Professional Grade DSLR Cameras: Why Would I want One?
To start off I'll just say that I will not be covering some aspects of what I consider important when buying a DSLR in this section, I will just be covering why someone should consider a professional grade DSLR camera body. If you want more information on this topic, such as basic principals to follow when picking a camera and lens system, read part two of the Camera Purchasing Guide. If you already own lenses from one brand, it would be a good idea to save money by staying with that brand, unless there is a feature(s) you simply must have for professional work.
There are a few reasons why some photographers should use or at least consider having a professional grade DSLR. In my opinion the primary reasons to consider a professional grade DSLR are, durability, flexibility and speed. I'll go into those reasons more more in depth in just a few minutes. There are other reasons to consider a higher end DSLR, most of which revolve around how you will be using your camera. If you are just using your camera to take photos of your pets and family, you might want to stay with a consumer grade camera, unless money is no object for you. In many cases the consumer grade cameras use the same, or similar sensors that the higher end cameras have, although they tend to get those sensors a year or so after the professional grade camera do. Now on to a break down of the reasons a photographer should consider a professional grade DSLR.
Build Quality: Is A Consumer DSLR Tough Enough for me?
If you are getting paid for your photography it is a good idea to have a camera that can stand up to the demand of a professional work. Most consumer cameras have shutters that are not designed to withstand the heavy usage of a professional photographer. If you find yourself taking 1,000 or more photos a week, you may find yourself wearing out your camera's shutter very quickly. Most consumer cameras shutters are designed to take 50,000-100,000 shots. Those cameras can, and do take more than that, but if you are getting paid for your work, why take the chance? Most professional cameras are rated for 150,000 - 300,000 shutter actions. That can make the difference between getting a new camera every year, or every three years, depending on how you shoot.
Most professional grade cameras also have water and dust seals built in to allow them to withstand outdoor use, better than consumer grade cameras. Even high end cameras can be destroyed by water getting inside, but thanks to water and dust seals around the buttons, and where areas the internal frame pieces join, they are less likely to fail than consumer grade cameras. At this time, the only consumer grade cameras that have any dust or water seals are mid-range Pentax DSLRs, while professional cameras from all the major brands (Canon, Nikon, Olympus, Pentax) have dust and water sealing of varying amounts. Another factor in the toughness of most professional cameras is what the camera chassis is made of, generally magnesium alloys, vs industrial strength plastics in consumer grade cameras.
There are some downsides to the toughness of professional grade cameras that you do have to keep in mind. Those though magnesium alloy chassis often make the professional grade cameras weigh 300-600 grams more than mid-range consumer grade cameras. That may not be an issue if you are shooting in a studio, where you can sit the camera down between shoots, but if you do a lot of hiking, those extra pounds could really slow you down. That weight issue is why many professional shooters have one or two professional cameras, and a low weight consumer camera for when they want to travel light. That being said, if you are going to carry around a bunch of pro lenses, those extra few pounds might not make much of a difference at all. Make sure you check the camera out before you buy it!
Speed: Is A Consumer Camera Not Fast Enough For Me?
Speed is another reason to consider a professional DSLR. I'm not just talking about the fact that most professional DSLRs have faster burst rates, although that can be an important factor in your choice. If you find that a consumer grade camera does not allow you to change settings fast enough for you subject, then maybe a professional grade camera is the answer to your problem. Many consumer cameras force you to dig through the menu system to change important settings, like whether you shoot in single frame or in continuous bursts. Now many higher end consumer cameras put those commands at your figure tips, they still might not be fast enough. For example, on my D80 I had to press a button several times to switch between single frame and burst mode. If I was out specifically to shoot a given subject that was not a major issue, but there were situations where it would have been beneficial to be able to change those settings faster. When I upgraded to the D300 I was easily able to change between those settings with the simple flick of the dial on the top of the camera.
What about frame rates? Many consumer cameras have slow continuous shooting speeds, often between 3-4.5FPS (FPS = Frames Per Second), which for many people is more than enough. That being said, I found being stuck with only 3FPS to be very limiting when I was shooting birds with my D80. Now my D300 can shoot 6FPS (or 8FPS with the grip), which does not sound like a big difference, but it can make the difference between getting the shot I want or not! If FPS are not important to you, then maybe you do not need a higher end DSLR? Then again, you don't have to use the full speed of your camera either, some people never take their cameras out of single frame mode.
Auto focus is another area where professional DSLRs are superior to their consumer counterparts. Generally speaking professional DSLR cameras are the first to see improvements in this area, while consumer cameras tend to be a generation or more more in some cases, behind. So if auto focus speed and accuracy are high on your list of features, you might want to consider a higher end DSLR. Many consumer DSLRs have very capable auto focus systems, so make sure you do some testing if possible before you buy for this reason alone. Some pro cameras have a auto focus system with 9+ focus points, up to 46 or even 51 points in some cases. That kind of AF system give you more flexibility in how you auto focus.
Flexibility: What Can I do with a Professional DSLR that I cannot without one?
That is an important question to ask before considering a pro DSLR. If you cannot think of any reasons why you would need a professional DSLR, there is a good chance that you might not need one. On the other hand, if you find yourself always running into limitations of your camera, such as not being able to change basic settings, like metering mode, drive mode, auto focus type, then maybe you need to start looking up. Most consumer cameras do not have the mirror lockup function, which can be extremely useful for long exposures, as locking up the mirror before such a shot is taken will reduce vibration and thus chances of causing blur in the final image.
14bit RAW files, now I have not talked a lot about RAW file shooting, but this could be a factor in your decision. 14bit RAW files give you more flexibility in post processing, giving you more room to recover blown highlights or shadows in the image. Some cameras are slowed down by using 14bit RAW files, so if you shoot a lot of action, you might find yourself avoiding the use of such RAW files. 14bit RAW files are also larger than 12bit RAW files, which means you use storage space on your computer a lot faster. Speaking of larger files, many flagship professional DSLRs have higher resolution, or larger image sensors than consumer cameras.
For example, Canon, Nikon and Sony's flagship (top of the line) DSLRs all have 20+ MP sensors, which at the time that this post was made, no consumer DSLR has. The closest consumer cameras are the Canon 7D and T2i at 18MP. Those large image sensors are great for capturing more detail, but again, the files take up a lot of hard drive space, even in jpg form, which means there are advantages and disadvantages to such high resolution.
There are other factors to consider, but I think that is all I will cover for now. I may revise this entry at a later date.