Bob Fowler
Richmond Hill Camera Club

The ‘Sweet Spot’ of a lens usually refers to the aperture at which its images are at their sharpest, but it can refer, additionally, to the optimal focal length for sharpness as well as for the absence of various distortions from the true characteristics of the scene in terms of factors like colour and symmetry.

All lenses differ considerably in what their sweet spot(s) is(are).  Let’s look at some of the very broad issues.  I’ll deal with aperture and sharpness in this discussion, but similar arguments can be made for the other ‘distortion’ factors as well.

Only your own testing will tell you where the sharpness sweet spot  of your lens is – i.e. what is the optimum aperture for the sharpest image.  Very often it is in the middle area of apertures between f5.6 and f11, and seldom at the extreme wide open end or the smallest aperture (highest number).  F1.8 on an f1.8 prime lens is unlikely to be the sweet spot for that lens.

Just as the price of a lens can vary significantly, so can the size of the range of sweet spots; and usually they vary together.  The most expensive lens will have more sweet spot apertures available than the cheapest lens, other things being equal.

The latter one-to-one correspondence will be most evident for full-frame cameras using lenses made specifically for their full-frame sensors.  Cheaper lenses will have fewer sweet spot options, both for sharpness and for the other distortion factors briefly mentioned above.

APS-C cameras sporting APS-C lenses will have similar issues.  However, APS-C cameras fitted with full-frame size lenses will have a distinct advantage.  Because their sensors address only the middle part of the lens and not the far edges, where the relevant quality  tends to vary; they experience less quality fall-off at apertures that would be noticeable on their full-frame camera cousins.  This means that cheaper lenses do not necessarily mean severe penalties in sharpness at certain apertures  that more expensive lenses would normally compensate for.  It means that a full-frame ‘kit lens’ on an APS-C camera can be a very good performer at the right apertures (and focal lengths).

What to take from this. 

1)    If you have a full-frame camera, invest in the best glass you can afford, or assemble a good collection of lenses, each tested to determine their fewer, but perfectly useful, sweet spots.

2)    If you have an APC-S camera do the same as 1) OR choose a full-frame version of the lenses you purchase rather than the APS-C.  Also, do not part too readily with your kit lens, for it too has sweet spot areas that can be just as sharp as expensive replacements.  Just test to find them.

Have a Great 2013!