First I'll talk about the strengths of SLR cameras and then proceed to discuss the weakness that I see in them. One of the greatest strengths of an SLR camera today, and in the past, is superior control of depth of field. The larger the format of camera that you use, the narrower the depth of field. For example, on a crop APS-C (1.5 or 1.6 crop) camera you do not have to stop the lens down as much as on a full frame (35mm frame) SLR for example, to gain more depth of field. Although the difference may not be as much as going from a compact camera, the full frame camera will allow more control over how much of the frame is in focus. This is where the strength of interchangeable lenses come in. Most compact cameras have reasonably wide apertures (F3.5, some even F2.5), but due to the small sensor, and the closeness of the lens to the sensor the depth of field becomes rather large, more like what would be achieved with F10 or F16 on a APS-C sensor on consumer and semi-pro DSLRs. The same goes for EVF cameras, that also use larger sensors.
Another strength of a SLR camera is auto focus speed, and accuracy. Modern DSLR cameras have extremely precise phase change auto focus system, which use distance information to determine whether or not the subject is in focus or not. This system is highly refined, because this technology has been in development for over 15 years at this point. The contrast detect system used by compact cameras, EVF, and DSLR cameras in live view does have some issues, namely speed. Have you ever used your compact digital camera and noticed that it can take a long time before the system focuses, that is because the system needs a lot of contrast to auto focus, which in extremely bright light or low light situations, can cause the camera to struggle to achieve focus. Although the phase change system used in SLR cameras can also struggle in bright or dark conditions, they are less likely to do so. The time it takes for the average DSLR to achieve focus is measured in milliseconds, while some compact cameras can take 5-10 seconds or more to achieve focus. The speed of focus may not be very important to you, if you shoot landscapes and posed portraits for example, but if you shoot any kind of sports, wildlife, then you would notice a big difference if you use a SLR system.
Control is another important advantage of an SLR, or EVL system. Many low cost compact camera give you little, or no control over how the camera will take the picture, which can be very frustrating. I can say that from experience, because I went through several mid range to high end compact cameras before moving to an SLR based system. Now, the mid-range and high end compact cameras will give you a lot more control, and some even give full manual control, they can still give you problems due to poor ISO performance, but that is another subject, tied more to sensor size. With an SLR system you tend to have more control over not just the exposure, but also things like how the system auto focuses, or whether the camera will only take a photo if the subject is in focus, for example.
Some disadvantages of an SLR system were mentioned in my last entry, namely weight, size and transportation. Even the most compact DSLRs, like the Olympus E series don't fit in your pocket very well, and if you can get it in there, with say a pancake prime attached, it wont be as easy as taking out a point and shoot. There are times when I find the weight of an SLR system to be less than ideal, such as when I went walking for a four hours last week, and carried my system and a few lenses in a camera backpack. I got some nice photos, but most likely not enough to justify carrying as much as I did. Of course that could have been remedied by not carrying as much gear! Still those weaknesses will turn a lot of people away from an SLR system. That is why the newer EVF cameras are very appealing, since they have the same APS-C or 4/3 sensor that their DSLR rivals have, the image quality is a big step up from a compact, without the weight and bulk of an SLR camera.
Like lenses, DSLR cameras are generally more expensive than compact point and shoot cameras. Some of the compact DSLRs, like the Olympus E series, Nikon's D3000 and Canon's Rebel line of cameras are priced reasonably low, but not low enough for some people. Sadly, at this time, EVF cameras do not solve the price issue either, at this time the cheapest EVF camera is $200 more than the average entry level DSLR.
What about the strengths and weaknesses of compact digital cameras? I will briefly say a few. For strengths, the small size and low weight are the greatest advantages of compact point and shoot cameras, no doubt about it. If you want a camera you can literally take everywhere you go, a compact point and shoot, or phone with a camera are your best bet. You also don't have to worry about damaging expensive lenses and other gadgets that go along with your camera. Some of the downsides of compacts have already been mentioned, such as poor high ISO performance, lack of control over exposure (low cost models generally) and little or no control over depth of field, unless you have a super-zoom camera with a long lens, at which point you start to loose the value and compact nature of a point and shoot anyway.
Next time I will talk about other photography gear.