Lens Characteristics and Specifi cations 237
when comparing the diagonal coverage of the square 36.7 36.7 sensor to the rectangular
6 4.5 fi lm size is only 1.3 not 1.5 as mentioned above for the 2¼ square.
Rather than determining the equivalent focal length as done above, it may be more valu-
able to determine the equivalent area coverage for the digital sensor. This can be helpful, for
example, when reading the V system close-up charts in Chapter 19. These charts apply to the
2
1
/
4
fi lm format. You can determine the area coverage along the longer dimension of the image
format for the smaller digital sensors as follows:
For the 36.7 49.0 sensor multiply the indicated coverage by 0.9
For the 33.1 44.0 sensor multiply the indicated coverage by 0.8
For the 36.7 36.7 sensor multiply the indicated coverage by 0.7
Such conversions are not necessary for close-up photography with H cameras. The Manual
includes charts for both the fi lm format and for the different digital sensor sizes in Chapter 19.
COVERING POWER
Lenses are optically designed to produce satisfactory image quality and illumination for a spe-
cifi c image format. This lens design characteristic is called covering power and is sometimes
called image circle (see Figure 14-1). The Hasselblad H system lenses, with one exception,
are designed to produce this quality and illumination on the 6 4.5 image format in fi lm
photography. These lenses produce superb corner-to-corner quality with all digital backs or
digital Hasselblad camera models since the sensors are smaller than the 6 4.5 fi lm size. The
exception is the HCD 28 mm f/4 lens which is designed specifi cally for digital imaging with
the H3D and H3DII camera and is designed to cover only the area of the 36.7 49.0 digital
sensor. Designing the lens for the smaller digital format resulted in a compact lens with an
extremely large 95 diagonal angle of view, larger than the angle of view of the Superwide
camera for fi lm photography. The Hasselblad V system lenses are designed to produce the
promised quality and illumination in the 2¼-in. (6 6 cm) square format.
The covering power of a lens depends on the lens design not the focal length. Lenses
with a 100 mm focal length, for instance, can be made to cover the 35 mm format, the medium
format, or the larger area of a 4 5-in. view camera.
Because lenses produce a circular image, the covering power of a lens is indicated by the
diameter of the circle within which defi nition and illumination are satisfactory (Figure 14-1).
Beyond that circle, the illumination or the sharpness or both are unacceptable. Lenses
designed for a larger format can be used for a smaller format.
LENS QUALITY
A single lens element produces an image affected by all kinds of faults, called aberrations, that
include spherical and chromatic aberration, coma, astigmatism, fi eld curvature, and distortion.
The aberrations can be reduced by combining various lens elements of different curvatures and
thicknesses made from different types of glass and arranged in different ways within the lens
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