The Camera 

The camera is the "earth" of the system. To choose the right camera, you have to consider various things: the first is surely the system itself, that is even more important than a single camera. With time, it is likely that you will upgrade the camera with a more advanced model, while it is not easy to change system if you discover that the one you have choosen doesn't satisfies your necessities.

A good system offers a wide range of lenses and cameras; has top level technology and is updated every year to offer the latest and most advanced tools to photographers. Your choice should not be based just on your current necessities, you should also consider what you will need in future. Nowadays, the best systems are Canon and Nikon, both produces excellent cameras and lenses, and overall they are on par. 

The image quality

The image quality is one of the most important aspects of every camera: even the most advanced camera of the world wouldn't be a good choice if it had poor image quality. The best way to know the image quality is to try the camera; if it is not possible, you can read reviews and download image samples from reliable websites, as the excellent DPreview. The various aspects of IQ are described in-depth in other articles, but here I'll summarize the three things to look at.

The resolution determines the amount of detail of the image, thus the maximum print size and the amount of cropping that you can do while maintaining a good quality. Even though megapixels are not the most precise way to measure resolution, they can give you an idea. Nowadays, even the cheapest cameras have al least 10-12 megapixels, thare are enough both for amateur and professional.

The noise is the "grain" that becomes more visible the more you raise the ISO sensitivity. A good camera must be usable throughout the entire range from ISO 100 to 1600; obviously, the highest ISO sensitivities are more noisy, but the noise and detail must be acceptable even at ISO 1600. Nowadays, the majority of SLR cameras are usable in this range, even though the noise level varies from camera to camera; the best SLRs, as the Nikon D700, Nikon D3, have so little noise that they can be used even at super high ISO sensitivities as 3200 or 6400 ISO.

The dynamic range is the range of brightness values that can be registered by the sensor: a wide dynamic range is better since it allows to capture more detail both in shadows and highlights. Nowadays, nearly all SLR cameras have a dynamic range of al least 7-8 stops, that is enough for the majority of the subjects.

Sensor size: Fullframe and APS-C

The sensor size is an important aspect of the camera. Currently, there are mainly two sizes: the "FullFrame" (24x36mm) and the APS-C (16x24mm). They have both advantages and disadvantages. The APS-C cameras are much cheaper than FF cameras: they range from the $600 of Nikon D60 to the $1600 of Nikon D300. Other than that, they are smaller and lighter than FF - the lightest APS-C weights just 500g (Canon 1000D), while the lightest FF weights 900g (Canon 5D). APS-C sensors usually have higher pixel density than FF sensors - for example, the Sony A350 has 5.1 µm pixels and a pixel density of 3.8 MP/cm², while the Nikon D700 has 8.4 µm pixels and a pixel density of 1.4 MP/cm², and the Canon 1DsIII has 6.4 µm pixels and pixel density of 2.4 MP/cm. This is both an advantage and a disadvantage. If you take photos of very distant subjects - for example birds - an higher pixel density helps to get more magnification of the subject; on the other hand, an higher density usually results in worse noise performance, and you need very high quality lenses to get the best from the sensor.
The Fullframe cameras are more expensive than APS-C: they range from the $2400 of Canon 5D to the $7800 of Canon 1DsIII. Other than that, they are heavier and larger. The main advantage of FF are image quality at high ISO and extreme wide angle. FF cameras usually have better image quality than APS - for example, both the Nikon D300 and the Nikon D700 have a 12 megapixel sensor, but the latter has much lower noise thanks for the FF sensor, that offers more surfare to "collect" the light and has larger pixels. If you use often sensitivities of ISO 1600 and above, the FF cameras are the right choice for you.  Other than that, there are some wide-angle lenses for FF that don't have any APS-C equivalent: for example, the Sigma 12-24 and the Canon 24 f/1.4 don't have any equivalent.

Continuos shooting and buffer

If you take photos of birds and wildlife, the continuos shooting is very important. The 2-3 FPS of entry level cameras as the Canon 450D or Nikon D60 are a bit slow for action; if you are interested in wildlife, I'd suggest to buy a camera that reaches at least 5 FPS. The majority of mid-range cameras reach this speed (the Canon 40D has 6.5FPS, the Nikon D300 6 FPS), and the top professional cameras, as the Nikon D3 and the Canon 1D MarkIII, reach even 10FPS.

Other than speed, you have to evaluate the buffer: entry level cameras have a poor buffer, usually 5-6 RAW photos, while mid-range and high end cameras have a buffer of 15-30 RAWs, than is enough to shoot at the maximum speed for some seconds.

Complete control on the exposure

Any serious camera must have the four essential exposure modes: P (Program), S ot Tv (Shutter priority), A or Av (Aperture priority) and M (Manual). I use Av for the 90% or my photos, and M for the remaining 10%: these modes are essential because they allows to control some of the most important aspects of the image, the exposure and the depth of field. The "scene modes" (landscapes, portrait, macro, ect) that are often found in consumer cameras, insted, are pretty useless.

Other than that, it is important to have an "Evaluative" or "Matrix" meter, that calculates the exposure analyzing the entire image with advanced algorithms. I don't consider essential the other types of meter, as center-weighted or spot, since Evaluative allows to get almost always good exposures once you learn to use it properly, in conjunction with the histogram. Histogram and Highlights warning are essential: once you have learnt to use these tools, you can judge the exposure at a glance and you will rarely miss a photo due to bad exposure. 

In many situations, you have to compensate the exposure. The best cameras allows to compensate the exposure in steps of 1/3 stop, in a range of +/- 5 stops. In the majority of situations a range of +/- 2 stop is enough, but in some particular scenes you need to apply strong exposure compensations, and if the range is limited you have to switch in manual mode and set the exposure manually.

Other than that, it is essential to have a wide range of shutter speed, at least from 30" to 1/4000, plus "B" (bulb) exposure; all current cameras have at least this range of speeds. The longest speeds are very useful for landscapes: in early morning or late afternoon, the best hours for landscapes, the light is very dim, and you need long exposures (of course, you should use the lowest ISO setting and a sturdy tripod). In some situation, you might even need the B pose, to take exposures of minutes: in this case, it is essential to have a remote release, to avoid the vibrations caused by the shutter release. The fast shutter speeds are useful for action and wildlife: if you like the photograph the animals in action, you need very fast SS to "freeze" the movements. Using wide apertures (f/4 or f/5.6) and ISO 400 I often get speeds between 1/1600 and 1/4000, that allows to create amazing action-stopping images. 

You should pay attention also to the sincro flash, that is the fastest shutter speed that you can use with the flash (at full power). A fast sincro flash is better because it allows to use flash even in bright light, to lighten the shadows: the best cameras have a sincro of 1/250, while entry-level cameras have a sincro flash of 1/125 or 1/160. 

LCD and Live View

The first digital SLRs had small, dim screens: now the technology has made big improvements, and the best SLRs have 3" LCD with a resolution ot 230,000 or even 900,000 pixels. The viewing angle has improved a lot, too: the most recent LCDs have a viewing angle of 160 degrees; they are very bright and easy to read even in direct sunlight. Even though the LCD screen does not allow to judge the sharpness of the image as precisely as your computer screen, it allows to determine if the image is noticeably blurred or misfocussed, using the zoom function during playback or live view.  Nowadays, nearly all cameras have Live View, that is the possibily to frame the photo throught the LCD. I consider LV essential and I'd never buy a camera that has not LV - for landscapes and macro, it is truly useful; it makes much, much easier to frame the photo and the judge the focus. Some cameras have even a tilting screen, than can be "extracted" and rotated - it is a very nice thing, that makes LV even more useful. Currently, the cameras with tilting screen are the Olympus E-3 (lateral, upward, downward tilt) and Sony A350 (upward, downward tilt). On the left: the Olympus E-3 has the most versatile LCD and Live View currently available in a SLR.

Wheater sealing

If you plan to use often the camera under the rain, in dusty or saindy environments, or in other "unfriendly" environments, I'd recommend to pay attention to the sturdiness of the camera body and to weather sealing. Weather seaded camera are not waterproof, you can not use them underwater, but they can be used without worries under the rain and in other difficult situations. The following table lists the weather sealed cameras currently in production. Canon: Canon 1D MarkIII ($4050), Canon 1Ds MarkIII ($7800) Nikon: Nikon D300 ($1600), Nikon D700 ($3000), Nikon D3 ($5000) Sony: Sony A700 ($1300) Olympus: Olympus E-3 ($1500) Pentax: Pentax K20D ($1050)

Other things to consider

The RAW mode and bit depth. All SLRs have RAW (if you want to get the best image quality, RAW is essential). Nowadays, the main difference between SLRs is that some allow to choose between 12 bit and 14 bit RAW (e.g. Nikon D300, D700, D3), while other cameras - as all Canon SLRs - have only 14 bit RAW. Personally, I consider the 14 bit just a waste of storage space, so I think that the possibility to choose 12 bit RAWs is an advantage.

Nowadays, all SLR cameras have at least an acceptable autofocus. The speed and reliability of the AF varies from camera to camera, but it is impossible difficult to judge the quality of AF by technical specifications - more AF points don't mean better AF. The only way to evaluate the AF is to try it :-)

Almost all cameras have a sensor cleaning function, that shakes a filter placed in front of the sensor to remove the dust. It does not guarantee to have always a clean sensor, but it is a nice help that allows to save some time and to change lenses without worrying about dust on the sensor (in the worst case, you can remove the dust by yourself with a blowing brush, it is not difficult).

The menu system should be well organized and easy to navigate, and the most important functions should be readily available through the buttons on the camera body or through the main menu. An essential function, if you are seriously interested in macro and landscape photography, is mirror lock up, that locks up the mirror some seconds before the actual exposure; it allows to avoid the blur caused by the mirror slap at slow shutter speeds. 

The size and weight of the body varies considerably: some consumer bodies, as the Canon 450D, are very small and light, while the top professional cameras are big and bulky. If a small camera body is important for you, I'd suggest to consider a consumer or mid-range body. 

Do you have comments or questions?

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