BetterPhoto Q&A
Category: New Answers

help with shutter speeds and f-stops numbers

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June 13, 2007

If you are going to shoot in manual, you'll need to understand the effects of various aperture settings as well as what shutter speed settings will will allow. (I recommend you read one of the many excellent books on just how to achieve correct exposure.)

For instance, on my Nikon D200 and D70, I determine the aperture I want to achieve the desired DOF. I then look thru the viewfinder and dial in the shutter speed that will give the correct exposure. I'll check the histogram and adjust the speed up or down until I get the desired result.

You may want to learn exposure by using the Av mode and make note of the effects on exposure as you vary the aperture. Also note what shutter speed your camera selects under various lighting conditions for selected apertures.

You don't need to go on a "photo shoot" to learn; just go out in your backyard and practice.

John

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June 13, 2007

First let's tackle f/numbers:

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 canvas from the chow wagon. It rains as expected and the pit begins to collect rainwater. By your experience and training at West Point, 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.
Alan Marcus
ammarcus@earthlink.net

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June 13, 2007