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06-07-2009, 08:55 AM   #1
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Lighting and Exposure

Any idea when set camera to aperture priority, if both a prime len (ie 50mm 1.4) and a super zoom len (ie 28-300mm f3.5-6.3 set to 50mm) are set to same aperture value (ie f6.3) and point at the same lighting condition, the camera will select the same shutter value ?

06-07-2009, 12:36 PM   #2
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In theory, yes. In practice, it's unlikely you'll get the zoom set to *exactly* the same focal length or pointed *exactly* the same way, so there might well be a slight difference in the scene that affects the results. And most lenses only approximate their maximum aperture, which can affect metering slightly. And the glass itself in one lens might transmit more light than the other even given the same aperture.

But we're talking about differences of a fraction of a stop. In cases where the camera would really like to set a shutter speed between two of the clicks, it might end up leaning one way with one lens and the other with the other.
06-07-2009, 09:45 PM   #3
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I tested this out with a (deep breath for the lens's full name) Tamron AF Aspherical XR LD [IF] 28-300mm 1:3.5-6.3 Macro θ62 and a Pentax-A 50mm f1.4. The results are exactly as Marc predicted. Most of the time the camera picked the same shutter speed, but I also got 1/3 stop higher and lower.

The Tamron can be set so it reports a focal length of 45mm or 53mm, not exactly 50mm, but I set it to closely match the field of view of the Pentax. It has 9 aperture blades vs. 8 on the Pentax. Actual test photos at the same settings were not exactly the same brightness either, for the reasons Marc listed above and possibly the different aperture shape.
06-08-2009, 06:22 AM   #4
ken
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Exposure

thanks for the answer and test result. i used to have the impression that fast len even if set to same aperture value as a slow len that my camera will select a much faster shutter speed for the fast len, that's why i almost always choose to bring along my sigma 18-50 2.8 than my sigma 28-300. now i can base my decision on which lens to bring along based on focal length requirement instead of lighting environment.

06-08-2009, 09:00 AM   #5
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QuoteOriginally posted by ken Quote
thanks for the answer and test result. i used to have the impression that fast len even if set to same aperture value as a slow len that my camera will select a much faster shutter speed for the fast len, that's why i almost always choose to bring along my sigma 18-50 2.8 than my sigma 28-300. now i can base my decision on which lens to bring along based on focal length requirement instead of lighting environment.
Even if you are using a smaller aperture than the max aperture of a faster lens, the faster lens will give you a better viewfinder (brighter) image and make composition and focus a lot easier. For that reason I go with the pro grade zooms that offer constant f2.8 ...they also have better build quality.
06-08-2009, 09:04 AM   #6
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something else to consider here is that super zooms do not have an accurate focal length at anything other than infinity so setting to 50mm may not be 50mm, and as a result the FOV of the lens, and the scene as interpreted by the metering may be different.

Also note that it is not uncommon to have a 1/3 stop error as this is the minimum metering step your camera makes. What is the metering increment you have set on the camera?
06-08-2009, 10:22 AM   #7
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QuoteOriginally posted by MikePerham Quote
Even if you are using a smaller aperture than the max aperture of a faster lens, the faster lens will give you a better viewfinder (brighter) image and make composition and focus a lot easier. For that reason I go with the pro grade zooms that offer constant f2.8 ...they also have better build quality.
I second that. I was comparing the lenses in low light and the Tamron is really dark. I have no complaints about build quality, though. It works and feels a lot better than the price tag would have you believe. I think I paid $135 for it.

The image quality is not going to match a prime lens, of course. In the example given, even though the superzoom is about one stop down from wide open and not at its zoom extremes, the prime is over four stops from wide open and probably at its best.
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