Exposure between two different lenses

[BrandonS]

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.

 

[Dubious Drewski]

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.

 

[BrandonS]

OK Found a link. http://www.great-landscape-photography.com/aperture.html

It is so. The actual aperture opening is larger, but you get less light because of the focal distance so it all evens out.

 

[Dubious Drewski]

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.

 

[BrandonS]

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.