 Michael Wasson |
Is Shutter speed an option for controlling DOF I read in My Nikon D50 Digital Field Guid that your sharpest resolution will be at twice the Maximum aperture. If thaths the case then you would always have greater DOF any time you wanted sharper portraits. Can this be offset by using a faster shutter speed?
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Mark Feldstein |
Yes. Because shutter speed in relation to f-stops is proportional. For example, if a scene calls for f8.0 at 1/250, you can get greater depth of field by shooting at f 11.0 and 1/125th, i.e., halving your shutter speed by one stop and doubling your aperture by one stop. Get the picture Michael?? ;>) Mark
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Jon Close |
Leaving aside the issue of depth of field... It is difficult to impossible to design a lens to have the same critical sharpness at all apertures. Typical is that performance at maximum and minimum apertures is just acceptible, and optimal sharpness is acheived at about 2 stops smaller than the maximum aperture. With respect to shutter speed - stopping down the lens requires longer shutter speeds than shooting wide open. Blur from camera shake and subject movement becomes an issue as shutter speeds get longer. So shooting at f/8 or 11 to maintain optimal lens sharpness may require the use of a tripod or shooting at higher ISO (which introduces its own problems of greater film grain or digital noise). So, just because a lens is sharpest at say f/8 does not mean that every photo should be shot at that aperture. If you want shallow depth of field, or action-stopping shutter speeds without shooting at high ISO, then you'll prefer to shoot with the lens wide open. If you want greater depth of field you'll shoot at smaller apertures.
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Alan N. Marcus |
Hi Michael, The camera’s lens functions as a wave guide that bends (refraction) light waves. These rays are redirected and caused to project a miniature image of the outside world at the image plane (surface of the film or sensor chip) inside the camera. As a rule of thumb, the sharpest f-number (aperture), will be an opening about two stops smaller (stopped down) than the maximum allowed by the lens. Generally this will be at about f.8. Larger openings like f/2 or f/2.8 are only slightly less sharp. This is because the larger lens diameter means light rays striking the peripheral of the lens must be bent more than rays that enter at the center. Stated another way; light rays that hit the lens near the edges are more likely to be misdirected. Misdirected rays degrade image quality. When we stop down a lens we make the functioning aperture smaller. The smaller diameter means that more and more of the peripheral portion of the lens is blocked and thus not used. As you might guess, utilizing mainly the central portion of the lens delivers superior sharpness. As we stop down we gain depth-of-field: The world of optics is not simple. To stop a lens down we use a mechanical aperture called a diaphragm (mimics the action of the colored portion of the human eye called the iris diaphragm). This lens entry hole is caused to become smaller as we stop down. At tiny settings around f/16 or so, a higher percentage of the light rays are forced to brush by the diaphragms edge. As they stream by, some rays are misdirected (diffraction). Now the image starts to degrade. As we continue to stop down, we get more and more depth-of-field but we sacrifice sharpness as we do so. Don’t worry! The camera maker stops you before much harm is realized. When depth-of-filed control is a called for, you must choose the appropriate aperture. As an example, in portraiture we generally want a shallow depth-of-field so we use f/5.6 or so. Having chosen an aperture, you must now adjust the lighting level to permit its use or choose an appropriate ISO sensitivity setting (film speed). Or choose an appropriate shutter speed. Or intermix these controls to achieve your goal. When electronic flash is used for lighting, the flash duration is so short and you are denied controlling exposure via the shutter speed adjustment. You alone must choose if you are going to use the portrait mode setting on your camera and allow the camera chip to do all the thinking or if you are going to study and learn and take command for yourself. More and more, as the camera logic exceeds my meager brain, I let it do the thinking for me. I wish you well, Alan Marcus
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Bob Chance |
Alan I am impressed! Did you actaully type all of that or did you copy and paste? LOL!!! Just kidding! Excellent explanation.
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Michael H. Cothran |
The shutter speed, by itself, has NO bearing on DOF. The entities that DO regulate DOF are the lens' focal length, the focusing distance, and the chosen aperture setting. One would obviously want to set the appropriate shutter speed to correspond with one's chosen aperture for exposure reasons, but in itself, this shutter speed has NO affect on DOF. Furthermore, do not confuse DOF with "motion blur." The shutter speed WILL affect the latter. Michael H. Cothran
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