Terminology - Hasselblad HTS 1.5 Manual

Terminology

Perspective (apparent perspective)
The appearance of perspective in an image is the result of a
number of factors including angle of view, focal length of the
lens, proximity to the subject etc, and is sometimes termed
apparent because it only appears that way in the image in those
circumstances.
Depth of field
Simply put, depth of field describes the amount of the subject
that is perceived as acceptably sharp. It is measured from a
calculated distance in front and a calculated distance behind
the point of focus (subject plane). It essentially expands and
contracts according to the point of focus and aperture setting
when in parallel to the image plane. That is to say, the closer
the subject is to the camera, the less the depth of field will be
and the smaller the aperture, the greater the depth of field will
be and so on, with all possible combinations in between.
In addition, the 'width' of the depth of field expands the further
it is away from the camera. See fig. 4 where it is so narrow
that the top of the red object is not included and therefore not
perceived as sharp, despite it being so close. Note that tilting
the lens does not increase or decrease the depth of field, it only
alters its orientation and shape.
It should be strongly emphasized that in practical terms the
perception of what is termed "acceptably sharp" can vary a
good deal depending on magnification, resolution, media etc
and should by no means taken to be an absolute. Depth of field
should therefore be seen as a relative term and is best judged
subjectively when all variables are taken into consideration.
Depth of field tables should be interpreted with this information
in mind and seen as guides.
Image circle
All lenses project a circular image termed an "image circle".
This has to be large enough to cover the whole of the sensor to
achieve a full image and avoid vignetting at the corners of the
sensor frame. The integral converter in the HTS 1.5 enlarges
the image circle enough to allow the lens to be shifted from its
central position while still projecting an image on the sensor.
Image plane / sensor plane / film plane / plane of focus
The plane of focus is the point in a camera where all the light
rays converge to create a sharp image. It is often referred to as
the film plane or more recently sensor plane.
Bokeh
Bokeh is the perceived quality of the out of focus areas in an
image, caused by the configuration of the elements in a lens.
Its appearance changes according to aperture setting, focus
setting and focal length. The bokeh could be particularly ap-
parent when using selective focusing with the HTS 1.5 and
contributes to the subjective quality of those areas.
Scheimpflug principle
This rule can be applied simply and rapidly to find a good start-
ing point for tilt calculations. It helps to optimise the situation
but it does not increase or decrease the depth of field. See
explanatory diagram in this manual.
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