The ‘Pan. Absolute’ ability, where enabled only pans by a few pixels left or right for
resolutions that are greater than VGA but panning by a larger amount is available when
the ‘Zoom, Absolute’ feature is used. The ‘Zoom, Absolute’ function is only enabled when
both height and width are smaller than the corresponding VGA values and even then the
number of zoom steps is limited to give a maximum zoom that is equivalent to VGA
resolution. There are a maximum of 4 zoom settings (0,1,2,3) that correspond to the
following zoom magnification factors: 0=x1, 1=x1.28, 2=1.6, 3=x2.0.
Although this zoom function is ‘digital’ in that is does not alter the actual lens position or
characteristics, it is not simply a mathematical upscaling of a low resolution image (as is
often the case with ‘digital zoom’ in consumer cameras). Rather it
does
give a genuine
increase in pixel resolution compared to the un-zoomed image. This is because the AF51
zoom mechanism works by sampling from the full 640x480 VGA output of the chip even
when the chosen video stream size is lower resolution than this. The higher the zoom
factor, the closer to 1:1 sampling of the full VGA image is produced (recall that zooming
only works for image formats lower than full VGA). However, capturing such a zoomed
image does not give quite as good an image quality as capturing in native VGA resolution
without zooming (see figure 3.7).
Figure 3.7 The effect of the ‘Zoom, Absolute’ factor on image quality for images that are below full VGA
(640x480) resolution. The pictures are of part of a computer screen photographed with the AF51 which is at
an angle to the screen – not face on). These show only a small part of a frame to emphasise the quality and
resolution differences and each picture has been scaled to the same size as the others for comparison in
this figure. While zooming does improve the pixel resolution of the QVGA (320x240) image, it is still not
quite as clear as the capture from the native 640x480 image stream.
OptArc AF51 Camera Page 31 of 99 User Guide v1.02