A simple question on depth of field

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plymman

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Hi,

I just got a 35mm 1.8 for my d5000 and have just started playing with it and have a query about depth of field and how to blur the background. Does the distance between the lens and object have a big effect on this? I had it set to 1.8 to get the maximum effect and when I was a few feet away the blur was minimal, I then got to within a few inches and the focus point was super sharp and the blur very pronounced. Is this how to control the effect - create/reduce distance?

If so how would I get a pronounced background blur when I am a distance from the object?
 
With the same aperture and same focal length, the more the key object near to you comparing to the background, the blurrer the background is.


You can get blurrer background simply by 3 ways:
With larger aperture
With longer focal length
With closer distance between you and the focus point.
 
By having the background even further back. Sorry :)

There's a calculator on this page, try it out.. http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

85mm/1.8 on a 1.6x crop body at "a few inches" or say 2.8 feet = 85cm which is the mimimum it will focus to gives 6mm for DoF. At 5m it's 24.5cm and for 10m = 1m DoF.

So the further you are, the further the gap between your subject and background has to be. Otherwise lenses wouldn't have such a thing as infinity focus.. and wouldn't it be nice to focus with a 6mm DoF from 10 meters :bonk:
 
I thought that was the case, so at 1.8 (largest aperture) and 35mm (fixed length) it's all down to distance.

Thanks
 
plymman said:
I thought that was the case, so at 1.8 (largest aperture) and 35mm (fixed length) it's all down to distance.

Thanks
Click to expand...

Yes.

Things to remember. Depth of field depends on f/number, and magnification. Longer lenses give more magnication at a given distance, hence shallower DoF. But if you move back so that the subject remains the same size, the magnification is the same and DoF is the same.

The other thing that affects bokeh and subject isolation if field of view. Longer lenses have a much narrower field of view so there is much less background visible behind the subject. It is less cluttered and less distracting, so the subject stands out more. It's not a DoF effect, but the result is similar.

Those wonderful bokeh effects you see taken with longer lenses are the result of both shallow DoF and narrow field of view.
 
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