If you want a 50mm lens for the 7d don't forget the crop factor!
If you shoot a Canon dslr camera with a crop sensor and want a lens with a 135mm focal length you do not want to go and buy a “135mm lens”. It may seem like I am spouting rubbish at the moment, but please bear with me on this and all will become crystal……..
A crop sensor camera doesn’t change the actual focal length of the lens you stick on the front of the camera but it does change the effective field of view. A crop sensor camera increases the effective focal length by 1.6 times, i.e. a 135mm lens effectively becomes a 216mm lens (being 135 x 1.6). This basically means the field of view of a 135mm lens on a crop sensor dslr camera is the same as the field of view of 216mm lens on a full frame camera.
An 85mm lens has an effective field of view as a 136mm lens on a crop sensor dslr camera, and whilst this is not the 135mm you’re after it is pretty damn close. Besides, you won’t get closer in the real world.
You may already be aware of the crop sensor effect, and if you are the above won’t be new to you. If you are not aware of the crop sensor effect you are not alone, from what I have discovered from reading around the various photography forums I actively participate in. It appears that many photographers don’t understand or appreciate the crop sensor effect and consequently end up buying an unsuitable lens. Before you part with your hard earned money and buy a lens make sure you consider the crop sensor effect and get a lens that is the “effective” focal length you need.
Keen photographer addicted to cameras, lenses and everything photography related. Feel free to follow me in my photography ramblings, and if you have any thoughts, comments, queries or anything else to add I would love to hear from you.