Mirror bokeh taken to extremes

[ron chappel]

Just for fun

[queenbee288]

no pic?????

[dougsmit]

Actually, I think the answer to mirrors is to accept their faults and make the most of them. Perhaps the more strange bokeh we have, the better? Another use is to shoot things (like spiders) that we would not ordinarily think of as a mirror job. Here's my mirror head and shoulders portrait. The 5mm depth of field does some strange things here and there.
http://www.pbase.com/dougsmit/image/56445596.jpg
Sigma 600mm mirror cropped at sides but full image top and bottom from 9 feet

[sharpfocus]

On the first picture I think the effect you are talking about is called the circle of confusion on the specular highlights and not bokeh, but I may be wrong.

Imagine a perfect white point in an empty black room. The point has no height, and no width. If you focus an optically perfect lens on that point, it forms a perfect point on the film as well. If, however, you focus slightly in front of or behind the point, the point will image on the film as a small blurry circle. If that circle is small enough, it will still look like a point when enlarged for printing. The "circle of confusion" is typically calculated as the largest on-film circle that you see as a point when you make an 8 × 12 print and view it from a "normal" viewing distance, typically 2-3 feet. Anything larger is seen as a small circle, and is therefore perceived as out of focus.

...Bob

[MiG82]

Please explain how bokeh isn't made up of large circles of confusion...
And a perfect lens won't give a point. It will give an Airy disk, which is just a name for the diffraction pattern of a point source diffracting through a circular aperture..

[dougsmit]

Bokeh is simply a recognition of the fact that the blurry circle made by a point may or may not be even in tone or round in shape. In a less than perfect world, the blur can be a bright center with tapering weak edges or a dim center with a bright rim. The mirror lens takes this last situation to an extreme producing doughnuts with completely dark centers and bright edges. Many people prefer blurs that are bright in the center and dark on the edge to the smooth kind since it blends well between the sharp and unsharp parts of the scene.

Bokeh 'grades' are subjective. In the early days of mirror lenses, some people thought the results were 'cool' but today most find them disturbing. There was even a time when people used 'Waterhouse Stop' aperture disks that slid into a lens and allowed shapes or even multiple holes (a large central hole surrounded by a ring of smaller ones was most common). Finding an example of this today would be next to impossible but in the 1970's I had a, then old, view camera lens with the capability. Some lenses were designed to have faults of correction and give images of poor sharpness but very specific bokeh characteristics. A famous example of this was the Leitz Thambar. When points get so far out of focus (easy to do with a mirror) that they lose their shapes and tonal identities, we lose the bokeh ID as well. My Titmouse shot shows this since the objectionable areas are in the feathers but the background is so far gone that it has lost the doughnuts.

It is not unusual for the evenness of the disk to be different in out of focus points behind sharp focus compared to those nearer than what is sharp. Most people prefer lenses with well handled spots in the distance since these appear in more photos than fuzzy foregrounds but this can be an issue in some shots.

Out of focus spots tend to show the shape of the iris diaphragm of the lens so 'better' lenses tend to have more blades and/or curved blades to approximate more closely a perfect circle. Today, the minimum number of blades is usually 5 which produced a pentagonal blur where we expect a circle. Some lenses go up to 9 blades but the difference between 7, 8 and 9 is lost on many people. There is a related matter where bright points in a scene (e.g. streetlights) take on a star shape with points matching the blade count. These tend to look better with an odd number of blades since the result of an 8 bladed diaphragm is more like two fours overlapped making seven look better to many of us.

One final point is that diaphragm shape is not the same at all apertures. Some lenses better approximate a circular diaphragm at some apertures than others and almost all give a circle when wide open so even the poster child for pentagonal bokeh, the 50mm f/1.8, looks round when used wide open.

[sharpfocus]

Sorry, I guess I was wrong. When I saw the picture I thought "circle of confusion" rather than "bokeh". Didn't consider that they were one and the same. My bad. ...Bob