Why can two lenses suggest the same shutter speed at different maximum apertures?
Asked 8/31/2011
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Using a Canon 1Ds Mark II at ISO 400 with no filters, I noticed that two lenses gave essentially the same metered shutter speed (about 0.4s) even though they were set to different maximum apertures:
- Canon 17-40mm f/4L at 17mm, f/4
- Canon 28mm f/2.8 at 28mm, f/2.8
The camera was in average metering, the scene was the same dark scene, and I changed lenses and retested to try to reproduce it. The shutter speed stayed almost the same, even when zooming the 17-40 through its range.
I expected the f/2.8 lens to meter a faster shutter speed than the f/4 lens. Why might the camera suggest the same exposure time?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
6
The apertures are different, but at only 1 stop I wouldn't call them very different. :)
Anyways, the only other thing that changes in the equation to make the exposure is the amount of light. If you're metering the whole scene then the zoom lens at the wider angle with the smaller aperture metered a larger area and the resulting average indicated the shutter speed. The 28mm, more open, but with a narrower field of view, metered less information and that resulted in less average light.
If you're center metering then you may have metered on a slightly different spot, a darker one, resulting in no change in shutter speed. In any case, it would strike me that the camera has different meter readings based on the scene it has through the lens.
Originally by user472. Source · Licensed CC BY-SA 4.0
user472
14y ago
0
Generated from our catalog & community — verify before relying on it.
Aperture is only one part of what the meter sees. The most likely reason is metering differences caused by the field of view: at 17mm the zoom includes a wider area, so the camera averages a different scene than the 28mm prime does. If the wider view includes darker areas, it can offset the one-stop aperture advantage of f/2.8.
A one-stop aperture difference is not huge, and small scene or metering changes can easily mask it. If your eye or stray light was entering the viewfinder during metering, that can also affect the reading; try covering the viewfinder when testing.
There is also a lens-transmission factor: f-stops describe geometry, not actual transmitted light. Different lenses can lose different amounts of light internally (T-stop), and zooms often lose a bit more than primes. That usually won’t explain a full stop by itself, but it can narrow the gap.
To test, photograph the same scene from a tripod, same framing, cover the viewfinder, and compare the RAW files. If brightness differs, the meter or lens transmission is the issue; if not, the metering was simply averaging different content.
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