A regular lens on a DSLR can be focused near or far, but the axis of the lens is always at a right angle to the sensor. The plane of focus (the slice of space that is in sharp focus) is always parallel to the sensor in the camera. No matter what, there is only one plane that is in absolute sharp focus, but apparent sharpness can be increased by using a small aperture to increase depth of field. With a regular lens, if you need more of the image in a single frame to appear in focus your only choice is to shoot at the smaller apertures. This has two major drawbacks: (1) smaller apertures mean slower shutter speeds. Try to photograph a field of flowers where you need f/22 for enough depth of field, but f/22 in turn means a shutter speed of 1/15 second, and that’s not fast enough to stop wind movement. Upping the ISO means the probability of increased noise. And (2) while you gain depth of field, small apertures introduce diffraction problems, reducing sharpness. Current high megapixel cameras, with their ability of record small detail, will all too readily show diffraction.
The solution is to reposition the plane of focus, to re-designate where it lies, and in order to do this you need a tilt/shift lens, which allows the front part of the lens to be moved compared to the sensor plane. The lens plane and the sensor plane no longer have to be parallel, but can be angled or displaced relative to each other.
Now, a bit of theory. Imagine three planes extending infinitely: the plane of the sensor, the plane of the subject (the part of the image where you want focus to fall), and the plane of the lens. With a standard lens only two of these planes can ever have a common intersection, because the plane of focus is always parallel to the sensor. But if you can tilt the lens all three planes can meet in a common intersection and a photographic miracle happens as now the focus plane has been repositioned.
You will often hear that using a tilt creates more depth of field. Not true. What a tilt movement does is reposition the plane of focus so that it no longer lies parallel to the sensor. This in turn creates a wedge-shaped depth of field which increases further away from the camera. By repositioning the plane of focus, by tilting the lens, part of the image closer to the camera will be in focus and part of the image further away will be in focus simultaneously. Depth of field is always on either side of the plane of focus, with less depth of field at close focus distances and more depth of field at far focus distances, hence a wedge shape. A really common belief is that using tilt movement will bring everything in a photo into sharp focus. Sorry, not true; you still need to stop down. The amount of tilt does not control depth of field, the aperture choice does that.
First you need to decide where you want to position the plane of focus. You always tilt the lens TOWARDS this plane, no matter the orientation of the camera body or the subject. But how do you figure our how much to tilt? How do you actually set focus? Setting a Nikon or Canon tilt movement is not the same as how you would work a tilt movement with a view camera due to the differences in the rotational point of the DSLR lenses. A lot of what you read on the Web applies mostly to view camera work.
So…step by step, here is what I do.
- Decide on the most important plane of my subject, where I want to position the focus.
- Chose a near point and a far point in this plane of focus.
- Set the tilt on my lens to zero (no tilt at all).
- Focus on the near point.
- Without refocusing, slowly tilt toward the far point until it appears in sharpest focus, and lock the tilt movement. The near point is now out of focus.
- Without changing the tilt, refocus on the near point. The far point is now out of focus.
- Unlock the tilt movement, and slowly move the tilt in the opposite direction than done earlier until the far point appears in sharpest focus. De-tilting, if you will.
- Repeat steps 5, 6, and 7 if necessary until both near and far appear in focus.
- Lock the tilt movement of the lens, and trim overall focus if needed.
- Select the aperture needed for depth of field.
The two biggest problems I see when people first use tilt/shift lenses are (1) working way too fast — using a tilt/shift lens should be slow and methodical — and (2) dialing in far more tilt than needed.