Lens Confusion o_O

[r3g]

So im reading a thread on flickr about peoples experiences with different lenses on their D40s. In the thread i came across a poster saying that he likes the 55-200mm Non-VR for portraits because:


remember 105mm F5.6 gives you a shallower DOF than 50 mm and f1.8!!! resulting in better bokeh, @50mm f1.8 you get the same DOF as at 200mm F22! [22 not 2.2]



How is this possible? And does this rule apply to all lenses at those focus lengths or just the 55-200 non-VR?




Here is the thread for all you other D40 owners. Its a pretty good read.

http://www.flickr.com/groups/d40slr/discuss/72157600167135514/

[Prospero]

What you read is about right. The depth of field depends on three things:

--> Focussing Distance
--> Focal Length
--> Apperture

Therefore at longer focal lenghts you get a lot less depth of field. If the focussing distance stays the same, a 200mm lens at f/22 will indeed give about the same depth of field as a 50mm lens at f/1.8.

On the following website you can find a depth of field calculator:
http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

In practice, though, the effect is not as bad as it may seem. IF the focussing distance stays the same, you indeed loose a lot of depth of field. But the thing is that increasing the focal length often means that the subject is further away. This compensates for the increase in focal lenght.

In fact, if you take a picture at 50mm and one at 200mm of the same object filling the frame, they will both have the same depth of field. The reason for that is that if you want the object to fill the frame at 200mm, you have to be 4 times as far away. This compensates the 4 times increase in focal length, so that both pictures have the exact same Depth of Field.

This is true for all lenses, not just the 55-200VR.

[r3g]

Thanks. Thats a very valuable piece of information. It also give me some piece of mind knowing i can use my VR @ 105mm F5.6 to get excellent results and you dont need to be that far away to focus (depending on how much of the subject you want in frame. and assuming you dont mind turning the camera sideways)