Crop factor in Camera is a term that can cause a lot of confusion when you're trying to pick a digital SLR camera. It doesn't help that it's referred to in several different ways.
For example, all of the following mean the exact same thing:
- Camera Z has a 1.5 times crop factor
- Camera Z has a focal length multiplier of 1.5 times
- Camera Z has a 1.5 times lens focal length conversion factor
- A 50mm lens frames like a 75mm lens on Camera Z
As most of you know, the standard full frame camera has a digital sensor of 35mm (which is the same size as the 35mm film frame).35 mm format cameras use roll film with an image dimension of 24X36 mm, which is the smallest format currently used in the professional market. The majority of cameras in the 35 mm market are single lens reflex (SLR) cameras, although the format also encompasses compact cameras and rangefinders.
In more recent years a type of hybrid cameras, the Semi-professional, more recently known as the "Prosumer" (Professional Consumer) camera has appeared. These SLRs tend to have more of the features of the professional ranges, but are cheaper and targeted at the serious amateur. Often, the lower price results in less professional features, one of which is the size of the digital sensor which seems to get smaller and smaller the less advanced the camera becomes. A couple examples of such sensors would be the digital APS-C sensor, and the four-thirds digital sensor.
Crop Factor in Camera, thus, is entirely dependent on the sensor size of camera, not the lens mounted on it. The smaller the sensor, the larger the crop factor, resulting in a narrower field of view.
Now, when the photographic film or digital sensor is exposed to light, an image is projected on that photographic plane where it comes into focus within the active area of that plane. In a full frame, the size of the active area of that plane is the same size of a 35mm film.
So if you’re using a full frame camera, the image projected would have the same dimensions and field of view as if you were using a film camera. If you’re not using a full frame camera, your digital sensor would not be of the 35mm size, but rather would be an APS-c or a four-thirds digital sensor or something similar as we’ve mentioned earlier, which is smaller in size. Consequently, an image captured by this kind of sensor would be smaller than that captured by a 35mm sensor. The projected image will partly fill the active area of the sensor, and partly fall off the active area of the sensor. So the resulting image will seem to have a narrower field of view (or a more zoomed-in view effect) than what the lens is actually projecting.
So a given lens will always project the same image onto the digital sensor or photographic film, but the part of the image that is actually recorded will depend on the active area of the digital sensor.
A full-frame sensor has a 43mm diagonal. An APS-c sensor has a 37mm diagonal. That’s a 1.6x ratio. Taking that into account, let’s assume you’re shooting with a 1.6x crop factor image plane at 50mm focal length. Your actual focal length would then be 50 X 1.6 = 80mm rather than 50mm.
On a full frame (35mm) digital camera sensor, the crop factor is 1. So the focal length of a given lens is the actual focal length you will get on such a full frame sensor.
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