DX does not get the same results as FX

[Shokinen]

Hey guys,

I thought this concept was a case closed, but I had to discuss about the differences between FX and DX to start the debate all over in my mind...

Last night, I place my D90 on a tripod, and took a picture at 70mm using my 16-85mm (DX).
I then changed the lens to my 70-300 (FX) and took the exact same picture at 70mm.


I was expecting the same picture, since the lens specification has nothing to do with the size of the sensor. Well... this is at least what I thought... The picture with the 70-300 (FX) as more magnification and the subject is bigger.

I know if I take I shot at 300mm on my D90 i'll need a 450mm on a D700 to get the same result. But this is the same body !!!?? Using the same focal lenght !!??

So here I am, this morning, wandering what I missed...

[autofocus]

...interesting, would it make sense to post both SOOC shots for us to view?

[johnske]

The Nikon DX lens is designed for use with an APS-C sensor and is guaranteed to cover only the area of an APS-C sensor so there is no crop factor involved.

The Nikon FX lens is designed for a full-frame sensor and there is a crop factor, when used on a camera with an APS-C sensor the FX lens will have more 'magnification' than the DX lens at the same focal length.

[scootermcq]

Nah.... 70mm is 70mm is 70mm. It is focal length, and it does not matter if the glass has been sized or optimized to fit a smaller sensor, the focal length is still 70mm.

My 85mm prime is DX, my 50mm is FX or by this logic 75mm. Believe me they are not close. one is 50 one is 85. Same I can set my 18-200 DX to 50mm and they are the same size as my 50

[OsmosisStudios]

Simple:
Focal length is indicated for a lens when focused to infinity. At closer focusing distances, you can see effects of focus breathing, where the lens shows a shorter apparent focal length (or magnification) than another even though they are at the same focal length.

Try doing the same with a 70-200 (the new version) and 200f/2, both focused at less than infinity. HUGE differences.

I would suspect thats the case here. Nothing to do with DX or FX, just individual lenses.

[inkista]

Highly recommend the lensrentals (it's now a blog!) entry, Things You Should Know About Your Lenses, But May Not.

[Shokinen]

Originally Posted by OsmosisStudios

At closer focusing distances, you can see effects of focus breathing

Thanks OS, totally forgot about focus breathing. This concept has been mentioned a few times in the forums but never clicked yesterday when I did my tests. All the subject were fairly close - inside 10feet.

I just tried it again focusing on something outside and you are right. It's the same thing.

Yay !!, I'm not that crazy after all.. hehe