[rbefly]

Hello Alamo500,
Some very good advice so far, I'll just try to add a bit, with a different slant;
Pictures, to illustrate a point.
DOF (depth of field) is the term we use to indicate how "far" the photo is in focus. By that I mean, if the main subject is in sharp focus, are objects closer and further away also in focus? Imagine a row of buildings, extending from near you (near your camera) to the horizon. OK?
You focus on the middle house, set your lens to f/2.8 at 1/500 second and click. The photo shows the middle house in sharp focus but everything else is more-or-less fuzzy.
Shallow depth of field/focus. Maximum bokeh.
Now you stop the lens down to f/22.0. This is 5 full stops "down", meaning you have let in much less light. Each full stop down lets in 1/2 the light.
Since the AMOUNT of light has been reduced, you have to allow more TIME for the light to reach the sensor. Shutter speed.
At f/4.0, you would have to increase your shutter speed one full stop (1/250s) for the same exposure. Half the light= twice the time.
At f/5.6, again you've cut the AMOUNT of light in half, so again you must allow twice the time, to achieve the same exposure. 1/125s
At f/8.0. once again the AMOUNT of light is reduced by 1/2, so the shutter speed is now 1/60s (roughly). Half the light, twice the time.
F/11.0 = 1/30s.
F/22.0 = 1/15s. Now we are in tripod, cable release territory!
But, what we've gained is DOF. Each stop down shows more houses in front and behind the main subject in sharp focus.
Maximum depth of field. Minimum bokeh.
Here's a group of photos that illustrate exactly what I've described. The post is by Matjazz, the 10th post on the thread;
SMC Pentax-DA* 200mm F2.8 ED [IF] SDM Reviews - DA Prime Lenses - Pentax Lens Reviews & Lens Database
Hope this helps!
Ron

[Anvh]

One of the most important features of Bokeh is the size of an out-of-focus pinpoint of light. The other feature is the character of the blur which I won't talk about here.

Read about what Bokeh is

[newarts]

Read about what Bokeh is

Thanks for the hint.

I have read much about Bokeh as the character of the out-of-focus areas of an image; see for example http://www.zeiss.com/C12567A8003B8B6F/EmbedTitelIntern/CLN_35_Bokeh_EN/$File/CLN35_Bokeh_en.pdf.

Two important parameters affect bokeh character: (1) the size and (2) the intensity distribution within the disk resulting from an out-of-focus point of light. Bokeh is the result of convolving the out-of-focus PSF with the image & the diameter is a domnant parameter of the PSF.

The size of the disk is fundamental and it turns out to depend only on a lens' F/N ratio (the size of the absolute aperture) - so as far as lens selection goes, if you want big Bokeh, you want big F/N.

[Anvh]

But big airy disk doesn't need to mean a pleasant bokeh.

Here is an example
135 at f/4.5


80-200 at f/2.8


80-200 at f/4.5


The light distribution is really important as you can see but no one seems to believe me

[newarts]

But big airy disk doesn't need to mean a pleasant bokeh.
The light distribution is really important as you can see but no one seems to believe me

Oh, I believe you completely! The size of the oof disk is only one obvious (and easy to quantify and control) aspect of Bokeh - to kill bad Bokeh increase the f-number.

But that's not to say the Point Spread Function (PSF) shape isn't important! If the PSF looks like a bright ring, the resultant Bokeh will be harsh and have the effect of turning single edges into double edges, separated by the diameter of the ring (in turn controlled by f-number.) On the other hand, if the PSF looks like a pile, deepest in the middle and diffuse at the perimeter, out-of-focus edges simply become blurrier as PSF diameter is increased.

The PSF's shape is what determines the qualitative character of Bokeh & size is only one parameter in describing shape.

Dave in Iowa

[Anvh]

Oh, I believe you completely! The size of the oof disk is only one obvious (and easy to quantify and control) aspect of Bokeh - to kill bad Bokeh increase the f-number.

Actually sometimes, decreasing the aperture helps but you need to have few blades and they need to be straight, they sometimes "cut" the blur-disks enough to decrease the bright edge.