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Posted by Doug Clifford on August 04, 1999 at 21:45:00:
In Reply to: Me again...new user camera questions posted by Basil on August 03, 1999 at 21:21:49:
The "F markings" on your lens refer to f. stops or aperture openings. Common f. stops found on 35mm cameras include f/1.8, f/2.8, f/4, f/5.6, f/8, f/11, f/16, and f/22. Most beginning photographers find these numbers confusing for two reasons. First, there doesn't seem to be a logical sequence to the numbers (and I am not going to explain the choice of numbers.) Second most people assume that the larger the number the larger the lens opening. Wrong! f/16 and f/22 are the smallest lens openings and f/1.8 is the largest lens opening on many 50mm lenses. All you have to remember is this, every time you change from one f. stop to another f. stop you have either cut the size of the lens opening in half or you have doubled the size of the lens opening. For example, changing the lens from f/5.6 to f/8 cuts the size of the aperture opening in half. Conversely, going from f/11 to f/8 doubles the size of the lens opening. When you adjust the aperture, you must also adjust the shutter speed on a manual camera (or when running an automatic camera in manual mode.)
Depth of field explained:
Focus your camera on a subject. Depth of field is the area in front of and behind the subject which is also in acceptable focus. Depth of field depends on several factors - the focal length of the lens you are using, the aperture (f. stop) you have set the camera on, and how near or far away you are from the subject.
For purposes of this explanation, I am limiting it to 35mm equipment. The numbers are different for medium and large format cameras.
First, the focal length of the lens. Lens are divided into three groups, normal, wide-angle and telephoto. In 35mm photography, a 50mm lens is considered the "normal" focal length. For many years, 35mm cameras were always sold with a 50mm lens. (Manufacturers and retailers now encourage buying a 28-70 zoom instead $$$.) The 50mm "normal" lens has fairly good depth of field starting at f/4 and this increases with each smaller f stop used, so with f/5.6, f/8, f/11, f/16 and f/22 the area of acceptable focus in front of and behind the subject increases with each step to a smaller aperture.
How much depth of field? This standard classroom example on depth of field is true for all 50mm lenses set at f/4 and used on a 35mm camera. Focusing on a subject 10 feet from the camera, everything will be in focus from 9 feet to 12 feet, in other words, one foot in front of the subject and two feet behind will be in focus. Remember this relationship: When you have depth of field, there will always be twice as much room behind the subject in acceptable focus as there is in front of the subject. This comes in handy when photographing a subject which has depth. For example, when photographing a group of people assembled into four rows, which row should you focus on? If you don't know the answer, you haven't been reading. :-)
Wide-angle lenses (those with focal lengths less than 50mm) have the most depth of field of all three lens groups. With a wide-angle lens you can photograph subjects only a few feet from the camera and have them and everything behind them - sometimes even miles away also in focus. Wide-angle lenses can be purchased in single focal lengths (24mm, 28mm, and 35mm are common wide-angle lenses) or they can be purchased as a wide-angle to short telephoto zoom (such as the very common 28-70mm or 28-80mm zoom lenses.)
Telephoto lenses (those with focal lengths greater than 50mm) have almost no depth of field at all; this is why focusing is critical with telephoto lenses, unless the subject is distant scenery. It's also why 35mm photographers prefer using an 85mm telephoto lens to do intimate portraits of people - it helps eliminate distracting foregrounds and backgrounds, throwing everything out of focus. Telephoto lenses can also be purchased in single focal lengths (85mm, 135mm, 200mm and 300mm are common) or they can be purchased as a zoom lens (when you use the 28-80 lens above and zoom past the 50mm mark you are using a telephoto lens. The 70-210mm is a common telephoto zoom.
Second the aperture or f stop you plan to use. The ability to choose an appropriate aperture is a major reason why many experienced photographers prefer to operate their camera on manual or in the aperture-preferred mode. Choice of aperture allows you to determine what else in the picture will be in focus besides the subject. Shoot with a lens wide open (f/3.5) and only what you focus on will be in focus. Shoot with a smaller aperture opening such as f/8 or f/11 and not only the subject but also some of the foreground and background will be in focus.
Third, how near or how far away the subject is from the camera. The closer you are to a subject, the more critical that depth of field and focusing becomes. When you are photographing flowers from only a foot away or closer (this is called macro photography) depth of field is very shallow, even when using a small aperture (f/11, f/16, f/22.) Sometimes when you focus on the stamen coming from the center of a flower, the petals will be out of focus, even at the smallest aperture.
There used to be depth of field scales engraved on the barrels of most lenses; it is far easier showing someone how to use the scale than it is explaining over the internet. The depth of field scale is characterized by an index mark with a tiny f/4 on each side of it, then f/8, f/11, f/16, and sometimes f/22 are engraved on each side of the index mark. Focus the lens at 10 feet and draw imaginary lines from the two engraved f/4 marks into the focusing scale, and you will see they point to roughly 9 and 12 feet (the classroom example I gave you above.) Notice how the depth of field expands with smaller apertures. Leaving the lens focused at 10 feet, draw imaginary lines from each f/16 mark and the depth of field is roughly from 6.5 feet to almost 30 feet.
Basil, you really need to buy the instruction book for that Minolta X-370. It will answer many of your questions. The internet is not the best place to learn how to take pictures. Take a class in beginning photography or check out a book from your local library.
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