Newbie Iris Question...

yzmotoxer807

yzmotoxer807

Well-known member
This is probably a stupid question and something well known but I cant find any info or figure it out myself. When the iris in a lens closes, it literally blocks the lens. Why does it not block off the image?(Why cant you see the iris in the shot?)

Just a question that popped into my head. Thanks.
 
I'm kinda wondering that too. But I guess it's like a magnifying glass. It's not like the lense is just flat. It's curved, and that's the trick I believe. Maybe. Someone could correct me.
 
I believe it's because the smallest iris setting it the exact same size as the back of the lens. The front of the lens is obviously bigger, therefore more light can come in and get focused through the smaller back glass piece. So the curvature of the front would redirect the light back to that smaller plane.

http://en.wikipedia.org/wiki/Aperture


I'm not 100% on that, but I believe that's the answer.
 
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That link still doesn't explain why, half the image isn't blocked. It just says if you block part of the lens then it stops down the light, but it doesn't mention how that works.


EDIT: Okay.... makes sense now.

Once you read the part about the iris, look back up at image 4 in the first row.... each of those arrows is the same part of the object just hitting the lense at a different point. Drop the iris, you drop the amount of rays from that object hitting the back plane of the lens; but it still seems like it would require the smallest iris setting to just about match the size of the back of the lens, so it would be one light ray per point hitting the lens, while not blocking any of those points. With an iris bigger than the back of the lens, it would start blocking points of the incoming light, therefore cutting it out of the image.
 
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Thanks guys it makes total sense now. Im glad to know that I wasnt the only one with this question:grin:.
 
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