LENS SELECTION
For many photographers, lens selections are as important or maybe more important than camera brands, or even formats. I will attempt to clear up a few issues about lenses that even some excellent photographers with much experience may understand more or less empirically, but have not thought out by the numbers as it were.
Lens selections have become more important recently due to the increased cost and exclusivity of interoperability even in the same brand of camera.
MOUNTS
Camera mounts are the first limiting factor. Some camera bodies can use adapters to mount non maker lenses. These adapters can work well unless they require an optical element to be inserted into the path. In this case, usually, the adapter will be the most limiting element in image quality. It is probably obvious that it would be better to find an equivalent performing lens in the correct mount, or to get a body that accepts the lens natively.
Large format lenses work better because the cameras they are designed to be used on are bigger, and have adequate flange distance to allow a reasonable adapter to be fitted.
Most adapters limit or completely eliminate automation features on lenses. These are things such as aperture control, full open metering, auto focus, and VR. They usually also limit the methods of control available in the camera, often require you to use stop down metering, or even an external light meter, and manual mode.
SHARPNESS AND DEPTH OF FIELD
Acceptable sharpness is not only a lens issue, but also a photographic concept that must be dealt with throughout the entire process of photography. Although there are some highly regarded, usually german or swiss, lenses that perform incredibly well, and cost a great deal, all lenses which are available from makers can be exceptionally good when used at specification.
Depth of field must be explained carefully because there are many misconceptions in the popular space associated with it. There is only one plane of sharp focus. This plane is based upon the distance the lens is moved away from the film plane. It should also be noted here that a lens must be a certain distance from the film plane. This distance is the focal length of the lens. It is where the lens is focused at infinity. Closer to the film plane is blurry, further away is focus closer, usually limited by some design criteria. Not that macro lenses, and any lens on extension tubes or bellows can focus down to the front element. [not really very useful] and at increased magnification – Mag ratios greater than life size are usually referred to as macro, whereas less than lifesize is usually referred to as closeup..
Between infinity and close up lie the plane of sharp focus. The degree to which subjects not exactly on the plane of focus become blurry is a continuously variable quantity. The degree to which this increases is controlled by the magnification ratio and the fstop.
If the plane of sharp focus could be imagined as a point source, the spot rendered on the film plane as we move out of focus becomes larger. If this blur spot is small enough so that it appears to be a spot on the print, [it remains invisible to human eyesight and appears to be a spot] we interpret that as sharp focus. This spot size is called a ‘circle of confusion’ The size of this spot depends on the magnification of the film plane image to the print and is fixed at the time of exposure. The circle of confusion specification is used to build depth of field tables. The ones on 35mm lenses are designed for an acceptable COC on a 8x10 inch print from a 35mm negative, or 8x, viewed by an average person from 10 inches. Clearly this may not be adequate for digital cameras.
It can be seen that the acceptable circle of confusion is dependant on the sensor size, the print magnification, and the fstop.
All of this is based upon the physical nature of light, and cannot be changed in the analog world. [This is an important disclaimer]
SHARPNESS AT APERTURE
When discussing sharpness it is important to note that this ‘sharpness’ is on the plane of focus, and that thisis a separate issue from depth of field discussed previously. It can be shown that at f64 for example there is a very wide range of ‘depth of field’ but at the same time, this fstop result in images so blurry all over the film plane, it is not usable on anything but very large cameras, such as 4x5 and 8x10 film cameras [think ansel adams group 64]. Most lenses available for 35mm format cameras don’t go past f22, and some end at f16 for this reason.
This is not to say that sharpness is the most important factor in image quality. As with everything in photography, there are compromises. Using a lens wide open at f1.2 or 1.4 may be valid in a low light situation even with the reduction in image quality. Sharpness may not be important as extended depth of field if print size is not large, or if prints are viewed from longer distances than the COC is set for.
Lens sharpness is affected by two factors. First is the lens imperfections that can to some degree be reduced by manufacturing techniques and design choices. More expensive, usually german and swiss lenses.
The second factor is diffraction. As the fstop is closed down, lens imperfections aberration is reduced, and diffraction is increased. At some point these two factors combine to produce the “least’ defective image. This is called the diffraction limited performance. Some very excellent lenses actually are diffraction limited at f4.0, and almost all are diffraction limited at f5.6. This is why, for sharpest performance at the focus point f5.6 is usually correct. Stopping down for extended depth of field decreases sharpness significantly. This is how pinhole cameras work…
The effects of diffraction are related to magnification ratio and fstop only and are part of physics, and cannot be changed in the analog world.
FOCAL LENGTH
Focal length is a simple discussion actually. One needs to get rid of the commercial concept of 35mm equivalent to understand the ideas. The focal length of a lens is the distance from the optical center of a lens to the film plane when the image is focused at infinity. A lens cannot bring into focus anything if it is moved closer to the film plane[that would be beyond infinity ]. To focus closer, the lens is moved further away from the film plane.
These distances can be measured in any unit you prefer. Usually in mm. on the film side and in America in feet on the subject side. This is useful if you want to do a little math with the optical formulas. We know that magnification ratio applies on both sides of a lens.
You need to think in terms of the film plane dimensions. For example if a camera has a 36mm wide film plane, [as 35mm cameras do] and we attach a lens which has a film to focal center distance of 36mm, in other words a 36mm lens, there must be a 1 to one ratio on the other side of the lens. So for our example, if we focus this 36mm lens at 10 feet distance, the subject will be 10 feet wide.
If we were to substitute a 72mm lens – our ratio would be one to two and at the same 10 foot distance, the subject plane would be 5 feet wide. OR we would have to move back to 20 feet distance to make the same 10 foot wide image.
Continuing this idea, an APS-C camera, say a Nikon DX, has a 24mm wide film plane, that makes our example square lens a 24mm rather than a 36mm. This may also give a clue as to why wide angle lenses are more difficult to build for small format cameras.
This works in all units – such as 4” lens on a 4x5 camera is a square
Also when thinking of lenses for a particular format – it might be useful to think in terms of squares
Ex
35mm Full frame camera
12mm, 24mm, 50mm, 100mm, 200mm
Some people prefer 1/3rds
