Exposure between two different lenses


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Feb 4, 2008
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Oklahoma City, OK
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Last year I went to the race track at night and was using my Nikkor 50mm 1.8. I went over the images again and the settings were coming out to 1/40s @ f/2.8 and ISO 1600.

Now if I pull out my Tamron 17-50mm f/2.8 and shoot a 50mm w/ ISO 1600 @ f/2.8 will my shutter speed be the same?

The reason I'm asking is because I currently have the above lens and I'm also looking to pick up a Tamron 70-200 f/2.8 shortly.

I'm assuming with all these lenses the shutter speed should be the same across all their focal lengths at the same ISO.

Am I right in thinking this? I wasn't sure if actual lenses would change this. If it is I'm hoping I can get acceptable images at 1600 on my D300, the D80 wasn't very nice at that ISO setting. I haven't really tried it's higher ISO settings yet. That's another thread though.
No, I believe you've got it right. f2.8 is the same amount of light now matter what lens is providing it. You should be able to use the exact same exposure settings.
Yeah, that used to throw me off too. I used to wonder how some point and shoot cameras could be f2.8 even though the lens was this tiny little piece of glass. Then I became aware of projected image size. A tiny point and shoot has a tiny sensor, so that minuscule lens provides enough light for it to be rated at f2.8.

Your 50mm lens is very likely a full frame lens, like almost all 50mm lenses are, so it has physically huge glass elements because the image it projects is intended for a surface much larger than a crop digital sensor.

The Tamron 17-50 f2.8 is a digital-only lens. So the image it provides for the sensor is a reduced size, so less light is required for it to provide that image at f2.8. And so the glass elements don't need to be as large.
It's still baffling me that at 2.8 the 50mm has a 17.xx mm aperture opening and at 200 f/2.8 the opening would be 71.xx mm. I understand it's farther away but it amazes me that light fall off is exactly proportionate to the focal length and opening size.

If that makes sense. I would think there would have to be some benefit of the larger opening; other than getting the same shutter speed at a longer focal length.

Or another example. An 85mm f/2.8 prime lense would have smaller front glass than the 70-200 f/2.8. The aperture opening would be the same at 85mm regardless, but the 70-200mm should collect more light through the front of it (bigger opening) which is directed to the center of the lens anyway. So I would think you'd get more light through the same opening.

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