Apeture changes when zoomed in? at DVinfo.net

Peter Butler · August 28th, 2002, 04:59 AM

Sorry for the amount of posts from me at the moment but I just keep finding new things. On the XM2(GL2) I was zooming in out, like you do and noticed that when nearly on full zoom the XM2 starts to close the aperture, and if you pull out it will open it up again, why is this? It was in a room lit with just a few lamps so it wasn't exactly bright but nothing that the XM2 couldn't cope with.

Peter B

Jeff Donald · August 28th, 2002, 07:09 AM

What mode are you using, Green Box, Auto or one of the semi-automatic or manual settings? The auto exposure system (AE) looks at the scene to determine proper lens opening (aperature, F-Stop) and shutter speed for correct exposure. When you zoom the lens, light sources change their size, shape and position relative to the pre zoom settings. The changes may require the AE system to open or close the lens opening to maintain proper exposure. Setting the camera to manual will force the camera to maintain the original lens opening. Of course this may result in the scene becoming too light (over exposed) or too dark (under exposed).

Jeff

Adrian Douglas · August 28th, 2002, 07:42 AM

Another reason for this Peter is that the lens is not a constant aperature lens. The max f-stop is f1.6-2.9, which means at the widest focal length the max available aperature is f1.6 and at the longest focal length the max available aperature is f2.9. This increments as you zoom from the widest to the longest.

As you were shooting in low light the zens would have been full open to allow as much light as possible in to the CCDs. The camera will maintain the wide open setting as you zoom but as the f-stop is not constantly f1.6 this will change depending on the focal length.

Peter Butler · August 28th, 2002, 07:51 AM

Cheers guys, Jeff I had it set to manual at 1.6 fstop I'm guessing Adrians right because it changes to 2. something or other when you go to full zoom, you can actually see the picture darken slightly as well. If you then try and put back up to the 1.6fstop while still zoomed in, it won't let you.

Peter B

Chris Hurd · August 28th, 2002, 09:58 AM

This is common for most cameras and camcorders... different aperture values for each end of the lens. They're pretty much all like that.

Peter Butler · August 28th, 2002, 10:00 AM

Excellent well thats cleared that one up, cheers everyone.

Peter B

August 28th, 2002, 04:59 AM	#1
Peter Butler Regular Crew Join Date: Aug 2002 Posts: 61	Apeture changes when zoomed in? Sorry for the amount of posts from me at the moment but I just keep finding new things. On the XM2(GL2) I was zooming in out, like you do and noticed that when nearly on full zoom the XM2 starts to close the aperture, and if you pull out it will open it up again, why is this? It was in a room lit with just a few lamps so it wasn't exactly bright but nothing that the XM2 couldn't cope with. Peter B

August 28th, 2002, 07:42 AM	#3
Adrian Douglas Trustee Join Date: Oct 2001 Location: Chigasaki, Japan. Posts: 1,660	Another reason for this Peter is that the lens is not a constant aperature lens. The max f-stop is f1.6-2.9, which means at the widest focal length the max available aperature is f1.6 and at the longest focal length the max available aperature is f2.9. This increments as you zoom from the widest to the longest. As you were shooting in low light the zens would have been full open to allow as much light as possible in to the CCDs. The camera will maintain the wide open setting as you zoom but as the f-stop is not constantly f1.6 this will change depending on the focal length. __________________ Adrian DVInfo.net Search for quick answers Where to buy? From the best in the business...DVInfo.net sponsors

August 28th, 2002, 09:58 AM	#5
Chris Hurd Obstreperous Rex Join Date: Jan 2001 Location: San Marcos, TX Posts: 27,368 Images: 513	This is common for most cameras and camcorders... different aperture values for each end of the lens. They're pretty much all like that. __________________ CH Search DV Info Net \| 20 years of DVi \| ...Tuesday is Soylent Green Day!

August 28th, 2002, 07:09 AM	#2
Jeff Donald Warden Join Date: Mar 2002 Location: Clearwater, FL Posts: 8,287	What mode are you using, Green Box, Auto or one of the semi-automatic or manual settings? The auto exposure system (AE) looks at the scene to determine proper lens opening (aperature, F-Stop) and shutter speed for correct exposure. When you zoom the lens, light sources change their size, shape and position relative to the pre zoom settings. The changes may require the AE system to open or close the lens opening to maintain proper exposure. Setting the camera to manual will force the camera to maintain the original lens opening. Of course this may result in the scene becoming too light (over exposed) or too dark (under exposed). Jeff

August 28th, 2002, 07:51 AM	#4
Peter Butler Regular Crew Join Date: Aug 2002 Posts: 61	Cheers guys, Jeff I had it set to manual at 1.6 fstop I'm guessing Adrians right because it changes to 2. something or other when you go to full zoom, you can actually see the picture darken slightly as well. If you then try and put back up to the 1.6fstop while still zoomed in, it won't let you. Peter B

August 28th, 2002, 10:00 AM	#6
Peter Butler Regular Crew Join Date: Aug 2002 Posts: 61	Excellent well thats cleared that one up, cheers everyone. Peter B