TILT/SHIFT in use.
Posted: Sat Sep 17, 2005 7:10 pmTILT/SHIFT lenses are an attempt by camera manufacturers to provide some of the image flexibility of the view camera (a fully adjustable monorail or flat bed camera - usually Large Format or Medium Format) to the user of the SLR camera.
Nikon and Canon produce TILT/SHIFT lenses (Canon 24mm, 45mm & 90mm - Nikon 85mm Micro) and there are also simpler devices which only have SHIFT made by those and several other makers.
The use of a shift lens is primarily to avert (or exagerate) diminishing perspective in vertical subjects such as buildings or product packets. By setting the camera level (using a spirit level in the flash shoe for instance) the lens is either raised or lowered by a worm-screw and dovetail mechanism to raise or lower the field of view. If the film plane remains parallel to the subject plane there will be no distortion (tapering) of the subject.
Shift can also be applied in a lateral orientation by rotating the lens on the camera and intermediate notches are provided to allow other settings.
There is a technique possible with SHIFTING on a DSLR (or on film for subsequent scanning) whereby the lens can be shifted all the way in one direction for a shot and then all the way in the opposite direction for another snap and the two stitched together for a distortion minimised panorama. ( Additional exposures in between might prove beneficial for smoothing the optical transition - and in critical work a macro stage used sideways will prevent any distortion at all by maintaining a constant position for the lens and an effective shifting of the camera (sensor).
Problems with SHIFT on DSLRs arise due to chromatic aberration because the angle of incidence of image forming light on the pixel array increases and faults may increase.
TILT has a different application entirely and it is used to alter the plane of sharp focus in the scene in accordance with the Scheimpflug Principle. In normal orientation with the optical axis precisely perpendicular to the image plane the plane of sharp focus is parallel to the image plane and depth of field is extended by stpping the lens down. Now, it is important to remember that DEPTH OF FIELD refers to an area of acceptable focus for an image of a given size and a given viewing distance. In other words there is still only one parallel plane of sharp focus but there is a greater or lesser range of stuff that is near as damn it to sharp.
If the optical axis is moved away from the perpendicular then the palne of sharp focus is no longer parallel to the image plane. Briefly, the plane of sharp focus follows a line that intersects with the junction of the new optical plane and the image plane.
In Paul's image of Mount Connor it may have been possible to leave the camera set as it was and to simply TILT the lens down to such an extent that the plane of sharp focus would extend from the scrub in the foreground, all the way to the mountain itself - EVEN WITH THE LENS WIDE OPEN at its maximum aperture. Obviously there would be issues wide open with the lens's resolving and vignetting characteristics but by stopping down to f/5.6 to f/8 an ideal optical performance could be achieved with perfectly sharp content from front to back (or near/far in photo parlance). The negates needing to stop right down to f/16 or f/22 where the optical performance of any lens will be adversely affected by DIFFRACTION - or a softening of the image by the ratio of the aperture opening becoming to close in value to the thickness of the aperture blades.
*********
Of course there is more to it than this and the movements of the TILT/SHIFT lens ca be used in concert with each other within the mechanical limitations of the device. But that should do for now.
Cheers,
Nikon and Canon produce TILT/SHIFT lenses (Canon 24mm, 45mm & 90mm - Nikon 85mm Micro) and there are also simpler devices which only have SHIFT made by those and several other makers.
- SHIFT
The use of a shift lens is primarily to avert (or exagerate) diminishing perspective in vertical subjects such as buildings or product packets. By setting the camera level (using a spirit level in the flash shoe for instance) the lens is either raised or lowered by a worm-screw and dovetail mechanism to raise or lower the field of view. If the film plane remains parallel to the subject plane there will be no distortion (tapering) of the subject.
Shift can also be applied in a lateral orientation by rotating the lens on the camera and intermediate notches are provided to allow other settings.
There is a technique possible with SHIFTING on a DSLR (or on film for subsequent scanning) whereby the lens can be shifted all the way in one direction for a shot and then all the way in the opposite direction for another snap and the two stitched together for a distortion minimised panorama. ( Additional exposures in between might prove beneficial for smoothing the optical transition - and in critical work a macro stage used sideways will prevent any distortion at all by maintaining a constant position for the lens and an effective shifting of the camera (sensor).
Problems with SHIFT on DSLRs arise due to chromatic aberration because the angle of incidence of image forming light on the pixel array increases and faults may increase.
- TILT
TILT has a different application entirely and it is used to alter the plane of sharp focus in the scene in accordance with the Scheimpflug Principle. In normal orientation with the optical axis precisely perpendicular to the image plane the plane of sharp focus is parallel to the image plane and depth of field is extended by stpping the lens down. Now, it is important to remember that DEPTH OF FIELD refers to an area of acceptable focus for an image of a given size and a given viewing distance. In other words there is still only one parallel plane of sharp focus but there is a greater or lesser range of stuff that is near as damn it to sharp.
If the optical axis is moved away from the perpendicular then the palne of sharp focus is no longer parallel to the image plane. Briefly, the plane of sharp focus follows a line that intersects with the junction of the new optical plane and the image plane.
In Paul's image of Mount Connor it may have been possible to leave the camera set as it was and to simply TILT the lens down to such an extent that the plane of sharp focus would extend from the scrub in the foreground, all the way to the mountain itself - EVEN WITH THE LENS WIDE OPEN at its maximum aperture. Obviously there would be issues wide open with the lens's resolving and vignetting characteristics but by stopping down to f/5.6 to f/8 an ideal optical performance could be achieved with perfectly sharp content from front to back (or near/far in photo parlance). The negates needing to stop right down to f/16 or f/22 where the optical performance of any lens will be adversely affected by DIFFRACTION - or a softening of the image by the ratio of the aperture opening becoming to close in value to the thickness of the aperture blades.
*********
Of course there is more to it than this and the movements of the TILT/SHIFT lens ca be used in concert with each other within the mechanical limitations of the device. But that should do for now.
Cheers,
