Zoom Lens For D3100

[thefonz78]

I got the kit lens at the moment but will be getting a tamron 17-50mm once i sell my d40x

[MattUK]

So where did you get "50 to 105mm" from?

It's either 55 to 70mm or 82.5 to 105, depending on how you look at it. Either way, it's only a 20mm or so gap.

[thefonz78]

If i got a 70 - 300mm its built to work on both FF and APS-C so the 70mm would have the field of view equiv of 105mm on a aps-c lens. The 17-50mm tamron is built for the aps-c crop and don't apply to the same field of view equation. So I would be missing equiv on 50/55mm to 105mm.

[greenwing]

Originally Posted by thefonz78

If i got a 70 - 300mm its built to work on both FF and APS-C so the 70mm would have the field of view equiv of 105mm on a aps-c lens. The 17-50mm tamron is built for the aps-c crop and don't apply to the same field of view equation. So I would be missing equiv on 50/55mm to 105mm.

Wrong. Focal length is focal length. 70-300 is 70-300, 17-50 is 17-50. You're missing the 20mm between 50 and 70. Ignore FOV calculations, they're obviously confusing you. Or apply them to all lenses, but I doubt if you think in terms of 'on 35mm I'd use a 75mm lens here so I need 50mm on DX', which makes the crop factor calculation superfluous.

Chris

[thefonz78]

I know focal length is focal length that is why i was careful not to say this. What ever you want to call if FOV etc.. 50mm on my kit lens which is aps-c lens is not the same as the my 50mm f1.4.

[greenwing]

Originally Posted by thefonz78

I know focal length is focal length that is why i was careful not to say this. What ever you want to call if FOV etc.. 50mm on my kit lens which is aps-c lens is not the same as the my 50mm f1.4.

I'd like to see proof of that. There might be small differences. 50mm primes, in the old days, were often not 50mm, and some lenses change focal length as you focus closer, but at the same aperture and at infinity there should be little to choose between them.

Chris

[thefonz78]

Sorry Greenwing you are right there was no difference. I guess i am little confused then. Am i right in thinking when the talk FF and aps-c that a 50mm on a FF camera will have a similar view to a 35mm on a aps-c. I was confused thinking somehow that FF lenses would act differently on a aps-c lens.

[MattUK]

Correct, full frame simply allows you to see a greater view at any given focal length than aps c

[thefonz78]

Thanks guys!

Matt how long through the focal range does the sigma 17-70mm stay at f2.8?

[MattUK]

Don't have my camera with me, but I'd guess 1/3 to 1/2 way through the focal range