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Building a Digital SLR System

by Philip Greenspun, June 1997 (updated February 2011)



Digital single-lens reflex (SLR) cameras are the standard tool for serious photographers. With the introduction of cameras such as the Nikon D40/D40x (current entry-level model is the Nikon D3000) and the Canon Digital Rebel XTi (current entry-level model is the Canon Rebel T3), the market for digital SLR cameras has expanded tremendously. A point-and-shoot compact digital camera can offer reasonably good image quality, but a digital SLR, which usually looks a lot like an old standard 35mm film camera and may use the same lenses, offers the following advantages:

Canon EOS 20D and 70-200/2.8L lens; note the blurred background from the f/2.8 aperture
  • accurate, large, and bright optical viewfinder
  • fast operation and large controls
  • excellent image quality in low "available" light situations when it is necessary to use higher ISO speeds
  • interchangeable lenses

For more information on what to consider when purchasing a DSLR, including details on lens compatibility, system expandibility, size and weight, ISO settings, noise levels, etc, take a look at Bob Atkins' article on Factors to Consider when Choosing a Digital SLR Camera.

With the digital SLR you have a good idea of what you're going to capture by looking through the viewfinder. When you press the shutter release the camera captures the image immediately. If you need to zoom or focus manually there are large rings that you can operate quickly by feel. If you see a beautifully-lit scene you can capture that beauty instead of using an on-camera flash to blast everything with harsh white light. If you need to make a specialized photo, you can buy or rent a specialized lens and attach it to the camera.

This article explains the different kinds of digital SLR cameras available, how to choose the right one for you, and what to do once you get it home from the shop. A digital SLR camera system, complete with lenses and accessories, can cost anywhere from $600 to $10,000. This article shows you how to choose and buy the basic items first and the more expensive and hard-to-use components later.

[If you don't want to read this article and are impatient to get started immediately, get a Canon Rebel T2i, (compare prices) (review) and Sigma 30mm f/1.4 EX DC for Canon, (compare prices); if you must have a zoom, the Canon EF-S 17-55 f/2.8 IS USM, (compare prices) (review) is a good choice.]

What is a single-lens reflex (SLR)?

Olympus E1 Four-Thirds System Camera and 14-54mm zoom lens

The single lens reflex (SLR) is most folks' idea of a serious camera. "SLR" means that the same lens is used for viewing and taking pictures. A mirror in the body directs the light from the lens up into a prism for viewing, then flips up out of the way just before an exposure is made. The standard photojournalist's Nikon from the 1960s or 1970s was an SLR with a roll of 35mm film behind the mirror. When the mirror came up the light passed through to the shutter, which opened to expose one frame of film for perhaps 1/60th of a second. A Canon Digital Rebel or Nikon D80 looks very similar and works in almost the same way. The only difference is that instead of a piece of film behind the shutter there is an electronic sensor.

The mirror and optical viewfinder are what enable a photographer to frame images more quickly and accurately than with a point-and-shoot camera. Regardless of what lens or filters you have attached to the camera you see what the sensor will see. The same can be said for the LCD displays on the back of a $200 point-and-shoot camera but those displays are difficult to interpret in sunlight. The typical digital SLR camera viewfinder offers additional information underneath the image, including all the most important camera settings.

The SLR is much larger and heavier than the point-and-shoot camera. If you are leaving the house to socialize and want a camera to keep in your pocket just in case an interesting photo presents itself, the SLR will seem cumbersome. If you are heading out specifically with a photographic project in mind you will appreciate how the SLR and its controls fit into your hands.

Because digital SLRs are more expensive than point-and-shoot cameras the manufacturers typically put in faster computers and better autofocus systems. This makes the cameras more responsive and you are more likely to catch the "decisive moment" as the baby's face lights up with a smile, the soccer ball leaves the player's foot, or the dog catches the frisbee.

A digital SLR may offer the same number of megapixels, individual image elements, as a high-end point-and-shoot. Not all pixels are created equal, however. Resolution is important if you intend to make large prints but dynamic range, the ability to capture detail within bright highlights and dark shadows, is more critical in many situations. The sensors in digital SLRs are typically much larger than those found in point-and-shoot cameras. The main advantage of a larger sensor is better performance in dim light. If there are 8 megapixels spread out over a sensor that is 4 times larger than the sensor in a point-and-shoot camera that means more photons of light will fall on any given pixel. If during an exposure 50 photons would fall on the small sensor then 200 photons would fall on the big sensor. If there is a small change in the light from one part of the scene to another the sensor in the point and shoot camera is trying to notice a single extra photon; the electronics in the camera with the big sensor have four extra photons that are much easier to detect.

First-time consumers of digital SLR cameras focus on the body. Long-time photographers, however, look at the system. An SLR system includes a body, multiple lenses, flash units, and various connecting cords. For most photographers the investment in lenses will come to dwarf the cost of a body. It is thus important to choose a system whose manufacturer makes the lenses that you need for all of your potential projects and, ideally, whose system is popular enough that you can rent special-purpose lenses for uncommon situations. Each camera system has its own lens mount design and a lens that works on, say, a Nikon camera cannot be attached to a Canon body.

Who makes digital SLR cameras?

The same companies that made 35mm film SLRs make digital SLRs. If you have a lot of Canon EOS lenses from your days as a film photographer, for example, you will probably want to buy a Canon EOS digital camera, because those lenses from the 1990s will work just fine on the new digital camera.

The market leader in the professional/advanced amateur photography world is Canon. If you don't have a major investment in lenses you will probably want to buy a Canon digital SLR. The number two spot is occupied by Nikon, which is also a reasonable choice. Fuji and Kodak have made digital SLRs that accept Canon- and Nikon-mount lenses. Once you get beyond Nikon and Canon it becomes very difficult to rent lenses and the companies that make the more obscure systems don't have a large enough market share to invest enough money to build competitive bodies. Leica, Minolta, Olympus, Pentax, and Sigma are the small vendors in the digital SLR market. Unless you have an enormous investment in lenses for one of these brands the only one of these worth considering for purchase is Olympus, due to its innovative Four-Thirds system, discussed below.

What kinds of digital SLRs are available?

There are three kinds of digital SLR systems being made as of September 2005:

  1. big lenses, big sensor
  2. big lenses, small sensor
  3. small lenses, small sensor

We will discuss each in turn.

100th Anniversary Boston Marathon (1996).

Big lenses, big sensor. Canon and Kodak have taken the most obvious approach to the challenge of transitioning from film to digital: build a digital sensor exactly the same size as one frame of 35mm film. The result is a chunk of silicon 24x36mm in size, which is vast compared to the sensor in a point-and-shoot digicam. The benefit of this vast sensor is reduced noise, which looks like grain, in low light/high-ISO situations. The drawback of a vast sensor is that manufacturing a flawless piece of silicon this big is very expensive. Consumer-priced cameras in this category include: Canon EOS 5D, (compare prices) (review), Canon EOS 5D Mark II, (compare prices) (review), Nikon D700, (compare prices) (review), Nikon D3, (compare prices) (review), and Sony Alpha A900, (compare prices) (review). If you have a strong back and an unlimited budget, the Canon EOS 1Ds Mark III, (compare prices) (review), is a great choice. It is probably the best digital camera made and produces image quality that rivals medium format film (e.g., 6x6cm Hasselblad).

The only other full-frame digital SLRs made were the discontinued Kodak DCS Pro SLR/n and SLR/c bodies. The Kodaks were cheaper than the Canon, but not quite as functional and the fact that they were discontinued is a good illustration of why you want to buy a digital SLR from a market leader. I own a 5D and have written a full review of the Canon EOS 5D.

Big lenses, small sensor. In order to keep the cost of the body within a range of $700-1500 and allow photographers to use their old 35mm system lenses most digital SLRs fall into this category. The front of the body has the same lens mount as an old film SLR. The back of the body has a sensor that is smaller than the 24x36mm standard frame of an old film SLR. The result is a camera that looks the same as the old film camera but multiplies the magnification of all the lenses. Having a smaller sensor is like cutting the center out of a drugstore proof print. You don't capture all the information on the left and right and top and bottom of the frame. It is as though you took the picture with a telephoto lens. The viewfinder has been adjusted so that what you see optically is what is captured in the digital file. If you're coming from the film world you will need to do a mental adjustment. A 50mm normal perspective lens on a big lens/small sensor camera behaves like an 80mm telephoto lens on a film camera. A 20mm ultra wide-angle lens behaves like a 30-32mm slightly wide angle lens on a film camera. Nearly all the popular digital SLRs fall into this category and their various merits will be discussed below.

Small lenses, small sensor. The biggest problem with the "big lens, small sensor" situation is that photographers are forced to cart around lenses that are much larger, heavier, and, theoretically, more expensive, than they need to be. A big heavy Canon telephoto lens is big and heavy mostly because it is built to cast an image circle large enough to cover a 24x36mm frame but the Canon EOS 30D body's sensor is only 15x22mm in size. Any engineer would look at this "big lens, small sensor" situation and say "Why not come up with a standard reasonable sensor size and then make lenses that are just large enough to cover that sensor with an image?" That's precisely what the Four Thirds consortium did. Olympus and Kodak seem to be the originators of the standard but Fuji, Panasonic, Sanyo, and Sigma have signed on as well according to www.four-thirds.org. This seemed like a great idea at the time (2002) but four years later only three Four Thirds system bodies have been built, all by Olympus, and only a handful of lenses, all from Olympus and Sigma.

If you have a a robust checking account and/or a lot of Canon EOS film camera lenses an unlimited budget the "full-frame" Canon EOS-5D (big lenses/big sensor; medium weight; $2900) is the obvious choice. If you don't need state-of-the-art performance and value compactness above all, the Olympus E System is a reasonable choice (see my review of the Olympus E1 More than 50% of all photographers find that the engineering compromise of "big lenses/small sensor" fits their budget and needs. This has led to the introduction of lenses that have the big lens mount for a 35mm film camera but optically cover only the small sensor of a mid-range digital SLR. These are sold as "digital-only lenses" or "digital camera lenses" but in fact they won't work on a full-frame digital SLR--the corners of the image would be black. Canon denotes these lenses as "EF-S", Nikon as "DX".


Text and photos Copyright 1995-2007 Philip Greenspun.

Article revised February 2011.