Are F-Stops subjective? at DVinfo.net

Jeremy Rank · March 25th, 2005, 05:23 PM

I've got two cameras...a Panasonic GS400 and a Sony VX2100. I was playing with them both in a fairly lit room (about 60lux) and noticed that the GS400 with an open iris and 18db is not as bright as the VX2100 at 4.8. Why is that? Granted it could have something to do with the ccd size...but I thought that the settings would be a little more objective rather than a subjective subject.

Aaron Shaw · March 25th, 2005, 05:53 PM

F-stops are a measure of the amount of light that is allowed past the iris in relation to the lens diameter. How that light is then handled by the CCD, DSP etc, determines what sort of image you get.

Boyd Ostroff · March 25th, 2005, 05:55 PM

F-stops are just calculated by the lens aperture and focal length and don't take anything into account for the sensitivity of the CCD's. As we know, the VX-2100 is the low light champ and has bigger CCD's than the GS-400. The GS-400 also has a higher pixel density on the small chips which make it even less sensitive. So in other words, your test would be like comparing two film cameras with different speed films in them.

I have a PDX-10 which has similar (if not identical) CCD's to your GS-400 (1/4.7" 1152x864 IIRC). In side by side tests I found about a 2.5 f-stop difference from my VX-2000.

A. J. deLange · March 28th, 2005, 05:03 PM

The F-stop setting is often referred to as the "speed" of the lens. Exposure also depends on the "speed" of the film or CCD and the speed of the shutter (short, fast exposure or long,slow exposure). Reducing the F-number by a factor of the square root of 2 (e.g. from f/5.6 to f/4) increases exposure 1 stop. Doubling the film or CCD speed (e.g. from 125 ISO to 250 ISO) increases the exposure by 1 stop. Doubling the shutter "speed" (e.g. from 1/60 to 1/30) increases the exposure by 1 stop.

March 25th, 2005, 05:23 PM	#1
Jeremy Rank Regular Crew Join Date: Dec 2004 Location: Visalia, CA Posts: 66	Are F-Stops subjective? I've got two cameras...a Panasonic GS400 and a Sony VX2100. I was playing with them both in a fairly lit room (about 60lux) and noticed that the GS400 with an open iris and 18db is not as bright as the VX2100 at 4.8. Why is that? Granted it could have something to do with the ccd size...but I thought that the settings would be a little more objective rather than a subjective subject.

March 25th, 2005, 05:53 PM	#2
Aaron Shaw Major Player Join Date: Jul 2004 Posts: 587	F-stops are a measure of the amount of light that is allowed past the iris in relation to the lens diameter. How that light is then handled by the CCD, DSP etc, determines what sort of image you get.

March 25th, 2005, 05:55 PM	#3
Boyd Ostroff Wrangler Join Date: Dec 2002 Location: Mays Landing, NJ Posts: 11,802	F-stops are just calculated by the lens aperture and focal length and don't take anything into account for the sensitivity of the CCD's. As we know, the VX-2100 is the low light champ and has bigger CCD's than the GS-400. The GS-400 also has a higher pixel density on the small chips which make it even less sensitive. So in other words, your test would be like comparing two film cameras with different speed films in them. I have a PDX-10 which has similar (if not identical) CCD's to your GS-400 (1/4.7" 1152x864 IIRC). In side by side tests I found about a 2.5 f-stop difference from my VX-2000.

March 28th, 2005, 05:03 PM	#4
A. J. deLange Major Player Join Date: Jun 2004 Location: McLean, VA United States Posts: 749	The F-stop setting is often referred to as the "speed" of the lens. Exposure also depends on the "speed" of the film or CCD and the speed of the shutter (short, fast exposure or long,slow exposure). Reducing the F-number by a factor of the square root of 2 (e.g. from f/5.6 to f/4) increases exposure 1 stop. Doubling the film or CCD speed (e.g. from 125 ISO to 250 ISO) increases the exposure by 1 stop. Doubling the shutter "speed" (e.g. from 1/60 to 1/30) increases the exposure by 1 stop.