What is the focus of the SLR camera?
Fixed focus lens because no need to zoom so the aperture can do a lot, the structure can be optimized very well, so the weight will be very light. General focus lens to the majority of portrait head, 50mm focal length can be achieved on the 1.8 or even 1.0 aperture, so the virtual effect is very good. Of course, there are other uses of fixed focus, such as Canon's "100 micro" is a very good macro lens, you can achieve a good imaging sharpness and magnification. Fixed focus head also wide-angle fixed focus, telephoto focus and so on.
Focus lens is the biggest advantage of large aperture, the second advantage is high quality, of course, is similar to the price and the focus of the zoom ratio.
Such as Nikon's 50 / 1.8 as long as less than 700 but can have a large aperture of 1.8 and 4000-5000 zoom lens sharpness.
In Nikon's 180 / 2.8, for example, Nikon has the highest sharpness in the head, but as long as more than 5000 pieces
In summary, the fixed focus is not change the focal length of the lens, it can not zoom for the cost in exchange for a better image quality, good structural features, a large aperture. Fixed focus lens is a very good choice for the lens.
