Does Focal Length Affect Exposure

**dms** · 07-13-2013, 07:26 PM

Imagine the often quoted example of a black cat against white background--and assume a ordinary averaged (center weighted) exposure. A wide angle sees mostly white background and thus underexposes. A telephoto sees mostly a black cat and thus overexposes. So yes the FL affets the metering. But if the scene is uniform then except for minor lens to lens variation (at mddle apertures likely +/- 1/3 stop about) they should all agree.

troika · 07-13-2013, 08:01 PM

I'm learning a lot from all of your responses. Thank you for taking the time to explain/debate this. It's helpful to me and maybe someone else who is reading.

**MSL** · 07-13-2013, 08:17 PM

Originally posted by Lowell Goudge

In A mode lenses are accurate because the non linearity I have found as a function of aperture and assumed scatter off the metering surface, is calibrated out. The camera knows in A mode the maximum aperture, and calculates exposures for camera controlled aperture. With manually controlled aperture the camera has only a light value but no knowledge of the actual aperture and since the metering appears to be non linear, there is nothing it can do

Lowell - maybe I'm reading more into this than I should, but it would seem to me that what you are saying is that part of the problem is that when stopped down, the smaller aperture changes the way the metering is done because of the way the area over which the light is entering has changed and cannot be related back to what the lens would be like when fully open. If I'm right or sort of right, does this mean that metering problems would lessen if you could use spot or center focus metering with a stopped down lens?

CodyShaw · 07-14-2013, 06:54 AM

The aperture doesn't stop down until the photo is taken anyway so what are you guys talking about?

**MSL** · 07-14-2013, 08:45 AM

Originally posted by CodyShaw

The aperture doesn't stop down until the photo is taken anyway so what are you guys talking about?

m42 lenses for example, where the lens is stopped down before the shot is taken

**Lowell Goudge** · 07-14-2013, 05:49 PM

Originally posted by MSL

Lowell - maybe I'm reading more into this than I should, but it would seem to me that what you are saying is that part of the problem is that when stopped down, the smaller aperture changes the way the metering is done because of the way the area over which the light is entering has changed and cannot be related back to what the lens would be like when fully open. If I'm right or sort of right, does this mean that metering problems would lessen if you could use spot or center focus metering with a stopped down lens?

It seems that the issue is or was partially due to the focusing screen. The problem is that some focusing screens seem to reflect light every where, spot metering makes no difference, it is just as bad.

troika · 07-14-2013, 08:19 PM

I have been using the magnified LV for focusing some things lately, especially my macro and tripod shots. But, I never meter in live view.

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07-13-2013, 07:26 PM	#16
dms Site Supporter Join Date: Aug 2011 Location: New York, NY Photos: Gallery Posts: 2,192	Imagine the often quoted example of a black cat against white background--and assume a ordinary averaged (center weighted) exposure. A wide angle sees mostly white background and thus underexposes. A telephoto sees mostly a black cat and thus overexposes. So yes the FL affets the metering. But if the scene is uniform then except for minor lens to lens variation (at mddle apertures likely +/- 1/3 stop about) they should all agree.

07-13-2013, 08:01 PM	#17
troika Pentaxian Join Date: Dec 2012 Location: Seattle, WA USA Posts: 1,753 Original Poster	I'm learning a lot from all of your responses. Thank you for taking the time to explain/debate this. It's helpful to me and maybe someone else who is reading.

07-13-2013, 08:17 PM	#18
MSL Site Supporter Join Date: Feb 2012 Location: Greater Toronto Area Photos: Gallery \| Albums Posts: 4,749	Originally posted by Lowell Goudge In A mode lenses are accurate because the non linearity I have found as a function of aperture and assumed scatter off the metering surface, is calibrated out. The camera knows in A mode the maximum aperture, and calculates exposures for camera controlled aperture. With manually controlled aperture the camera has only a light value but no knowledge of the actual aperture and since the metering appears to be non linear, there is nothing it can do Lowell - maybe I'm reading more into this than I should, but it would seem to me that what you are saying is that part of the problem is that when stopped down, the smaller aperture changes the way the metering is done because of the way the area over which the light is entering has changed and cannot be related back to what the lens would be like when fully open. If I'm right or sort of right, does this mean that metering problems would lessen if you could use spot or center focus metering with a stopped down lens?

07-14-2013, 06:54 AM	#19
CodyShaw New Member Join Date: Oct 2012 Photos: Gallery Posts: 18	The aperture doesn't stop down until the photo is taken anyway so what are you guys talking about?

07-14-2013, 08:45 AM	#20
MSL Site Supporter Join Date: Feb 2012 Location: Greater Toronto Area Photos: Gallery \| Albums Posts: 4,749	Originally posted by CodyShaw The aperture doesn't stop down until the photo is taken anyway so what are you guys talking about? m42 lenses for example, where the lens is stopped down before the shot is taken

07-14-2013, 05:49 PM	#21
Lowell Goudge Site Supporter Join Date: Jan 2007 Location: Toronto Photos: Gallery \| Albums Posts: 17,887	Originally posted by MSL Lowell - maybe I'm reading more into this than I should, but it would seem to me that what you are saying is that part of the problem is that when stopped down, the smaller aperture changes the way the metering is done because of the way the area over which the light is entering has changed and cannot be related back to what the lens would be like when fully open. If I'm right or sort of right, does this mean that metering problems would lessen if you could use spot or center focus metering with a stopped down lens? It seems that the issue is or was partially due to the focusing screen. The problem is that some focusing screens seem to reflect light every where, spot metering makes no difference, it is just as bad.

07-14-2013, 08:19 PM	#22
troika Pentaxian Join Date: Dec 2012 Location: Seattle, WA USA Posts: 1,753 Original Poster	I have been using the magnified LV for focusing some things lately, especially my macro and tripod shots. But, I never meter in live view.