Very good article for newbies. You may have been taught bad ideas.
http://www.amasci.com/miscon/lens1.html
http://www.amasci.com/miscon/lens1.html
good essay
Yes, I've seen plenty of those drawings that are just completely wrong for illustrating image projection. But they DO show what a lens does with a bundle of parallel rays (eg. from the sun): It focusses all the rays to a single point at the focal distance of the lens.
This would only apply to projecting a single tiny pixel LCD onto a screen at an infinite distance. All real projectors use a screen that is closer than infinity, so the LCD has to be further from the lens than the focal distance.
This perpetuated misinformation is just like the "common knowledge" from art classes & books. To this day, some people in that field will argue that red, yellow, and blue are the primary pigment colors, even though printers MUST use magenta, yellow, and cyan to print good color pictures! Duh......
Yes, I've seen plenty of those drawings that are just completely wrong for illustrating image projection. But they DO show what a lens does with a bundle of parallel rays (eg. from the sun): It focusses all the rays to a single point at the focal distance of the lens.
This would only apply to projecting a single tiny pixel LCD onto a screen at an infinite distance. All real projectors use a screen that is closer than infinity, so the LCD has to be further from the lens than the focal distance.
This perpetuated misinformation is just like the "common knowledge" from art classes & books. To this day, some people in that field will argue that red, yellow, and blue are the primary pigment colors, even though printers MUST use magenta, yellow, and cyan to print good color pictures! Duh......
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