Recently I have started to notice that we get an increasing number of product and people shots that are NOT out of focus, but are kind of soft anyway. As the review tool shows conveniently the EXIF of the photograph, a correlation started to emerge. Namely, the correlation between the aperture and softness of the image.
Now, most of you would think that - yes, but of course, the wider the aperture the softer the image and the shallower given depth of field. Right you are, but interestingly the same is true if you approach the other end of the possible aperture values.
How is this possible? Isn't it so, that the bigger "f-number" I have, the deeper DOF I achieve?
Correct. But in addition to depth of field, other characteristics come into play. So far so, that I say the DOF does not play a role in the sharpness of an image other than it just defines the spatial amount of image that is in focus. Extrapolating from this, the image can theoretically be "in focus", but it can still be unsharp if the quality of the lens itself is low - starting to get the picture?
One of these aperture characteristics has a nickname "sweet spot". This sweet spot moves around a bit from lens to lens, and without going too technical, depends upon the arrangement and groups of small lenses inside the housing.
Well, how to determine the sweet spot of your particular lens then?
There are several ways to do that, depending on your commitment and technical understanding. There are trial-and-error methods on one hand, and there are MTF charts, Nyquist frequencies and what not, at the other.
The rule of thumb, not the absolute truth, is that the lens is at its sharpest, from EDGE-TO-EDGE at apertures of some 3-4 f-stops smaller than the widest maximum. (Now, take note of this edge-to-edge, since the majority of "Poor optical performance" reject reason is for blurry edges!)
If you would like to understand what on earth the aperture is, I found an excellent blog already posted
here on Dreamstime - thanks, Brad Calkin for taking the time to explain all this in layman's terms!
You can of course Google the "sweet spot of a lens", but in my quest I found one that gets you started:
Finding the sweet spot »
Since I am a Nikon-guy, I will post a few shortcuts to the MTF charts of most popular Nikon lenses:
Nikkor 18-55mm/f3.5-5.6 VR (kit lens)
Nikkor 80-200mm/f2.8 VR
What you conclude from these charts (move the zoom and aperture sliders around at the bottom), is that LENS SHARPNESS GENERALLY DROPS UP TO 50% WHEN STOPPED DOWN TO THE SMALLEST APERTURE!
Next time you shoot an object in your light-tent, take a few samples to see if you really need that f32 or is just as well in DOF with f8-f11? It certainly will be SHARPER at the latter settings.
If you MUST use those extreme aperture (shooting macro or the object is long on the axis of shooting) you have to sharpen the image afterwards in photoshop.
What we, reviewers, hate most is a professional looking and nicely composed image that is visibly soft at 100% :)
You can easily pick other makes and models from the huge database of the Dpreview specialists. To my knowledge it is the most respected independent photography equipment review site. Of course, Canon, Sigma, Tokina, etc are all covered so chances are that you'll find the lens review you are looking for.