I read about some differences between spherical and aspherical lenses. Mostly about geometry though. I was wondering what is the difference on the pictures.

  1. How are the pictures of an aspherical lens different?
  2. I see some lenses being advertised as aspherical (Samyang 85mm Samsung-NX fit). However the specs of other lenses do not specify if they are spherical or aspherical. For instance the Samsung 85mm f/1.4 ED NX Lens. Why is that? Is the latter spherical?

Geometry is a big part of optics. Don't think of it as something other than the picture, it's what makes the picture.

  1. An aspherical lens does not suffer (so much) from spherical aberration. That's a fancy term for "the rays of light do not meet exactly in one distance from the lens". But what's exactly in one distance from the lens? Right, the sensor or film of the camera. If you have spherical aberration in your image, some parts of it will be blurry (out of focus) even if they should be sharp because the rays do not meet each other exactly on the sensor (they meet a bit in front or behind it). The further from the sensor the rays come into the lens, the more pronounced will this problem be.

    That's why the cool kids are always bragging about the corner sharpness of their lenses. That's where it pays off to have aspherical lens.

  2. Well, "advertised as" plays an important role here. It sounds technical and can attract buyers. Lenses are often lens systems that are made out of several elements. Usually not all elements are aspherical, because the production of such elements is more expensive than the spherical ones. Optical systems have a lot of properties you don't even hear about or only hear about insufficiently making them useless for comparison.

    For example: a lot of lenses have a stabiliser, to prevent camera shake. But how does one compare to the other? The information is limited to them having a stabiliser. What exactly does it mean to have weather sealing? Can it be dropped in the mud, used in a hail storm or what?

    But sure enough, not mentioning them will make your product look worse than those of other vendors.

Don't worry so much about these marketing buzz words. If you are not sure what lens to buy, read about them. This will give you an estimate of their strengths and weaknesses. If you still can't decide, rent both. Preferably at the same time and compare them for yourself.

And remember that it's you who takes the picture, not the asphericitillityness* of your lens.

*that totally is a word

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Use of aspherical elements in a lens is one of the ways to correct spherical aberration. Some differences that you can observe are haze/glow (or lack thereof) at very wide apertures and some differences in bokeh (quality of blur in out of focus areas).

Lenses with uncorrected SA:

  • Smooth background blur
  • Harsh foreground blur
  • Not very sharp on wide apertures (e.g. f/1.4)
  • Produce haze/glow at wide apertures

Corrected lenses with aspherical elements:

  • Background and foreground blur look similar to each other. Background blur usually harsher than spherical lenses.
  • Lenses with cheap aspherical elements may have blur circles filled with concentric rings
  • Lack of the haze/glow that is typical for spherical lenses
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