[net.rec.photo] Confused About F Stops

sml (03/24/83)

F4 should be the same on any lense if you are metering the same light source.
When you change lenses on a camera and focus on an object, you are not seeing
the same picture with both lenses.  This will cause a difference in light
reading.  Try your experiment by metering off a uniformly lit wall, the
resultant shutter speeds should be the same.  You can even meter off something
just in front of the lens, it doesn't need to be in focus.  Kodak sells
"gray" cards if you wish to get scientific about your metering.

The various pins and levers found in your lens serve to tell the camera
about the f-stop setting of the lens.


				Steve Lazarus
				Ford Aerospace
				...!fortune!wdl1

wdr (03/29/83)

It is not true that any two lenses with the same marked Fstop set MUST
meter identically. As pointed out in previous articles, Fstop measures
only light gathering ability of the lens. It is a CRUDE approximation of
the actual transmission of light to film.
	(1)	The internal glass etc. absrobes light, i.e., doesn't
		transmit 100%. As you might guess, the  heavier the
		lens, the more absorbtion or less transmission.
		I would not be surprised if someone metered a f/1.8 
		lens at f/2 and a f/2.0 at f/2 by the same factory and
		found that the f/2.0 at f/2 was brighter. It has less
		glass in the way.

	(2) 	The focal length the Fstops are calibrated against is 
		focus-at-infinity length.  If you are metering on a
		closer object, there is small error here if the two
		lenses are different lengths -- they have different
		errors.

	(3) 	Modern cheap zooms do not maitain their labled f-stop
		during zoom. These are the f/3.5-4.5 marked zooms you've
		seen adds for, which are amazingly light. Because the 
		aperature doesn't adjust on cams while everything else
		moves. 

	(4) 	Individual lenses of the same Product (so & so's f/x Nmm)
		will vary in transmission, by experimental error.  If it
		is within the latitude of the film THEY expect you to use,
		they don't worry about it. (Ditto the shutter and meter.
		hopefully all these cancel out, they USUALLY do.)


		Bill ricker 

		wdr@security.uucp