Camera Lenses: a Crash Course

Focal lengths 2: digital variations

No larger version

Fig. 4. Relative sensor sizes for some common digital camera formats

All this changed when digital came on the scene. The omnipresent compact digitals have a "normal" focal length about 1/5th or 1/6th that of 35mm, depending on the model. So 8 to 10mm (which was extreme telephoto for 35mm) is normal for that size camera. For example, the popular Canon Powershot A-series cameras, such as the Canon A720 IS have lenses with focal length ranges like 5.8mm to 34.8mm. This would go from ultra wide angle to moderate wide angle on a 35mm film camera, but on the A-series these focal lengths have the same reach as 35mm to a whopping 210mm on a 35mm camera. In fact, the actual 5.8 to 34.8 numbers are rarely used for such small cameras, while the 35mm to 210mm numbers are used instead with the understanding that these are the 35mm format equivalents.

The markings on the lens barrel of a Canon A720 say 5.8-34.8.
We know the conversion factor to 35mm is 6.
So 5.8mm x 6 = 35mm and 34.8 x 6 = 210mm.

The majority of dSLRs (often called APS-C or DX format cameras) have sensors that are 1/3 smaller than a 35mm film frame, and so have a 1/3 smaller normal focal length of 33mm. In practice this means multiplying the actual focal lengths on the lens by 1.5 in order to get the familiar 35mm equivalent values. For example, most APS-C format kit lenses seem to be 18-55mm zooms like the one the ecstatic young lady in Fig. 1 is using. On a 35mm camera this would be ultra-wide angle to normal, but because of the 1.5 multiplier, they are in fact the equivalent of 27-82mm.

The markings on the barrel of a typical kit zoom lens say 18-55mm
We know the conversion factor to 35mm is 1.5.
So 18mm x 1.5 = 27mm and 55mm x 1.5 = 82.5mm.

Many Canon dSLRs have a slightly smaller sensor, requiring that we use a 1.6 conversion factor, so an 18-55mm zoom would be a 29-88 equivalent lens. Olympus and Panasonic market cameras with even smaller sensors called 4/3rds and now micro 4/3rds, which have a simple 2x conversion factor. The kit lenses for 4/3rds have so far been 14-45mm, equating to 28-90mm.

Finally, dSLRs are becoming available in the original 35mm format, now often called full frame, or FF. On an FF camera, a 75mm lens is 75mm-e, no multiplier is required.

Note: Because of the complexity of having to re-think the practical meaning of a given focal length for each digital camera format, it is very common to talk about focal lengths in all formats in their 35mm format equivalent, unless the actual focal length is important. Mike Johnston introduced a useful shorthand for this, which I'll use here. I'll write mm-e when referring to the 35mm equivalent, for example: the 14-45mm kit lens for a 4/3rds camera would be 28-90mm-e.