Help control our exposure
Control the angle of view and magnification
Change the perspective of the scene
Focus properly
Control the Depth of Field Control Exposure. There are two controls to adjust the amount of light that reaches the film. Last week we talked a lot about shutter speeds, and introduced the lens portion of the equation, f-stops. We measure the size of the lens opening in f-stops (usually called “stops”). Opening the lens, or making the aperture larger, one stop, doubles the amount of light that passes through the lens. Closing the lens, or making the aperture smaller, halves the amount of light that passes through the lens. Whole stops end up being funny numbers, i.e., 1.0, 1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45 and 64. Notice that if you allow a bit of fudging of the numbers, that the numbers double every second step, i.e., 8, 11, 16, 22, 32, 45. Remember, little numbers mean a big opening, and therefore a lot of light. Big numbers represent a small opening, and therefore less light. Remember equivalent exposures! If you open the lens one stop, you have to make the shutter speed one stop faster to get the same amount of light on the film. You can change the shutter speed any way you want, as long as you change the shutter speed the same amount in the opposite direction. Opening the lens a whole stop doubles the amount of light that reaches the film. Closing the lens a whole stop halves the light reaching the film. Whole stops end up being funny numbers, i.e., 1.0, 1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45 and 64. Your lens probably doesn’t have all of those numbers. We often define our lenses by the largest f-stop. Commonly, the largest opening on our ‘standard’ 50 mm lenses is 1.4, 1.7 or 1.8 (lenses don’t have to be built to match whole stops). Commonly, zoom lenses have smaller largest openings, often f 3.5, 4.0, 4.5 or even 5.6. Most 35 mm lenses have a smallest lens opening of either f-16 or f-22. Control the angle of view or magnification. This is a fancy way to describe “zooming in” on a subject. As you increase the magnification of the subject with your lens, you narrow the angle of vision. That is, you don’t see off to the sides as much. Telephoto lenses increase the magnification, they make things appear closer. Wide-angle lenses decrease the magnification; they make things appear to be further away. The trade off is how much you see off to the sides. With a telephoto lens, you see little to either side, with a wide-angle lens, you can bring in more to each side, but everything looks further away. We measure focal length in millimeters. We can compare focal length to our eyes. We define what our eyes see as “normal.” A lens of about 50 mm gives the same angle of view and magnification as our eyes, so we call them “normal” lenses. For telephoto lenses, we can get a rough idea of the magnification by dividing the focal length by the normal length of 50 mm. Therefore, a 200 mm lens would give about 4 times magnification, and a 400 mm would give about 8 times magnification. Common wide-angle lenses are 20 mm, 24 mm, 28 mm, and 35 mm. Short telephoto lenses are 85 mm, 100 mm, and 135 mm. Long telephoto lenses are 180 mm, 200 mm, 300 mm, and 400 mm. Some camera manufactures even make 1,200 mm lenses! What we really change when we change focal length is the Perspective. Perspective is the how we perceive depth relationships in a photo. Another way to put it is; it is the apparent relationship between how far things appear to be from the viewer. We say long lenses have strong perspective; they make things appear closer to the viewer, and give an apparent ‘compression’ of the space between distant objects. Short lenses are said to have weak perspective, that is they make distant objects look further away than they really are. We can learn to use this to our advantage when composing photos. We also rely on lenses to focus the subject the way we want. Almost always we want to have the sharpest possible image on the film. Occasionally, for artistic reasons, we want to have a ‘softer’ or less sharp image. Frequent uses of soft focus include ‘dreamy’ scenes (check any wedding photographer’s portfolio), or help hide facial wrinkles when photographing women over 19. My recommendation for the occasions you need soft focus is to either use a UV filter with a bit of petroleum jelly around the edges. How we set the aperture in the lens controls our depth of field. We tend to think of lenses as focusing on only one exact point, usually the subject we’ve put in the middle of the frame. If you look closely at photos, you’ll see there is a zone both in front of and behind the subject that appears to be in focus as well. This zone is called “Depth of Field.” Sometimes we want as much as possible in focus, like when we’re close in on something like a single flower. Sometimes having a ‘shallow’ depth of field, that is very little in focus in front of or behind the subject, is the best artistic approach. Many portraits are shot with a very shallow depth of field. Often the subject’s eyes are in focus, but the tip of her nose and her ears are out of focus. If you have a distracting background, using a shallow depth of field to throw it way out of focus can help accentuate your subject. We control the amount in focus in front of and behind the subject by the f-stop we pick. Large f-numbers, those with a small lens opening, provide the most depth of field. The larger the number, the more of the scene that will appear in focus. Small f-numbers, those with a large lens opening, provide very shallow depth of field. As a general rule of thumb, the zone in focus in front of the subject is roughly a third as deep as the zone in focus behind the subject. Often the zone behind the subject will reach to ‘infinity,’ making everything behind the subject appear in focus. With practice, and the Depth of Field scale on your lens, you can find the “hyperfocal” distance. The hyperfocal distance the closest distance you can focus and still have infinity in focus, or the point where you have the greatest amount of the scene in focus. Long lenses appear to have less depth of field than shorter lenses. Wide-angle lenses often appear to increase the depth of field. While not technically correct, it often appears to work in practice. All lens designs are compromises of very different competing demands. No lens is perfect. You are likely to discover your lens is sharpest and has the best contrast about two stops down from wide open. Our project this week will give us practice with depth of field.
Getting the Correct Exposure Your light meter (and the automatic settings on your camera) are useful, but dumb tools. With a little understanding of how they work, and a tiny bit of practice, you will soon be much smarter than they are. You’ll get better-exposed photos and you’ll be able to use what we learned about controlling your shutter speeds and depth of field to your advantage. Light meters think the entire world is 18% gray. 18% gray is visually halfway between what your eye perceives as white, and what it perceives as black. 18% gray is also a very close approximation to how your meter sees an “average” scene. If you happen to take only average photos, you’ll do well putting your camera on auto and shooting away. If you meter a scene with a large portion of light colors (white sand at the beach, snow, bright water, light colored walls), your meter “sees” a lot of light being reflected, and believes the scene is lit much more brightly than it is. Therefore, it recommends a shutter speed & f-stop combination that lets less light than needed hit the film. Your photo turns out dark. The opposite is also true. If you meter a scene that is mostly dark, such as, someone under a spot light on stage, your light meter “sees” very little light being reflected, and recommends letting a lot of light reach the film. Your photo ends up being gray, with the person in the spotlight is overexposed or too bright. You can learn to compensate for your meters failings. Experience is the best teacher, but there are some hints:
Get close enough to your subject so that they are filling the frame in the camera.
Carry a gray card. Kodak sells four 8x10 cards for about $20; other companies are more reasonable. Cut them into quarters, stick one in your bag and carry it with you. If you have a question about the exposure, hold the gray card under the same light as your subject and take a meter reading off that.
Watch for substitute gray cards in the world. Light pavement makes a good approximation for 18% gray. Grass in sunlight is usually pretty close.
Learn the exposure difference between your hand and a gray card. Hopefully, you’ll have your hands nearby when you’re metering. The best way to avoid being fooled is to learn to have a sense of your camera settings for the types of photos you take. Most of us take many of the same sorts of subjects again and again. Knowing what has worked before is a great guide.
Depth of Field Project We’re going to put what we learned about lenses and depth of field to work for this week’s project. We want to end up with two photos of the same scene. The first with the minimum depth of field for your lens, the other with the maximum depth of field for the same lens. This will, dramatically, show us the difference. How to do it: Find a scene that has some depth to it. (Hint something with a repeating pattern may work well. Parking meters in a row, headstones in a row, a fence). For the first shot, open your lens to the smallest number on the dial. Take a light meter reading, and set the shutter speed to give you the proper exposure. Take the photo. Now, without changing anything else, stop the lens down to the smallest aperture- the biggest number. Adjust your shutter speed to give you the proper exposure. Take the photo. (A tripod may be helpful). Watch for subjects that are a moderate distance from the camera. This project won’t work very well with subjects very close because the amount of depth of field is so limited. Subjects far away from the camera are likely to be in the hyperfocal zone and the difference won’t be noticeable. OK, this one can be a little dull. Use your imagination for a subject and have some fun to liven it up!
