Digital cameras are like a film camera, except the film is replaced with an electronic sensor, formerly of vastly inferior quality to slide film.
The things that make a film camera good also make a digital camera good. This means good optical quality lenses, zoom lenses, single lens reflex design, robust construction, ease of use, fast repeat shots, appropriate film speed, good selection of exposure time and aperture, and so on. Unfortunately few digital cameras could be considered really good by comparison with good film cameras (in 1999, when I first wrote this). By 2004, digital cameras had almost killed Kodak's film business. More than half of all cameras sold were digital. By 2005, Kodak were basically out of the film camera business.
If digital cameras were bad, then why get them? They are great for photos for web sites. The quality is often sufficient for sales photos for real estate and other goods. You never have to buy film or processing again (although the digital storage cards were typically expensive). You get results right away, no waiting for processing (however you may need a computer on hand for printing). Indeed, only a tiny percentage of all digital photos ever get printed (the rest get deleted, or stored only electronically).
Why did I write this page, when I'm an amateur at photography? Because I had a lot of trouble back in 1999 coming to grips with what to look for in a digital camera for web site use. It seemed to me that writing up what I discovered would help me avoid making too many mistakes in selecting a camera, and act as a guide to evaluating cameras, and reviews of cameras.
The relatively small charge coupled devices (CCD) that replace the film have a resolution typically between a low of say 640x480 up to about 1760x1168 (some very expensive - five figure - specialist cameras have higher resolution). In late 2000, some cameras started to have considerably better resolutions than I have quoted, usually around 3 million pixels. By 2002, this figure was 6 million pixels. In 2003, there were a large number of medium price digital cameras offering between 3 and 5 million pixels.
A typical 35 millimetre film measures 24mm x 35mm (much larger than a CCD), and has a resolution of at least 100 lines per millimetre, or at least 2400x3500. This is a minimum of 8 million pixels, against a former maximum of about 3 million for a digital camera. A friend in quality optics says you can get up to 300 lines per millimetre in specialist equipment. This isn't three times better; this is nine times better. By 2003 you could actually get some really decent digital cameras at competitive prices.
In short, a cheap disposable film camera could probably take better photos than a good digital camera, circa 1999. By 2003, it was a tie, but digital cameras cost more initially.
Whether resolution is important depends a whole lot on what you intend to do with the photos. Digital camera are a very good match for web site production, and for quick advertisements for window displays produced by Real Estate agents. Newspapers frequently use them now, for speed of transmission back to the office.
Image sensors in cameras are gradually changing from CCD to CMOS. Low resolution CMOS was formerly used in very cheap cameras. A fancier version was used in high price cameras. These CMOS are gradually squeezing CCD image sensors out of mid-range cameras. CCD is typically not particularly light sensitive, and is thus limited to 50-400ASA equivalent. At more sensitive settings, noise artefacts in the image are likely. CMOS can function much faster and at lower light levels.
35mm film is 36mm by 24mm, a 3:2 aspect ratio, like 6x4 prints. Many consumer digital cameras use image capture devices with a 4:3 aspect ratio. If printing photos, the best print sizes may not match what you expect. Parts of the picture may be discarded for printing.
I gather that an interchangeable lens in a single lens reflex camera is the most desirable option in film cameras. I am not certain this is a good technique for digital cameras. Each time you remove the lens, you risk having dirt and dust settle on the light sensing element. My advice would be to stick with a non-removeable lens for most purposes.
Optical zoom lenses give you a lot of flexibility in capturing photos. You want at least a 3 times zoom (equivalent to say a 35mm to 105mm telephoto lens). By 2003 a few cameras were offering 8 times optical zoom, by 2005 some offered 12 times. Really great, but remember you probably need a tripod to get a decent picture at maximum zoom, due to the effects of vibration.
Some cameras offer optical image stabilisation. The light path or sensor is manipulated to reduce the effect of vibration. Optical image stabilisation is much better than electronic image stabilisation, which reduces the image resolution while it operates.
Digital zoom is close to useless when taking photos. It just multiplies existing pixels and manipulates the image. You can do that with many desktop image packets, usually including those supplied with the camera. What you need is a genuine optical zoom - accept no substitutes. Many cameras don't indicate what they use, so be suspicious, and check the manuals, not what the sales people say. I always switch off digital zoom, and wouldn't accept a camera which doesn't allow you to switch off digital zoom. Digital zoom can however be handy when viewing images taken with the camera.
Fixed focus vs autofocus. Cheaper cameras don't focus. If you need closeups, this can be a problem. Check what the closest macro setting is, especially if you might take photos of small objects. The typically small image sensors in digital cameras ensure that most digital cameras have a great depth of field, far greater than typical 35mm film cameras.
Focal length can be listed either in real terms, or the equivalent with regard to a typical 35 mm camera. Since the optical sensor is a different size to film, it sometimes make more sense to use the equivalent focal length.
An optical viewfinder is needed, just like on a regular camera, as well as the LCD display. You often can't see the LCD display well in daylight. LCD displays that you can move may well make it easier to use the camera, but can be fragile. LCD displays use up a lot of battery power.
Many camera viewfinders do a terrible job in terms of matching what image is captured. They often show only 80% or so of what the final picture will be like. Totally unreliable.
Digital cameras have speed problems in a bunch of areas.
The aperture of the lens helps determine how dark it can be before you fail to get a photo. However large apertures take expensive lenses. There are no world record apertures in the digital cameras I've seen.
The electronic film speed tends to be very low, typical the equivalent of 100 to 400 ASA. This again means poor results in low light (or long exposure times, which can cause blurred results). Forcing the electronic ASA rating high results in images with artefacts in the result.
Most digital cameras take a fair while to process a photo and get ready to make the next photo. If you always leave ten seconds between snaps this is not a problem, but if you are after action scenes, check the speed to the next photo. Cameras in 2003 often offer fast sequences of photos, but the number of photos in a sequence can be very limited.
Most digital cameras take a fair while to prepare for their first photo. If you like capturing action scenes, then check the speed to the first photo. At the camera price range I use, it took five seconds or more in 2003.
There is a noticeable delay between pressing the button and actually capturing the picture. This can be a half second. Static subjects are easier. If taking a moving subject, keep the subject centered for a second after pressing the button, because the camera is still working on it.
Flash for low light
While many cameras have a built in flash, many are pretty wimpy in actual use. If you do much photography inside or in marginal conditions, then check out the flash unit actually has the range you will need.
Ease of Use
In my experience, most digital cameras are a pigs arse in terms of ease of use. Lots can't be really used except for snaps without memorising the equally confusing manual. Even when you set up the camera the way you want (in terms of default settings), many lose these settings as soon as you shut them down. This is a real pain. In 2003 I got a Pentax that was pretty nice in handling this problem (you could tell it what settings to memorise). I'd expect cameras will start getting a lot better in this area.
You need to get your pictures into a computer, to work on them, and for loading on the internet. File transfer is the most technical thing you do with the camera, and the makers try to shield you from the problems. I like to see the pictures in JPEG format, ready to go on the internet. I would never accept a camera that uses only a proprietary format. Luckily almost all provide jpg.
Some professional and semi-professional cameras allow access to raw formats, for which there is no real standard (although Photoshop is promoting a standard). These images occupy more space on your camera, but allow far superior manipulation of the image without loss of quality. Unless you are a pro photographer, you probably should ignore this option.
Here are the options I know, in the order I prefer them. Many cameras include more than one method. Make sure that you can get your money back if their transfer method doesn't work.
Memory cards mean that you can carry spare film. Offload to a notebook or desktop computer each evening. I prefer Compact Flash (CF), because that is the most like a standard hard drive interface. If the camera slot is a Type II, then it will also accept larger CF cards containing a hard drive (although this will hit battery life badly). Many cameras that use CF come with a CF to PCMCIA adaptor that will fit notebook computers. By 2003 there were many cheap (under A$100) memory card readers that attach via USB to a computer. I dislike all the other styles of storage card, especially those proprietary to only a few brands of camera.
Given that there are a large number of card readers that accept multiple types of memory card, I can absolutely no good reason why camera should not do the same. A camera should accept any type of memory card (or at least most). Complain to dealers and manufacturers about every camera that tries to lock you into one memory card format, especially if it uses a proprietary card.
Serial RS232 cable to a PC. Not as fast as some methods. Computers all formerly had a serial port. My second choice. As at 2000, some computers do not include serial ports. My suggestion - never buy such a computer (there are lots of other peripherals only available with serial connections, including GPS devices). In 2005, I have to admit that serial ports are dead, and image file sizes are too great for serial.
Infrared, or specifically IrDA. Equivalent of a serial cable, available on many notebook computers and PDAs. However Windows 2000 and later versions (deliberately) broke some camera IrDA transfers. Make sure your camera has IrTrans, if interested (most do not, except for Casio).
Firewire is a very fast peer to peer wire network. Appropriate to very fast movie transfers as well. Several Apple models include it, and relatively cheap Firewire add on cards are available for desktop systems. Don't know of it in any PC notebook, which is a pity because it seems technically superior to USB.
USB has become more common in desktop computers since about 1997. Lets you plug several different devices in. I found it unreliable in the mid 1990's. Also, PDAs do not have the server version. I'd prefer to see the better designed Firewire as a replacement, and would resist being pushed into the USB camp (USB efforts have been described as "polishing a turd" - I tend to agree). Unfortunately, USB has become standard in digital cameras, most of which use non-standard USB cable connectors.
Video out plugs the camera into a TV for viewing. Handy for an audience, but I don't know of many computers accepting it. Some cameras only provide USA type NTSC, and not the PAL video that most places use.
Bluetooth is a forthcoming wireless data network. The people who would like to sell it say it will be wonderful. I'd wait until they have something to sell that actually works. My Bluetooth page has more material.
Parallel printer cable. Used by many computer gadgets, with drivers in the computer to accept stuff from the camera. Faster than serial. Two problems. Lots of other gadgets, like scanners, may already be on the printer port. Some recent computers don't have a printer port (as with serial ports, I wouldn't buy any such computer).
None. This isn't an option. This is a joke. Any camera that can not transfer photos to a computer isn't worth having as a gift. This includes film cameras!
Memory Card and physical media
Since it is easy to make multiple card readers, I see absolutely no good reason why a digital camera should not accept any type of memory card. Complain to manufacturers until they fix this!
Compact Flash (Canon, Epson, Hewlett-Packard, formerly Kodak, Konica, Minolta, Nikon, Olympus) is an industry standard, and is also used by Windows CE and Psion organisers. Unless you have good reasons, reject cameras that do not make Compact Flash memory cards available. Also, the camera needs to save the files in FAT format (as used by MS-DOS) as JPEG pictures (as used by web browsers). This way lots of computers can easily read it.
Smart Media - SSFDC (Agfa, Fuji, Hanimex, Olympus, Ricoh, Sanyo, Toshiba) square with visible connectors. Smaller than CF. I dislike it. It is more expensive and slower. I believe this media card has now (2005) disappeared.
Secure Digital and Multi Media Cards. Small square cards. I don't like them as much as Compact Flash. Capacity is smaller, costs may be slightly higher than Compact Flash, and load times slower (they compensate by loading photos to camera memory before loading to the card).
Memory Stick (Sony, and perhaps Olympus and Casio) like a stick of chewing gum. Proprietary, and not used by PDAs (except Sony brand). I'd avoid it. There also seem to be about three or four different form factors in this type of card.
XD. Another proprietary format. I'd avoid any camera that uses XD.
Floppy Disk (Sony Mavica). Very standard, but very limited storage. Very cheap, but the cameras are rather large. I think they have given up now, so I'd avoid even the second hand models.
CD-W (Sony). Very standard, good storage capacity. Write once, just like film. The cameras are exceedingly large. I don't think this will survive.
Power consumption tend to be heavy, with all that electronics. I think there is only one reasonable choice. Rechargable AA cells, in a camera that can also take ordinary AAs. That way you can pick up AA batteries anywhere you travel, but if you have time, you recharge the batteries. Kodak got that perfect on some of their DC models. You should note that high camera power drain characteristics kill off alkaline batteries pretty quick. The rechargeables are the best option for normal use.
Remember the LCD viewer screen chews power, so you can expect it to have an automatic shutdown. I turn mine off, since I have a decent optical viewfinder.
The flash chews power. Make sure you have a choice of on, off, and auto.
I wouldn't buy any camera that used a custom rechargeable battery, especially one that can't be easily replaced. This is not a negotiable decision.
Many cameras have flimsy openings and covers, despite frequent access. I'd expect bits to break. The more traditional cameras seem to have worked out how to do this right.
Prints (from an ink jet printer) fade quickly. Using a fancy paper and specialist inks to get good results can be very expensive.
Image Format on Compact Flash
I've mentioned that you generally want the digital pictures you take to be in a standard format, such as JPEG (Joint Photographic Expert Group). JPEG is what is commonly used on web sites to display photos. It is a lossy format, in that it compresses an image, and may discard some information. Some cameras give you an option to also save in a non-lossy format, at the expense of far larger photos.
Cameras generally save photos on Compact Flash in MS-DOS FAT format with a filename consisting of a camera identifier and a sequential number. The date of the file is generally the date it was transferred to your computer, not the date it was taken. Neither are convenient for tracking your photos.
Luckily, many cameras bury photo information in the JPEG file, as you would see if you search the image file for ASCII strings. This topic is covered by ISO standard 12234, Electronic Still Picture Imaging - Removeable Memory. The format is called Exif (you will find this string early in the image file) and comes from the Japanese Electronic Industry Development Association (JEIDA).
The Exif file contains a JPEG comment, followed by a JPEG image. The JPEG comment in the case of a camera image is a TIFF format list of data about the image. TIFF is a Tagged Image File Format, a format owned by Abobe, which lets you store arbitrary data in an image.
TIFF identifies a file as little-endian byte ordered by starting with II, or as big-endian by starting with MM. Then is has a short containing 42, followed by a long showing the offset of the first image file directory.
The image file directory contains a short count of the number of entries, and then the entries. Each entry contains a short tag identifier, a short tag type, a long count for the tag, and a long tag data or pointer to tag data terminated by a null. The image file directory ends with a long pointing to the next image file directory.
Exif has lots of neat ideas buried in it, including stuff like provision to store descriptions, photo details, and GPS readings for each photo. I expect some nice photo handling programs either are or will be available.
There is a Perl exif.pm that will extract tags, available from gPhoto, a GNU digital camera application at gphoto.fix.no