Digital cameras are confusing to a lot of new users. In this basic
guide to digital camera technology we hope to try to give digital
beginners at least some basis to use in deciding which digital camera is
appropriate for them. When shopping for a digital camera it's at least good to
know what the basic terms like white balance, pixel, ppi and
dpi mean and how they affect image and print quality. It's also
important to know the difference between things like optical zoom and
digital zoom as well as the advantages and disadvantages between storage
formats such as Compact Flash (CF), Microdrives, Sony
Memory Stick, Secure Digital (SD), Multimedia and camera
interface technologies such as USB 1.1, USB 2.0 and
Firewire IEEE 1394.
A pixel is a contraction if the term PIcture
ELement. Digital images are made up of small squares, just like
a tile mosaic on your kitchen or bathroom wall. Though a digital photograph looks
smooth and continuous just like a regular photograph, it's actually composed of
millions of tiny squares as shown below.
On the left the full image, on the right the area in the
red square magnified to show individual pixels
Each pixel in the image has a numerical value of between 0 and
255 and is made up of three color channels. So for example a pixel could be
37-red, 76-green and 125-blue and it would then look like this . If it was 162-red, 27-green and 12-blue, it would look like this
. There are over 16 million possible combinations using this scheme
and each one represents a different color. Computer savvy readers will note that
each color in this scheme can be represented by an 8-bit number (byte), so the
color of each pixel is defined by three color bytes. This scheme can be expanded,
for example to use 16-bits (two 8-bit bytes) for each color. Images using three
8-bit values are sometimes called 24-bit color images. Images using
three 12-bit values for color definition are called 36-bit color images,
and those using three 16-bit values are called 48-bit color images.
One of the main ways that manufacturers categorize their digital cameras is in
terms of pixel count. What this is is the number of individual pixels
that go into making each image. Today this number varies between 1 million (1
Megapixel) to around 14 million (14 Megapixels). A million pixels
is abbreviated to MP, so a 1MP camera has 1 million pixels and a 3MP camera has 3
million pixels. Currently most popular consumer digital cameras have between 2MP
and 5MP. A 3MP camera can make excellent 4"x6" prints and very good 5"x7" prints.
If you intend to make lots of 8"x10" prints, then perhaps a 4MP or 5MP camera
would be a better choice. Sometimes two numbers are given, total pixels and
effective pixels. Total pixels count every pixel on the sensor surface. Usually
the very edge pixels aren't used in the final image. Effective pixels are the
number of pixels actually used in the image after the edge pixels have been
Largest Image (typical)
2048 x 1536
2272 x 1712
2592 x 1944
Print size at 320dpi
6.5" x 4.8"
7.1" x 5.4"
8.1" x 6.1"
Print size at 240dpi
8.5" x 6.4"
9.5" x 7.1"
10.8" x 8.1"
Typical maximum image size vs. nominal Pixel Count. See
below for comments on dpi and print size
The aspect ratio of a camera is the ratio of the length of the sides of the
images. For example, a traditional 35mm film frame is approximately 36mm wide and
24mm HIGH. This has an aspect ratio of 36:24, which can equally well be expressed
as 3:2. Some digicams use the same aspect ratio for their digital images. For
example most digital SLR (single lens reflex) cameras have a 3:2 aspect ratio.
However, video monitors typically use a 4:3 aspect ratio. For example a monitor
with a 800x600 display has a 4:3 aspect ratio. With this in mind, most consumer
level digicams use a 4:3 aspect ratio for their images.
The size of the digital sensor element (which is equivalent to the size of the
negative for film cameras) is pretty small in all consumer digicams - typically
around the size of a fingernail (and a small fingernail at that!). As I said
above, a 35mm film frame is 24mm high by 36mm wide but most digital cameras use
sensors very much smaller than this. Here are some typical digicam sensor sizes.
The "name" of the sensor is based on specification for old TV tubes used in the
1950s. Nobody is quite sure why it's being used for modern digital sensors since
the "sizes" don't really relate in any consistent way to the actual physical size
of the sensor. However these names are widely used, so it's best to know what
they are. They are often listed in digital camera spec sheets.
Relative size of various digital camera sensors
Most of the current small 5MP digital cameras use 1/1.8" sensors which are
about 7mm x 5mm. They have an area 25x smaller than 35mm film and about 9.5x
smaller than a small sensor digital SLR like the Canon EOS 10D. You might wonder
why sensor size matters and that's a pretty complex issue. The bottom line is
that, for a given pixel count, the larger the sensor (and hence the larger the
area of the individual pixels) the better the image quality and the lower the
noise level. While large sensor cameras like the EOS 10D can operate at the
equivalent of ISO 3200 (though the image does get noisy), many consumer digicams
with small sensors cannot operate above ISO 400 before the noise becomes
excessive. For a full treatment why all this is so, see my article here on photo
Another factor in quality here is that small sensors tend to be of a different
type than large sensors. Small sensors, and the sensors used on all consumer
digital cameras, use a scheme which can read the data from the sensor in real
time using a scheme called "interline transfer" and the CCD electronics
control exposure rather than a mechanical shutter. Large sensors used on more
expensive Digital SLRs are often of a different design known as full
frame - which doesn't refer to their size, but their design - and which
require the use of a mechanical shutter. They don't read out and the display the
data in real time, only after the exposure so they can't give real time LCD
displays or record video. The advantage of this scheme is that the whole pixel
area can be used to capture light while interline transfer CCDs use part
of each pixels to store charge. Since smaller pixel areas generate more noise
and interline transfer CCDs are not only smaller to start with but
use some of their pixel area for charge storage, their noise level is
significantly higher. So the smaller interline transfer sensors in consumer
digital cameras yield lower quality images than those used in higher end DSLRs,
they can do more "tricks" like recording video clips and giving a live image
display on their LCD screen. The lack of a mechanical shutter also makes the
cameras cheaper and simplifies construction.
Small sensors mean that short focal length lenses are needed to give the same
field of view as cameras using larger sensors or 35mm film. So, for example, a
typical consumer digicam may need a 7mm lens to give the same view as you would
get using a 35mm focal length lens on a 35mm camera. This has consequences on
depth of field and means that most consumer digicams have a vary large depth of
field. Great if you want everything in focus, not so great if you want a blurred
background. This is covered in detail in my article here titled
DIGITAL DEPTH OF FIELD
With film you can buy "daylight balanced film" for shooting outdoors or
"tungsten balanced film" for shooting indoors under normal domestic lighting (not
fluorescents!). If you use daylight film under tungsten light the images will be
very yellow. If you use tungsten film in daylight the images will be very blue.
With film you have to correct for the "color temperature" of the light using
filters or by the right choice of film.
With digital you can pick your white balance to suit your light
source, so that white looks white, not yellow or blue. Normally there is an
automatic setting and the camera decides what white balance setting to use.
However if you know what your light source is you can usually set the camera to
it and this may give better results. Most digital cameras have settings for
sunlight, shade, electronic flash, fluorescent lighting and tungsten
lighting. Some have a manual or custom setting where you
point the camera at a white card and let the camera figure out what setting to
use to make it white.
Sensitivity settings on digital cameras are the equivalent of ISO ratings on
film. Just about every digital camera will have settings with a sensitivity
equivalent to ISO 100 film and ISO 200 film. Many will have an ISO 400 setting,
but above that the images from cameras with small sensors gets pretty noisy. The
more expensive digital SLRs with much larger sensors have much higher sensitivity
settings. At ISO 400 they are virtually noise free and some can go as high as ISO
3200 or even ISO 6400! Very few cameras have ISO setting lower than ISO 100
because noise levels are so low at ISO 100 there would be no real advantage in a
slower setting. Quite a few digital cameras have an "auto" ISO setting, where the
camera will pick from ISO 100, ISO 200 and sometimes ISO 400, depending on the
light level and the mode in which the camera is operating.
Digital Zoom and Optical Zoom
Most cameras have both optical zoom and digital zoom. Optical zoom works just
like a zoom lens on a film camera. The lens changes focal length and
magnification as it is zoomed. Image quality stays high throughout the zoom
range. Digital zoom simply crops the image to a smaller size, then enlarges the
cropped portion to fill the frame again. Digital zoom results in a significant
loss of quality as is clear from the examples below. It's pretty much a last
resort, and if you don't have it in camera, you can do a similar job using almost
any image editing program.
Comparison of optical zoom and digital zoom
JPEG, TIFF and RAW
The size of the digital file corresponding to the image which the camera
produces depends on the pixel count. In most consumer digicams each pixel
generates 3 bytes of data (so called "8-bit data"). One for red, one for green
and one for blue. This means that a 3MP camera, which has 3 million pixels,
generates 9 million bytes of data, or 9MB (megabytes). A few cameras can generate
extra data for extra quality, and some of these cameras generate files which
correspond to 2 bytes of data for each color ("16-bit"), so a 3MP camera
which is capable of generating 16-bit data will produce an 18MB image file.
Now these files are pretty big and they can be compressed quite a lot without
a significant drop in quality. This is where JPEG (Joint Photo Experts Group)
comes in. JPEG is an algorithm designed to work with continuous tone photographic
images) which takes image data and compresses it in a lossy manner (this means
you do lose some information). The more you compress, the smaller the file but
the more information you lose. However, you can reduce file size by a factor of
10 or so and still get a very high quality image, just about as good as the
uncompressed image for most purposes. You can reduce the file size by a factor of
40 - or even more - but the image starts to look really bad!
On the left, 10:1 JPEG compression. On the Right 40:1
Uncompressed the image would look virtually identical to the 10:1 JPEG on the
With 10:1 compression the 8-bit files generated by a 3MP camera
would be 900Kbytes in size rather than 9Mbytes, which is a big saving with little
quality loss. The smaller files take up much less storage space and are much
faster to send between computers or from the digital camera or memory card to a
There are also lossless ways of saving files using TIFF (Tagged
Image File Format) . These keep all the original information, but at the cost of
much bigger files. TIFF files can be compressed in a non-lossy way, but they
don't get very much smaller. For example, compare the file sizes for the rabbit
image above: TIFF files can also be used to save 16-bit data (those these files
are twice the size of 8-bit data files), JPEG files can only save 8-bit data.
JPEG at 10:1 compression
JPEG at 40:1 compression
Some cameras offer a third option, that of saving the actual data generated by
the sensor in a proprietary format. Canon calls their version of this "RAW",
Nikon call it "NEF". These files are compressed, but in a non-lossy manner. They
are significantly smaller than equivalent TIFF files, but larger than JPEGs.
Typically they achieve a compression of around 6:1 using 16-bit data, so files
are 1/6 the size of equivalent TIFF files. The only disadvantage of these formats
is that the image must be converted to either JPEG or TIFF for most software to
be able to display them. The conversion is quite a complex process and can be
time consuming if you have a lot of images to convert and a PC that's not very
fast. Since the RAW and NEF formats contain more information than JPEGs (and in
fact often more than TIFF files) you can do some degree of exposure
compensation during conversion to JPEG to rescue otherwise improperly exposed
images. You can also make white balance corrections during conversion, so if you
shot with the wrong white balance, you can fix your error.
Display, Printing, DPI and PPI
There's lots of confusion here so I'll try to go slowly!
When you display a digital image on a monitor, the only thing that determines
the size of the image is the pixel count and aspect ratio. DPI and PPI (and I'll
explain them later) mean absolutely nothing. If your image is a 480Kbyte file
which is 800 pixels wide by 600 pixels wide, it will display as a full screen
image if you are using an 800x600 display. It doesn't matter if your DPI is set
to 1 or 1000 or if your PPI is set to 1 or 1000. This is 100% true as far as web
display goes and as far as any monitor display goes - unless some software
intervenes. For example the IE6 browser will take large images and resize them so
they fit on the screen. However DPI and PPI are still ignored. A few advanced
page layout programs and advanced image editors may be capable of taking DPI and
PPI into account when displaying images.
So I'll say this once again. The way you control how large an image
appears on someone's monitor screen when viewing your images on the web is by
changing the pixel count. If your original image is 1600x1200 pixels
it will probably be too large to see all at once on 95% of the video monitors out
there. It will also be slow to load since it will be a large file. If you want
someone using an 800x600 display to be able to see your image clearly, you need
to change the size to, say, 600x400 pixels (remember the browser window is
smaller than the full monitor display). You change image size in software. All
image editing programs can do this. Sometimes it's called "downsampling" or
"downsizing". See your image processing software manual for details on what
options your software offers.
PPI stands for "Pixels per inch" and is almost exclusively used for
printing, not video display. If you take an image that is 800 pixels
wide and 600 pixels high, and you print it with a PPI setting of 100 pixels per
inch, the print will be 8 inches wide by 6 inches high. If you print at 200 PPI
you get a print 4" wide by 3" high. Now the print at 200 PPI will be higher in
quality but smaller. Most people seem to agree that around 320 PPI is the highest
number you really need. Above that it's very hard to see any improvement in image
quality. 240 PPI is often used and even that is often regarded as high quality.
Most people notice a quality drop when they go below 180 PPI.
DPI stands for "dots per inch" and is a property of a printer, not a digital
image. It's a measure of how finely spaced the droplets of ink can be in a print.
However the number is a bit misleading since it's not always measured in the way
you think it might be! Printer settings of 360dpi, 720dpi, 1440dpi and 2880dpi
are often found. However the difference between then is subtle at best. Most
people probably couldn't tell the difference and 360dpi usually looks great.
Changing DPI does not change the size of the print. PPI controls that.
DPI controls print quality (though as I said, over 360dpi you don't see much
There are quite a few different (and incompatible) memory cards used in
Compact Flash (CF) - The original memory card. 42mm x 36mm x 3mm. Somewhat
larger than the others, but used on all high end DSLRs. Available in capacities
up to 2GB. There are also miniature hard drives (Microdrives) with almost the
same form factor as CF cards (CF type II, 5mm thick)) which are available in
capacities from 340MB to 4GB. Microdrives used to be cheaper than solid state CF
cards, though there is not a big difference today up to about 1GB. The 4GB
Microdrives are actually cheaper than the 2GB CF cards though. Of course prices
change pretty fast these days! Overall CF cards tend to be cheaper than any of
the other forms of solid state memory - though this too could change. CF cards
and microdrives contain their own disk controller, so that makes the camera
Secure Digital (SD) - Very small - about 24mm x 32mm and 2mm thick. They have
a built in write protect switch to prevent accidental erasure and certain
encryption capabilities of little interest to digital camera owners.
Multimedia - Same size as SD but with less features and no encryption
capability. There are some that can be used in some SD cameras but they aren't
100% compatible with SD cards in all applications.
Smart Media - Thinner than CF cards, but lacking an on-card memory
controller. Despite the name, they're pretty dumb!
Memory Stick - Introduced by Sony and used only by Sony(?)
XD - Developed and used by Fuji, Olympus and Toshiba - even smaller than SD.
20mm x 25mm by 1.7mm thick
Is there any real difference in performance? No, not really. The CF cards are
the cheapest per megabyte and are available in higher capacity models than the
other (of course that may change with time). Most high end DSLRs use them. The
smaller cards tend to be used in the smaller consumer digicams. There's really no
reason to pick a camera with one type over another unless you have multiple
cameras or other devices (MP3 players for example) which also use memory cards -
then it's convenient if they can share cards. It may also be difficult (and/or
expensive) to find really high capacity cards (1GB and up) in formats other than
CF, but that's probably not a concern for most digicam users.
The following table gives the approximate number of shots you can expect to
get using low JPEG compression using various pixel count cameras in conjunction
with various sized memory cards at the lowest ISO speed settings of a typical
camera. The exact numbers depend on how much compression the camera applies and
the ISO speed used. Higher ISO settings result in more noise and noise is hard to
compress and so leads to larger files and less images per card. If you're
shooting in a RAW or NEF format you can divide these numbers by 3. If you're
shooting TIFF files you'd have to divide these numbers by 8.
Approximate number of shots per memory card for various
digital camera pixel counts using high quality JPEGs for storage
Digital Camera Interface
Once you've got the images stored in your camera on the memory card you need a
way to get them into your computer! There are several ways to connect digital
cameras to a PC as well as external card readers.
Serial - The earliest digital cameras had a serial interface, but no current
cameras use this since it is so slow
USB 1.1 - USB was the first widespread high speed method of data transfer
from cameras. It is theoretically capable of transfer speeds up to 11
megabits/second (note megabits not
USB 2.0 - A development of USB but much faster - up to 480 megabits/second.
USB devices are compatible with USB1.1 ports on a PC, but will only work with
them at the lower data rate.
IEEE 1394 (Firewire) - Though this is an older interface than USB, it was
originally only really used much on Apple computers. It's capable of high speed
transfer (400 megabits/second) and it's now found on some PCs or it can be added
to them via a plug-in card. More common on digital video cameras than still
Just about all cameras can connect to a PC, but it's sometimes easier to
remove the memory card from the camera and insert it into a dedicated card
reader. Even if your camera only has USB 1.1. if your computer has a USB 2.0 you
can use a USB 2.0 card reader for faster transfer. Card readers are cheap,
anywhere from $15 to $40.
Buying a Digital Camera
Sad to say there are more dishonest discount camera stores than you'll find in
almost any other business. Some of the popular photography magazines are cram
full of ads advertising very low prices. What they don't tell you is that you
won't ever actually get the camera for that price. Either they will add
on $75 in shipping charges or they will be "out of stock" on that model, but they
will have a more expensive model available of course. Sometimes they'll tell you
that the advertised camera is plastic in made in Taiwan, but for another $50 you
can get the model made in Japan. Sometimes they'll ship you the wrong item in the
hope that it will be too much trouble to send it back. Shop in the ads at the
back of magazines based only on the lowest price you see and most of the time
you'll be sorry.
However, there are reputable discount dealers and photo.net is associated with
a few of them. These dealers do give photo.net a small commission on sales made
through their website via the links below, so please use them if theu have what
you want at a good price. They're honest, they stock what they advertise, they
have low shipping charges, good prices and responsive customer service. If they
didn't we wouldn't be associated with them and we wouldn't recommend shopping
Here are a few links to other articles on this website which might be of
interest to readers looking for digital camera information.