Understanding a reason to care probably is helped by some 35 mm experience. THIS camera always does only exactly whatever its specific sensor and lens determines it does. After we enlarge the smaller sensor image more, then the view is zoomed "as if" it had been a longer focal length on FX. DX has to be enlarged half again more, which is not an equal comparison of what we have. The first obvious reaction looking in a FX view finder is that (compared to DX with same lens), it shows the subject smaller, but the overall scene is larger, a larger area, visually appearing as if from a greater distance. For example, the FX sensor is 36 mm wide, and the Nikon DX sensor is 24 mm wide. Simplistically the crop factor is just the ratio between the sensor width (or height) of a system relative to the full format (e.g. K.I.S.S.
But the smaller DX view, when enlarged to show it same size, is magnified 1.5x more than the FX, which is the same effect seen when FX uses a lens that is 1.5x longer focal length (more telephoto than FX). In comparison on APS, the cropped view from the same lens looks like 300 mm (would look on Full Frame or 35 mm). It is still a smaller image, but the same field of view in this case. Sports and wildlife photographers may prefer the DSLR 1.5x crop camera (over full frame) because its smaller cropped area makes their lens appear 1.5x longer (only as compared to using that Equivalent focal length on a full frame sensor, like 35 mm film). 300 mm lens on a full frame camera result is a 300 mm view, so to speak (in terms of 35 mm film frame size).18 mm lens on a full frame camera result is an 18 mm view (in terms of 35 mm film frame size). But the lens mountings were not normally interchangeable among those film formats (often not removable at all), so other than in the darkroom enlarger, using the same lens on different film formats was not often possible. The Nikon Full Frame (FX) models give you both choices so you can shoot DX or FX mode, for both Raw and JPG. (or the Below are D300 DX and D800 FX images, using the same 105 mm lens, on the same stationary tripod, at same distance.
Using a longer lens focal length magnifies size to crop the field of view smaller, and a smaller sensor simply captures a smaller image and smaller field of view. FWIW, a typical compact camera with a tiny 7x5 mm sensor and shooting with maybe a 9 mm lens does even more of this, with a crop factor maybe 5 to 1 compared to 35 mm film FX size. Crop factor computes the diagonals, but these are the same aspect ratio (same shape, so crop ratio applies to W and H too). Then a smaller sensor simply crops it smaller, for a smaller Field of View.
So this is a comparison of 1x Full Frame size sensors to APS size cropped sensors (1.5x or 1.6x). The cropped sensor simply captures a smaller cropped image. The camera viewfinder shows these same views. But many people have considerable 35 mm film experience using many familiar lenses, and this can tell them exactly what view to expect from the new smaller camera. Just a quick note: Changing where we stand with the camera to get the same view may give the same angular view, but it does still change a few things: Or, if both use the same focal length lens, standing in the same place, then the larger sensor simply sees a larger field of view. It measures 101mm diagonally. But if the larger sensor instead used the longer 75 mm "equivalent" lens, then the meaning is that it would see the same field of view that the 1.5x cropped sensor sees with its 50 mm lens. Chances are, you might not care what the full frame format may show, unless of course, you had significant 35 mm film experience to be familiar with it. And some poor descriptions do confuse it that way, but there is no magic, and that cannot happen, nothing can change in the lens. It is only the "cropped view" that is different. It is about as wide as you see before moving into panoramic cameras, which I’m not covering for the purposes of crop factor comparisons. When we enlarge the cropped image to be displayed at same size, it appears to be telescopic, as if with a longer lens, or as if standing closer. So, the DX camera view "looks same as" if a 1.5x focal length were used on FX. The "Equivalent" or "effectiveâ focal length number (due to the crop factor) is simply the comparison to FX size, which is the same size as 35 mm film, which many of us were used to for many years. Nikon's name of 1.5x cropped APS-size sensor is called DX, and Canon calls them APS-C for 1.6x crop. The difference in these sensor sizes causes different visual effects. The "equivalent" focal length is used on the other full frame sensor.
Or of course, we can always crop FX to the DX view and size anytime later. Now you may notice that this is actually not so easy for Micro-Four-Thirds because the image ratio is different (4:3 vs 3:2).
Obviously, the top picture below is the view of the same scene, with the same lens, at same distance, but using the two sizes of sensors.
A Crop Factor of 1.5 means that (if both are using the same lens with same focal length) the larger Full Frame sensor sees a Field of View 1.5x larger dimensions than the small sensor (orange sensor case in diagram). A 24 mm lens on Full Frame is serious wide angle, twice wider than a normal lens. Log-log graphs of focal length vs crop factor vs diagonal, horizontal and vertical angles of view for film or sensors of 3:2 and 4:3 aspect ratios by CMG Lee. The lens and its image does not change, but APS sees a cropped view on the smaller sensor (which when enlarged later, appears telephoto). The lens is NOT changed, but DX merely crops to see a different VIEW from it.