What's the difference between a cropped (APS-C or micro four thirds) DSLR sensor and a "full frame" 35mm sensor?
Every article I read online seems to miss the three most glaring points that differentiates the larger full frame sensor from their cropped counterparts.
Here are what I feel are the best arguments for a full frame sensor. It's not what you think. And it's unlikely what you have read on the internets.
A Bit of Basic Knowledge
Aperture refers to the opening of the lens, Think of it like your iris in your eye. It closes (becomes smaller) when its bright and opens when it's dark. It is measured as a reciprocal (ie. "one over x") so larger numbers equals smaller apertures. f/1.8 is bigger than f/4.0.
Focal length gives you your field of view. Longer focal lengths give you a narrower field of view. Eg. A 200mm lens gives you a more zoomed in view of the world than a 50mm lens.
Perspective is determined by the distance between the camera and the subject. Standing further away from a subject makes things look flat (think paparazi photos of celebrities!). Standing closer to a subject makes things look "3D" (use your camera phone for a selfie!). You will notice your facial features are exaggerated (big nose?) when the camera gets too close.
Depth of field simply refers to the area which is in focus. These three things will give you a shallower depth of field:
1) Longer focal lengths.
2) Larger apertures. (smaller f/ number)
3) Lower distance (nearer) to subject.
The Why of Full Frame
In the camera world, full frame simply refers to the size of the sensor used. Full frame = same size as traditional old skool 35mm film. It is 864mm sq in a 3:2 width:height ratio. Digital SLR's mostly use APS-C, or "cropped" sensors. They are smaller, and measure 370mm sq. So what are the benefits of a full frame camera? Everyone online harps on how it has better ISO performance and how it gives lenses better depth of field etc. Oh how they completely missed the point. Below are my reasons for going full frame.
1) The Viewfinder.
Ever used a film camera? The viewfinder is huge compared to the APS-C DSLRs. Twice the size, in fact. It makes a world of difference to shooting because ultimately, you shoot with your eyes.
2) That "Magical" quality - Better Perspective vs Depth of Field
This is a bit hard to explain, so please bear with me. It took me awhile to realize why my 35mm film shots (with a 50mm f/1.4 manual lens) had a certain magical quality to them. It wasnt the resolution (I scan at 6MP). It wasnt the dynamic range (it's noticeable even on a low contrast projector).
At normal few-arms-length distances, especially indoors, a 50mm f/1.4 lens gives just the right amount of depth of field and field of view to give images a certain look that I really like.
On an APS-C cropped sensor camera, I could technically take the same shot with a 35mm f/0.95 lens (it will look exactly the same). But such a lens does not exist. If I were to use a 50mm f/1.4 lens, the crop would mean that I would be standing further away from the subject. Moving further away does two things: It changes perspective (loses that '3D'ness) and increases depth of field (less background blur). This is why cropped sensor cameras will always look different for low light indoor shots and street photography.
3) Lower pixel density = Better Sharpness = Better Quality Pixels
ISO performance is enhanced by simple physics, but it's NOT the main reason why a bigger sensor produces sharper images. New cropped sensor DSLRs have very good ISO performance!
Real reason: The smaller your sensor, the more pixels are crammed into a smaller space. That also means that the resolving power of the lens is pushed to the limit, in addition to exacerbating the effects of other factors like physical vibration/movement. A bigger sensor means that the lens quite literally does not need to generate an ultra sharp small image. Images captures on a bigger sensor are simply sharper.
Ever wondered why those Full HD (1080p, read: 2 megapixel) youtube music videos look so great while your 8 megapixel cameraphone takes pictures that look muddy in comparison? Simple. They were shot using professional gear with relatively huge sensors. Even with the extreme compression youtube uses to make videos small enough to stream, the difference is still palpable.
It's not just resolving power. The lens is also not pushed to the limit for simple things like color differentiation. Colors will pop more, and the added contrast adds that extra dimension to your photos.
That "magical quality" is even more apparent in medium and large format photography. The film used is many many many times larger than a 'full frame' camera. Film is still shot today by serious professional landscape photographers and hardcore hobbyists.
If you are a beginner reading this article...
You do not need a full frame camera. Any camera with enough manual controls will do. Choose any camera that gives you manual control over aperture, shurtterspeed, ISO and white balance and you're set. Until you hit a wall with your photography, these differences dont really matter to a novice, and FF is simply not worth the cost.