Hasselblad Digital Camera, Manual, Page: 258 / 432

Lens Characteristics and Specifi cations
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usually two f stops (Figure 14-4). This allows you to judge to what extent image sharpness
improves in the center and at the corners of the image when stopping down the aperture.
Image Quality at Small Apertures
When lens apertures become very small, a loss of sharpness may be caused by the nature of
light, not the lens or lens design. Light going through a small opening spreads slightly at the
edges, an effect known as diffraction, possibly affecting image sharpness at small apertures.
To reduce this problem, manufacturers of high quality lenses, like Hasselblad, limit the mini-
mum aperture to a point where the quality loss is not objectionable or even noticeable. This
explains why some of the lenses may stop down only to
f /16 or f /22 so you do not have to
be concerned about closing the aperture too far. Stop it down completely if the depth-of-fi eld
requires it. However, use a somewhat larger aperture setting with any lens if the smallest aper-
ture is not needed for depth-of-fi eld.
Your Own Quality Tests
There are probably occasions when you want to test a lens yourself. You can use the readily
available lens test charts for that purpose, but other objects with fi ne detail and sharp out-
lines, such as good quality printed material, are even better. Brick walls are often suggested for
outdoor tests, but they are not exceptionally good because they do not have sharp outlines or
much contrast. An old building made from stone or weathered wood is a better choice.
Here are a few other points that you must consider:
●
The test target must be absolutely fl at so that you do not have to worry about depth-of-fi eld.
●
The image plane must be parallel to the test target.
● If you compare different focal length lenses, each lens must cover the same size area.
●
Eliminate any possibility of camera motion.
● Make the test with the aperture wide open and closed down about two stops.
● Keep specifi c lens designs in mind. Macro lenses are designed to produce best quality at closer
distances. Retrofocus types perform better at long distances.
● Evaluate the center and the corner of the image.
● If done on fi lm, each negative or transparency must have the same exposure and must be
developed identically. Evaluate the original negative or transparency, not a print or a projected
transparency.
Color Correction and Apochromatic Lenses
Most photographic lenses are corrected so that mainly the red and blue light rays form the
image at the same point. Such achromatic lenses can produce images of superb sharpness,
at least within the wide angle, standard, and short telephoto range, but long telephotos may
produce images with color fringes. As a result, some long telephoto lenses are of the apochro-
matic design. They are corrected not only for red and blue but also the colors in between and
may carry the letters “apo” or the word “apochromatic” as part of their name.
Apochromatic lenses may have elements made from special optical materials (not glass)
that are sensitive to changes in temperature and affect the focus setting. As a result, the focus-
ing ring goes beyond the infi nity setting. Such a lens must always be focused visually on the