The crop factor of a sensor is the ratio of the diagonal dimension of the reference format to the diagonal dimension of the sensor. 35mm film is the common reference format. What is it for a photographer? Let’s understand what is crop factor in photography, and why is it important to be familiar with different crop formats/modes.
Crop Factor & Sensors in Photography
The above image lists the dimensions of different sensors available in the market. To keep it all simple, we will stick to the most used APS-C, APS-C DX, and full-frame sensors. Keep in mind that full-frame sensors are the digital equivalent of the classical analog 35mm films. Let’s see what these formats are.
APS-C stands for Advanced Photo System – Type C. An APS-C sensor dimensions range from 20.7×13.8 mm to 28.7×19.1 mm. Canon uses a 22.2×14.8 mm sensor for its APS-C format. Other manufacturers, like Nikon, & Sony, use a 23.7×15.6 mm sensor for their APS-C formats, also called DX formats. Before talking about crop factors, it is crucial to understand what is focal length, and the field of view. If you’re already familiar with that terminology, you can scroll down to “Crop Factor in Photography“.
Focal Length of a Lens
Focal length is the basic parameter that defines a photography lens. It is usually measured in mm. Focal length is the distance between the camera sensor and the lens. In other words, it is the distance between where the image is formed and the lens when the subject is in focus. The focal lens also has an impact on the field of view and magnification factor.
Field of View of a Camera
Before we start, the red rectangle in the above image represents the 35mm (or full-frame) sensor, whereas the blue color box represents an APS-C sensor. Now, the front end of a lens is circular. Hence, the projected image will be a circular image like the one shown above. If you study the image, you will see that a part of the image is falling outside the 35mm sensor, and that part is clipped in the final image. This is how we get a rectangular image from a circular lens.
Now, the APS-C sensor is smaller than the full-frame format sensor. Hence, they see a smaller part of the vision than the full-frame sensors. The field of vision (a.k.a field of view) is the coverage of the scene that a sensor can actually see. That is, a smaller sensor has a lower field of vision than that of the full-frame sensor. Also, increasing focal length in a variable focal length lens will reduce the field of view of the sensor due to their manufacturing constraints.
Crop Factor in Photography
Understanding the field of view and focal length is crucial to learn the significance of crop factor. As defined earlier, the crop factor of a sensor is the ratio of the diagonal length between the full-frame lens and the sensor diagonal length. The diagonal length sensing area of an APS-C/DX-format lens is 29mm approximately. Similarly, the diagonal length of the sensing area of a 35 mm full format lens is 43 mm. Now, the crop factor of an APS-C/DX sensor is 43/29=1.5(approx.). But, what is the significance of this value?
The above image shows a setup of a full-frame sensor and cropped sensor(any sensor smaller than a full-frame sensor), a fixed focal length lens, and the subject. The image also marks the angle of view of the lens and the field of view of the sensor. Now, you can see that the cropped sensor has a cropped field of view, unlike the full-frame sensor. We have already discussed that field of view is affected by focal length. From the above scenario, the field of view is also dependent on the sensor size, even though there is no change in focal length.
In effect, different sensors see a different field of view of the same subject at the same focal length. Crop factor helps you analyze the field of view with respect to that of the reference format, full-frame sensors in this case. APS-C/DX-formats have a crop factor of 1.5x. So, if you’re using a 50mm full-frame lens on an APS-C/DX camera, you’ll get a focal length equivalent to 75mm (50mm x 1.5x = 75 mm focal length). In other words, a 75mm lens on a full-frame camera and a 50mm on an APS-C/DX camera will provide you the same field of view.
Crop factor in Full Frame Cameras
The crop factor of full-frame sensors is 1x (So no cropping happens here). However, if we move on to video recording even some full-frame sensors can have a crop factor. Strange? Let’s take an example of Canon EOS R. It has a crop factor of 1.7x for shooting 4K videos. What does this mean? Why is there a non-unit crop factor for full-frame cameras?
All full-frame cameras have a crop factor value 1x while shooting still images. While shooting high-resolution (such as 4K) videos, modern cameras use the entire sensor to record videos at even higher resolution and then oversample the video. This method provides superior video quality. Or, another method to record high-resolution videos is to only read a part of the large sensor. Canon EOS R uses this method as it has a higher resolution(30 MP). The 1.7x crop factor while recording 4K accounts for the change in the field of view from the perspective of a full-frame readout.
So why do manufacturer’s do not specify a crop factor for high-resolution video recordings? There are other methods to store lower format videos apart from partial readout from the sensor. One such technique is called Pixel Binning. Canon EOS R uses this technique to record FHD and HD videos.
Practical Concerns | Crop Factor in Photography
Crop factor will not be an issue for most still photographers who landed straight into APS-C sensors. It is most significant for those who have any experience or deal with a full-frame camera. The crop factor helps you in identifying the right lens for the field of vision you compose from the perspective of a full-frame camera, in terms of still photography.
In filmmaking, selecting the right lens is crucial to match the right field of view for a scene. If a full-frame camera uses crop factor while recording in a specific resolution, make sure that you use the lens compensating for the crop factor to get the right field of view while making movies in that specific resolution.
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