I’m in the market for a digital SLR camera before this year’s big trip. I have a pair of SLR film cameras with five lenses that I thoroughly enjoy shooting with but they don’t offer the convenience of digital photos.

I checked in with a few friends who are either into regular film photography or digital photography. No one had any decent advice for the transition from film to digital SLR, so I’m doing the research myself, Fisher style — a exhaustive, anal style of research in which one nearly researches the research itself in an effort to become a subject expert.

As my research has expanded so has this article. Therefore I’ve gone through and tried to organize the information better and give readers a way to hit the parts they want without having to sift through everything.

Here are quick links to the meat and potatoes:

1. Types of Digital SLR (DSLR) cameras
2. Lens physics for DSLRs (yes, its different from film)
3. How DSLRs convert light to data
4. Factors that determine image resolution
5. Size matters pixel size, that is
6. Resources

I started my quest for knowledge at a local camera shop here in Chicago by telling a salesperson I was new to digital SLR photography and would like a primer. I had a $1,300 Nikon D70 and lens shoved in front of me and a sales pitch about how it took better pictures than the Canon and even some inaccurate technical information about the unit (I found it inaccurate after I went home and Googled the D70). He didn’t even hit on the subject of transitioning from film to digital SLRs, any little nuances of digital versus film, terminology or nomenclature; none of it. For example, I assumed that since no film is used, I wouldn’t have to factor ASA (ISO) sensitivity. However, ISO sensitivity is a setting on digital SLR’s.

DSLR Types

Google saved the day again and I eventually found Philip Greenspun’s site Building A Digital SLR System. Here Philip starts with some quick basics like what is “SLR”, who makes them in digital format, and most important; the types of digital SLRs.

• big lenses, big sensor
• big lenses, small sensor
• small lenses, small sensor

A camera can be oversimplified as a lens and film. The lens captures a scene (in the form of light) and directs it to the medium which records the scene (the light). In a digital camera, the medium is silicone, which is very expensive¹. To match a 35 mm frame of film requires the same sized chunk of silicone. This is the big lens, big sensor solution but ends up being a $7,000 camera. Hardly what I call a hobby.

The big lens, small sensor solution is today’s hobbyist’s digital SLR camera. The sensor is smaller than the old 35 mm film, which means the recorded image is smaller on the digital medium than it would be on a film medium. This means we have some waste in digital format when using conventional film lenses.

Back to Philips site, he then goes into the differences between a Canon platform and a Nikon platform and completely side-steps any sales pitches, although he does provide links to amazon for all the products he recommends.

After scouring Philip’s site I decided I wanted more technical reviews of the various models of cameras. I’m on a budget for my transition but it isn’t so small I have to skimp on quality and I certainly don’t want a shite rig after spending $1,500!

I hit Google again and found Steves-Digicams.com. Here you’ll find technical reviews of the major players in digital photography. They have a fairly comprehensive overview of each camera, to include sample photos, Steve’s conclusion, multiple pictures of the product, etc. This is a great resource to hit before you buy.

As recommended by Philip Greenspun, I plan on renting the Canon Digital Rebel XTand then the Nikon D70 as these are the two models I’m interested in. I’m leaning heavily towards the Canon simply because that is what Sam Javanrouh of Canada has been using lately on his photo blog; daily dose of imagery. This has allowed me to see what is possible with the system. He’s promised a “mini-review” of the Canon Rebel XT (Canon 350D in non-US markets) but he hasn’t had time yet.

Lens Physics

What has my research provided thus far? First and foremost, due to the “big lens, small sensor” approach, lens physics means something a bit different in the digital world. Because the sensor receiving the image from the lens is smaller than a 35 mm frame of film, you don’t get the full image captured by your lens. However, the view finder correctly frames the picture you’ll get.

If you’re a die-hard photographer who looks at a scene and knows a 50 mm lens is needed, you need to relearn your lens physics when switching to digital. I’m lucky, I’m less of a technical photographer and more of a “this feels like a 50 mm lens, let me see what it looks like through the view finder” amateur.

Also, as with normal film photography the lens makes the difference. Conventional wisdom then leads one to assume that you want to go with the leader in lens production, which is arguably Nikon. Though Canon seems to have the upper hand in the digital realm and also has competitive lenses. Canon has been the top seller of digital SLR cameras but I’m not sure if that is because more people raised on digital point-and-shoot cameras are moving up to SLR or because more film SLR people are moving over to digital. My guess is the former.

Digitizing Light

How does a DSLR convert light to data? Oversimplified again; light is gathered with the lens and directed to the sensor. The sensor has millions of pixels (1 megapixel = 1 million pixels). Pixels convert the light to an electrical charge. This charge is then sent to the ADC.

The ADC (Analogue to Digital Converter) of a digital camera translates (classifies) the analog electrical charge, which it receives from the sensor, to a digital signal of 1’s and 0’s representing a level of brightness. A 1 bit ADC would classify the analog signal into 1’s for light and 0’s for dark. A two bit (2 to the power of 2) ADC renders four levels of brightness; dark, light, and two shades in-between. Most consumer digital SLR cameras use an 8 bit ADC, which renders 256 levels of brightness (2 to the 8th power) per pixel.

Image Resolution

Image resolution (minimum bit rate) of an ADC is determined by the dynamic range of the sensor, expressed as a ratio of the maximum possible signal divided by the minimum possible single it can generate, i.e. 1000:1.

If a sensor has a dynamic range of 1000:1, it should have a bit rate of at least 10 bits (2 to the power of 10 = 1,024 levels of brightness). This way it can process the maximum input of 1,000 levels of brightness without loss of information².

Therefore you want a high dynamic range along with a high bit rate for optimal image clarity and sharpness when using RAW format versus JPEG format, which is restricted to 8 bits of data per channel.

Pixel Size

The size of the pixels in the sensor also affects image clarity and sharpness. As the sensor gathers light, the light fills the pixels like little containers. Larger pixels allow the sensor to capture a wider spectrum of light. Once a pixel has reached its capacity, any additional information “overflows” and is discarded, creating an “over-exposed” situation. High frequency information fills pixels quicker than low frequency information. To compensate, one would use a quicker shutter speed to decrease the amount of information collected. However, for the lower-frequency channels, this means they have less time to be collected and may not be collected at all, narrowing the spectrum of light collected. Larger pixels allow the higher frequency channels to fill without overflowing and thus affords more time for the lower frequency channels to actually collect information³.

This means that a 6.2 megapixel sensor with large pixels is able to compete with an 8.0 megapixel sensor with smaller pixels. Think of 6.2 one-gallon jugs versus eight 3/4-gallon jugs (8 X .75 = 6 versus 6.2 X 1 = 6.2).

As I continue my research and test the units I will of course post my findings here.

Resources:

Glossary of digital photography terms:
http://www.dpreview.com/learn/?/glossary/

Camera reviews:
DPreview.com (sponsored by affiliate retailers, careful!)
Steves-Digicams.com

¹ A great explanation of how light gets converted to data is here
²DPreview.com: AD Converter
³DPreview.com: Pixel Size