Confused - 70mm isn't the same between lenses

[budfox]

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/categories/products/Sigma/70-300mm%20f4-5.6%20DG%20Macro%20(Nikon%20AF%20Including%20D40D4 0x)%20-73537/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.

[Blue Eagle]

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.

[cyclone]

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.

[DigitalRelish]

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.

[GarynLea]

Are the front lens diameters the same?

[budfox]

Well here's a pair of images to show what I mean. Quite a difference:

http://www.budfox.co.uk/nikon_18to70_at70mm.jpg

http://www.budfox.co.uk/sigma_70to300_at70mm.jpg

Whilst I wouldn't expect them to be identical, I'm surprised at the difference.

[cyclone]

That's remarkably close!
I'd be more concerned what happened to all the contrast on the Sigma?

[budfox]

Are the front lens diameters the same?

No, the Sigma looks smaller. Can this make a difference?

[davekiddle]

I think it's to do with internal focusing versus non internal focusing.

See this link (http://www.kenrockwell.com/nikon/18200/18200-focal-length.htm) about the Nikon 18-200 compared to other zooms.

[postcardcv]

I'd be more concerned what happened to all the contrast on the Sigma?

indeed - the difference is amazing

[HoppyUK]

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.

[budfox]

Thanks for the replies. At least I'm not as stupid as I thought I might be...