Digital Photography from 20,000 Feet

Lenses on Digital SLR Cameras

While digital point-and-shoot makers normally quote their lenses in familiar 35mm terms, no such convention exists in the digital SLR market. Here, everything is the opposite, as all specs are defined in 35mm terms even though the digital SLRs themselves use a smaller APS C size sensor. This is even carried so far as using 35mm equivalents to define lenses designed for the Digital SLR - even though the "digital-design" lenses can't even cover the frame on a full-size 35mm camera. Thus lenses like the common 18-55mm Canon and Pentax kit zooms and the 18-70mm Nikon and Sony (Minolta) kit zooms really describe those lenses as if they were 35mm.

To know what a lens can do on a digital SLR you have to know the lens factor. Nikon, Pentax, Sony, Minolta, Samsung (Sony sensor) and most others use a sensor about 23.6 x 15.8mm with a lens factor of 1.5. This means that if you multiply the stated lens focal length by 1.5 you will get a focal length of how that lens behaves on your Digital SLR. Let's see how that translates.

SLR Lens Factor Conversions
35mm Focal Length	1.5 Lens Factor Nikon, Pentax, Sony, Minolta, Samsung	1.6 Lens Factor Canon	2.0 Lens Factor Olympus
10-20mm	15-30mm	16-32mm	20-40mm
14-45mm	21-68mm	22-72mm	28-90mm
18-55mm	27-83mm	29-88mm	36-110mm
18-70mm	27-105mm	29-112mm	36-140mm
24 mm	36 mm	38 mm	48mm
28 mm	42 mm	45 mm	56mm
35 mm	53 mm	56mm	70mm
50 mm	75 mm	80 mm	100mm
85 mm	128 mm	136 mm	170 mm
70-210mm	105-315mm	112-336mm	140-420mm
135 mm	203 mm	216 mm	270 mm
75-300mm	113-450mm	120-480mm	150-600mm
500 mm	750 mm	800 mm	1000 mm

Canon uses a CMOS sensor that is slightly smaller at 22.1x14.8mm, and therefore requires a slightly higher 1.6 lens factor. This makes a slight difference in the real focal length of the lens on a digital SLR, but both 1.5 and 1.6 factors are close in value.

Olympus has taken a totally different approach to the sensor used in their digital SLR cameras and has adopted a new digital-only lens mount and lens system called the four thirds system. There is a consortium of 4/3 members that support the 4/3 standard. They include Kodak, Fujifilm, Leica, Panasonic, Olympus, Sanyo, and Sigma. Olympus introduced the first 4/3 system camera and lenses in 2003. Panasonic recently added their own 4/3 camera in the Lumix L1, along with several Leica 4/3 lenses.

It should also be mentioned that Canon has introduced a professional and a premium grade full-frame digital SLRs. There is no lens factor required for these SLRs with a 35mm-size CMOS sensor. These two cameras are, however, in a different category, with the EOS 1DS Mark II selling for about $8000 and the newer EOS 5D for about $3300. Nikon, another recognized top pro brand, has stayed with the DX/APS size sensor in their top pro camera, the Nikon D2X.

Technology has certainly reached the point where 35mm size sensors could be manufactured for a relatively reasonable cost; however, there is no clear movement at the present time to a 35mm size sensor. Most in the industry seem content with the APS C to DX size digital sensor. Perhaps in the near future, we may see some movement to full-frame sensor for the top Pro cameras with APS C/DX for mainstream SLRs. It's just a bit too early to do anything but speculate at this point.

Lens Angle of View

Now that you have a good idea of how to figure out lenses on digital SLR cameras, it is worthwhile to remember why we change lenses.


Wide angle, normal and telephoto lenses see different points of view as you can clearly see in this same scene taken with 35mm, 70mm, and 105mm lenses on 35mm film. From the chart above you can see this would be equivalent to 23mm, 47mm, and 70mm lenses designed for 35mm photography and shot on a Nikon or Sony (1.5 factor) digital SLR. This is a very important distinction and critical to understanding how 35mm lenses behave on today's digital SLR cameras. A 28mm lens designed for a 35mm camera "sees" as if it is a normal lens on a digital SLR, a 50mm normal lens "sees" like a 75mm short telephoto (portrait) lens on a digital SLR. Zoom lenses have been around for quite a while now and most will be familiar with the different view captured with each type of lens. However, they may be surprised to find that the 28mm-80mm wide-angle to short telephoto kit lens they bought for 35mm "sees" like a normal to medium telephoto 42mm-120mm lens on the digital SLR. That is why the new kit lenses that sound so exotic like the 18-55mm and 18-70mm are nothing more than the APS C size version of the old reliable 28-80 and 28-105.

Once you get accustomed to the new focal length definitions for digital SLR cameras, things will fall into place. The shorter the focal length the more extreme the wide angle and the larger the "view" included in the image. The larger the focal length value the more "magnified" the image. Wide angle ranges are very useful for interiors and shots of groups of people. Telephotos are great for shooting from an audience or nature photography like birds. Normal of course is a good all-around focal length.

Because existing 35mm lenses make up the bulk of available lenses on digital SLR cameras, and because the real lens value is a multiple of 1.5 or 1.6, it is easy to see that wide-angle lenses are hard to find on digital SLRs. The 35 wideangle 28mm is a normal lens on a digital SLR, and the super-wides usually stop at specialty fisheye lenses around 15mm, which is still a normal wide 23 to 24mm on digital SLR cameras. As a result, almost any extreme wide-angle lenses you will find for digital SLR cameras will be designed just for digital cameras. These include the Sigma and Tamron lenses in the 10-20mm range, and lenses from the major lens makers like the Nikon 11-18mm.


The Olympus four thirds system lens factor is included mostly for reference, since Olympus manufactures a whole new lens line for their digital SLR. None of the older Olympus lenses from 35mm work on the new digital cameras unless you use a special converter. Even with the converter they will work in full manual mode only. The 2.0 lens factor does come into play for third-party lenses designed to work on the Olympus 4/3 cameras, so you can determine the effective focal lengths with the 2.0 multiplier. However, independent lens makers are mainly designing for a 1.5 to 1.6 lens factor, and the 4/3 equivalents are not always very attractive. Sigma markets basic and specialized fast digital lenses in the 18-55mm range. That's a desirable 27/29 to 83/88 on a 1.5 or 1.6 camera. However, on Olympus 4/3 that is a 36-100mm lens. That is still useful, but there is almost no wide-angle when mounted on a 4/3 camera. This is the problem Olympus faces. Until others adopt this open standard four thirds mount, the only lenses designed specifically for the 4/3 system will come from Olympus.

It should now be clear why the digital SLR, with the ability to see through the taking lens, is a more flexible platform for digital photography. Today's Digital SLRs focus and meter exposure through that same taking lens. With auto-exposure and autofocus, a digital SLR can be as simple as any point-and-shoot camera. Plus there is the added flexibility of a much larger selection of interchangeable lenses that are available to allow the user the best chance to capture what they want in any situation. However, despite the fact you don't need to know anything about F-stops, shutter speed, and the light sensitivity of the sensor, we will tell you with absolute certainty that you will take better images if you do understand a bit about what is going on behind those automatic functions.