What causes a lens to be "soft" when wide open?

For example, I hear that the 50 1.4 tends to be soft wide open. Are all lenses like that or is is unique to this lens and primes in general?

Generally true of all lenses. Wide open they use the glass right to the edges. As you stop down you use a more precise section of the lens. Then eventually diffraction (the bending of light as it passes an object) sets in.

Law of physics are laws not guidelines.

That said lenses can be designed to work wide open or at a particular distance. For example Leica once made a 90 f/1 that only focused at one point (they made three extension tubes to change the focus point). It was optimized to work at f/1 as best as they were able to. A general purpose lens usually is designed to work best around f/8.

Hmmm, interesting. Thanks for your explanation. I'd like to read more about defraction and it's relationship to how sharp a lens is but not something that is too technical.

Here's nontechnical:

Think of it this way. Why does wide open yield less depth of field?

Think of a bucket with no lid, or wide open. Pour water into the bucket. It goes in diffused, or in a big blob, so that very little of the water hits the bottom of the bucket in focus.

Now, place a lid on the bucket with just a small hole in the middle of the lid. Now pour the water in. The water hits the bottom of the bucket in a much more focused manner. Capice?

mark

In a few words:

"Wide open the lens has quite low contrast and the definition of fine detail is very soft, creating blurred details and a generally soft image"

Generally, it is easier to construct a prime lense that gives sharp images than a zoom lens. Most lenses are sharper stopped down than wide open, with the exception of a few - and quite expensive - lenses (e.g. the Canon 85/1.2L)

However, f/1.4 is quite an extreme aperture ... which makes lens construction quite a challenge. Still, stop down the 50/1.4 to f/2.0, and it will beat many consumer lenses in sharpness. It certainly is sharp enough for my requirements at that aperture.

Take a look at the lens tests at http://www.photozone.d​e/8Reviews/index.html and review the resolution of that lens with the resolution of zoom lenses ... it compares quite favourably.

Best regards,
Andy