Before we go too deep into focal lengths, let’s review the impact of the sensor. On single lens reflex (SLR) cameras, the standard has been the 35mm film for decades. SLR lenses were calibrated for the 35mm film size. Then, we got the digital SLR which doesn’t use film, but has a sensor. Since everyone was comfortable with the 35mm camera's form factor, the camera companies kept it. Full frame cameras have sensors very close to in size to 35mm camera image size, hence the name. Full frame cameras tend to be much more expensive, in part, because of their large sensor size. “Prosumer” SLR cameras (because no one wanted to use the term “amateur”) use smaller sensors and are often called “crop” sensors. Crop sensor cameras will show a smaller field of view than a full frame camera even if you use the exact same lens. This crop factor will vary between cameras, so you should know yours. They are typically 1.3, 1.5, or 1.6. Therefore, a 35mm lens on a Nikon D300 (crop factor 1.5) will have the same field of view as a 53mm on a Nikon D4 (35 x 1.5 =52.5). A 100mm on a Canon T3i (crop factor 1.6) has the same field of view as a 160mm on a Canon 1D. There are other differences between the sensors, but we’re going to ignore them completely for this discussion. When you see focal lengths in this series, we are talking full frame sensor focal lengths. The differences caused by sensor size are only confusing if you try to go beyond the simple math, so don’t. Some photographers care about this distinction because their parents didn’t love them. ;-) I just wanted you to know the difference so you can be prepared when they come to pester you. The general ideas we’ll go through work for both types of cameras.
We begin our lens focal length discussion with a “normal” lens (40-50mm). This lens is called the normal lens because it renders a photo with about the same field of view as your own eyes. This was the lens that came with your new camera, back in the old days. New photographers were instructed to learn their camera’s capabilities with this lens before moving on to anything else. “Learn to see. Practice until you know exactly how your camera and that lens will expose the frame of film.” Still good advice
This is a good lens for people and general photography. It is compact and light. It is very inexpensive in the f 1.8 variety ($125-200). The f 1.4 is pricier ($375-400), but you get an extra stop of light and better build quality. This fast lens (f 1.8 or f 1.4) gives you the opportunity to try your hand at learning to use the bokeh effect (blurred background). Every other lens with an aperture this wide usually costs more than $1,000. The extra aperture width will also come in handy when you need some extra shutter speed for fast subjects or low light environments.
Because the 50mm has no zoom capability (called a prime lens), you zoom with your feet. It's shorter than most portrait lenses, so you'll have to move in to fill the frame for a portrait, and for group photos, make sure you have room to move back far enough to get everyone in. Try not to cut off the top of the flag staff with your Confederate re-enactors. Doh!
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