Explanation of terms


cher-cher

Link Posted 13/04/2011 - 14:21
Hi, I need a little help in understanding some terms. I saw in the Gallery section, when looking at the Exif details, that two terms appear:
F-Stop: f/6.7
Aperture Value: f/5.5
(the values differ from case to case, of course)
I am confused. I knew that F-stop is the ratio of the focal length to the aperture of a lens, and the aperture is the size of the hole that allows light in. For example, having a focal lenght of 50 mm, and a f-stop of 5.6, will result an aperture of 8.93 mm. So, what does it mean that term appearing in the forum called aperture value? I presume the f-stop is the parameter we fix on the camera (if we do, or is automatically set according to the camera program adjustment we use).
Thank you.

grahamwalton

Link Posted 13/04/2011 - 17:11
F-Stop and Aperture Value refer to the same thing.
If you set you camera to Aperture Priority (AV on the dial) then you select the F Stop and the camera sets the Shutter Speed.
If you set Shutter Priority (TV on the dial) then you select the Shutter Speed and the camera sets the F stop.
In Manual Mode (M) you select both F Stop and Shutter Speed.
In Program Mode the camera selects both according to its progam line.

A detailed explanation of F Stops can be found here: http://en.wikipedia.org/wiki/F-number
Friendly Regards
Graham

cher-cher

Link Posted 13/04/2011 - 21:58
Well, you say they refer to the same thing. Then why are they often different, for the same photo?
I know how to set those parameters, but I do not know why, for a picture, the exif there shows bothe F-stop and Ap value, and the values are different?

greynolds999

Link Posted 13/04/2011 - 22:15
I think in any mode when the camera is setting the Aperture the F-Stop is the value being displayed by the camera but the aperture value is the actual aperture.

This can be similar to (when using Av) the displayed shutter speed may be (say) 1/500 but the actual shutter speed is 1/400.
My Photobucket

bforbes

Link Posted 13/04/2011 - 22:46
Womble found this in a previous thread on the same subject..link I don't understand it but that is not unusual.
Barrie
Too Old To Die Young

http://www.pentaxphotogallery.com/artists/barrieforbes
https://www.flickr.com/photos/189482630@N03/

pentaxian450

Link Posted 14/04/2011 - 00:35
f:stop is the actual physical opening of the lens, e.g. f:2 on a 50 mm. lens is 25 mm.

Aperture value is generally the physical f:stop less the light loss due to lens extension, light absorption due to the glass, dispertion and diffraction due to internal reflection.

In other words, aperture value is the physical opening minus everything that keep some of the light from reaching the sensor/film plane.
Yves (another one of those crazy Canucks)

cher-cher

Link Posted 14/04/2011 - 09:34
Thank you for the answers.
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