March 7, 2006
A Spot Meter for the EOS 20D?
I finally got tired of battling with the light meter on Canon's EOS 20D. The problem is that, with the default setting of "Evaluative Metering", the EOS 20D looks at the light hitting the entire CMOS sensor, and takes at stab at what the correct exposure should be. Of course, you can change the exposure by adjusting the aperture or the shutter speed, but the problem is, it's hard to know how much to compensate, because the EOS 20D doesn't have a true spot meter.
The EOS 20D has three different settings for light metering.
- Evaluative metering - This is the default. Looks at all light hitting the sensor.
- Partial metering - looks at all of the light hitting within the partial metering circle that covers about 9% of the sensor.
- Centerweighted average metering. Weights the points in the center more than the other points on the sensor.
What the EOS 20D really needs is a "spot meter". A "spot meter" looks at a very small section of the sensor. A true spot meter should cover an angle of only approx 1 degree. Unfortunately, the EOS 20D is widely panned for not having one. (See here, here , here, and here. It was such an issue, that they included it in the newly released EOS 30D.
Only, as it turns out, the EOS 20D may actually have a true spot meter. On page 71 of the Canon EOS 20D Instruction Manual, in Chapter 4: "Setting the AF, Metering, and Drive Modes" under "Selecting Metering Mode", the Instruction Manuals says:
During manual focusing, evaluative metering is based on the Center AF point.
So, the way I read this, if you turn off your autofocus, then the evaluative metering uses only the center AF point, and it's smaller than a gnat's ass. So, that would make it a true spot meter, although I should probably do some tests on this, as it is possible I'm misinterpreting the text.
Screen print from Instruction Manual in extended entry.
I submitted the following question to Canon tonight:
I'm confused by the Instruction Manual for my EOS 20D. On page 71 of the Canon EOS 20D Instruction Manual, in Chapter 4: "Setting the AF, Metering, and Drive Modes" under "Selecting Metering Mode", the Instruction Manuals says:
"During manual focusing, evaluative metering is based on the Center AF point."
What, exactly, does this mean? Is the metering based solely on the light entering through the Center AF point? If so, this would make it a "spot meter". Or, does it mean something different? Why does it specifically mention "during manual focusing"? Does it act differently during autofocus? I'm confused.
Reply from Canon 03/08/06:
Thank you for contacting Canon product support. We value you as a Canon customer and appreciate the opportunity to assist you.
During autofocus operation preference is given to the active autofocus and the metering information in that area when the evaluative average of the scene is done. During manual focus operation the camera uses only the center focusing point when choosing preference in the equation.
We hope this information is helpful to you. Please feel free to contact us again if you have any other questions or concerns.
Thank you for choosing Canon.
Technical Support Representative
Posted by Peenie Wallie on March 7, 2006 at 8:40 PM
Sadly, I doubt very much that the actual metering of light in manual focusing mode is being done through the central, miniscule AF point.
I think the paragraph in the manual is simply stating that when manually focusing, if you try to use the 20D's ability to select a specific AF point in order to help you frame a subject away from the normal centre of frame, the evaluative light metering process will be measured from the central AF point regardless of which AF point you have selected.
The only real way to enable the 20D to mimic spot metering is with a zoom lens. For instance, I have a EF70-200mm F2.8L IS USM lens. If I'm using the lower zoom ranges for a subject (say 70mm) and I wish to somehow get a spot meter reading of my subject, I would simply use partial metering mode and zoom in to the maximum 200mm lens setting and take a reading. Zooming in effectively narrows down the 9% partial meter circle area at 70mm, to a smaller circumference when at 200mm releative to my subject.
Posted by: Darren Evans on April 4, 2006 at 11:23 AM
Canon confirms what you have said. That, it isn't a spot meter and it doesn't truly only use the light coming through the center AF point, as I had hoped. However, the manual is misleading then, at best. I assume that something changes when you're in "Evaluative Metering" mode and switch to "manual focusing" otherwise, their paragraph makes no sense what-so-ever. However, if it doesn't switch to a true "spot meter", it must switch to one of the other modes (i.e. "partial metering" or "center weighted average" metering). It would help if they had clarified this in the manual.
Your suggestion of zooming in using partial metering is a good one. Thanks.
Posted by: Peenie Wallie on April 4, 2006 at 1:38 PM