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Gene M
31-Dec-2003, 14:58
Here we go again with the sharpness questions. I realize that "sharpness" is a subjective term and that perceived sharpness depends on many factors so I'll try to ask an answerable question.

In searching for posts relating to optimum taking lens aperture I found this. It was written by a knowledgable person associated with this website.

"Stopping down more than necessary will result in loss of sharpness due to diffraction. All lenses are limited in resolution to about 1500/N line pairs per mm (the actual resolution on film will be slightly lower since it is a product of lens and film resolutions). At f16, this is 93 lpm, f22:68, f32:46, f45:33, f64:23."

I realize that smaller lens openings are often necessary for good DOF and I routinely shoot between f22 and f32 with my LF lenses. I do a lot of landscapes and I love the look of near to far "sharpness."

Last weekend I used my new Symmar XL 80 and I'm pleased with its performance. I got to wondering if there's an optimum lens opening for this lens (and my others.) I looked at the Schneider website and found lots of graphs and stuff but I'll be damned if I can find anything that helps me.

(Shut up and ask the questions !)

Is there an optimum lens opening for this lens ? Is it f16 as stated two paragraphs ago ? If all lenses have the same resolving power why not simply buy less expensive lenses ?

Steve Hamley
31-Dec-2003, 16:45
Gene,

Try here: http://www.hevanet.com/cperez/testing.html#65mm_thru_125mm

Thanks!

Steve

Ellis Vener
31-Dec-2003, 18:44
As a general rule I have found it is best to best individual lenses to find that lenses sweet zone.

Ernest Purdum
31-Dec-2003, 19:41
Although diffraction places the same absolute limit on all lenses, resolving power depends on many additional factors. All lens design is compromise. Rather than just designing for ultimate resolving power, factors such as angle of coverage, aperture, ability to fit into a particular shutter size, etc., have to be taken into account. In use, the photographer will often have to compromise in order to select the amount of depth of field wanted, to use a high enough shutter speed to eliminate subject movement or obtain a moving water effect. Shooting at two or three stops down usually works well when there is no reason to select some other aperture. You could do some testing to find out what aperture of your particular lens yields the highest resolving power, but I doubt if you would notice any d8ifference in your final results.



There is no optimum aperture that will suit all circumstances, only an optimum aperture for each photograph.

John D Gerndt
1-Jan-2004, 08:06
If you are out to get maximum resolution (I have been doing an informal research project for 10 years now) donít throw too much money at lenses! NOTHING beats going up in format! This assumes using decent lenses and that you have done all the aligning/dimensional/vibration checking that can be done. If you are stuck at 4x5 then experimenting within your operating circumstances may yield some improvement; it costs very little and makes you better at all aspects of photography: do that.

The XL series is about as good as it gets. Playing with the aperature will result in compromises you will have to judge in your own prints. Look towards larger negatives for gaining that elusive resolution.

Best of luck in the New Year,

Alan Davenport
1-Jan-2004, 14:07
In practice, sharpness loss due to diffraction must often be ignored in order to have sufficient DOF for the subject. You can find the "optimum" f/stop for a given set-up by measuring the focus spread (distance the lens must be moved to focus the nearest and farthest points you need to have sharp) and plugging that number into a simple formula.
The result is the f/stop that will give the smallest circle of confusion over the entire focused range and takes into account both defocus and diffraction.

See Paul Hansma's excellent article for all the math, it's linked elsewhere on this site.

The other answer to your question, finding the sweet spot for a particular lens, is probably best approached by running your own tests, IMO.

Cap Frank, Narragansett
8-Jan-2004, 17:19
To find the sweet spot of my lenses for B&W, I tape a page of the classified section of my local newspaper to a northfacing wall (for color photography lens sweet spot I use a Sunday color comics page). I will take one picture of the newspage at each aperture setting using B&W slide film while holding the shutter speed constant. (with slide film I find I can also check the accuracy of the shutter since the exposure lattitude is so narrow). Then I will examine the slides on a light table and pick the best apertures for the particular lens (the best aperture will be the one that allows you to read the most letters edge to edge).

With color slide film I will also compare the slide against the original page to judge color accuracy. Sometimes some apertures are better at resolving colors than others. Once I find the best aperture, I will redo the sunday comics test while this time holding the optimum aperture constant and varying the shutter speed. At the light table, poor shutter speeds and blade performance stand out dramatically when the color slides are placed side by side and up against the Sunday comics page photographed.

Over the past 30 years of doing these test on all new purchases, I have found pretty much that for B&W photography, stopping down 2 aperture stops generally hits the lens' sweet spot.

Hope this helps. Cap.