Spherical lens vs aspherical lens

"A lens marketed as "aspherical" will usually have only one surface (one side of one piece of glass) aspheric, and all the other surfaces will be spherical (or flat).So the vast majority of glass in any lens, even those lenses marketed as aspherical, is spherical anyway.

An aspheric surface can help correct for spherical aberration, which can make the image look soft when the aperture is wide open. But it's not an exact science, because compromises have to be made. For example, the spherical aberration correction can sometimes make background bokeh more harsh, which isn't great for portraits.

Aspheric lens surfaces are much more expensive to produce, because they can't be ground by a natural rotary motion."
 
As far as I understand, aspherical elements make it easier to make the lens sharp with fewer elements (smaller lens), but they have a negative impact on the bokeh (quality of the out of focus blur).

The Tamron lens you are looking at is a typical example of a lens that is sharp but has ugly bokeh.
 
Generally aspherical lenses help with falloff and vignetting in the corners and maintain sharpness, but as mentioned above, it does diminish the bokeh qualities in the wide or low stops like 2.8 or less. So it's some give and take. Generally you can use a non aspherical lens and get quality shots with them on crop sensors, as most don't take full advantage of the lens in all its corners, where as a full frame does. For example a 50mm 1.4 on a D7000 won't use all of the glass on say a D700.
 
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