Increasing focal length, its effect on DOF/bokeh

Dinomania

Dinomania

Dinomania

Soldato
Joined
21 Apr 2003
Posts
4,328
So, right, I seem to be hunting for a legacy (i.e. canon) 'nifty fifty' plus adaptor for my GF1.

Yes I know it's 'equivalent' mm-age will be 100mm, compared to its native position on an SLR. So the focal length will be significantly longer than that of the 20mm (40mm equiv.) pancake I use most of the time currently.

Sooooooo the deal is, I'm trying to take lots of good pictures of dancers, and currently if I manage to get the whole couple in shot, the focus is too far away from the lens to blur/knock out the background as much as I'd like.

Occasionally I do get a little bit of blur in the background if the dancers are close enough/the people in the background are far enough away.

Am I right in thinking that when Margerie in the background behind the dancing couple is just-about blurred when using my 20mm pancake, if I use a 50mm lens with the same/similar aperture, she in herself won't be any more blurred (DoF remains the same) but she will be much enlarged, so will /appear/ more blurred-out (more bokehlicious)?

Examples of distractions in the background - often half-people:

P1020028 by sarawallen, on Flickr

P1020054 by sarawallen, on Flickr
 
Ahhh ohh so 'focal length' is used to talk about the lens 'length' as well as the distance from lens to focal point?

I understand the latter example, that to knock out the background it needs to be a decent proportion away from the object-in-focus compared to the distance my lens is from the object.

OK so f-stop = lens focal length/diameter (http://en.wikipedia.org/wiki/F-number#Notation).... so with a longer focal length, say 50 as opposed to 20, the aperture diameter itself will have to be be bigger for the same f-number. Soooo...

...thinks....

At the same f-number (say 1.8), the 50mm lens will open wider, let more light in, there'll be larger circles of confusion (?) and possibly less DOF (?)?

So I may get what I want, buying a longer prime lens?
 
Oh I know that. But I am happy with the camera and can't afford to change it even if I wasn't!

Have found the calculator towards the bottom of this page that confirms that I get a smaller DOF with a longer lens when the object is the same distance away - just remains to see whether this makes much difference once the subject of the image is made the same size.... as that will change the distance away that I stand. Hmm.
http://www.cambridgeincolour.com/tutorials/depth-of-field.htm
 
Fair enough.

I'm not too fussed about the 'quality' of bokeh (it's not something I notice) - I'm more concerned about knocking out the background to render it less distracting... It shouldn't be the object of my picture!
 
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