Increasing focal length, its effect on DOF/bokeh

Dinomania

Dinomania

Dinomania

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21 Apr 2003
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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
 
We're often led to believe that the Depth of Field is indicated by the aperture size, which is also often referred to as the F-number. So you might think therefore that the DoF would be equivalent for an object at a set distance from the lens, if the F-number is the same, eg. F4 on a 20mm lens, or F4 on a 50mm lens.

However, the F-number is actually the ratio of focal length of the lens over the aperture size. So if you are comparing the two different focal length lenses at the same F-number, one actually has a much greater aperture size than the other. Its this aperture size which really determines the DoF.

Also, a second factor in determining the DoF is the distance the lens is from the focal point, and I like to think of this effect in terms of the percentage difference in focal point. For example:

Focal length 10cm. A object 1cm further back is 10% further away.

Focal length 100cm. An object 1cm further back is 1% further away.

Take it to an extreme:
Focal length infinite. An object 1cm further back is 0% difference, ie. still in focus.

Hope that helps :)
 
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?
 
Another factor to think (worry) about is the sensor size. I don't know the physics of it but the smaller the sensor, the greater the DOF. So a 1x or 1.5x crop camera can produce a lower DOF with a given aperture and focal length than a 2x crop like the GF1.
 
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
 
You should also not confuse Shallow DoF with Bokeh as they are not the same, Bokeh merely refers to the way the out of focus components are rendered and not to a shallow DoF per se. You can get pleasing Bokeh from a lens stopped down to f/22 (common in Macro), and many fast lenses that offer shallow DoF provide very ugly Bokeh. The Nifty Fifty type lenses are a classic example, yes they can allow a shallow DoF for a small price but the Bokeh is plain ugly.
 
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!
 
Dinomania said:
What nobody has said, is that if I will benefit from getting a longer lens at all or will it be a waste of money?
Click to expand...

If you buy a lens with twice the focal length and the same aperture, and you are trying to keep the subject at the same magnification, then there will be nor meaningful difference in the acceptable DoF. However, there will be a change in perspective in the background which will have important effects.

Wiki is good source: http://en.wikipedia.org/wiki/Depth_of_field


Also you can play with this http://www.dofmaster.com/dofjs.html
 
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