Wildlife photographers, and especially bird photographers, have specific requirements in the equipment they purchase. They are always looking for more reach to fill the frame with their subjects, but that is not the only factor. The main factors to consider are:
These factors must be balanced to achieve the best reach while maintaining acceptable image quality. Image quality is a relative matter, and depends on the photographer's standards and the planned use for the image. It is probably worthwhile for me to describe what I consider to be acceptable image quality, and show how changing certain factors can impact image quality. Many photographers assess image quality by how the image looks in a given size print viewed from the appropriate distance. While this is a valid approach, I do not subscribe to it because too many extra variables are introduced. I assess images by viewing them at 100% on a monitor. This means that each image pixel is represented by a single monitor pixel 1:1. When an image has excellent technical quality it looks good at 100%. It reaches out and says "you nailed it".
I took this image under optimal conditions with a D700, 600VR, TC14 at f/5.6, 1/640s and ISO 200. Perhaps if I had closed down to f/8 and shot without the TC14 then I could have improved the image quality, but for me, this is acceptable.
The 100% crop above shows fairly good detail while not over-sharpened. Note the small amount of noise in the green background. Green backgrounds are particularily susceptable to showing noise, and all images have some noise. Noise impacts image detail. In this case, the impact is minimal.
I expect this type of quality from a super telephoto system. There will be losses due to imperfect technique and higher ISO but I will be deciding to sacrifice image quality, not dictated lower image quality due to equipment capabilities.
Keep this thought in mind as you read ahead and look at balancing budgets, weight, sensors, bodies and lenses.
This shot, on the other hand, is less than optimal. It was also taken with a D700, but using a 500mm, TC14 at f/5.6, 1/4000s, ISO 1250. The D700 is pretty good at ISO 1250, but I under-exposed this shot by 0.5 EV and compensated in lightroom, which emphasized the noise some more. In this shot I would have been better advised to shoot at 1/1600s and ISO 640. I do not view this quality as acceptable.
Both of these images were taken with pretty good camera/lens combinations and show how image quality can degrade as image noise increases. The second image is perfectly acceptable for web use, or to make a 5x7 print, but would not hold up in a 20x30 inch print.
In DSLR cameras sensors currently come in four main sizes and can be described by their focal length multiplier (FLM). The size of the sensor, compared to full frame (36mm horizontal) is also called the crop factor. The standard APS-C sensor is 24mm wide, or 2/3 of the full frame sensor. This reduces the field of view (FOV) and makes the same object, viewed at the same distance with the same focal length, appear 1.5 times larger than a camera with a full frame sensor. In this case FLM = 1.5x. The four main sensor sizes and the cameras that use them are:
This image of a Bufflehead illustrates what you would see in the viewfinder with a camera at the various sensor sizes using the same lens focal length. The full frame shows the Bufflehead in its habitat while the APS-C produces a frame filler. One thing to consider is that the depth of field (DOF) remains the same no matter which camera you used. DOF is a function of distance to subject, focal length and aperture, and it is not a function of sensor size. However, if you were to move closer with the same lens and aperture setting on a full frame camera to duplicate the FOV seen with the APS-C sensor the DOF would be reduced. Therefore, for the same field of view, larger sensor sizes result in shallower DOF. Something to consider.
Another key sensor characteristic is the pixel density, which is a function of the sensor size and the number of sensor pixels or sensor elements (sensels). The higher the density, the greater the detail that can be captured, as there will be more pixels on the subject. This is not all roses as smaller sensels exact a penalty in image quality. The smaller the individual sensels, the less photons can be accumulated in a given shutter exposure time. The sensels capacity to collect photons is proportional to the area, which is a square function of the diameter. Currently diameters vary between 4.3 μm (7D) and 8.5 μm (D3s), or 14 μm2 to 57 μm2. This means a Nikon D3s captures 4 times as many photons as a Canon 7D in each sensel. The greater the number of photons collected, the lower the ratio of the noise to the total light, and the higher the perceived quality of the image. You will find a thorough explanation of this phenomenon here. The larger sensels, with lower noise, allow exposures at higher ISO values before the noise becomes an issue. Another characteristic of smaller sensels is increased diffraction at medium apertures. A D3s will show hardly any softening from diffraction at f/11, while the 7D will. You can learn more about diffraction here.
Smaller, high density sensels also require more rigorous shooting techniques to avoid image blur due to camera movement. As the sensels get smaller, the slightest jiggle is magnified more and more. Also, the smaller sensels quickly show deficiencies in all but the very best lenses. A benefit of smaller sensels is that they only use the central sweet spot of a lens, while full frame sensors stretch out to the corners of the lens circle, where the lens tends to be softer, distorted and subject to chromatic aberratiion. However, these concerns tend to be more evident in wide angle lenses, and the wildlife photographer will likely be using a super telephoto lens.
In summary, smaller sensors with high pixel counts will produce more pixels on the subject and utilize the center of the lens circle, but you will pay a price in the quality of those pixels, which will be noisier and perhaps softer, especially as the ISO is increased.
If you are photographing birds then you will want the longest lens you can afford and can carry. Nikon and Canon are the leaders in long lenses, with a possible edge to Canon, who make an 800mm f/5.6 lens that is lighter than the 600mm f/4 lenses and f/4 or f/5.6 versions for most focal lengths. A word of advice on lenses. Many photographers start out with something in the 300mm range and then decide that is not enough focal length, but suffer sticker shock when they see the prices for the 500mm and up lenses from Nikon and Canon. Suddenly the third party lenses such as Sigma look pretty attractive. Well, this is where you get what you pay for, and the very best lenses hold their value better than the cheaper alternatives. Save yourself a step and buy the best. You are better off buying a last generation Nikon or Canon lens, which may not have the latest image stabilization, but will have superb optics and very fast focusing.
Most wildlife photographers add a teleconverter to their super telephoto lens to stretch the reach. Adding any additional glass/air interfaces will impact image quality, but the impact is minimal for a 1.4x convertor. The Nikon 1.7x converter works quite well, with only a small amount of image softening. While both Nikon and Canon have come out with a new generation of 2x converters stay away from them. They significantly reduce image quality, except perhaps at near distances, up to 10 meters. Current camera bodies cannot rapidly and reliably focus at apertures slower than f/5.6. At apertures up to f/8 you will be able to acquire focus on static birds, and birds in flight (BIF) against a bright sky, but you will lose them when they move to a low contrast background such as a hillside or forest and the lens will hunt back and forth. This means you are limited to using a 1.4 converter on a 500mm or 600mm lens and no converter on the 800mm lens if you need this type of focus acquisition and tracking.
Super telephotos benefit tremendously from image stabilization and the best support systems. Some camera manufacturers include image stabilization in the camera body. Aside from the claims that in-lens stabilization is superior to in-body stabilization, especially for long lenses, the in-lens stabilization also stabilizes the image in the viewfinder, which is a real benefit when using a super telephoto.
I am defining the field of view (FOV) as the equivalent focal length to see the same view in a full frame camera. For example, the FOV for a APS-C 1.5x camera such as a D300 with a 500mm lens would be 750mm. When maximizing reach, the current equipment provides options between 1000mm and 1500mm. Quite often you will find 1000mm to be too much, but if you are using a lens and teleconverter you can remove the teleconverter. The greater the FOV focal length the more difficult it is to initially find the subject and to track the subject. Also, for subjects under 10 meters, shooting at 1000mm or greater will result in a very shallow DOF. Finally, as you push the FOV it becomes more demanding for excellent camera technique. It is challenging to get a critically sharp image shooting at 1500mm. You might have to resort to weighing down the lens to dampen vibrations and using mirror lockup.
When working with a super telephoto lens large, hefty camera bodies are preferable to balance the mass of the lens mounted on a Wimberley gymbol head and have room to grab on with both hands when tracking animals or birds. Heavier bodies have more inertia and result in sharper images at slower shutter speeds. Camera controls that facilitate exposure and focus adjustments without taking your eye away from the viewfinder are also very useful. It is much easier to shoot in portrait mode on a gymbol mount with a vertical grip integrated into the camera body. The top level professional bodies are all heavy, ergonomic and have integrated grips. The lesser models all have accessory grips, so almost all camera bodies meet these criteria to some extent, with the possible exception of ergonomics.
Full frame cameras have bigger and brighter viewfinders. This is important because, as you develop as a wildlife photographer, you will use manual focus more and more with super telephoto lenses. It is much easier to manually focus with a larger, brighter viewfinder. Another benefit of full frame high megapixel cameras is that they allow you to see the area around the subject and crop later. This makes it easier to track animals and birds that are moving rapidly.
When shooting action photography you are more likely to capture the moment if you shoot a burst of frames so the frames per second (FPS) and the burst duration or buffer size can be a factor. In general, when shooting action, 5 FPS is glacially slow and 10 FPS is pretty darn good. The buffer determines how many frames you can shoot at maximum FPS before the camera starts to slow down as the buffer fills and has to transfer to the memory card. If the buffer is less than 15 frames you will run into issues from time to time. A buffer of 30 or greater should satisfy most shooters. Don't forget you are going to have to assess all those frames and agonize over which ones to keep.
Perhaps the most important camera feature is how rugged it is. You need a camera that can survive the occasional bump, rain and snow, dust, heat and vibrations: all the things that inevitably happen when you are out and about.
In the following table I have attempted to quantify the two key reach factors: pixel density and pixel quality. The pixel density is simple to calculate, but the pixel quality depends on many factors and can be somewhat subjective. I have based pixel quality on the DxO Mark sensor sports (low ISO) rankings. My experience is that I start to lose optimal quality at about one half of the DxO Mark score. For example, the DxO ISO score for the D700 is a lofty (at this time anyway) 2300. However, when you start to peer at 100% magnification pixels, you have to be doing everything right at ISO 1250. There is a dramatic difference in the DxO marks compared to the pixel density values, so I have taken the square root of the Dxo ISO scores and referenced them to the D3s and multplied this by the pixel density to come up with a resolving quality evaluation. The resolving quality* is a combined weighting of the pixel density and the pixel quality.
When you rank the cameras by pixel density the larger FLM sensors tend to lead and result in getting the most pixels on the subject. This is probably a reasonable ranking if you are able to photograph under ideal conditions at base ISO or one stop above base ISO.
When the pixel quality is added to the mix, the ranking changes considerably. This ranking is probably more realistic when you factor in real world shooting conditions.
If you find yourself shooting at or over ISO 800 on a regular basis, then based on this then you would probably choose the 1DMK4, D3s or D3X. If you are more of a fair weather shooter or on a budget then the 7D or D7000 looks like the best.
The other factor to consider is the available lenses. This is where the 1DMK4 becomes very attractive when paired with the 800/5.6. Lots to consider :*)
|*||I doubt if many will agree with my method of determining a resolving quality value but you are of course welcome to invent your own, perhaps inspired by my feeble efforts.|