Confused - 70mm isn't the same between lenses

B

budfox

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Hi all.

I have an 18-70mm AF-S Nikkor 3.5-4.5 G ED kit lens that came with my D70s. It's a DX lens.

I also have a Sigma 70-300 4-5.6 DG. I assume it's a DX lens???

(here it is: http://www.jessops.com/online.store...(Nikon AF Including D40D40x) -73537/Show.html)

When I look through the Nikon lens at 70mm I see a considerably wider view than when I look through the Sigma at 70mm. I have a funny feeling that the Nikon is about 1.4 times wider...

I'm missing something stupid, and I know it must be to do with DX format, but I don't understand why.

Could someone enlighten me please.

Thanks.
 
they should be the same....no?

try taking two shots on a tripod, without moving the tripod, and make sure both are at the 70mm end obv.
 
It's nothing to do with the DX format, 70mm is 70mm.

It's probably focus breathing, try comparing at infinity you'll probably see a closer match, the 18-200 does the same thing at close focus.
Also consider that the quoted focal lengths are probably more nominal that factual.
 
I believe there may be some variation of reported focal lengths between different lenses and that a 70mm lens is a 70mm lens whether it's DX or FX.
 
Are the front lens diameters the same?
 
Well here's a pair of images to show what I mean. Quite a difference:

nikon_18to70_at70mm.jpg


sigma_70to300_at70mm.jpg


Whilst I wouldn't expect them to be identical, I'm surprised at the difference.
 
That's remarkably close!
I'd be more concerned what happened to all the contrast on the Sigma?
 
In practise, only prime lenses are really close to their marked focal length. Many zooms frankly take the mick and are always a bit longer at the short end, and shorter at the long end. Focus close and the difference between a prime and a zoom can be even more dramatic. I don't know why so few lens tests do not check the actual focal length; it would be very revealing.
 
Thanks for the replies. At least I'm not as stupid as I thought I might be...
 
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