Aperture/Bokeh Ratios?

[Hippo_MC]

Hi, learning the ropes at the moment. Is there a general ratio that can be applied with the aperture and the distance the subject has to be from the background to get decent bokeh?

i.e. If you have a 50mm lens with f1.4 you need the the subject to be at least x distance from the background, but if you have a 100mm lens that has a max f4.0 then the distance needs to be x to ensure the background is blurred?

Or is it a on a shot by shot basis and ensuring that your focus it set correctly on the subject?

TIA

 

[HoppyUK]

Hi, learning the ropes at the moment. Is there a general ratio that can be applied with the aperture and the distance the subject has to be from the background to get decent bokeh?

i.e. If you have a 50mm lens with f1.4 you need the the subject to be at least x distance from the background, but if you have a 100mm lens that has a max f4.0 then the distance needs to be x to ensure the background is blurred?

Or is it a on a shot by shot basis and ensuring that your focus it set correctly on the subject?

TIA

It's on a shot by shot basis I'm afraid. There is a lot more to bokeh than f/number.

Bear in mind that if you keep the size of the main subject the same, depth of field remains the same f/number for f/number, but field of view changes dramatically, which makes a very big difference.

Physical shape of the aperture has a lot to do with how out of focus areas look. Also different lens designs look different to a lesser extent, eg zooms vs primes.

Generally speaking you often don't have much control over the distance between your subject and the background, unless you can move them. Usually, the main variable is camera to subject distance, as you can always move yourself.

Depth of field tables tell you about what is in focus, they don't tell you about what is not in focus.

If you know the variables that affect bokeh, you can adjust things on a shot by shot basis as far as is possible by chimping a quick test snap. I would have thought this was a better way of doing it anyway, than trying to follow a set of tables. Bokeh is a very subjective thing and depending on the subject and background, a combination of various factors that look great in one shot will probably look quite different next time you try it.

 

[Hippo_MC]

Thanks for the reply Hoppy.