Although there has been information concerning digital zooms on megapixel.net for
nearly 4 years, digital zooms have continued to evolve. Moreover, to
this day, one of the more frequent questions we get asked remains "what
is a digital zoom?".
The concept behind the digital zoom is actually very simple, but often,
the way it is presented as an extension of the optical zoom, creates
confusion. Examples of this are common: a 10X optical zoom suddenly becomes
a whopping 60X zoom, as a 6X digital zoom is tacked on
to the actual focal length of the lens. This puffery may make the numbers
more impressive, but are quite misleading for the uninitiated.
The digital zoom is primarily a cropping system and not as labelled:
a "zoom". A real optical zoom is a lens which can bring the
photographer closer to the subject, much like a telescope can. A digital
zoom on the other hand is merely a system that cuts the central part
of a digital image in such a way that it corresponds approximately to
the field of view that a longer optical lens would show.
Here is a simple example.
The photo shown at right is captured with an optical zoom
which, in this case, corresponds to a 42mm focal length.
The original image is captured with a 6 megapixel camera, which yields
an image size of 3024 x 2016 pixels, and is reduced to a size of 400 x
267 pixels, to show here.
The subject at the centre
of the frame is some distance away, but if we crop the original
image to the size shown above, a 400 x 267 pixel frame allows
us to show the subject in the same physical space on this
page.
This is exactly what the digital zoom does. It simply crops the image.
At this stage, the image quality remains the same as it was in the original
image.
Obviously, a system that allows this type of cropping to be done directly
with the camera can be useful, but most digital zooms take the process
one step further. Indeed, what happens afterwards is critical to the
image quality, either it remains the same as it was in the original full-size
frame, or it gets degraded. Currently, there are 2 very different
types of digital zooms. One type interpolates the image back
to the size of the image the crop was taken from, the other leaves
the cropped image as is. We'll start by looking at the first type
of digital zoom.
With most digital zooms, the cropped section from the full size frame is
interpolated to create an image that has a horizontal and vertical size
that is same as the image size currently selected on the camera. The interpolation
process that takes that small cropped section and brings it up to a much
larger size involves the use of complex algorithms, and takes place in
the camera. The sizing up process is done by adding pixels in between the
existing ones, matching their colour. Since only the original pixels in
the cropped section recorded actual information, the interpolation
process cannot add detail.
Since the pixels that made
up detail for the image are multiplied, the inevitable result
is that the image becomes coarser. In fact, the larger the
image size increase achieved by interpolation, the greater
the degradation of the image.
With our example above, if we were to take the crop we made and increased
its size to match the size of the frame it came from, we would need to
blow it up by 756%.
Shown here is a comparably size section (400 x 267 pixel) taken from
the crop shown above which has first been interpolated up to 3024
x 2016 pixels, resulting in a severe loss of detail.
Although most digital zooms don't go to this extreme — a 756% magnification
would be equivalent to a 7.56X digital zoom— the principle is nevertheless
the same, and the image quality invariably decreases. Commonly, digital
zooms apply a lesser interpolation, as otherwise the image quality declines
precipitously.
With the majority of cameras, the digital zoom crops from the optically magnified
image. In other words, the crop is made from what the camera captures at
the full telephoto setting of the lens. The examples below show the process:
The entire image shown above
and to the left is what is captured with the camera's optical
zoom. The yellow frame is equivalent to the area a 3X digital
zoom would select from this frame.
In the image above on the right, the digital zoom "discards" the
rest of the image, shown by a yellow overlay, and selects the centre.
The last photo, shown at left, is the resulting digitally zoomed image.
The cropped section has been interpolated by 300%, resulting in an image
that has the same dimensions as the one it came from.
The other type of digital zoom has appeared more recently, and is often
referred to as the "Smart Zoom". The Smart Zoom is an
evolution of the concept, and became possible as the image resolution
of cameras increased. Nowadays, 4 and 5 megapixel image sizes are common,
and this allows a smaller, but still useable section of the full resolution
to be used without interpolation. With this system however, for the Smart
Zoom to be available, the user must select an image size inferior to
the camera's maximum image size.
With all cameras, when a smaller image size than the full resolution of
a camera is selected, the image captured by the sensor, whatever that may
be, is "downsized" by the camera to match the selected image
size. The downsizing process can be thought of as the opposite of interpolation,
the image size is reduced by extracting pixels (data) from the full size
image resulting in the desired image size. In this case however, although
data has been extracted from it, the image generally retains a sharp and
detailed appearance.
It is this process the Smart
Zoom takes advantage of, by using the full resolution
of the sensor and cropping the central part of the image
it captures.
As long as the image size is less than the full resolution
of the sensor, the Smart Zoom can work from the full image, avoid any interpolation,
and the attendant degradation of the image quality that the interpolation
process entails.
Therefore, starting with a
camera that offers a 5-megapixel resolution (a full resolution
of 2592 x 1944), when the the camera is set to capture a
640 x 480 pixel image, the full size image is internally
downsized to 640 x 480. If the optical zoom is used, up to
its maximum magnification, the captured photo is downsized
to 640 x 480. But, when the Smart Zoom is used, the difference
between the selected image size (640 x 480) and the actual
resolution of the sensor (2592 x 1944), allows capturing
an image that seems to have been taken with an even longer
lens, but is simply cropped and at most downsized a slight
bit.
One could assume that this
same principle holds true with all digital
zooms. As long as the image size is set to a smaller size
than the full resolution of which the sensor is capable,
the camera will reduce the degree of interpolation, or
avoid it altogether. However, such may not be the case.
Just as likely, the camera will interpolate the digitally
zoomed image, and then re-size it downwards to save it;
in effect "blurring" the digitally zoomed image
through interpolation first, and then "re-sharpening" it
through the downsizing process. Not quite the same as a
Smart Zoom.
Conclusions:
Until the arrival of the Smart Zoom, the Digital Zoom was probably the
most misleading, and frankly, useless element found on a digital camera.
As it was originally implemented, the digital zoom simply degraded the
picture quality. Moreover, it was regularly used to boost the apparent
capability of a camera, which only served to confuse those unfamiliar
with its function. This is changing. Most newer cameras — with
some exceptions — are not marketed with the digital zoom lumped
in with the optical zoom, and others are adopting the Smart Zoom concept.
