The most important decision you’ll need to make, whether you’re buying your first SLR camera or you decided to switch your lens lineup is the lens mount and sensor size (full frame / crop). Of course this decision will be based greatly on existing camera models but you also need to consider differences in the lenses offered by different manufacturers.
The sensor size traditionally was (and still is) important because bigger sensor can give better picture quality. With the latest cameras, though, even the small four-thirds sensors will produce high-quality photos -- also at high ISO levels -- so this is not as important as it once was. Of course if shooting in extreme conditions (very dark) this may still be an important factor. Very often, though, the sensor size will automatically determine which lenses you can choose from. You can’t use crop lenses on full-format cameras and using full-format lenses on crop cameras is of course possible but is not ideal: the shifted focal range may not make a lot of sense and you’ll be carrying around a heavier / bigger lens than one made specifically for the smaller sensor size. Of course you may want to buy full frame lenses and a crop camera if you’re short on money and don’t want to replace your lenses when full frame cameras get cheaper. Either way, don’t count on the manufacturers to develop a wide range of new crop lenses, especially bright ones.
The lens mount is of course the most important decision. Here much will depend on your shooting style. Whether you prefer primes or constant-aperture zooms or perhaps zooms with a huge focal range will most likely be the most important factor in determining the lens mount you should decide for. You’ll see big differences between some manufacturers, here are two examples:
1. Let’s say you like primes as much as I do and need a basic set of 20 / 35 / 50 / 85mm (which is also what I use) which covers the range from very wide angle up to short telephoto with four relatively cheap lenses. Let’s take a look at the prices (from a single store) of this set for a few mounts:
Nikon full-frame: $1396
Canon full-frame: $1261
Sony full-frame: $1196
Note: Whenever there was a choice I opted for the cheaper version in this price comparison e.g. 50mm f1.8 and not f1.4.
Now let’s see what happens when you go with Pentax. Here you’ll use a crop sensor so the lenses will need to have different focal lengths: for a similar experience I chose 14 / 21 / 35 / 50mm which come close to their full-size counterparts -- usually not as bright as the full frame lenses above but, on the positive side, smaller in size. The total price for the Pentax lineup is $2135.
As you see, the differences between Nikon / Canon / Sony are not shocking but you still might prefer one over the other (e.g. because of the differences in lens sizes or built). Choosing a different mount yet (e.g. four-thirds) makes such a line-up impossible altogether.
2. If you’re after bright constant-aperture zooms, you may be interested in getting two lenses: 24-70mm and 70-200mm (both f2.8) to cover everything from wide angle to very long telephoto. Here are the prices for different mounts:
Nikon full-frame: $3627
Canon full-frame: $3227
Sony full-frame: $3297
If you go with the four-thirds lenses (14-35mm and 35-100mm respectively) the price will add up to $3752.
As you see the difference in price here is already higher. Adding an ultra-wide zoom will make the differences even bigger.
Another thing to consider is how you like the controls / menus of different manufacturers. Bodies made by the same manufacturer tend to have a similar logic so you’ll need to see what you prefer. If you only used cameras from a single manufacturer, you may be surprised how different the controls can be.
Of course if you like buying second-hand equipment, mounts such as Nikon / Pentax / Sony=Konica Minolta may be advantageous because of a huge number of used lenses for film cameras (whereas Canon’s mount changed more recently from FD and four-thirds didn’t even exist a few years ago).
If you mostly buy third-party lenses (Tamron / Tokina / Sigma), they exist for multiple mounts so here you don’t have to worry about the lenses offered by the camera manufacturer. The decision between the crop and full-frame sensor size still needs to be made, though.