From: Exercise 1 Focal Length and Angle of View

I posed my self the following:

NOTE: I need to do some reasearch into my camera to establish the sensor size and the relationship to the focal length.

QUESTION: What is the standard focal length mathematically and how do I calculate it?

The first thing I have discovered is that My Canon EOS 7D has a 18MP APS-C CMOS sensor which is not full frame.


A full-frame digital SLR is a digital single-lens reflex camera (DSLR) fitted with an image sensor that is the same size as a 35 mm (36×24 mm) film frame (Source) (Source 2 Canon White Paper)

From the course we determined that a normal or standard focal length is one that gives an image in the viewfinder equivalent to the view seen by the human eye

I found this definition

Focal Length

The technical definition of a standard lens is one whose focal length roughly matches the diagonal or the film or image sensor. For a standard full-frame 35mm camera this gives a focal length of around 43mm.

In reality, the actual focal lengths chosen by manufacturers tend to be slightly longer than this. For a 35mm sensor, 50mm is the most common standard lens, although some companies do sell lenses which are closer to 43mm focal length.

I found this here at photography mad: which also states:

Crop Factor

Because the “ideal” focal length depends on the sensor size, cameras which are less than full-frame will require lenses with shorter focal lengths. For example, a camera with a 1.5x “crop factor” will require a lens which is 1.5 times shorter than the standard 50mm, which works out at 33mm. A number of manufacturers produce 35mm standard lenses to meet this requirement.

On Digital Picture .com I found a useful table and an explanation of crop factors which suggests my camera has a crop factor of 1.6 my maths tells me that a standard focal length for my camera should be 50 / 1.6 which rounds down to a 31 mm lens.

Next question is why does the camera show 50 mm on the zoom scale when the image looks normal? It appears from this Canon article that the focal length is correct or true as it says at the bottom of the article:

It is important to realise that there is no change to the actual focal length of the lens when it is used on cameras with different sensor sizes. Focal length is a characteristic of the lens and cannot be altered simply by moving it to a different camera.

As my lens is an EF lens and is intended to be used on full frame Canons such as the 1D or the 5D, it would be rather odd for them to have marked it with the equivalent focal length, so I assume that the magnification is the same but the angle of view is not, thus I see an image that is equivalent in magnification but more cropped than a 35mm frame.

I wondered why any of this mattered at all if the result was much the same then it occurred to me that this would not be an issue for telephoto users but would be a problem for wide-angle users as the standard 28mm wide-angle lens would not show as much of the scene. It seems that canon has remedied the problem by producing lenses such as the EF-S 10-22mm which has framing equivalent to a 16-35mm zoom on a full frame.

I think the reason that this question originally stuck out for me is that my normal working lens is a Canon EF 28-200mm USM lens and by my recollections of the old days of my Pentax SP2 and my Canon A1 28mm would have given a very wide effect that this lens does not seem to replicate, I know why now and have ordered a canon EF-S 10-22mm so life should be perfect now.