Established 1988

Category: Camera

The ‘Sweet Spot’ of your Lens

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!


The ‘Crop Factor’

Bob Fowler
Richmond Hill Camera Club

Recently at Photokina in Germany it became apparent that some major camera manufacturers are concentrating on bringing full-frame digital cameras to market at more affordable prices.  Why might you be interested in a ‘full-frame’ (i.e. large sensor-equipped) camera?

Two reasons, mainly:

  1. Generally  larger information advantages of a larger sensor (more and larger pixels with positive consequences for image quality and less noise in low-light situations).
  2. Better control of depth of field.

This time I have concentrated on how the larger sensor helped achieve the second aim, namely how it affected the camera’s depth of field (dof).

All other factors being equal [focal length, distance from subject, and aperture setting (f-stop)] the size of the sensor makes a major difference in the dof possible in an image.

If one uses the full-frame dof as a base, one can calculate the equivalent f-stop effect of the other sensor sizes by multiplying the f-stop  of the chosen camera by its crop factor.  Thus, if your APS-C camera has a crop factor of 1.5 and the full-frame camera’s f-stop is f8, your equivalent f-stop effect will be f11, and if your camera phone’s crop factor is 5.6, your equivalent f-stop effect will be f22, probably its highest possible setting and enough to get from camera to infinity as a dof.  Similarly a 4-thirds style camera would have an equivalent f16 result on its hands. This matters a great deal if you are trying to achieve a narrow depth of field for purposes of portrait photography or just to separate out your subject from distracting background matter.

As the sensor  size decreases, the need to manipulate the other dof factors becomes increasingly necessary and can make the desirable result more difficult to achieve, though to some degree possible.

This forms part, but not the only part by any means, of the decision to spend the extra money for a full-frame digital camera.