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Ryan Emanuel

Sensor Size F Stop Question

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I'm a lil confused and wanted to ask for some clarification.  From my understanding the f stop is calculated from the ratio of the focal length divided by the entrance pupil size.  So a 50mm lens at a f2 has an entrance pupil of 25mm, and a 100mm lens has a entrance pupil of 50mm at an f2, so F stop's entrance pupil size are relative to the focal length.  An f2 is a different size opening at different mm's.  The reason why a 50mm at f2 has the same exposure to a 100mm at f2 is that the 100mm proportionally projects a larger image.  The pupil diameter is twice as big on the 100mm, so the surface area of the hole is 4 times bigger letting in 2 extra stops of light at a f2 but the projected image of the 100mm is proportionally larger so the exposure evens out.   When comparing large format vs smaller formats, that sensor size is not fixed so that extra light of the 100mm vs the 50mm would be captured by the sensor.  So lets say you are comparing micro 4/3 (2x crop) vs full frame, to get the same angle of view you need a 50mm to roughly match the 100mm on the full frame camera.  But f2 on both those lenses will not be the same amount of light.  So will the full frame camera be brighter at the same F stop? Will you need to adjust the f stop proportionally to match the change in sensor size, so a f2 for the 50mm  on micro 4/3 would be around a f4 for the 100mm on full frame to match exposure and depth of field?  Thanks for the help. 

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No they are the same. The micro 4/3 may have less light coming in overall but it has the same amount of light relative to the surface area of the sensor as the full frame setup. Another way to look at it is the F stop is the density of light produced by the lens, different lenses on different formats may have different amounts of light coming in but the denisty of light at the sensor/film plane is the same if the F stop of the lenses have is the same.

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// edit: ninjaed.... replying to the original post, not David's.

the lens projects the image as multiple very small points of light. Also the sensor reads millions of small areas separately. So no, it does not work that way.

IF the sensor only had one pixel in it AND the lens would be totally out of focus only projecting a single point of light, then it would be another matter

Edited by aapo lettinen

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21 minutes ago, David Hessel said:

No they are the same. The micro 4/3 may have less light coming in overall but it has the same amount of light relative to the surface area of the sensor as the full frame setup. Another way to look at it is the F stop is the density of light produced by the lens, different lenses on different formats may have different amounts of light coming in but the denisty of light at the sensor/film plane is the same if the F stop of the lenses have is the same.

I'm really just trying to understand Yedlin's article here http://yedlin.net/lens_blur.html  So if the exposure on different sensor sizes are the same, then are the examples from the article for Alexa are lit to a f4 while the imax shots were lit to a f11 to match the depth of field?

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Yes. He probably lit to f/11 and used ND filters on the smaller sensor cameras so that the lighting was the same but he could open up the iris to match depth of field. Or instead of ND filters, one could change the frame rate / shutter time.

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