There's a lot more to digital image quality than a sharp lens,
avoiding digital zoom, and capturing a lot of pixels. Another crucial
component is how you save your file.
For instance, a 3 MP digicam's sensor will have 6 million green
pixels, 3 million red ones, and 3 million blue ones. Each pixel may
record 2 bytes of data (10-16 bits per color channel). Twelve million
pixels at two bytes each means that a 3 MP digicam might be working
with 24 MB of raw data. If it recorded that directly to your memory
card, you'd get at most five images on a 128 MB card. (This is
not how most digicams store raw images.)
That's one reason most photographers never save a raw image file.
Another is that the camera works to combine this data into a file with
three color channels, not two separate ones for green. That gives us 3
million pixels, each with one or two bytes of red, green, and blue
information. Our best quality image is 18 MB, and we can get just seven
of these on a 128 MB memory card.
By reducing the image to 8 bits per channel, we can cut that in half
- 9 MB of data per shot allows 14 uncompressed images on a 128 MB
card. Things are getting better.
But most digicams ship with a lot less memory and promise to store
dozens of images. How in the world do they do it?
Compression
The answer is file compression, but don't confuse this with zipping
or stuffing a file on your computer. Those processes are designed to
compress and decompress files with no loss of information. When dealing
with photos, we can handle some loss of data and still have an image
that looks good.
Digital photos are usually stored in JPEG format, which was created
especially for dealing with photos. The JPEG protocol allows for
various levels of file compression. At lower levels of compression, you
might be hard pressed to see any difference between a JPEG file and a
RAW one, but artifacts rear their ugly head at high levels of
compression and make it quite obvious that the image has been
compressed.
Although most digicams offer three or more quality settings, my
advice is to leave your camera set to the highest quality. You can
always compress it more in Photoshop once you've put your pictures on
your computer, but you can't restore lost detail and quality after the
fact.
For the same reason, I used to always recommend shooting at your
camera's full resolution unless you knew you don't need that level of
detail. With cameras going well past the 6 MB mark, I no longer
advise that. Unless you're likely to crop extensively or make prints
larger than 8x12, you should find shooting at 3 MP more than
satisfactory. That's the setting I use on my 5 MP and 8 MP
cameras. Unless you know you won't want anything larger than 4x6,
there's really no reason to use a lower setting than 3 MP. The
other big benefit of shooting at 3 MP instead of 5 MP or higher is
that you can fit a lot more 3 MP images on your memory card than
higher resolution ones.
Remember, you can always reduce a file after the fact, but you can't
put in detail that wasn't there to begin with, so if you do plan to
crop or might print really large, go ahead and use full resolution.
File Size
Every image is different, so the file size will vary from shot to
shot using the same compression setting. Because of this, the
information on various websites and in digicam manuals is either an
estimate or a range. For instance, the Canon PowerShot G1, a 3 MP
digicam, notes that 3 MP images will range from 468 KB to 1.7 MB
depending on compression (vs. 2.4 MB for a RAW file). That's quite a
range, and those numbers are unique to this model.
As a general rule of thumb, figure that a 3 MP digicam will produce
a 2 MB image at the highest JPEG quality. Your mileage will vary,
so check the manual that comes with your camera, but this will give you
a ballpark figure.
Memory
I'm not going to tell you that Compact Flash, Smart Media, Memory
Stick, or Secure Digital is best. They all work pretty much the same,
although there are some differences you should be aware of.
Be aware that memory cards come in many different speeds: lower than
20x (3 MB/sec.) is considered standard speed, and over 40x (6 MB/sec.)
is considered high speed. The fastest memory cards on the market are
300x card from Lexar, which have a maximum speed of 45 MB/sec. Keep in
mind that digital cameras can only read and write data so fast, so
using the highest speed cards may not benefit your camera at all. Check
the owner's manual to determine the maximum read and write speeds, then
buy a memory card at least that fast.
Compact Flash
My first digicam used Compact Flash (CF), so that's what I compare
everything with. As the name implies, CF cards are fairly compact,
although less so than Smart Media and more recent formats. CF is still
fairly popular, showing up on a lot of high-end digicams because CF has
higher capacity than any other format.
The biggest advantages of CF are capacity (up to 64 GB), speed
(twice as fast as SD cards), popularity, and the fact that adapters for
CF tend to cost less than those for SmartMedia.
SmartMedia
SmartMedia is about half the thickness of Compact Flash and came to
market at the same time (1995). By 2001, it was found in about half of
all digital cameras, the rest using Compact Flash. It's been downhill
ever since, and I don't know of any current cameras that use
SmartMedia. Capacity is far lower than CF (128 MB maximum, and many
older digicams are limited to 16 or 32 MB), and speed is limited to 2
MB/sec. (13x).
SmartMedia is essentially obsolete, and nobody makes these memory
cards any longer.
Memory Stick
Sony decided neither CF nor SmartMedia was good enough for them, so
in addition to cameras that used floppy drives and mini CD-Rs, they
created their own proprietary flash memory technology, Memory Stick, in
1998. The original Memory Stick specification supported capacities to
128 MB, and Memory Stick Select allowed two banks (256 MB total) on a
single card. Sony's newer Memory Stick Pro specification supports up to
32 GB.
Sony has created some very clever, innovative products using the
Memory Stick, which is used on their digicams, DV camcorders, and
Palm-like handheld computers. It may be the most versatile memory
technology of the bunch, but it is almost completely limited to Sony
hardware.
Sony has since introduced smaller Memory Stick Duo cards, which are
about 2/3 the size of regular Memory Sticks, and Memory Stick Micro
cards are tiny: 15 x 12.5 x 1.2mm.
The original Memory Stick supported speeds to 2.5 MB/sec. (16x), Duo
and Pro to 10 MB/sec. (66x), and Micro 20 MB/sec. (133x).
Secure Digital
Perhaps the most popular memory format today is Secure Digital (SD),
a secure version of MMC, a compact memory card designed for MP3 players
and other very compact devices. SD was introduced in 1999. Minolta's
very compact Dimage X was one of the first digicams to ship with SD,
which was done primarily to keep size down.
As with Compact Flash and Memory Stick, SD cards come in a wide
range of capacities and speeds. Maximum theoretical capacity is 128 GB,
although 8 GB seems to be maximum capacity available at present. The
basic transfer speed is 6x (0.9 MB/sec.), and cards as fast as 133x are
available.
The original SD format has been supplemented by mini SD (about 2/3
as large) and micro SD (11 x 15mm).
xD-Picture Card
The newest standard is the xD-Picture Card, which was codeveloped by
Olympus and Fujifilm when they realized SmartMedia had run its course.
xD-Picture is much smaller than SmartMedia - even smaller than Secure
Digital. Maximum theoretical capacity is 8 GB, although no cards larger
than 2 GB are available at present.
The original xD-Picture card was joined by Type M, slower and
cheaper, and later by Type H, a higher speed card initially available
only in Japan. Transfer speeds for the original xD-Picture Cards were
5 MB/sec. (33x), Type M dropped that to 4 MB/sec. (26x), and
Type H tops out at 5 MB/sec. read speeds, but writes one-third
faster than the original xD cards.
Memory Capacity
I can't think of a single digicam that ships with enough memory. Of
course, "enough" is a very flexible term, but I think it only makes
sense to have enough memory for at least dozens and dozens of pictures
at full resolution and minimum compression. For a 2 MP camera, that
means a 32 MB card might be adequate, and for 3 MP, buying 64 MB right
off the bat is a minimum.
The there's the question of vacation. Plan on shooting more pictures
with digital than you would with film - and of deleting a lot more
before you head home. If you usually shoot a 36 exposure roll a day on
vacation with film, figure that you'll shoot 50-60 and keep 30-40 each
day. With a 3 P camera and an average of 1 MB per image,
that's 30-40 MB per day and 200-300 over a one week vacation. On my
honeymoon last summer, we kepts over 650 photos shot over 8-9 days.
Fortunately I bought a 1 GB memory card (and I have 512 MB in my
"snapshot" camera).
What's the best way to get the capacity you need - a single card, a
couple smaller cards, or several even smaller cards? Again, there's no
single right answer. As with computer memory, there's a sweet spot
somewhere between the highest capacity (where you pay a premium because
it's the highest capacity) and the lowest capacity (where the cost of
packaging the memory becomes a factor). And then there's the issue of
convenience.
If cost per MB is close, I'd go with a single high capacity card -
and be sure to take alongthe card that came with the camera
(increasingly rare) just in case I fill up that card.
Backup and Backup Again
This is a good a place as any to tell you that the most important
thing for you to do after you copy your images to your hard drive is
archive them to CD and/or your iPod. In fact, CD-R media is so cheap
that I recommend making two backup copies and keeping one in a drawer
at work, at a friend's house, etc. just in case of a worst case
scenario.
Unlike film, where you need the original negative for the best
quality prints, every digital copy will be just as good as your
original picture. (As I keep saying, shoot at the highest reasonable
resolution with the least compression so your original is the best that
it can be.) Multiple backups are like having a spare set of
negatives.
In other words, you want a CD burner on your computer. If you don't
already have one, get one. Consider it an essential accessory for your
digicam. Hard drives eventually fail, so make backups. By having one or
more copies of your images stored somewhere else, you preserve your
digital memories.
I don't care if you never burn a music CD: You need to back up your
images - not to mention your other important files. Backup is an
essential part of computing, and most of your images can probably never
be replaced.
This also gives you peace of mind if you start messing with one of
your pictures in Photoshop and really botch things. Just go to your
backup CD and start over. Cover your digital assets.
Speed matters less than the fact that you're backing up, but I
suspect you'll quickly tire of waiting for anything slower than an 24x
burner. Save money if you must, but try to avoid anything slower than
24x.
Next: Finally, Picking a Digicam
