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Category: New Questions

Photography Question 
Marissa D. S
 

Apeture


Can anyone guide me to a detailed description of Apeture settings? I'm a little rusty and people always assume you know what they are talking about. Thanx!!


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11/11/2006 2:50:18 AM

 
Alan N. Marcus   Hi Marissa,

The job of the lens is to project an image of the outside world onto the film or digital chip. Thus the camera system acts like a slide projector backwards, the film/chip being the screen. Now how bright the image on the screen will be is a function of several factors. For this discussion we are only interested in the lens’s working diameter which is defined as the lens’s aperture. We need the ability to change the working diameter to make the screen image brighter or dimmer. Years ago it was determined that the best way to do this was to use an increment that either doubles or halves the screen brightness.

Now changing the subject (maybe). You are the captain of Cavalry “A” Troop. One hundred men with horses marching through the desert. Water is a problem. You bivouac for the night and you expect rain. You order the men to dig a circular pit 8 feet in diameter and line it with their canvas tent material. It rains as expected and the pit begins to collect rainwater. By your experience, you know an 8 foot diameter pit is adequate to collect rain water for your needs. Unexpectedly a lookout spots “B” Troop approaching --another 100 men with horses. You order your men to expand the diameter of the circular pit to accumulate water for 200 men and horses.

How big must the revised pit be to double the amount of collected rain water? Answer: You multiply the pit diameter (8 feet) by 1.4142. This value is the square root of 2. The answer is 11.3 (rounded it’s 11 feet). You order the pit expanded to 11 feet diameter. Surprise, this new value causes the pit to accumulate twice as much water as before. Why? The surface area (catch basin) now has double the surface area; thus it can capture twice the amount of rain.

The lens opening or aperture is also a circular geometric figure. The area of any circle (thus its ability to collect rain or light) is doubled if you multiply its diameter by 1.4 (1.4142 rounded). Using this factor a number set emerges:

1 – 1.4 – 2 – 2.8 – 4 -5.6 – 8 – 11 – 16 – 22 – 32 – 45 – 64

Note each number to the right is its neighbor on the left multiplied by 1.4 and then rounded. Each number to the left is its neighbor on the right divided by 1.4 and then rounded.

These are the mysterious values engraved on the lens barrel. With geometric precision they allow the adjustment of the working diameter of a lens, making it smaller or larger. We need this number set because it allows even and logical and predictable changes to be made in image brightness. On a camera the aperture or f/number is a ratio.

Maybe next time we can expand on the ratio concept.

Alan Marcus
ammarcus@earthlink.net


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11/11/2006 7:57:38 AM

 
Marissa D. S   Thank you!! THis makes more sense to me than any other answer people have given me. I'm used to reading blueprints not camera manuals. Thanx for putting in terms that make sense. I appreciate your help.


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11/11/2006 9:39:06 PM

 
Dennis Flanagan
BetterPhoto Member Since: 12/31/2005
  Although I understand all I need to know about f-stops and shutter speed relationships, I have never heard the analogy you used Alan. Excellent explanation.


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11/12/2006 12:54:31 AM

 
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