Assignment • The Lens
You know we can take images with just a pinhole but we give up a lot of control and our exposures would take more light and longer exposure times. Adding a lens makes serious photography possible. With a zoom lens we can cover a range of focal lengths like this 24-70. This is popular range if you only have or only want one lens.
Focal length
Two kinds of lenses that you hear about are zooms, and prime lenses. A prime lens is only one focal length instead of a range like the zoom. A prime lens is usually a faster lens with a larger aperture to shoot at faster speeds and a shallower depth of field. We also talk about wide angle, normal and telephoto lenses. It's nice to have some variety to be able to record what you need. Wide angle lenses have a wide view while a telephoto brings you closer to your subject. Wide angle lenses will have a greater depth of field than a telephoto. A telephoto will compress the image and reduce distortion. The zoom lens that came with your camera goes from wide to telephoto.
Photograph a person as a portrait with the telephoto setting and without moving, change to wide angle and shoot another shot. Now move closer to frame the subject like the first shot only using the wide angle setting. Take the shot and again without moving go back to telephoto and take another shot. Compare the first and third shots. If you are creating a portrait, you know the second and fourth shots don't work. Which of these two do you prefer?
You are going to look for or create a vacation shot. Pick one or all the kids and stand them in front of something. Where are you? What are you presenting Follow the rule of thirds and use the widest lens you have. You can use no more than 1/2 of the frame for people.
Auto-focus
A lens is more than just a piece of glass. It is many pieces of glass working independently and in combination to focus, zoom, reduce flare and insures that all the colors focus on the same plane. Inside this complex assembly the lens also holds and controls an aperture which controls the amount of light entering the camera, and depth of field.
The camera is the brains that powers the AF controller and talks to the lens. It is very handy and quick in average situations to focus. Used lenses with and without auto focus can be found easily on line and at camera stores at a savings.
Most cameras today are auto-focus and have the option to focus manually. When you depend upon auto-focus all the time, you may be disappointed. Auto-focus depends upon contrast of a specific point in the image, so it may take time to find focus when the contrast is low. Auto-focus or AF is getting better and faster all the time. When focus is critical, use manual.
Aperture
The aperture is a function of the lens and not the camera. In combination with ISO and shutter speed, correct exposure is achieved. The aperture allows or reduces the amount of light entering the camera. A wide aperture or ƒ-stop allows more light but also reduces depth of field.
A wide angle lens stopped down to a small aperture (ƒ-16) will have a great deal of depth of field while a telephoto at ƒ-16 will have much less. Some lenses have an aperture dial on the lens, while some cameras, being electronic, have a dial on the camera to change ƒ-stops.
Set your mode to (A) use a wide angle lens and take a picture of a kitchen table. Shoot low from the corner down the length of the table. Line up 6 cans or bottles with labels near the edge. Get close and focus on the second can in line. Use your largest aperture or ƒ-stop (2.8). Ok second shot, stop down the lens two full stops (ƒ-5.6). For your third shot stop down to ƒ16
Back up from the table and do the same shots except use your telephoto. Repeat the same ƒ-stops and compare images. What we are interested in, is the depth of field. Is the depth of field equal front to back? Do you see distortion or compression in the images?
I hope you have a tripod, you'll need it for this one. We are going to take a night picture. Set everything up before you leave, cause it's hard working in the dark. Consult your manual under shutter speed for Time or Bulb setting. Use Manual mode M, ISO 100, ƒ-16 or higher, a busy street with lots of street lights and traffic. You'll do several shots starting at 2 seconds. Remember that doubling is one stop so the second shot will be 4 seconds, then 8 and then 16. If the image is still too dark you can continue. View these images and see which exposure you like. Use that exposure and try several different perspectives.
Streaks of light from the traffic and ghosting and hey, how about those highlights. Long exposures can be fun. ƒ-16 will create stars out of small shiny lights. Christmas lighting works really well too. And try this on moving water like your favorite waterfall, or even the faucet.