 Jean Walley |
How to Calculate Hyperfocal Distance I am having a great deal of difficulty in understanding how to work out hyperfocal distance at different f settings. Is there an easy way or am I thick?
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John A. Lind |
Jean, you're not thick! Hyperfocal distance is based on the concept of the largest acceptable "Circle of Confusion" for the image (on the film), the lens focal length, and the lens f-stop setting being used. It is the first step in computing the "Depth of Field" for a specific f-stop setting and subject distance. It is the "circle of confusion" that usually causes the most confusion about hyperfocal distance and depth of field. The basic equation for Hyperfocal Distance is: If you set the lens focus to the hyperfocal distance, everything from half of "H" (hyperfocal distance) to infinity should appear to be in focus in prints up to about 8x10 size or for slides projected on a 40-50 inch screen. The "largest acceptable Circle of Confusion" is based on human perception of how big an "out of focus" point can be on the *film* (not on the print or projection screen) and still be seen as being "in focus" by a person viewing a print or projected slide. In other words, things have to be "sufficiently" out of focus before the average human eye can detect it. Example: H = 50mm^2/(8 * 0.025mm) Set the focus on the lens at 41 feet and everything should appear in focus in the print or projected slide from about 20 feet to infinity. If you change the f-stop being used to f/16, "H" becomes about 20 feet and setting the focus at that should have everything from 10 feet to infinity appear in focus. If you are using a zoom lens, every time you zoom (change focal length) the hyperfocal distance will change! The aperture is also *not* the lens speed (widest possible aperture for the lens). It is the actual f-stop setting being used when making the photo.
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Jean Walley |
Thank you, John. You made it sound easy. Why have I spent a whole week trying to work it out, many thanks!
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BetterPhoto Member |
Hello Jean, To answer your question in a very simple way is to use the basic principles of depth of field 1. The farther you focus the greater the depth of field In combining both these principles you get the greatest depth of field, ie., step down your aperture to the smallest on your lens and focus at slightly less than infinity, please note - not infinity. If your lens has a depth of field scale marked on it then with this setting you can get the maximum range with this setting. If you look at your lens you will note that the you have to rotate the barrel more when changing your focussing distance from 5 feet to 10 feet than between 15feet and infinity. This explains the principle of the depth of field at any point being 1/3rd before and 2/3 after the point of focus. Even if the depth of field scale is not marked the reading will give you this depth. You can have hyperfocal distance for any aperture by focussing at slightly less than infinity only the depth of field will vary. I hope this will solve your problem. Reen
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John A. Lind |
Thank you Jean! Several tips in practical application of hyperfocal focusing: 1. Check the accuracy of the distance markings on your lens(es). Not all lenses are accurately marked. [They're put on a lens for computing correct aperture using the Guide Number of a fixed output flash; great accuracy isn't required for that purpose.] For a 50mm lens pick a couple of distances marked between about 10 and 40 feet, measure that distance from something you can easily focus on, and focus the lens as you normally would. Then look at what the lens markings say the distance is. It will tell you how much, and in which direction you might need to correct for when setting a hyperfocal distance on the lens. To check marking accuracy on a wide-angle use shorter distances, and longer distances for telephotos. This will help make setting your lens to a hyperfocal distance more accurate. 2. Be careful about cropping the negative much for a print, and how much you try to enlarge it if you shot it using hyperfocal focusing. Remember it is human perception about what is and isn't in focus. If you crop a negative much for a print, you cannot enlarge it as much before very distant objects will start to appear out of focus. You might only be able to extract a good 4x6 or 5x7 under those conditions. Attempting to make huge prints (11x14 or 16x20) from even the entire negative is likewise risky. 3. For the lenses (focal lengths) you would use most with this, compute the hyperfocal distances for at least f/8, f/11 and f/16. Write these on a 3x5 card and stuff it in a pocket in your camera bag. Then you won't have to do the math or hunt for your calculator when you're trying to make a photograph in the field. After using the technique some and judging various print sizes afterward you will gain some experience in how much you can crop and enlarge a hyperfocal negative. Best Wishes!
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