A blast from the past than some new members may enjoy, inspired by recent topics and requests:
--Reposted from SPORRS 09/06/96-- Subject: SPORRS: Depth of field calculations >>From Dave Cohen: Here is some info on depth of field, strongly biased from Ernst Wildi's notes in his Hasselblad Manual and my own limited experience. When you focus on a subject at any given distance from the lens, theoretically only the subject or part of that subject at that distance will be recorded sharply on your film. Sharpness will gradually fall off in front of and behind the set distance that you have focused on. On your slide (or print) though, some degree of "blurriness" is acceptable as it still appears fairly sharp. This range of acceptable sharpness is designated "depth of field". At normal focusing distances (i.e. not macro), the reduced sharpness decreases more rapidly in front of the subject and more gradually behind it. As a result of this, approximately 33% of the total depth of field is in front of the point of focus and approximately 67% behind it. You can use the depth of field scales on your lenses to see this more clearly. An example would be if you focused your 50mm lens on a subject about 15 feet away, then your depth of field would be approximately 5 feet in front of as well as 10 feet behind your point of focus (on the subject). Another example: With your lens focused on a subject at 20 feet and the aperture wide open, the depth of field may extend from 19 to 22 feet with one third in front and two thirds behind your focused distance. With the same lens "stopped" down, the depth of field increases, for instance from 17 to 26 feet, but there is still one third in front and two thirds behind the point on the object that you focused on. Depth of field is not a range of ABSOLUTE sharpness with any abrupt fall to "blurriness", but a gradual decrease in sharpness in front as well as behind the set focused distance. This is why it is sometimes difficult, if not impossible, to accurately see depth of field on our focusing screens in our viewfinders even when we are using our depth of field preview camera function, (if your camera even has one - almost all do now). The figures printed on depth of field charts and on your lenses were calculated with a formula that included the lenses focal length, aperture and the "circle of confusion" (which is the acceptable degree of blurriness). These depth of field figures therefore have nothing to do with the type (or brand), design or quality of any particular lens. Thoroughly "confused" now? Using your depth of field preview function (stopping down your lens to check the depth of field) can be helpful in helping you decide where and on what to focus on, but the "one and then two thirds" rule always applies. When you evaluate your slides on your light table under your loupe, you will see that you will undoubtedly discover that there is a difference of sharpness even within the depth of field range. This is always normal, and cannot be avoided. The iris' of our eyes work the same way, but we don't notice it because our brain calculates depth perception using information "recorded" by both eyes, and our eyes move and "refocus" faster than any autofocus camera ever will. It is also true that we can see better (or judge depth better) in bright light because our eyes are "stopped down" more, much like a camera lens works. This is one reason that it is more difficult for us to focus in dim light, even if we can still see our subject clearly. Depth of field is "acceptable" sharpness. If after evaluating your slides with a loupe that your shots don't have enough depth of field, don't blame the lens. It probably just means that you are working with an exceptionally sharp lens (and probably also very high resolution film as well). A sharp lens will record the subject that you have focused on with such exceptional degree of sharpness that anything else in your composition will appear blurred, or at least less sharp. Because subjects recorded on film with an inferior lens do not have that exceptional sharpness, it is harder to see the difference in sharpness from one point of the image to the next, THEREFORE such a lens may only APPEAR to have a wider range of acceptable sharpness. If you want to "increase" the sharpness range of any good lens, or if you plan on blowing up a print to a fairly large size, say 11"x14" or larger (in the 35mm format), then don't use the entire depth of field range. Stop the lens down two stops further than that required for depth of field. If one specific subject is to be critically sharp, then focus your lens on that subject. Don't rely exclusively on depth of field. The image that you see on your focusing screen will always give you SOME indication of the depth of field, but it will never show it ACCURATELY for several reasons. Foremost, when you stop your lens down to check depth of field (such as when shooting at f8 or f11 or so) the image in any SLR camera (regardless of the focusing screen used) will be too dark to see the subject accurately, much less judge it's depth of field to any degree of usefulness. Another reason is always the "low" magnification at which the image is being viewed. Unless your viewfinder magnifies the focused image more than about five times (most 35mm cameras are only about .75 with a 50mm lens), then you will never be able to determine what may appear sharp at 8x or even 20x (slide projection as an example), especially with the focusing screen itself contributing to some perceived image degradation in the viewfinder. The most important reason (again) is that depth of field is a very gradual fall-off in sharpness, not a complete or abrupt change from "sharp" to "unsharp" at or near the depth of field limits. When depth of field (and focus) is important such as with a high resolution film like Kodachrome 25, it is imperative that your intended composition subject be within the depth of field of your shooting aperture (or at least the part of it that you want to appear "sharp" on your slide), or it will not appear "sharp" on your slide, no matter what shutter speed that you use or how secure your equipment was from vibration (such as on a tripod/monopod). If the range of distances between (or beyond) the depth of field indicators on your lenses do not show you that you will have a sufficient depth of field (acceptable range of sharpness around your subject) with your selected shutter speed, then a compromise MUST be made. You must then decide whether it is more important to have the background or the foreground sharp in your composition or whether it is better to have BOTH beyond the depth of field range. This obviously will not work some of the time, so a slower shutter speed is required in order to gain more needed depth of field by stopping your lens down additionally. Then of course, we risk motion blur of the focused subject, so we must use our own best judgment on this compromise, often at the spur of the moment when a headlight is rapidly closing down upon us. No one ever said that true railroad photography (or photography in general) was easy, but it can still be a lot of fun, as well as challenging and frustrating. If you would like to try to achieve focus of more than one subject in your composition, especially at varying distances from the lens, then you must consider the "hyperfocal distance". This will be the subject of another posting soon to follow. (Note: that posting never happened. Any interest?). I hope that some of this information above can be useful to some of us, and further comments and experiences are much appreciated. Dave --end of repost-- Dave Cohen Photographer [EMAIL PROTECTED] --> SPORRS: Serious Photographers of Railroad Related Subjects
