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The lenses are the basic elements of photography. To choose the right lenses and use them in the best way, you have to know their basic attributes: the sharpness, the angle of view, the depth of field and the perspective.
Choosing the Right Lenses
The sharpness is one of the most important aspect of every lens. You can correct distortion, vignetting, chromatic aberration and other lens aberrations, but you can't improve a lot the sharpness - the Smart Sharpen or the Unsharp Mask improves only slightly soft images, but they don't help with photos that are very soft due to bad lenses.
If a lens was perfect, it should give the sharpest results wide open. Actually, only the best lenses (as some Canon "L") are truly sharp at the widest aperture, while the majority of lenses are more or less soft wide open, and they improve by stopping down by 1 or 2 stops. For example, in my lineup the Canon 24-105 L IS and the Canon 600 f4 L IS are very sharp even wide open at f/4, while the Sigma 180 Macro is excellent at f/8, but it is a bit soft wide open at f/3.5.
Stopping down the aperture improves the sharpness because when you use a small aperture you are using only the light that pass through the central portion of the lens, that is the sharpest area. When you get a new lens, I'd recommend to take test shots at various apertures to judge the sharpness. If you need wide aperture, it is essential to choose a lens that is sharp even wide open: I'd never buy a tele or a standard lens that is not useable wide open, while I don't care much about the wide-open performance in wide-angle and macro lenses, that I often use at small apertures (between f/8 and f/16).
After reading this, you might think that the more you stop down, the better the results. This holds true until you stop down to f8 or f11, then the sharpness diminished noticeably, and at the smallest apertures - as f/32 - all lenses are so soft that they are nearly unusable, for my standards. The reason is the diffraction: it affects all lenses (even the best ones) because it is a physical phenomenon; it is not an aberration and it can not be completely avoided. What is exactly diffraction?
When a wave pass through an hole that has a width similar to the wave's length, it changes its angle of propagation. Since the light is a wave, and the aperture is an hole, the lenses are affected by diffraction. The amount of diffraction depends by the diameter of the aperture. With large apertures the diffraction is negligible, while with small apertures that diffraction becomes a serious problem: generally, I prefer to avoid apertures smaller than f/16; few times I've used f/22, and I never use f/32 or smaller.
The visibility of diffraction is influenced by the pixel pitch: a sensor with small photosites (as the 18 mp APS-C sensor of the Canon 7D that has 4.3µm photosites) show more the diffraction than as sensor with larger photosites, as the 12mp fullframe sensor of the Nikon D3s that has 8.2µm photosites. I suggest to take test shots at various aperture to determine what is the smallest useable aperture, depending by your camera and your standards.
Comparison between the angles of view of different lenses.
The angle of view
The angle of view is determined by two variables: the focal length and the sensor size. The major camera systems offers a wide range of focal lengths between 12 and 600mm; there are mainly four formats of professional (SLR) cameras: 4/3 (18x13.5mm sensors), APS-C (25x16.7mm), 35mm (24x36mm), digital medium format (36x48mm). The formula to calculate the angle of view is Angle = 2*arctan(d/2f), where d is the diagonal of the sensor, f is the focal length.
In every format, the focal length that gives and angle of view of nearly 46° is considered the "standard" focal, because it has approximately the same angle of view of the human eye. The lenses that have a shorter focal are wide-angles ( they gives a wider angle of view) and the lenses that have a longer focal are called telephoto (and they give a narrower angle of view).
The following table lists the angle of view (degrees) given by various focal lengths in the four main formats, and the photos illustrates the practical difference between the angle of view of different focal lengths.
The depth of field
The depth of field is one of the basic principles of photography. When you focus an image, only a determinate plane (distance) will be really focused. Everything that is before or behind that plane will become gradually more out of focus; the areas near the focus plane that still have an acceptable sharpness constitute the depth of field.
There are three factors that influence the DOF (depth of field): the first one is the aperture. Wide apertures, as f/2.8 or f/4, give shallow DOF, while small apertures (at f/16 and f/22) give wide DOF. You have to choose the aperture depending by the result that you want to achieve. If you want to separate the subject from background, as I like to do with wildlife, you need a wide aperture; on the other hand, if you want the entire photo in sharp focus (as I want in landscapes) you need a small aperture, as f/16.
Left: at f/4, the DOF is quite shallow ; right: at f/16, the DOF is much more extended.
The focal length is related with depth of field and background. If the subject size is the same, the depth of field is the same for every lens. For example, let's say that you want to photograph a butterfly: at the same aperture, you get exactly the same depth or field both with a 50mm and a 200mm. The difference is that the 200mm, thanks to the narrower angle of view, gives a much cleaner background.
If you take a photo of the same subject with different focal lengths, the depth of field is the same, but the longer focal gives a cleaner background.
The size of the subject is the third variable. If you take photos of large subjects you will have proportionally more DOF. For example, if you take a photo of a mountain at f5.6 you will have a lot of DOF, but if you take a photo of a butterfly with the same aperture the DOF will be very shallow.
Left: aperture f/5.6 with a 5 millimeters subject; right, aperture f/4 with a subject measuring several hundred meters.
How to get the best from depth of field? When you photograph an animal, you should always focus on the eye, and choose the aperture that gives the right DOF for your purposes. For small animals, as a tit or a robin, I use apertures of f/8 or f/11, for larger subjects I often shoot at f/4 or f/5.6. In macro photography, if you want to have the entire subject in focus, you should try to stay perfectly parallel to its body.
Left: the subject is perfectly parallel to the sensor and the entire wings are in focus, because they stay into the plane of focus. Right: the subject is not perfectly parallel to the sensor so the tips of the wings are not sharp (they are outside the plane of focus).
In landscape photography, the technique is different. Some photographers try to calculate the depth of field and the hyperfocal distance (the hyperfocal is distance that gives the largest depth of field at a given aperture); personally, I consider it a waste of time, and I prefer to use easier and more intuitive ways of focus.
With a wide-angle and an aperture of f/16, you have an extremely wide depth of field, and it is easy to get the entire photo in focus. If the closest element of your composition is at 2-3 meters from the lens focus at 6-8 meters and stop down to f/16 to get everything sharp from the closest element to "infinity"; if the closest element is at 1 meter or less focus on 1.5-2 meters and stop down to f/16 (or f/22, if the closest element is really close). If you want to check the focus, you can give a look to the photo in live view, magnifying a detail of the image up to 10x.
In theory, if you maintain constant the distance camera-subject, the perspective is the same for every lens. In practice, the wide angles are often used to include into the composition subjects that are very close to the camera, while telephoto lenses are used to capture distant subjects.
As a result, the wide angles tend to exaggerate the perspective, while telephotos give a "compressed" or "flatter" perspective. The perspective is a very important creative element that has a strong impact on the aspect of the image: in landscape photography, the wide angles are often used to give a sense of depth to the image, thanks to their peculiar perspective. On the other hand, if you want to focus the attention on a detail, the longer focal length allows to take more bi-dimensional, intimate photos.
The two photos above are a good example of the different perspective given by a wide-angle and a telephoto lens : the first photo is taken with a 12mm, and the distances are exaggerated, giving depth to the image. The second photo, taken with a 105mm, has a much quieter mood, the branches in foreground and the tower seems to be in the same plane, even though actually they were quite distant.
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