Haha -- exactly. I had a baby manhandle (babyhandle?) the 50mm lens on my Canon, and I love the way it looks now. Lower contrast, gorgeous flares, and less overall harsh "bite."Originally Posted by Christopher Broadbent
I always drool looking at lenses -- maybe I should be drooling ON lenses.
This was rejected. The other version was a reshoot.
Last edited by cjbroadbent; 28-Feb-2009 at 15:27.
As for diffraction and sharpness, they converge when you get to pinhole photography. Anyone compare a F256 shot with and without a lens? When, if ever, does the lens stop mattering?
Ed Richards
http://www.epr-art.com
I always thought the "optimum" fstop was the one that works for the shot?
The grissini look like some kind of strange growth from the ham, and the "greenery" reinforces the effect.
I can see why they preferred the version you showed first.
Ole, Right on. The original took half a day and I thought I could overbear an uninteresting ham with wild breadboards and grissini and a bit of shadow. The reshoot took much longer and I couldn't afford to make mistakes so everything is just so (like an illustration). The original was on 5x7 and sharper, the reshoot on 8x10 and softer for the DOF problems raised by this thread.
Some comments.
First, it is not at all mysterious how Rockwell came up with his formula. He tells us that he derived it by assuming that things work best when the diameter of the Airy disc is equal to the diameter of the circle of confusion arising from defocus. This is a plausible assumption. Hansma derived it by assuming the total effect of diffraction and defocus could be obtained by taking the square root of the sum of the squares of these two quantities. He then applied elementary calculus to minimize it. But you get exactly the same formula if you assume the total effect is obtained simply by adding these quantities directly without taking squares and you minimize that. These correspond roughly to the two rules of thumb commonly used to determine the combined effect on resolution from more than one comment. The more commonly used one says you should take the square root of the reciprocal of the sum of the squares of the reciprocals of the component resolutions. The other says you should simply take the reciprocal of the sum of the reciprocals of the component resolutions.
From the assumption that the diameters should be equal, the formula follows from some simple algebra.
I find that Jeff Conrad's article is the most illuminating thing I've read on the subject. The rules of thumb described above are just that and don't enlighten us much about what is actually going on.
Personally, I find that the use of the Hansma (-Rockwell) formula tends to lead to very conservative apertures, i.e., to overkill. So, in large format practice, if your aim is to be sure you have sufficient depth of field, you are unlikely to be proved wrong using it. Also, you automatically stop down enough to compensate for the inevitable errors resulting from lens aberrations, and inaccuracies in focusing. (But by using such small apertures you are likely to run into subject motion problems ) I find that it works just as well, as least for small to moderate focus spread, to base the aperture simply on defocus* , followed by stopping down another stop or so. Many large format photographers agree that in large format photography it is unwise to let diffraction concerns dominate what you do.
Note also that Hansma's method is format indepdentt, which seems a bit implausible to me.
* The rule I use is to multiply the focus spread by 10 and divide the result by 2 to get the target f-number, and then to stop down some more as noted above. This is based on an assumed maximum acceptable circle of confusion of diameter 0.1 mm at the level of film in a 4 x 5 camera, or 0.2 mm for an 8 x 10 print viewed from 10-12 inches. I also sometimes look at the Hansma recommendation for the same focus spread for comparison.
Haha... When I saw this, it almost made me squirt beer out my nose.
And now I really want to know!
*** SITE ETIQUETTE VIOLATION !!! ***
You are only allowed to squirt wine out of said nose.,.
Ref: ...en.wikipedia.org/wiki/File:Camera_obscura2.jpg
Etiquette aside, any guesses as to what the transfer function is for the
transition from the lens maker model to the camera-obscura model should
be ?
The attached sketch suggests a (no surprise) conic section, and perhaps
an artist holding a canvas in place of ground glass.
Vectors also seem implied (little lines from the pin hole to the canvas).
Line of sight is pretty clear (90 to about 70 degrees) from canvas to
pinhole.
When does diffraction dominate an image? Good question. Here is an answer.
I was shooting a number of still life tabletops with a pinhole on a Tachihara 810HD view camera. It occured to me to shoot the set-ups both with a lens, a Fujinon 300 f5.6, and a pinhole to REALLY see what the differences were.
The lens picture was always sharper until the stop hit f 700. Beyond f 700 the photographs came out the same whether glass was present or not! To get f 700 on a Fujinon 300 f5.6 was easy. All I did was to unscrew the front group of the lens, insert a disc with the tiny hole in it to mimic the iris diaphragm, and replaced the front lens group. I even allowed for the fact that the entrance pupil of the lens is the size of the iris (or pinhole) magnified by the front group.
Strictly speaking this result applies to 8x10 film intended for contact work but extrapolating the principle to other formats might be fun.
Photography:first utterance. Sir John Herschel, 14 March 1839 at the Royal Society. "...Photography or the application of the Chemical rays of light to the purpose of pictorial representation,..".
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