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Old Mar 16, 2010, 11:33 AM   #21
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Originally Posted by Mark R. View Post
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I thought that you guys equate soft to shallow DoF, and sharp to deep DoF. ...
No, sharpness does not equate to DoF at all. DoF translates to how much of the background is in focus or out of focus. If in one hand it is true that the wider the aperture the less sharper the image is, on the other, you need that wide aperture to create a shallow DoF. The optical quality of a lens determines how sharp the image will be at its widest aperture. Those are the more expensive lenses you find (and we are talking DSLR lenses). Just because the lens does not produce the sharpest images at the widest aperture it does not mean the image is not good. They are still sharp, just not as sharp as if shot @ f8-f11 (DSLR) or f4-f5.6 (P&S).

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...But then, Tullio wrote: "... very good lenses, which will produce reasonably sharp images at their wider aperture - but still not a very shallow DoF".
Please keep in mind that I'm referring to P&S (fixed) lenses here. A f2.8 on a P&S (fixed lens) translates to a f5.6 in DSLR terms. At this aperture (in DSLR terms), images will not have a very blurred background at all. In order to obtain a very shallow DoF, the lens must be at f1.9 or less. This can never be accomplished by P&S lenses.

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...What, then, is meant with "sharp" and "soft"?...
There is, of course, a very technical description of sharpness/softness but to make things simple, these terms refer to edge definition in terms of contrast. When you go to Photoshop and increase the sharpness of an image, the software is basically adding more definition along the edges, which in turn makes the image look sharper to the eyes. It does absolutely nothing to the depth of field. Now, why the image looks soft is a different story. Lets not get confused between soft and out of focus images. Soft simply means that the outline of each element on the image is not very well defined. This is usually caused by either a poor quality lens, inadequate sensor or the camera's image processor doing a poor job while out of focus can be caused by camera shake, inaccurate AF system or incorrect MF, etc. A soft image can usually be (somewhat) fixed in post processing. The focus is OK but the edges need more definition, but an out-of-focus image can't. You can not re-focus an image that was taken with an incorrect focus.

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And if diffraction does not really play a role at F8, as Tullio maintains, why was Gordon's F8 image fuzzier than the F4 image? According to Tullio, an F8 image should be sharper!
Are we talking P&S or DSLR? On a P&S, f4 to f5.6 will produce the sharpest images while on DSLRs, f8-f11 are the best apertures for most lenses. That's not to say that a P&S will not produce good images at f8. It will. In fact, a P&S may produce better images at f8 than a DSLR at f32 because f8 does not quite translate to f32. The amount of diffraction on a P&S lens at f8 is significantly less than on a DSLR at f32.

I hope my explanations clarified things a bit.
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Old Mar 17, 2010, 8:30 AM   #22
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Hi Tullio,

Your explanation has clarified things somewhat.

The camera review I was referring to, was indeed a P&S, not a DSLR. (In fact, I've just ordered this camera and am awaiting it eagerly.)

Here's the specific page on which Gordon was referring to diffraction issues:
http://www.cameralabs.com/reviews/Pa..._results.shtml

The diffraction issue is discussed close to the bottom of the page, referring to the three pictures in the second-last row on the page.

The finding was that the image at F4 was sharpest, while that at F8 was softest. (F8 is the smallest aperture available on that Leica lens.)

Regards,
Mark
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Old Mar 17, 2010, 8:56 AM   #23
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Here's a page discussing Diffraction limits. You'll see a calculator where you can plug in values. The sensor in that Panasonic is 1/2.33". So, the results should be somewhere in between what you'd get with the 1/2" and 1/3" choices you'll see for camera type.

http://www.cambridgeincolour.com/tut...hotography.htm
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Old Mar 17, 2010, 11:37 AM   #24
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Quote:
Originally Posted by Mark R. View Post
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The finding was that the image at F4 was sharpest, while that at F8 was softest. (F8 is the smallest aperture available on that Leica lens.)...
Yes, that's what I said previously. On P&S, the ideal aperture for sharpest images is between f4-f5.6 while on DSLRs it's between f8-f11. So, no surprises here. The thing is, although you get the sharpest images at these apertures, you won't get a shallow DoF as these apertures will produce well defined backgrounds.

I recommend a book by Brian Peterson called "Understanding Exposure". It's simple, easy to understand (but well written at the same time) and well illustrated. In this book he talks about the differences between fixed lens and removable lens in terms of aperture and how they relate.
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