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ixoye25 wrote:
Can someone help me understand why you would opt for a lens with a 1.4 aperture over a lens with 1.8 or smaller (smaller/larger always screw this up)?
When speaking of f-number, I say "greater" or "less than" to refer to the value of the number. So an f-number of 1.4 is less than an f-number of 2. But this can still be a bit confusing, so when comparing apertures I prefer to use the terms "fast" and "slow." A fast aperture admits more light than a "slow" aperture, so this description is unambiguous. f/1.4 is faster than f/2.
It seems to me that 1.4 DOF is too shallow for most subject matter and the price difference seems to be exponential.
Not always. Depth of field also depends on focal length and the subject distance. A 24mm lens shot at f/1.4 focused 10 meters away can have quite a lot of DOF. By contrast, a 400mm lens shot at f/2.8 focused 10 meters away can have very narrow DOF. And a 100mm macro lens shot at f/16 focused 30 cm away can have millimeters of DOF.
Regarding price, yes, the faster the lens is, the more it tends to cost. The EF 50/1.8 II is $100, the EF 50/1.4 is about $350, and the EF 50/1.2L is $1350. On the telephoto end, the EF 300/4L IS is $1250, and the EF 300/2.8L IS is $4400. You pay a lot for that extra stop of light. The reason is simple--faster lenses use more glass, and the larger diameter elements are harder to manufacture precisely. The construction of the lens tends to be more complex, and its overall build is held to a higher standard. This is true for many products--the high-end models tend to cost much more than the mid-range, even though the performance difference is not proportionally better.
What type of photography benefits from this aperture?
A fast aperture has two primary effects. First, it admits more light. So f/1.4 will admit twice as much light as f/2, meaning one can shoot in darker conditions without having to set the camera's sensitivity higher (and thus increasing noise), or using a longer exposure time (which increases the chance of blur due to camera shake or subject motion). It is also beneficial even when light is apparently plentiful, because one might wish to capture a very fast-moving event.
Concrete examples include low-light photography without flash such as wedding ceremonies and performance arts; sports photography such as indoor gymnastics, basketball, etc.
Second, a fast aperture reduces depth of field for a fixed focal length and subject distance. This is often beneficial for situations where the photographer wishes to emphasize the subject and de-emphasize the background. If all objects in the frame were rendered sharp, this could be distracting because it draws the viewer's attention to portions of the image that may not be as visually appealing.
Concrete examples include field sports such as football, baseball, and soccer; and portraiture, such as candids, street photography, and headshots.
If you are using an aperture of 1.4 how do you use the shallow DOF for the 'look' you are trying to achieve?
One can write a whole book on this subject. I think the best way to understand this is to look at the images that other photographers produce using such fast apertures, as this has more to do with understanding composition, especially as it pertains to the tonal relationship of visual elements in the scene.
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