tag:blogger.com,1999:blog-1924948806084793655.post8376913655162809436..comments2023-09-26T11:36:56.423-04:00Comments on Machine Vision 4 Users: Bigger pixels or smaller?Unknownnoreply@blogger.comBlogger2125tag:blogger.com,1999:blog-1924948806084793655.post-35600759151532483372010-11-30T16:43:42.299-05:002010-11-30T16:43:42.299-05:00Hi Vladimir,
Actually, one must not forget to tak...Hi Vladimir,<br /><br />Actually, one must not forget to take into account f-number and object distance in this discussion.<br /><br />Let's say you start with a 50mm fl. lens and you are imaging your object at 1:1 onto a detector that is exactly the size of your object. Object distance will be 100mm in this case.<br /><br />If the lens is set at F/8, the aperture size will be 50mm/8 = 6.25mm diameter. A point at the center of the object will send a cone of light rays to the lens, with a half angle of about 1.79 degrees. [ atan((6.25mm/2)/100mm ) ]<br /><br />On the other hand, if you wanted to image the same object onto a half-sized detector (so mag. = 0.5X now), the same lens would put the object at a distance of 150mm. For the same f-number, the half-angle of the cone would be 1.19 degrees -- less light would be collected.<br /><br />If you wanted to keep the same object distance, then you would use a lens with a focal length of 33.3mm. In this case the object distance is held at 100mm, but the 33.3mm fl. lens *at F/8*, would have an aperture size of only 33.3mm/8 = 4.16mm diameter. Again, the lens would collect a smaller cone of light to be projected onto the detector.<br /><br />The way to have the same light collection for either case is to use a smaller f-number.<br /><br />Spencer Luster<br />LIGHT WORKS, LLCSpencer Lusterhttp://www.LW4U.comnoreply@blogger.comtag:blogger.com,1999:blog-1924948806084793655.post-5005199880255840542010-11-24T03:19:40.622-05:002010-11-24T03:19:40.622-05:00Hi,
I'm not sure that the statement "sma...Hi,<br /><br />I'm not sure that the statement "smaller pixel=less light" is exactly true. In absolute terms, definitely. But think about this situation. You have two cameras with the same chip resolution, but different pixel sizes. You are observing the same FOV. Naturally, you must use different focal lengths of the objective lenses. The result is that in both cameras, the lens projects the same amount of light from the FOV to the area of the chip. In other words, in the camera with the smaller chip, the light is more concentrated to the smaller chip area. As a result, each single pixel gets the same amount of light, in both cameras. This is of course true only if their resolutions are the same.<br /><br />The other issue is the fill factor, which is really usually lower for smaller pixels. But that's a different technical issue.Vladimír Držíkhttps://www.blogger.com/profile/12257405408489709779noreply@blogger.com