About me:

On an amateur level, I practice photography with passion and delight. I am an autodidact; everything I know I have learned from books and magazines. I prefer "slow" photography, such as landscapes, scenery, still life, architecture, the passive type of animals - generally spoken, subjects that don't mind the time it takes me to first dig out the proper lens from my camera bag, and then to the best of my knowledge master to set camera parameters such as focal distance, exposure measurement method, white balance, aperture, exposure time (*sigh*) ... and to eventually release the shutter. Action or sports photography do not belong to my preferences.

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Take my word - I never ever intended to ridicule myself by exposing schoolmaster-like behavior on this website. But I just have to contribute my little share in straightening out an unfortunately widespread and even in serious photo magazines by now argumentatively well-established nonsense that I simply cannot take/hear/read any longer! Apparently it cannot be repeated often enough.

"Focal length extension" or "crop factor". Why 200 mm focal length never metamorphose to 320 mm.

The lens of a camera provides us with a circular image... the so-called image circle. The camera's viewfinder then shows us a rectangular cut-out of this image circle. This extract depends on the camera sensor's format.

The sensor format

A so-called "full-frame" camera with a sensor size of 36 mm x 24 mm (aspect ratio 3:2) produces a rectangular image circle cut-out whose size approximately corresponds to the diameter of the image circle.

However, most digital cameras on the market today are equipped with sensors that are smaller than full-frame sensors, for example sensors of the so-called APS-C size (sensor size approx 22.5 mm x 15 mm, aspect ratio also 3:2).

Full-frame versus APS-C sensor

Click the image to enlarge it.

Referring to the resulting impact the use of a smaller sensor has on the picture, the term "focal length extension" is quite often and light-heartedly used (particularly in the German language where its termed Brennweitenverlängerung)... a completely misleading and cardinally wrong term, since a lens's focal length is by no means extended just because the lens is used on a camera with a sensor smaller than full-frame. A smaller sensor just covers a smaller area of the image circle; it simply creates a correspondingly smaller cut-out if the image circle. The rest around this cut-out is lost! The real representation size of the subject is always the same - regardless of the sensor!

The full-frame sensor of a Canon EOS 5D MkII, for example, has a side length of about 1.6 times longer than the APS-C sensor of a Canon EOS 7D or EOS 60D. This fact is then falsely called a "focal length extension by 1.6", just because a look through the viewfinder or at the LCD display creates the subjective impression of using a lens with a longer focal length.

This impression of a "focal length extension", however, simply results from the fact that the APS-C camera represents its smaller cut-out on the same, for example, 3 inch LCD display as a full-frame camera would represent its larger image on. The image reproduced on the APS-C camera's display is simply enlarged by the factor of 1.6.

Quite often this lack of understanding is getting to a point where someone using an APS-C camera multiplies himself a 50 mm lens to a "light telephoto" lens of 80 mm, or a 200 mm lens to a "super-tele" with a focal length of 320 mm. This is stark nonsense! An APS-C sensor does not zoom in on the subject 1.6 times; it is just lacking image information because that is cropped all-around.

Therefore, a far more accurate term than "focal length extension" would be "crop factor" — as the English language unmistakably terms it. In German: Beschnittfaktor.

So, what is to be done?

If I want to shoot with an APS-C sensor the exact same image as with a full-frame sensor, I inevitably have to reduce the image. There are two options to do so:

  1. I stay where I am. In that case I need a 1.6 times wider angle, ergo a 1.6 times shorter focal length (30 mm instead of 50 mm, for example). Now my image representation is identical to the one of a full-frame camera. The image, however, is physically smaller; the depth of field is greater. The perspective does not change because of the the shorter focal length; it depends solely on the the photographer's position!
  2. I do not change the lens but move away from the subject 1.6 times. Now I have the same image representation with the larger depth of field again, but also a different perspective because I changed my position.

Far too complicated?

Hmmm... perhaps a simple mnemonic aid? APSC = narrow-minded!
Got it now?