Bokeh & Aperture Size ?

[jun]

i am interested in taking pictures with blurry backgrounds for portraits and macro shots. from reading i know that a smaller aperture size (f1.8,f1.f,1.2) will give excellent bokeh blurs.
but from what i see in sample shots in flickr an aperture of f3.4 or f4 in a telephoto or zoom lens would also get the same bokeh effect.

anyone want to shed some light on my question/
thanks.

[Laurentiu Cristofor]

[Himself]

Here is one

http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

[Klypen]

The focal lengths of tele lenses compress the background of the subject and create blur, you got more blur the nearer the subject you are (a very short & easy explaination).

[iangreenhalgh1]

The wider the aperture the narrower the depth of field. Therefore a wide open aperture makes the background further out of focus if the subject is in focus and this gives a stronger blurring effect on the out of focus elements of the image which comprise the background. Usually the further out of focus the more pleasing the result is on the eye as it is less distracting. This quality is known as 'bokeh' and good bokeh is considered to be highly blurred with smooth, subtle variations of shape, tone and colour. Bad bokeh is the opposite - sharp changes in tone and colour that distract the eye from the subject. The idea of bokeh is to give a pleasing and artistic effect that does not detract from the focus on the subject.

Think of a painting like The Mona Lisa, Leonardo painted the background with little detail and soft, flat colour scheme so that the eyes were drawn to the face of La Giaconda and the background just served to compelte the picture without detracting from it's intended visual focus.

Here is an example of bad bokeh - distracting jagged shapes with high contrast meaning the background is distracting and the eye isn't naturally drawn to the branches that are in focus. The culprit is the Meyer Domiplan 1.8 50mm, a lens known for having bad bokeh.



Here is an example of good bokeh, this time from a Helios 44-2 58mm, a lens renowned for good bokeh as it gives such a soft, smooth and painterly rendering of the out of focus background.



In general, the rounder the aperture in the lens the better the bokeh, therefore the more aperture blades the lens has the better the bokeh is likely to be. This is why lenses like the Meyer Orestegon 135mm with 15 blades is known as a 'bokeh monster' - it's aperture is very round a all settings on the scale.

Hope that helps.

[ZoneV]

I never make a proper derivation, but I think the maximum bokeh circle diameter of a small unsharp lightsource / reflex is the same as the entrance pupil in the sharp object plane.

So if you use a 50mm @ f/2.0 with entrance pupil = 25 mm @ f/2.0, lightsources in the background will have 25 mm diameter in the sharp foccused area.
If you use a 50mm @ 1.0 the blurred light will have 50 mm diameter.
And if you use a 300mm @2 the diameter in the sharp object plane has 150mm