Read these 19 Digital Photograhy Basics Tips tips to make your life smarter, better, faster and wiser. Each tip is approved by our Editors and created by expert writers so great we call them Gurus. LifeTips is the place to go when you need to know about Digital Photography tips and hundreds of other topics.
An astute reader of this site pointed out that if CCD sensors generally produce better quality than CMOS sensors... and CMOS sensors are less expensive to produce... why is it that you find CCD's in so many lower-end digital cameras and CMOS sensors in so many "prosumer" digital SLR's?
Well, I don't pretend to know everything about digital cameras, but I think I have a pretty good idea of how this seeming contradiction works.
By most accounts, CCD sensors still generally produce somewhat better images than CMOS sensors. But there are a number of variables. One of those variables is pixel size. The smaller the pixels, the more likely your images will have noise.
Typical digital point-and-shoots have tiny image sensors. And the demand for higher resolution images has led manufacturers to cram more an more pixels onto these tiny chips. The result? More noise. The greater image quality available from a CCD makes it the obvious choice for these tiny sensors.
On the other hand, CMOS sensors are much faster than CCD's. They translate light into a digital signal right on the pixel itself. CCD's have to send a signal to the edge of the sensor before the translation occurs. When you have a full-frame (24 mm x 36 mm) sensor - one with realtively large pixels - the trade off in image quality isn't as great, and speed can take a front seat.
Cost may play a role here, too. The quality of a full-frame CMOS images is darn good - even if not as good as a full-frame CCD - but the cost to manufacture is so much lower, the CMOS makes economic sense.
The proof of the superiority of CCD image quality is clear when you look at products from high-end digital camera makers such as Hasselblad and Mamiya, where image quality is paramount. Their digital camera backs use CCD sensors.
Several digital camera models have recently added a "double exposure" feature. It's one of those extras that 99% of photographers will use only once... and then forget about it. Here's how double exposures work:
In the days long before digital cameras, double exposures were most common when someone forgot to advance their film. When the same frame of film was exposed twice, a double exposure resulted.
As cameras developed, the shutter was often linked to the film advance, which made this mistake nearly impossible. But some folks - pros and creative types - wanted to be able to make double exposures.
That's when SLR manufacturers introduced a button that, when depressed, allowed the shutter to be cocked, but the film didn't advance. This feature was found mostly on higher-end cameras.
Then came digital, and the entire formula changed. That's because there's no film to expose or advance. When a digital camera takes a shot, the image is immediately stored on the camera's memory card. With digital, you can't make a true double exposure.
Digital pros use their photo software - typically Photoshop - to create virtual double exposures. They simply "sandwich" two different images together. (This is something photographers did with slides and negatives for many, many years beforehand.) Combining images digitally offers the advantage of complete control - so the image always looks the way the photographer wants it to.
The double exposure feature on newer digital cameras simply does in-camera what digital pros have been doing with their software: it combines two digital images into one.
But the bottom line is that in-camera double exposures don't offer the creative control that your photo software does. If you're interested in creating "double exposure" effects, you'll be better off doing it on your desktop than in your camera.
Does this scenario sound familar...
You walk up to a store's camera counter. A clerk approaches and offers to help you. You say you're looking for a digital camera, and the clerk immediately asks, "How much did you want to spend?"
Needless to say, this scenario nearly always ends up with the discovery that the "perfect camera" for you costs just about exactly what you have to spend. (Surprise, surprise!)
Unfortunately, it also often leaves you with a digital camera that's far less - or far more - than you need.
To get the best camera for your needs, you should answer a few questions before you walk into the store. They're simple questions... but they can save you a bundle of money.
First, ask yourself how you'll use the camera. For snapshots at family outings and other occasional use, a compact point-and-shoot digital camera is all you'll need. If you like to shoot candids on the fly and capture spontaneous moments, then a pocket camera is probably most appropriate. If you're into sports or wildlife, a digital SLR or "superzoom" model will be more appropriate.
Next, ask yourself how you'll use the pictures. If you'll mostly send them to family and friends or post them in an Internet photo album, a low-resolution camera will be fine. On the other hand, if you want poster-sized prints, you need a high-resolution model. For most people, the ideal is somewhere in between.
Then ask which features are important to you. Today's cameras offer lots of features... but very few people need all of them. Selecting just those that are important to you can help keep your camera cost down. Here's a quick review of some key features:
* Lens. Most digital cameras come with either a fixed focal length lens or a zoom lens. Fixed focal length lenses often have the advantage of being "fast." That is, able to shoot in lower-light situations. Zoom lenses have the advantage of being able to make far-away object seem closer. With point-and-shoot cameras, the most comon zoom length is 3x, though there are some with longer zooms. Some "superzoom" cameras now sport zooms over 20x - though these lenses tend to be relatively slow. With a digital SLR, you can choose from a variety of fixed focal length or zoom lenses, and change them at will.
* Manual control. Most compact and pocket-sized digital cameras offer only automatic operation. For most people, this is fine. But if you like to get creative with your photography, options such as manual exposure and manual focus may be critical to your buying decision.
* Expandability. Digital SLR's are "system" cameras. That is, you can add more capabilities to your camera with various lenses, flashes, filters, power grips, etc. Most other cameras don't offer much in the way of expansion. These cameras are limited to their built-in features.
* Size and weight. This is an important consideration that most people overlook. Consider this: The Canon EOS 50D with Canon's 70 - 300 mm zoom lens weighs three pounds. To get an idea of how comfortable that much weight will be to carry, put a rope on a 3# barbell, hang it around your neck. Now try walking around for two or three hours with the dead weight hanging there. If you find that tiring, you'll have the same problem with the camera. On the other hand, Samsung's NV4 is a tiny 2.3" x 3.8" x 0.7". That's not an awful lot larger than a credit card... and possibly too small to be comfortable in large hands.
Of course, there are many other features, but these are some of the most important to consider.
Finally, your last question to yourself should be, "How much do I have to spend?" And that's how you should approach your purchase. When the clerk asks you the question, take a pass and simply say, "What I'm looking for is..." and tell them the type of camera and resolution you need, plus the features you want. Then ask them to show you the cameras that fit your specifications.
That way, you can choose the camera that fits you best - and probably come out spending less than you'd planned to.
Imagine you're inside a big cardboard box, and you're trying to read a book. That's pretty hard to do in the dark, right?
Okay, so let's poke a pinhole in the box. Not much better, is it?
But what if you cut a hole that's 4" in diameter? You'd get a lot more light inside your box, wouldn't you? And that, of course, would allow you to read your book.
Well, that's what the lens speed is all about: the largest amount of light the lens will let into your camera (measured by the lens's "maximum aperture"). The wider the lens opens, the more light it allows into the camera.
The maximum aperture of a lens is expressed as an "f-number." The smaller the f-number, the more light the lens can let into the camera. (I'm skipping most of the technical stuff here.) So, for example, an f-2.8 lens can allow more light in than an f-3.5 lens.
And here's where it becomes important: A faster lens can shoot a low light scene at a lower ISO rating than a slower lens. And that means you'll see less noise and less "grain" in the resulting picture. All other things being equal, a faster lens will always yield better pictures in lower light.
Have you ever tried to catch a kid in motion with your digital camera? Or take a picture of a waterall? Chances are, you ended up with more blur than you wanted. And missing that perfect shot can be frustrating.
With many digital cameras, you can easily eliminate that frustration. The trick is to switch your camera off it's "auto" or "programmed" setting.
Nearly all digital SLR cameras, most super-zoom models and many mid- to upper-range point-and-shoots offer a setting called "shutter-priority automation." Here's how using this exposure mode gives you control of the action:
When you leave your camera on the full automatic or programmed exposure modes, a complex algorythm in the camera's software decides two things. It decides how much light to let into the camera (aperture, or f-stop) and how long to let that light in (shutter speed).
This is great for most average shooting situations. But not all situations are average. For example, most digital cameras won't make the right exposure decisions to capture sports action when they're set to programmed or full auto. That's why many entry-level digital cameras have a "sports" shooting mode. It simply tells the camera to favor fast shutter speeds.
You see, the faster the shutter speed, the less time light is let into the camera. And that means that less motion has occurred while the shutter was open. Thus, you get less blur.
When you set your camera to shutter-priority automation, you can achieve the same effect as a sports mode - with one important difference. The shutter-priority exposure mode allows you to choose precisely how much blur or motion you get in your pictures.
With shutter-priority, you can add just a little blur in that shot of your daughter on ice skates. Just a little blur shows that she was in motion. Or you can choose to freeze the action altogether to capture the expression on her face in sharp detail.
On the other hand, you might want to shoot a waterfall. By setting the shutter to a slower speed, your picture will show more motion. And that can create a much stronger feeling, because viewers will see that the water was flowing.
So take your digital camera out and experiment with shutter-priority automation. It can open up a whole new range of creative possibilities.
Most digital cameras can save files as JPEG (JPG) files. In fact, most point-and-shoot models only offer a JPEG option. But as you explore your camera's menus, you'll notice that you still have some control over the files you save.
First, most cameras offer various resolution options. These are usually expressed in terms of so many pixels by so many pixels (For example, 1500 x 2000, which equals 3 megapixels.) or by megapixel count (For example, 5 MP.). Simply put, the bigger the number, the more detail the resulting image will have - and that means better quality.
I recommend always shooting at the highest resolution your camera offers. You can always use your image-editing software to make a file smaller, but you can't increase the quality of an image beyond it's original size.
The second option digital cameras offer for JPEG files is compression. Typically, you'll have three options, large, medium and small. Large files retain more data, while small files discard more of the image data. And because of this, I recommend you always save files using the least compression possible. (In other words, save using the "large" option.) Here's why:
JPEG is a "lossy" file format. During compression, "redundant" data is discarded to allow for a smaller file. Once the data has been discarded, it can't be recovered. So, to ensure the highest quality images your camera can offer, save your JPEG files with the least compression possible.
But JPEG's have another problem with "lossiness." Every time you resave a JPEG file, it loses a little more image data. In theory, you'll eventually degrade the file to where all the data is lost. (Of course, this would require a lot of saving and resaving.) But you can get around this problem easily.
You can open and close a JPEG file as much as you'd like without losing data. It's only when you save a JPEG that data is lost. So, if you don't overwrite your original files, they won't suffer any further loss in quality. Instead, if you edit your JPEG files, save the edited version under a new file name. That way, you'll always have the original - highest quality - image file to work from.
Follow these three simple guidelines, and you'll always get the best out of your JPEG images.
Nearly all digital cameras use one of two types of sensor: either a CCD (charge-coupled device) or CMOS (complimentary metal-oxide semiconductor).
In the early years of digital cameras, CCD sensors had a clear edge in image quality, and were the overwhelming choice of camera manufacturers. CMOS sensors, though less expensive to manufacture, simply couldn't match the picture quality of CCD's. That's because they were prone to "noise" - digital artifacts that marred the picture.
However, advances in the quality of CMOS images have outpaced decreases in the cost of manufacturing CCD's. And CMOS sensors use just a fraction of the power required for CCD's, resulting in longer battery life in your camera.
Today, most manufacturers use CMOS senors in their higher-end cameras, while most point-and-shoot cameras have CCD sensors. But in terms of image quality, for most snapshooters, there's not enough difference to worry about.
It's the digital camera industry's dirty secret: All pixels are not created equal. They don't tell you, because they want you to suffer from pixel envy. That's how they get you to buy the newest digital camera models.
But is a 10-megapixel camera really better than an 8-megapixel model? Or even a lowly 5-megapixel model? Not necessarily. Here's a case in point:
The 12-megapixel Nikon D300 retails for a little under $1,500. Nikon's D700 is also a 12-megapixel camera... but it sells for about $3,000. Since both are DSLR's, if you don't need the extra features on the D700, you'll get the same quality pictures for $1,500 less, right?
Wrong. Though both cameras offer roughly the same nominal resolution, at 23.6 x 15.8 mm, the D300's sensor is less than half the size of the D700's. That means each pixel in the D700 sensor is able to absorb more light... and thus can provide greater detail and richer color.
But don't knock the D300... it's sensor is still about twice the size of the average point-and-shoot camera's sensor.
Greater nominal resolutions can also create a disadvantage. Smaller pixels tend to increase "noise" - off-color or grainy spots in your photos. That's why your old 6-megapixel camera may have produced better looking photos than your new 10-megapixel model.
So when you're looking for a digital camera, look at two numbers: resolution ("megapixels") and sensor size. Because resolution alone is just half the story.
Back in the dark ages, when film ruled - you know, about 10 years ago - most photographers thought before they shot. You had 24 or 36 frames, and that was it for the roll. And then you had to pay to have those photos developed.
With digital cameras, the cost of an additional shot is virtually nothing. And that's led to photographers shooting like crazy - and thinking less about each shot.
Unfortunately, this shuteer-crazy mentality has a negative effect. People think less about their shots - and the overall quality of photography has suffered.
Even though you're shooting with a digital camera, light matters... composition matters.
Don't go shutter crazy just because you can. Think about your shots. Consider the composition; consider the effect of light. Will changing your angle or perspective make a more interesting photograph? Will aiting two minutes for the sun to move give you better light?
You can make your pictures stand out from the crowd simply by thinking a little more about your shot. While the rest of the world goes shutter crazy, give yourself a minute to think, and you'll create better digital photos.
Secure Digital memory cards are by far the most common cards used in digital cameras today. But unlike Compact Flash, Sony's Memory Stick and xD cards, SD and SDHC cards have two data transfer ratings.
An SD card's speed referes to how quickly it can accept still image data from your digital camera. But the rapid improvement of digital video has spawned a second measure: class.
On an SD card, class refers to how quickly it can accept video data from your digital camera.This number is critical if you're shooting HD video.
So when you're buying new SD cards, check both the speed and the class - to be sure they match your camera's requirements. That's the easiest way to get the best performance from your digital camera.
Most of us know that bright overcast is ideal for shooting... but we don't get those conditions as often as we'd like. So here are a few quick tips to improve your chances of getting better shots in real-world weather.
When you're shooting at the beach (with light reflecting off sand and water) or in winter (with light reflecting off snow and ice), your camera's light meter will often be fooled. Most digital camera's light meters will set the exposure as if the sand, snow or other bright surface is a medium gray tone... and your pictures will be overexposed.
You may have a couple of options here. Many cameras offer "exposure compensation." This feature lets you set the camera to expose for up to two or three stops less or more light than the meter reads. Using exposure compensation, you can tone down those bright situations for a more accurate photo.
The second option may be found in your digital camera's "scene modes." Many digital point-and-shoot cameras offer a snow scene mode, beach scene mode or similar setting that compensates for the excessive brightness of these situations.
Another common weather-related challenge is shooting in a snowstorm. Here, you need to be aware that using flash will cause all the nearby snowflakes to reflect light back into your camera's lens. Depending on how heavily the snow is falling, you could end up with shots that look like thousands of fireflies were buzzing your lens... and little else in your image.
And speaking of snow... don't forget that most batteries perform poorly in cold weather. To keep your batteries working longer, you keep your digtal camera tucked inside your coat. Your body's warmth can extend battery life on a chilly day.
Finally, beware of humidity. On very humid days, taking your camera in and out of air conditioning may result in internal condensation. It's bad enough when your lens fogs up. But when moisture condenses inside the camera, you may have to wait many hours for the situation to resolve.
Keep these tips in mind, and you'll get better results from your digital camera... no matter what the weather.
Today's auto-everything cameras have made taking snapshots easier... but not necessarily better. And that's because cameras can't think. They don't understand what part should be in focus.
That's where depth of field comes in.
Depth of field is the distance in front of your camera that's in focus. For example, if you focus on a person 20 feet in front of your camera, there's an area closer and further than 20 feet that's actually in focus. This is your depth of field.
Three factors play major roles in determining depth of field.
First, there's the distance between your camera and your subject. All other things being equal, the closer your subject is to the camera, the narrower your depth of field. So, if you move the person in the example above to just 10 feet from the camera, there will be less in front and behind them that's in focus.
Second, there's the focal length of your lens. (This is the "lens length.") The longer the lens's focal length, the narrower the depth of field tends to be. So, all other things being equal, a 50 mm lens generally has a greater depth of field than a 200 mm lens. (And the 70mm setting on a zoom lens has a greater depth of field than the 200 mm setting on the same lens.)
Finally there's the "f-stop" or "f-number." This is a measure of how much light the lens allows in. Although f-stops are actually ratios, they're expressed as single numbers, such as f 1.4, f 2.8, f4.5, etc. The smaller the f-number, the narrower the depth of field.
Now, here's where all this knowledge comes in.
Let's say you're shooting an outdoor portrait, but the background is a busy street. To blur more of the background, you can shoot with your subject standing closer to your camera. Or, you can switch to a telephoto lens (or zoom your lens out). Or, you can set your camera to a smaller f-number.
On the other hand, you may be shooting a landscape scene, and want to get as much in focus as possible. In that case, you may not be able to move your subject. But you could still switch to a wider angle lens (or zoom out) or use a larger f-stop setting.
After you've practiced these tricks a few times, they'll become second nature - and your pictures will stand out from the pack.
Buying from a brick-and-mortar retailer has its advantages, particuarly if you are buying your first, second or third digital camera. But, in all fairness, shopping online has advantages, too.
The first and foremost advantage to shopping online is selection. Very few brick-and-mortar retailers even come close to the selection of digital cameras online. If you're looking for a camera with specific features, this could be an important consideration.
For example, only a handful of cameras have a fast lens that zooms out 10x or more. And because these models tend to be relatively expensive, many retailers prefer not to tie up inventory dollars stocking them. After all, a brick-and-mortar store serves a limited population.
On the other hand, an online retailer can afford to carry these cameras because they can sell them to customers anywhere. And that means quicker turnover of their inventory. So you're more likely to find the special features you're looking for online.
Price can be an advantage to buying online... but exercise caution. Sometimes online retailers achieve "bargain" prices by selling parallel imports - that is, gray market merchandise that doesn't carry the manufacturer's US warranty. Other retailers simply inflate prices unbelievably. For example, the Canon EOS 40D has a suggested retail of $1,099. But I found online retailers selling it for as much as $1,434.68
I was also able to find several reputable online retailers selling the same camera for under $850 - with the US warranty. So the good deals are out there. You just have to exercise a little caution.
Digital photography really works on much the same principle as traditional film phtography. When the camera's shutter is tripped, it allows light to enter the camera very briefly - sometimes for just 1/1000th of a second or less. The light strikes a light-sensitive surface inside the camera, and the "pattern" - colors, tones, etc. - is recorded on that surface.
In film photography, the light-sensitive surface inside the camera is film. Silver halide crystals in the film's emulsion react to the light, recording the image. Later, in film processing, the image is "fixed," or made permanent.
Digital photography's process is a bit more complex. In place of film, digital cameras use sensors. The image is recorded by an array of tiny receptors on the sensor, and the information from each receptor is transmitted to the camera's memory. The information from one receptor is one pixel.
Most digital cameras store the information from each exposure as a digital picture file on a solid-state "flash memory" device. (A few cameras can use a "micro drive" - a tiny mechanical hard drive.) You can then display the individual picture files on the camera's LCD screen... or download them to your computer or another digital storage device.
There he was, right in front of me. The King of Trees - or something like that. Bedecked in foliage with his face and hands painted to match, he cut an impressive figure. But how could I get the shot?
You see, I was at a Renaissance Faire, and this character had the most impressive costume I'd seen yet. But the background included cars and people in modern dress. They'd spoil the mood of my shot.
Fortunately, I understand selective focus... and the shot was saved. The King came out in sharp focus, while the distracting background elements were a pleasant blur.
How can you do the same? It's easy.
First, get comfortable using your camera's manual settings. Then you can use a wide aperture to narrow your depth of field. This little trick lets you decide how much of what's in front of your lens will be sharp. (This is one reason higher-end digital SLR's have a depth-of-field preview button.)
Second, you can use a telephoto setting or long lens. The longer the focal length setting of your lens, the narrower the depth of field. When you're at a greater distance from your subject, "pulling them closer" with your zoom will result in a narrower depth of field.
Either way, the narrower depth of field will allow you to blur elements that might distract from your image.
Some digital cameras have lenses with a fixed maximum aperture. For example, f-2.8 or f-4.0. Other cameras have variable maximum apertures. This is usually marked on the camera or lens as f-3.5 - 5.6, or something similar. So what's going on here?
It's simply the need for affordable zoom lenses.
You see, the f-number is actually a ratio. It essentially compares how wide the lens opens to the focal length of the lens. For most people, that's a little confusing, so let's look at my favorite metaphor:
Let's say you're inside a huge cardboard box. You're sitting in a chair eight feet from the box's far wall and you want to read a book.
Now let's say I cut a 4" hole in that far wall. It isn't going to let a lot of light into an eight-foot box, is it? You probably wouldn't be able to read your book. But a three-foot hole in the side of the box would let in a lot more light. And then maybe you could read.
But let's say the box is only three feet long. In a box that short, you just may be able to read your book by the light from a 4" hole.
It's very much the same with lenses. The longer the lens, the wider it has to be able to open to admit a useful amount of light. And that can mean a lot of expensive glass. How expensive? Well, a Nikon 80 - 200 mm f-2.8 zoom lens will run you about $1,000 or more.
Imagine tacking a few hundred dollars onto the cost of a digital point-and-shoot camera. It would be an affordable option for a lot less people. And that's why digital camera manufacturers use variable maximum apertures: less glass and less cost.
So, when you see a camera with an f-3.5 - 5.6 (for example) lens, you know the zoom will be a lot faster at the wide-angle end than at the telephoto end. That's a good point to keep in mind when you're choosing your new digital camera.
You want quality photos... crisp images with high resolution. You know the tiny image sensors on compact point-and-shoot cameras won't do the job for you. But digital SLRs are just so bulky.
You may have heard there's a compromise out there: micro four-thirds. These cameras offer higher quality than compact point-and-shoots, but not the bulk of a digital SLR.
And they do offer interchangable lenses and many of the other features of a DSLR.
Is a micro four-thirds camera the one for you?
Maybe. But micro four-thirds isn't just a DSLR without the mirror. You see, the sensor in a micro four-thirds camera is still a fair amount smaller than the sensor in even a low-end digital SLR. And that low-end DSLR probably costs quite a bit less.
Another consideration is your choice of lenses. This system offers far fewer dedicated lenses. (Olympus offers a lens adapter... but then you have another expensive piece of equipment.)
If you're looking for a bump up in quality, you'll get it with a micro four-thirds camera. But they can't match most digtal SLRs.
Most point-and-shoot digital cameras make saving files easy: the only option they offer is a JPG (or JPEG) file. We'll discuss JPG files in a moment.
But first, when you move up to a digital camera with more features and options - such as a superzoom model or digital SLR - you'll often encounter other file options.
The two most common are TIFF (Tagged Image File Format) and RAW files.
RAW files are unprocessed image files - the camera saves the image information exactly as it's recorded. RAW files tend to be large, because they're also uncompressed. The advantage of saving your digital images as RAW files is that you can then exercise complete control in your "digital darkroom" - the image-editing software you use. (Note: Not all image-editing software can import RAW files.)
TIFF files save information with each pixel. So, even though TIFF files are compressed, they tend to be very large. In fact, depending on the type of compression used a TIFF file of a digital image can actually be larger than an uncompressed RAW version of the same file! As the use of RAW files has increased, fewer and fewer cameras have been offered with the option of saving TIFF files.
Virtually all of today's digital cameras offer the option of saving files in JPG format. (JPEG is short for "Joint Photography Experts Group.) This file format is popular - in part - because you can save a JPEG file using different levels of compression.
But compression is also JPEG's Achilles' heel. That's because a JPEG file is compressed by tossing out data. The format is configured to delete redundant data, but what's considered "redundant" changes with the degree of compression requested.
But don't panic if you've been saving files as JPEG's. There are ways to maintain the quality of a JPG file... and we'll cover them in another tip.
I enjoy shopping online - and, chances are, you do, too. It's fast, convenient and - if you exercise just a little common sense - safe. But it also presents some disadvantages - especially when buying something as personal as a digital camera.
"Personal" may sound like an odd word to use for an item like a camera, but it's accurate. And it's also why you may want to consider buying your digital camera from a brick-and-mortar store.
The size and weight of a camera makes a big difference. For example, the Casio Exilim EX-S10BE weighs in at less than four ounces (without battery). And it's a tiny 3.71” x 2.15” x .59”. If you want a small camera, that may sound ideal... but folks with big hands are likely to find it too small to use comfortably.
Near the other end of the spectrum is Nikon's D300 digital SLR. At 23.9 ounces (about 1-1/2 pounds) - and that's without a lens - lugging the D300 around for a day of shooting would wear many people out.
That's why "trying a camera on" is important. You won't use a camera that isn't a comfortable fit in both size and weight as much as you'll use one that fits you well. And that's wasted money.
The second reason to consider buying from a physical store is design. Sure, you can see what a camera looks like online, but you won't know how a camera feels in your hand until it's actually in your hand. Are all the controls conveniently placed? Is the grip comfortable?
Finally, there's performance, and that's tied closely to how you use a camera. Shutter lag (the time between when you press the shutter release and when the exposure is actually made) , write time (how long it takes the camera to transfer data to memory), auto-focus speed... these are all critical differences.
Many important performance numbers are available... but it's hard to translate a number into experience. For instance, I could tell you that a camera's shutter lag is 0.1 second, but what does that mean when you're shooting?
"0.1 second" sounds pretty small, doesn't it? Sure, but it's the difference between getting a picture of your child blowing out her birthday candles - or a shot of smoke streaming upward from the candles she just blew out.
Shopping online is convenient. And sometimes it saves you money. But when you're looking for your ideal digital camera, you're better off going to a store and "trying them on."