Lenses, Sensors, Pixel Density and Resolution
By: Gary Gray
Here are my thoughts on different cameras and resolutions vs different lens focal lengths.
You may find this interesting.
Canon 7D ~ 18 megapixels, Crop sensor = 1.6x field of view. APS-C
Canon 5D MK III ~ 22 megapixels, Full Frame Sensor = 1x field of view. Full Frame
Canon 1Dx ~ 18 megapixels, Full Frame Sensor = 1x field of view. Full Frame
Canon 1D MK IV ~ 16.1 megapixels, Crop Sensor = 1.3x field of view. APS-H
Canon Crop Sample Image
The dimensional relationship between Full Frame, APS-H and APS-C sensors
What is the equalized Field Of View and equivalent resolution on each body with a 500mm Lens?
7D = Field of View = 800mm @ 18 Megapixels
5D MK III = Field of View = 800mm @ 8.6 Megapixels
1DX = Field of View = 800mm @ 7.03 Megapixels
1DMK IV = 800mm @ 8.83 Megapixels
What does this mean?
Well, if you're looking to get the most reach out of your lens, meaning the narrowest effective field of view, you have to calculate the field of view as it's modified by your camera.
A full frame camera uses the same field of view as the lens, so a 500mm lens on the 5D MK III or 1DX gives you a 500mm field of view. On crop bodies, the formula is FOV = Crop factor x Focal Length. So, on a 1DMK IV, the crop factor is 1.3, so a 500 mm lens gives you an effective focal length of 500 x 1.3 = 650mm. On A Canon 7D, the formula is 500 x 1.6 = 800mm effective focal length.
The second consideration is the pixel density of the camera for any given field of view. If we equalize all cameras to the same field of view, you'll see that the 7D has an almost 10 megapixel advantage over the other three, more expensive cameras, when it comes to throwing raw pixels at the image.
This is a handy thing when trying to determine what the maximum print size will be off of any camera. This pixel density difference is what makes the Canon 7D a very desirable camera for wildlife. You can throw a 400mm lens on a 7D (or any APS-C sensor body) and get an effective field of view of having a 640mm lens coupled with 18 megapixels of resolution, compared to throwing a 500mm lens on a 1D MK IV which will give you a slightly longer 675mm effective field of view with 16 megapixels. The bottom line in the Canon lineup, the EOS 7D is the best bang for the buck (not necessarily the best camera) if you're looking for a high performance camera with good telephoto results, because it gives you more resolution and more reach with any given lens than any of the other bodies and it allows you to buy a more affordable lens in the process. A 400mm prime lens will cost you about $1,400 vs the 500mm costing you $10,500. The quality of the image between these cameras is not significantly different. Also, the more you crop an image in post processing, the less detail you get. So, those full frame super high pixel count bodies will blur quickly when cropping them down to the same FOV as the 7D, because you're going to be reducing the number of overall pixels in the image.
The main differences between the cameras will be frame rates and high ISO performance. The Canon 7D will perform quite well up to ISO 2500, whereas the full frame bodies will perform well up to ISO 6400. The MK 4 is excellent up to ISO 3200. The slowest frame rate is on the 5D MK III at 6 fps, the 7d comes in at 8 fps and the other cameras will perform at or above 10-12 fps. 6 frames per second is pretty fast, so you could use any of these cameras effectively as a wildlife or action camera. However, resolution trumps noise. You can eliminate noise more effectively if you have more resolution because you can afford to lose more detail in the noise reduction process. So, the 18 Megapixels in the 7D with noise reduction will still provide a cleaner image with more resolution than a heavily cropped full frame camera. When you hear "gear heads" talk about the 7D noise levels being too high, they don't understand what they are talking about. Their eyes are glued to the display and they aren't thinking about applying noise reduction and making prints. This is what makes a professional grade crop sensor body such as the 7D a worthy camera for wildlife photography.
You can do the same comparisons with Nikon bodies too, just use a 1.5 crop factor for their APS-C bodies.
To equalize their full frame to crop pixel density, simply divide the full frame pixels / 1.5 / 1.5 So, a camera such as the D800 with 36.3 megapixels equalized to a 1.5 crop body such as the D7100, you'd have the equivalent field of view on the D800 with 16 Megapixels.