Despite digital's ease, flexibility and accessibility, I had a lot to learn after I made the conversion from film. To my dismay, much of the learning entailed even more math than what I had to learn when I cut my chops on film photography in college. This was disappointing because of my aversion to math ever since high school algebra and geometry; I made it through college -- with a degree -- without taking a single math class. Now that sidestepping was coming back to haunt me.
Digital still required a knowledge and familiarity of numerical things such as aperture settings (f/2.8, f/4, f/8, etc.), shutter speeds (1/30, 1/60, 1/125, etc.) and focal lengths (10 millimeters, 50mm, 100mm, 300mm, etc.). It just introduced a new layer of numerical labyrinths.
One of those was important for those who could afford only the less expensive small-sensor DSLR camera bodies: something called "crop factor" and its corresponding impact on a lens' focal length, a/k/a, multiplier. Film images captured with the widely popular 35mm bodies are in 36x24 proportions, whereas frames captured by the image sensors used in manufacturing the first wave of lower-cost DSLRs -- for Canon shooters, that involves the 20D and Digital Rebel -- were in varying dimensions, but none the same as film's 36x24. My 300D, for example, shot frames at 24x18. What does that mean? I found out the hard way when ordering my first 5x7 and 8x10 prints from jpegs taken with the 300D -- and making no allowances for the crop factor. The 4x6 prints were fine; but on the 5x7 and 8x10 prints, it looked like someone had taken an ax to my precious art and whacked the tops or the sides indiscriminately. I couldn't figure out the rhyme or reason. I complained to the retailer, and to the store's credit, the salesperson patiently explained the crop factor ramifications to me.
Here's what they said: 5x7 and 8x10 prints don't translate precisely in digital because those dimensions are not multiples of 18x24, the dimensions of my 300D's images. And besides, digital print retailers base their machine settings on the old 36x24 dimensions of film. Remember, nowadays, retailers simply push film and digital image files through machines -- no man/woman at an enlarger ensuring logical cropping anymore. So if I really wanted my digital prints in those traditional 5x7 or 8x10 sizes, the responsibility was on me to convert my digital jpeg files to those dimensions first before bringing them in for prints. How? Image software on your home computer.
I rarely order 5x7s anymore, so I wasn't worried about those. It was the 8x10 prints that concerned me the most. So when I wanted them, I'd first make a copy of the original jpeg and convert that copy (or crop it) from 24x18 to 5x7 or 8x10 with editing software. When I learned my photo print retailer also offered 8x12 prints (because 8x12 is a multiple of 24x36, it required no preliminary cropping on my end), I started ordering my larger prints in that size; then, if I wanted to use it in an 8x10 picture frame, I'd just grab a scissors and make the 2 inches of trim myself, rather than have a machine do it. Lately, I've noticed a greater number of 8x12 picture frames for sale in stores, and I've been purchasing those instead of messing with scissors to make them fit in an 8x10 frame.
After going to digital, I also learned that the same 36x24 vs. 24x18 discrepancy figures into how a lens' focal length (the number etched onto the lens) really didn't accurately represent the distance I was getting when using my 300D anymore. Lens focal lengths on digital camera bodies with small sensors like my 300D actually were 1.6 times the listed distances, hence the reference to 1.6 being the crop "multiplier." This meant that my 50mm prime lens actually reflected shots taken at about 80mm. It also meant that, at its farthest "listed" distance of 300mm, my 75-300mm telephoto lens actually was giving me about 420mm worth of distance.
This turned out to be a good and bad thing. The good: For shooters who never think a lens gives them enough distance, you now got more distance for your glass.
The bad: Well, sometimes, you really want the shorter distance stated on the lens. You need that wider angle to get more subject matter into a frame at a closer distance. Even more critical is that in the case of the aforementioned telephoto lens (the 75-300mm f/4.5-6) , the maximum length now was actually about 420mm, which means it required an even faster minimum shutter speed to ensure hand-held images without blur from camera shake.
How fast? The prevailing rule is that to ensure no blur from camera shake with hand-held shots, your shutter speed should never be slower than the number equal to the lens' effective focal length. Computing that isn't hard when using a 35mm film camera (or if you can afford them, the very expensive high-end full-sensor digital bodies like the Canon Mark or 5D series). Because with those, the numbers are as plain as what you read on the lens: with a 50mm lens, the slowest shutter you should use is 1/50, or 1/60, which is the closest equivalent on most cameras). For a 100mm lens, the slowest shutter would be 1/100, or 1/125, the closest equivalent on most cameras).
However, these small-sensor cameras like the 300D are a different matter. In the example of the aforementioned telephoto lens, to figure the maximum effective focal length, you start with the listed 300mm and multiply it by the 1.6 crop factor, or about 420mm. This means to avoid camera shake blur while hand-holding the camera, you'd need to be using a shutter speed of 1/420, or 1/500, the closest equivalent on most cameras. And because you don't often have bright enough lighting conditions (certainly not indoors) to allow for shutter speeds that fast, it means that at its maximum focal length, for any light conditions other than sunshine, you'd need a tripod or monopod to use that lens successfully. A tripod is worthless for shoots where anything is moving (sports, birds in the air, kids at play, etc.), which is why many professionals, when they go to invest in lenses, won't look at anything that doesn't give them at least f/2.8 at its maximum aperture, whether it's a prime lens (one set length, e.g., 50mm f/1.8) or telephoto (variable-distance) lens, but one in which f/2.8 is available at ALL of focal lengths. This gives them more opportunity to ensure adequate exposure on the aperture end, which in turn permits them to use faster shutter speeds, thereby avoiding the blur from camera shake. In the case of Canon and Nikon, having image stability (IS/Canon) or vibration reduction (VR/Nikon) in lenses also is a big deal for professionals.
Alas, there is a trade-off for having lenses equipped with wider apertures (as low as f/2.8) and IS/VR: They are significantly more costly, bulky and heavy.