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Photopsycho
6-Dec-2006, 05:24
Can anyone tell me how I can figure out the length of zoom on a lens? I've tried measurement conversions from MM to Feet or Inches and it makes no sense. Say if I have a lens that is 35-108mm how far is that in feet, yards or what?

Thank you.

naturephoto1
6-Dec-2006, 05:57
Can anyone tell me how I can figure out the length of zoom on a lens? I've tried measurement conversions from MM to Feet or Inches and it makes no sense. Say if I have a lens that is 35-108mm how far is that in feet, yards or what?

Thank you.

To convert mm to inches divide the number of mm by 25.4 to determine the number of inches. So 35mm divided by 25.4 equals 1.378 inches; 108mm divided by 25.4 equals 4.252 inches.

Rich

Ernest Purdum
6-Dec-2006, 11:33
I'm not at all sure that I understand the question, but I'll try to give you some help.

By stating the lens goes from 35 to 108mm, the maker is giving you the focal length range of the lens. 35mm is a moderate wide- angle lens on 35mm cameras. It could be useful for a shot of a group of people standing together or perhaps of a landscape of a very big subject that you are close to. 108mm, again on a 35mm camera, is a moderately long lens, that is, one that gives a rather narrow view. It can be useful for close portraits because of perspective, or when you want a picture of something some distance away. The subject will then fill the film more than if a shorter focal length lens had been used.

If this isn't the sort of information you are looking for, let us know.

Photopsycho
7-Dec-2006, 03:44
Actually that's about what I was looking for, I thought about the question later and thought of a better way I could have asked it. Say I want to take a picture of a single person with the 108mm zoom, about how far can they be standing away from me and still be able to capture the detail in their face? Thank you very much.

I'm not at all sure that I understand the question, but I'll try to give you some help.

By stating the lens goes from 35 to 108mm, the maker is giving you the focal length range of the lens. 35mm is a moderate wide- angle lens on 35mm cameras. It could be useful for a shot of a group of people standing together or perhaps of a landscape of a very big subject that you are close to. 108mm, again on a 35mm camera, is a moderately long lens, that is, one that gives a rather narrow view. It can be useful for close portraits because of perspective, or when you want a picture of something some distance away. The subject will then fill the film more than if a shorter focal length lens had been used.

If this isn't the sort of information you are looking for, let us know.

Ernest Purdum
7-Dec-2006, 07:47
I( can't really give you much of an answer on this one because there are too many variables. The grain size and other characteristics of the film is perhaps the most important. The quality of the lens is another. If the lighting is good enough to enable a very short exposure, or a tripod is used, the results will be more likely to be successful.

OIne way of looking at it is that at the 108mm setting, you should be able to get a recognizable likeness from just over three times the distance at which you could while at the 35mm setting. (108 divided by 35 = 3.086)

Leonard Evens
7-Dec-2006, 08:01
Actually that's about what I was looking for, I thought about the question later and thought of a better way I could have asked it. Say I want to take a picture of a single person with the 108mm zoom, about how far can they be standing away from me and still be able to capture the detail in their face? Thank you very much.

I think you are in the wrong forum and you need some instruction in the fundamentals of photography. But let me try to answer your question. There is a good chance you won't understand what I say, in which case, you should find a book or tutorial on photography fundamentals to study before proceding.

When you take a picture of a person at some distance from the lens, either the camera focuses automatically on the subject, or you focus manually. Focusing moves the lens either nearer or further from the film plane (or sensor plane for a digital camera). The closer the subject, the further lens is from the film plane. If you don't understand that, it would be hard for you to understand the rest of what I say, so make sure you do understand the idea of focusing.

In practice, there is a furthest distance you can move the lens from the film plane, and that limits how close a subject you can focus on. For a simple point and shoot camera, this will usually be a few feet, but it will depend on the focal length. For longer zoom settings the dlosest focusing distance will be further away. Just what that distance is for your camera at a particular zoom setting is something you should be able to determine, either from the manual or by experimentation.

Whether or not there is sufficeint detail in the face will depend on a variety of factors. The further away the subject, the harder it will be to distinguish detail. Usually, if the subject is between something like 3 and 10 feet, there will be adequate detail in the face. What will change is how large the face is. The closer the subject is to the camera, the larger the image of the face, and the easier to see detail. Also, the size of the image will depend on the focal length or zoom setting. If you fix the distance of the subject from the lens, the longer the focal length, the larger the image will be.

Since there is usually sufficient detail at a variety of distances, you usually want to keep the subject at a somewhat greater distance in order to avoid overemphasizing some facial features such as the nose. Generally, I would suggest something like 4 to 6 feet. For the image to be sufficiently large, you will want to use a relatively long focal length. But, of course, you can't focus any closer than the minimum distance allowed by your particular camera and lens at the specified zoom setting. In that case, it usually won't hurt to put the subject even further away. If you can't get the subject that far away for your longest zoom setting, use a lesser setting.

Finally there is the issue of depth of field. Only one plane is in exact focus, but there there is a region about that, called the depth of field, which for all practical purposes is in focus. If you concentrate on keeping the size of the face in the image approximately the same through choice of subject diatance and zoom setting, the size of that region of adequate focus depends only on the relative aperture or f-number. If you are using an automatic camera, that will typically be set by the camera, but it usually possible to set it manually yourself. For typical small format point and shoot cameras, there is usually adequate depth of field at typical portrait distances, but in some cases you may have to choose a larger f-number in order to keep everything in focus from the tip of the nose to the ears.