Digital Photography from 20,000 Feet

Understanding 35mm and Digital Lenses

If you have no interest in understanding what different lenses do and when to use them, then don't buy a Digital SLR. The main reason SLR exists is to offer a flexible platform for using a wide range of lenses. You could justify the quality aspect and through the lens viewing, but you can find those features on fixed lens cameras. People who buy a digital SLR with one lens they never change and never intend to change are basically wasting their money - they should buy a quality point-and-shoot or fixed lens SLR.

Image Size

To understand how different lenses behave and which lens to use in digital photography we need to go back to 35mm basics. Early 35mm cameras generally shipped with a single focal length "normal" lens. "Normal" is defined as the approximate point of view of your single eye when looking at a scene. That is, if you keep both eyes open when looking through a viewfinder, a camera with a normal lens will look about the same as what the other eye not looking through the lens sees. It is easy to determine the "normal" lens focal length for any image size - it is approximately equal to the diagonal of the image size, which is obviously about the same as the field of view if you consider that as a circle.

Traditional Film Image and Lens Characteristics
Film format	Image dimensions	Image diagonal	Normal lens focal length
APS C	1.67 cm x 2.51 cm	30.15 mm	28 mm, 35 mm
135	2.4 cm x 3.6 cm	43.27 mm	50 mm, 45 mm
120/220, 6 x 4.5 (645)	5.6 cm x 4.2 cm	70.00 mm	75 mm
120/220, 6 x 6	5.6 cm x 5.6 cm	79.20 mm	80 mm
120/220, 6 x 7	5.6 cm x 6.8 cm	88.09 mm	90 mm
120/220, 6 x 9	5.6 cm x 8.2 cm	99.30 mm	105 mm
large format 4 x 5 sheet film	10.16 cm x 12.7 cm (4" x 5")	162.64 mm	150 mm
large format 8 x 10 sheet film	20.32 cm x 25.4 cm (8" x 10")	325.27 mm	355 mm (14")

This means a normal lens for 35mm would be about 45mm. For those who enjoy history you might be interested to know that the 35mm normal lens got defined as 50mm by Oskar Barnack, the creator of the Leica camera. At the time lens technology could produce sharper lenses if they were slightly longer than normal, so Barnack defined normal as 50mm on the early Leicas. The definition stuck, but most would agree normal on 35mm is anywhere between 40 and 58mm. Lenses with shorter focal lengths than normal are called wide angle, because they see more than the normal lens, and longer focal lengths are called telephoto.

You can see from the chart above that "normal" for APS C size, used in most digital SLRs, is about 28mm - and not the 45mm to 50mm typical of 35mm. The digital normal is a little more complicated than the simple math of film normal, because it got defined in the 1950s by TV tube size (normal is about 2/3 TV tube size). However, the real image size for APS C in digital ranges from about 22.7mm x 15.1mm to 23.7 x 15.8mm (sometimes called DX). This means that 28mm is close enough for this discussion.

For those of you who have looked at Digital Camera specs and wonder what a 1/1.8" sensor means in point-and-shoot digital specifications, it means an image size of 7.18mm x 5.32mm - MUCH smaller than the 22.7mm x 15.1mm of APS C. You can see the translations in the chart below of digital.

Digital Image and Lens Characteristics
Sensor type	TV-tube diameter	Image dimensions	Image diagonal	Normal lens focal length
1/3.6"	7.1 mm	4.00 x 3.00 mm	5.00 mm	5 mm
1/3.2"	7.9 mm	4.54 x 3.42 mm	5.68 mm	5.7 mm
1/3"	8.5 mm	4.80 x 3.60 mm	6.00 mm	6 mm
1/2.7"	9.4 mm	5.37 x 4.04 mm	6.72 mm	6.7 mm
1/2.5"	10.2 mm	5.76 x 4.29 mm	7.2 mm	7 mm
1/2"	12.7 mm	6.40 x 4.80 mm	8.00 mm	8 mm
1/1.8"	14.1 mm	7.18 x 5.32 mm	8.93 mm	9 mm
1/1.7"	14.9 mm	7.60 x 5.70 mm	9.50 mm	9.5 mm
1/1.6"	15.9 mm			10.5 mm
2/3"	16.9 mm	8.80 x 6.60 mm	11.00 mm	11 mm
1"	25.4 mm	12.80 x 9.60 mm	16.00 mm	16 mm
4/3"	33.9 mm	18.00 x 13.50 mm	22.50 mm	23 mm
(APS-C) 1/8"	45.7 mm	22.70 x 15.10 mm	27.3 mm	27 mm
DX	n/a	23.7 x 15.8	28.40 mm	28 mm
FF (35 mm film)	n/a	36 x 24 mm	43.30 mm	50 mm

Since the imaging companies are convinced that consumers understand 35mm lens ranges you will find most point-and-shoot cameras define their zoom or fixed lenses as equivalent 35mm specifications. When you see a Kodak P880, for example, defined as a 24-140mm zoom lens you might also notice it uses a 1/1.8" sensor. Since that sensor is about 9mm at normal, the true focal length of the lens is somewhere around 4-28mm. By calling it 24-140mm the manufacturer hopes the potential buyer understands the zoom range in common terms.