uses a simple average this trick isn't particularly worthwhile, but it also works extremely well for a circular Gaussian blur. This means that you can reduce the number of calculations quite quickly – instead of being proportional to the square of the box side, it's now proportional to the length of the box side.
However, there is yet another trick up the graphics algorithmist's sleeve. Using the Central Limit Theorem, one of the major results of probability theory, it turns out that if you apply a standard box blur three or perhaps four times in a row, the result approximates a Gaussian blur extremely well.
Since a box blur is extremely quick to calculate (it's a summation followed by a division after all – no weightings in sight), this makes a pseudo-Gaussian blur very quick to calculate.
Focus on blurring
Back to the diorama effect.
Now we can simply select a narrow area or band of the photograph to be in focus, and everything above or below that band to be Gaussian blurred.
If we are clever we can 'band' the blurs themselves so that the closer we are to the focused band the smaller the radius of the blur, whereas the further away, the larger the radius. This will give a pretty good diorama effect.
The diorama effect can't be applied to all photos.
For a start the photo must be a fairly wide angle of view, like a landscape, with no large objects in the foreground. The eye would use them to gauge distances, which would negate the blurring effect.
The more eligible photos would also be taken at a slightly elevated angle.
Nevertheless, diorama or tilt effects are extremely addictive to make using post-processing of images, and don't require expensive gadgets like view cameras or tilt-shift lenses.