When Size Matters
There is often this idea around the interwebs that an APS-C camera is good enough for many professionals and that the difference between a full frame and APS-C sensor is almost negligible. So why would I want an full frame camera vs an APS-C camera and what is the difference?
A full frame sensor is roughly the size of 35mm film, and an APS-C sensor is slightly smaller. (see the diagram)
It's ultimately this size differences that is responsible for any change in visible quality between the two sensors. It's also important to note that megapixels do not matter when comparing sensor size, since the differences is entirely how it works with the optics.
Enlarged to show texture.
I've also included the Micro 4/3 sensor size since it's similar to APS-C and also very popular.
Full Frame Features
Because a full frame sensor is larger it covers more surface area and takes in more light. So naturally a full frame sensor will perform better in a lower light situation. On the Canon cameras the difference is about one stop and with the new 5Dmkiii, it's even a little more. This isn't anything major, but for someone shooting weddings or sporting events you might want that additional stop of light. This would allow you to close down your aperture a bit more or increase your shutter speed slightly.
The other major advantage of a full frame sensor is how it works with the optics. Because the full frame sensor covers more surface area and is hit by more light, it allows for a shallower depth of field. (How blurry background gets from your point of focus.) It's hot a huge difference but is noticeable. It works this way because of math. Yeah, I'll leave it at that.
Say for example you were shooting at an f2.8 on a full frame camera, the background would be much blurrier than if you were shooting an f2.8 on a APS-C sensor. It would perform more like an f4. This can be a pretty substantial advantage if you were shooting medium or even low quality lenses. Lenses tend to look better at an f5.6-f8 so with a full frame sensor you could close down more to hit that sweet spot without losing to much of that shallow depth of field.
The opposite is also true. Because the lens covers more surface area of light through the lens, it tends to accentuate lenses of less quality a bit more. So you'll notice the poor resolution of a cheap lens slightly more on a full frame camera. The same is true as our megapixel count goes up.
APS-C Features
One of the nice features of an APS-C sensor is how it works with lenses designed for full frame cameras. An APS-C lens has typically a 1.3-1.5x crop. The 7D is a 1.4x crop. Meaning, the lens might project light to cover the surface area of a full frame sensor, but because the APS-C sensor is smaller, you get an optical crop. This causes the APS-C lens to perform slightly more telephoto than full frame because of the 1.4x crop. A 50mm will look more like 70mm, 10mm will look more like 15mm, 200mm will look like 280mm and so on. So if you're looking to shoot landscapes, you'll need to go even wider on an APS-C camera. Like a 10mm for great landscapes.
Another advantage to this crop is that you're capture the light mostly from the center of the lenses designed for FF cameras. So cheaper lenses that don't have good corner and edge resolution get cropped out on an APS-C sensor. This of course becomes irrelevant if you're using lenses designed for the cropped sensors like Canon's EF-S lenses.
APS-C cameras are usually smaller, significantly less expensive and in some cases they shoot faster frames per second.
Lenses for APS-C camera systems are also usually a bit cheaper.
Summary
Full Frame Sensors
-Shallower Depth of Field
-Better in low light
-Size of 35mm film
-High Megapixel Count
-More Expensive
APS-C Sensors
-Cheaper
-Smaller
-Often 1.3-1.5x crop
-Better reach, 200mm = 300mm
-Need wider lenses for landscapes 10mm = 15mm
-Less Depth of Field compared to Full Frame
Which Camera System Is Best For You?
That's up to you decide. If you're shooting portraits, weddings or sports where you need as much light as possible and want to get that super shallow depth of field, then you might want a full frame camera. But if you want to get a bit more reach out of your lenses, say with sports or nature photography, or even if you're shooting a lot of landscapes and travel photography where depth of field is irrelevant, than I'd consider an APS-C camera. I personally prefer full frame since they usually have more megapixels so I can print as large as possible. But I still would love to have a compact mirrorless APS-C camera for travel and fun.
