Digital Image Quality

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