50mm on 1.6x body VS 85mm on FF body

[futureal33]

This has been asked before, but I still don't really know the answer!

Basically I have a Canon 40D (1.6x crop) and a Canon 5Dmk2.
I have a Sigma 50mm f1.4 lens which I love, and happily use wide open on either body. I love the framing on both bodies, and think that on the 40D it makes a perfect portrait focal length, whereas on the 5D (Full frame) it is quite a bit wider - and acts as my general use lens.

Is there any point in me getting an 85mm 1.4 to use on my 5Dmk2, when effectively, sticking my 50mm 1.4 on my 40D body achieves the same focal length?

I also have a few other primes around this focal length, albeit not at f1.4, with the 100mm f2.0 and 135mm f2.0 L - so Im not 100% convinced I need it but having seen the pictures the Sigma 85mm 1.4 takes, I want one badly!!

I am also thinking that if I put the 85mm 1.4 onto a 40D crop body, it would effectively become a 135mm f1.4?

Thoughts?

 

[redddraggon]

You'll be able to achieve a far smaller DOF with the 85 1.4 on FF.

When you put the 85 1.4 on the 40, you'll get the 135mm equivalent focal length, with a light gathering ability of 1.4 but an equivalent (compared to your 5d) DOF producing aperture of around f/2.3.

 

[aeronic]

Of course you can also take advantage of advantage of better iso performance on the 5d2

 

[woof woof]

Is there any point in me getting an 85mm 1.4 to use on my 5Dmk2, when effectively, sticking my 50mm 1.4 on my 40D body achieves the same focal length?

Yes. I have the Siggy 50 and 85mm f1.4's and although the 50mm is a good lens the 85mm is IMVHO even better.

I've played about quite a bit with lenses on APS-C, FF and also MFT and IMVHO it really is hard work getting exactly the same "look" by changing lenses and formats, you can get close, maybe even almost identical but if you are a pixel peeping geek and if you look closely there are differences (for better or for worse) and IMVHO if you like the "look" that 85mm gives you on full frame then the way to get it is to use a 85mm lens on a full frame camera And then there's the look and character of the lens itself and you don't get that by using a different lens unless it's an identical design.

Once you accept that it's easy to justify buying the Sigma 85mm f1.4. It's a cracking lens.