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Crop factor (format factor, focal length extension)

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That is what the term crop factor meant

Literally translated, the term means something like image detail factor. Unfortunately, this term is often translated as "focal length extension" and is therefore often misunderstood. With this article we want to shed some light on the darkness of the crop factor.

Right at the beginning some information about the focal length: Apart from some compact and bridge cameras, where an equivalent focal length is actually indicated, the focal length indicated on the lens is an unchangeable physical property (size) of the lens.

This means: If a lens has a focal length of 50mm, nothing changes in that. Regardless of whether you attach the lens to a camera with a small or large sensor format. 50mm focal length remains 50mm focal length.

Common names for sensor formats

  • FX - (Nikon) - 35 mm format 36 x 24 mm
  • DX - (Nikon) - "Crop" format 23.7 x 15.6mm - factor 1: 1.5
  • Small picture (Canon) - small picture format 36 x 24 mm
  • APS-C (Canon) - "Crop" format 22.2 x 14.8mm - factor 1: 1.6
  • MFT (e.g. Panasonic) - Micro-Four-Thirds - Factor 1: 2

Sensor sizes are responsible for the crop factor; the actual focal length of a lens is always unaffected by the sensor format. The image section used changes depending on the size of the sensor in the camera. What exactly this means, we want to explain in more detail with the following illustrations:

The APS-C sensor of a Canon DSLR of the 1200D, 700D or 70D series, for example, is smaller than a sensor in 35mm format, such as the 1Dx or 5D use. In the first case, the sensor diagonal measures around 27 millimeters, while the 35mm format is around 43 millimeters. This means that the APS-C sensor is 1.6 times smaller than the full format sensor.

Basically, you can achieve the same effect if you print out a photo and cut off a wide border around it. However, the result is then correspondingly smaller. In most cases, however, the photos will be printed in identical sizes for comparison or shown online. Due to the smaller sensor, the image appears larger after enlargement to the same format than a photo taken with a camera with the same focal length at the same distance from the subject. The magnification factor corresponds exactly to the factor of 1.6 (Canon) or 1.5 (Nikon).

The term full format

One often reads the term full format when writing about DSLRs that have the larger sensor.

This term is not entirely correct, because there are cameras with significantly larger image sensors.

Full format refers to the fact that the camera size discussed here used the classic 35mm film at analog times (24 x 36 mm) and the APS-C cameras, for example, have a smaller sensor. So you want to differentiate between the small sensor (APS-C / DX) and the "full" format, such as the 5D series. The designation small image format or KB is therefore correct and less misleading.

By the way, APS-C is a name that has its origins in the APS film cassettes. The APS stands for Advanced Photo System. These cassettes offered the possibility of recording three different image formats:

  • APS-H with 30.2mm x 16.7mm in 16: 9 aspect ratio
  • APS-C with 25.1mm x 16.7mm in 3: 2 aspect ratio
  • APS-P with 30.2mm x 9.5mm in an aspect ratio of 3: 1.

This film format was mainly used in compact cameras because of its ease of use. Films for APS cameras have not been produced since the end of 2011.

Since the full format (35mm format) was the standard format for hobby photography for a long time, many photographers still compare the image effect by extending the focal length by the crop factor.

Exactly this fact often leads to confusion, because this "conversion" only makes sense if the photographer has an actual reference to the ratio of image detail to focal length in the 35mm format. Anyone who has never worked with the 35mm format will find no reference to the statement that a lens with 50mm focal length on an APS-C camera works like a lens with 80mm focal length on a camera with 35mm format.

In fact, the crop factor only describes the image section related to the smaller sensor that is perceived as an enlargement. In fact, it is not enough to just convert the focal length for comparability; for example, the aperture must also be changed in order to be able to really compare.

This is where the difference becomes clear at the latest. KB format has a higher release potential due to the open aperture than APS-C. In practice, however, you don't have to worry about it very much.

We would now like to explain the difference a little using the following examples in order to make the topic more transparent:

If the effect of the change in the angle of view is not taken into account, the crop factor has an advantageous effect on telephoto lenses. In the case of wide-angle lenses, on the other hand, the reduction of the section loses some of the “wide-angle”, which is why there are lenses especially for the smaller APS-C sensors that start at a focal length of 8 mm. These lenses can then only be used on cameras with a small sensor. The image circle generated is not sufficient to illuminate the sensor of, for example, a Canon EOS 5D or Nikon D800. The advantage: Due to the smaller image circle, these lenses can be made more compact and therefore cheaper.

A Sigma 12–24 mm for full format sensors costs around € 800, while the more compact Sigma 10–20 mm is available for less than € 400. So if you limit yourself to cameras with APS-C sensors, the smaller and cheaper Sigma will always be the better choice. If, on the other hand, you work with both formats (or plan to do so), the investment in the more expensive optics is worthwhile, as it fits both types of cameras (the 10-20, however, not).

At Nikon it is a little different: there some cameras in FX format can be switched to DX (or do this automatically if appropriate lenses are connected). However, the outer part of the sensor is then simply switched off and only the center of the sensor is used.

Reading tip: Online photography course "Technical basics of lenses"

 

If you would like to deal with topics such as focal length extension, circle of confusion, crop factor and angle of view in more detail, we recommend our online photo course Technical Basics of Lenses. In this photography course you will learn, among other things, which focal lengths you are still missing for your photographic motifs and which lens class is sufficient for your purposes.

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