dalka (11/09/82)
One interesting point about auto focus cameras: Don't try to take pictures thru a window!!!! Ken Dalka ihuxe!dalka
hamilton (11/09/82)
#R:ihuxe:-14000:uicsovax:17200001:000:439 uicsovax!hamilton Nov 8 23:04:00 1982 that's not really true, unless you're talking about "sonar" types (eg polaroid). the new auto-focus mechanisms (there quite a few now) actually "look" at the image produced and "recognize" sharp focus. what you have to watch out for are scenes without the edge features that the algorithms look for. (don't ask me for more details; i don't fully understand how they work). wayne ({decvax,ucbvax,harpo}!pur-ee!uiucdcs!uicsovax!)hamilton
mem (11/10/82)
c As I understand it, most auto-focusing mechanisms work via a maximum contrast recognition facility. That is, there is some measure of the average maximum difference between light and dark spots (does that make sense?). (This, by the way, is in response to a reply from wayne hamilton.) So shooting through glass should still work, unless the glass diffuses the incoming light. Mark Mallett
rsx (11/10/82)
Actually, I believe the auto-focus works on the grain of the negatives as reflected back to the head from the print board... Jim Morton GenRad Inc. Concord, Mass. (...decvax!genradbolton!grkermit!jim)
malcolm (11/22/82)
#R:ihuxe:-14000:pur-ee:8400006:000:1908 pur-ee!malcolm Nov 10 13:35:00 1982 The lab that I work in has been looking at the technology that is available for measuring depth for the past few months. We have found three different technologies used in cameras today. Everybody is most familiar with the Ultrasound technology used by Polaroid. Their approach is to simply transmit a 1mS chirp of 50-60KHz sound waves and to time their return. The timing is very simple since there is a 177 uS delay per inch of distance. As has been pointed out before do not try to have the camera focus through a window. Glass is a excellant reflector of sound waves at this frequency. Canon is using an infared technique in their Auto-Focus camera. I have been told that they scan a sharply focused infared light source through a series of angles. They use a receiver on the far side of the camera to receive the scattered light. When the light source is pointing directly at the object the light received by the receiver will be the greatest and the angle of the light source can be used to estimate the distance to the object. The whole thing becomes a simple triangulation problem. This approach also has the advantage of being more sensitive at closer distances (where the focusing is more difficult) because the angles changes faster. I have also seen some talk of using contrast to measure sharpness of a picture. I believe it was somebody like Honeywell that was exploiting this approach. The idea is that by using three light detectors they can form an estimate of the contrast of the image (just take the variance of the light received at the three detectors). It is easy to see that the focus will be sharpest when the contrast is maximized. I have never used any of these cameras so I can't tell you how well they work. I hope this answers everybody's questions. Malcolm Slaney Purdue EE Dept. {decvax,harpo,ihnss,ucbvax} !pur-ee!malcolm