Always a source of confusion: the so-called crop-factor. “On APS-C a 35mm lens becomes a 50mm lens. Because, well, crop, you know.” Why that’s not true at all but not that wrong either is a thing I’ll try to explain.
Some decades ago, when the hobby of photography reached the masses, there was mainly one kind of film (you know film? These rolls they used to put into a camera): the 135 or 35mm film. This went into the leica, the canon, the zenith, the minox 35, or simply put: when an amateur put a roll of film into his camera you could bet it was a 35mm roll. The negative of a 35mm film was about 35mm x 24mm and there you already have the name explained.
Now imagine: the year’s 1972. You’re walking around with your shiny new Canon F-1 and a 35mm lens attached . Suddenly you spot a beautiful Ferrari Dino at the gas station on the other side of the street. You take the camera to your eye and shoot. But you can already see in the finder that the image won’t be a keeper: In addition to the Dino there’s also a fat dude on one side of the car and a heap of empty oil canisters on the other side. So you take off the 35mm lens and put a 50mm lens on the camera and take another picture just before the owner (remember the fat dude?) get’s into the car and takes off.
At home you go into your darkroom and develop the images. Yep, the image taken with the 35mm lens is no keeper, too much clutter on it. The 50mm one would by perfect, but it’s blurred! What now? Well, not all is lost. You are a master of the darkroom and develop only part of the negative of the image taken with the 35mm lens. Let’s say an area in the middle – about 15mm x 23mm, cut (or crop) the rest and blow it up to the same size as your other pictures.
You then put your zoomed-in, new image alongside your blurry 50mm image and realize: except for blurryness the images look astonishingly similar! The field of view of theses two images is nearly identical, you can’t distinguish the 35mm image of the 50mm one.
So did you just magically transform a 35mm lens to a 50mm one? Of course not, the lens is still the same, the physical properties didn’t change. But because you only developed part of the negative with a side length difference of 1.5 to the original negative, the field of view also changed by a factor of 1.5. The ‘cropped’ (that’s what it’s called) image taken with the 35mm lens looks like it was taken with a 52mm lens.
So, let’s get to APS-C vs full frame. When digital SLRs came to be, sensors were very expensive. A bigger sensor was not only more expensive, it was also a lot more difficult to build without defects, so a DSLR with a full 35mm x 24mm sensor would have been prohibitly expensive. Solution? Build smaller sensors, name them APS-C size and call it a day. These smaller sensors were used behind the same bayonets as the analogue films, and thus behind the same lenses. But as the side lengths of the light sensitive area was about 1.5 times smaller, the field of view also changed by the same factor of 1.5. If you used a DSLR and an analogue SLR side by side, the images with the 35mm lens on the DSLR showed roughly about the same field of view as one shot with a 50mm lens on the SLR.
And so the crop factor was born.
Should you mind the crop-factor? Well, not really. Problems arise only if you want to compare different camera systems because the field of view und thus the resulting image derives from a combination of sensor size and focal length. On point-and-shoot cameras it became custom practice not to print the correct focal length on the lenses but the equivalent focal length of a 35mm camera. And as sensor sizes are different the nomenclature also differs. Whereas a 35mm lens is considered wide angle on a full frame camera, it’s a normal lens on an APS-C and a portrait lens on a Micro-Four-Thirds camera (crop factor 2 ).
But if you’re just looking for a new lens – don’t bother. Just make sure the lens will work on your camera and shows the field of view you desire.
So, why are there full-frame and APS-C lenses anyway? Well, the focal length is one thing, the image circle another. A lens that can project an image onto a 35×24 mm surface area needs wider lenses then one that only has to project onto a 23×15 mm area (see the tiny lenses on your mobile phone? That’s because it also has a really tiny sensor). So an APS-C lens can be build smaller, lighter and often also cheaper than a full frame lens.