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Exposure Stops and F aperture values
http://www.x404.co.uk/forum/viewtopic.php?f=17&t=626		 Page 1 of 1
Author:  Fogmeister [ Sun May 17, 2009 11:50 am ]
Post subject:  Exposure Stops and F aperture values
Following my past few queries I've been wondering about stops and F values and stuff.

I know that 1 stop up or down is allowing twice or half the amount of light onto the sensor respectively and this can be done by changing the aperture or shutter speed.

With the shutter speed it's fairly easy to know what it means i.e. 1/1000 means the shutter is open for one one thousandth of a second. So using the same aperture and a speed of 1/500 is one stop higher.

With the aperture I know that F4 is one stop higher than F8 but what does the number equate to? Is it just an arbitrary number for that specific lens or if I use 1/500 shutter speed and F4 with one lens will it give the same results as using those settings with another lens? If so the number must mean something.

Also, my camera dial goes up and down in 1/3 stop intervals. Does this mean that if I change the aperture up 1/3 of a stop and the shutter speed down 1/3 of a stop will I be at the same exposure that I started with?

Lots and lots of questions :D Not too important really but it's something I'd quite like to know.

Thanks
Author:  nickminers [ Sun May 17, 2009 12:14 pm ]
Post subject:  Re: Exposure Stops and F aperture values
Quote:
With the aperture I know that F4 is one stop higher than F8

Not true - it's two stops. The aperture value is the number by which you must divide the focal length of the lens to get the diameter of the aperture itself. So, for example, f/4 on a 100mm lens is 100mm/4 = 25mm. That's why it's written as f/x (f divided by x), as f = focal length. Since this is the diameter of the aperture, if you double the number (e.g. to f/8) you are halving the diameter and so quartering the area, meaning only a quarter of the light reaches the sensor or film, so it's two stops darker. This is why a single stop is a factor of about 1.4 (roughly equal to the square root of 2).

Quote:
Also, my camera dial goes up and down in 1/3 stop intervals. Does this mean that if I change the aperture up 1/3 of a stop and the shutter speed down 1/3 of a stop will I be at the same exposure that I started with?

This holds true for most situations, yes. It's called reciprocity, and it only tends to break down at very long exposures on film (1 minute or longer), beyond which point you need to increase the shutter speed by even more than normal.
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