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Sharpness - Bokeh
Foto tutorial (English)
Foto tutorial (Espaņol)
Equipment recommendations US-ES
Flaat for Canon
Flaat for Nikon
Flaat for the BMC
Flaat for NEX-5N
Old Picture Style Tests
550D video lineskip
APS-C vs Full Frame
Badly assembled lenses and image quality
Lens mount compatibility chart
ISO on different cameras
High ISO on the 5D3
DIY: DR test chart
RGBWK Bayer sensors
Notes on DoF-FoV
Notes on crop-DoF-FoV
Custom Cropmarks for Magic Lantern on the Canon 550D
How many megapixels do I want?
How many megapixels can I see?
Quick Monitor Calibration Chart
A lens that can change its focal length is called a zoom. Nearly all lenses used nowadays are zooms: they allow the user to change the focal length, "bringing stuff closer" or "getting wider". For example, the kit lens that came with my camera was a 18-55mm zoom, a very common range for cameras with a 1.6x crop factor, that includes both a moderate wide angle (18mm), normal (35mm) and moderate telephoto (55mm).
In that lens, the ratio between the maximum and minimum focal lengths is approximately 3 (it is a 3x zoom). There are zooms with much higher ratios: for example, a 18-200 has a ratio of 11. This is very convenient, because you don't have to change lenses, but if you care about image quality it is better to avoid it: nearly no zoom with a ratio over 3 gives a nice image quality (that's why there are none of those in my equipment recommendations).
Lenses that are not zooms are called primes. For example, the lens that I use most often is a 35mm. These lenses cannot "bring stuff closer" or "get wider" by pressing a button or twisting a ring: if the group of people doesn't fit in the image, I have to move and get a couple of steps farther away; if the building looks too small, I have to walk and get closer (or I can change lenses and use a different one, with a longer focal length, for example a 90mm).
The advantage of zoom lenses is that they are very convenient: changing lenses is risky (dust is your enemy) and slow (by the time you're done, that bird you wanted to take a picture of will probably be gone).
The advantages of primes are (if you don't know some of these concepts, visit this list again when you've finished the tutorial):
* they're usually faster: there are very few zooms with an aperture above f/2.8, but it is not difficult to find primes with apertures of f/1.4 or even f/1.2; this is good for low light situations (we'll see it here), and when we want a shallow depth of field (we'll see it here)
* they're usually sharper: only the best zooms can compare in terms of sharpness to a half-decent prime lens (which will usually cost about a fourth of the price of one of those good zooms)
* they usually have less distortion: zooms tend to have barrel distortion in the wide end (if you take a picture of a brick wall, it seems to bump out), and the reverse one on the telephoto end (pinchusion distortion)
* they usually have better bokeh (we'll see it here): only some zooms can compare in bokeh quality to a half-decent prime lens (and even then those primes, if they are faster, will have a quantity of bokeh advantage)
Lastly, another advantage of prime lenses, very subjective, is that they reduce lazyness: you have to move your feet to look for the desired framing, or change lenses to get a different perspective. They make you work to get what you want. And that is good: few interesting images are taken without effort.