Depth of Field
Focus is fundamental to photography, both in terms of what is and what is not in focus. In the photographs below, the left one is focussed on the Teddy, but the background is also in focus - the depth of field is large. In the right hand picture, the camera is again focussed on the Teddy, but this time the background has been prevented from coming into focus by using the widest aperture available - the depth of field is small. This is a common technique for isolating the subject from busy backgrounds. Conversely, the effect can be used in the opposite sense to improve the depth of field, particularly for close up work, when stopping down (using a small aperture) will increase the depth of field. Depth of field is related to focal length of the lens used and the aperture. Essentially the shorter the focal length of the lens - the greater its depth of field for any given aperture. Depth of field varies constantly across the focussing range of the lens too, its depth of field at 3ft will be measured in inches whilst at 30ft it will be many feet either side. Finally, the smaller the aperture the greater the depth of field for that lens.
For cameras fitted with ground glass screen or reflex viewing the determination of depth of field is easy - just look at the image with the camera stopped down - i.e. the camera set at the intended aperture. If the depth of field is too small, use a smaller aperture (and double the exposure time for each stop you go down) until you get what you are after, or vice versa if you wish to go the other way. When it comes to viewfinder or rangefinder cameras, it gets a tad more awkward. These cameras will usually have a depth of field chart to read off the distance against aperture either side of the point of focus that the image will be acceptably sharp. Below is one such chart as found on a Universal Mercury II. In this case it is a matter of measuring or guessing the depth that is required, but the picture taking experience won't be instant
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