Hi Mike:

I do not consider the test by, 'luminous-landscape', as being valid, in
this discusion, as regards to considering diffraction.

A 180mm, LF lens is not optimized for providing its sharpest image in
the center. It is optimized for images ranging from 4 x 5 to 5 x 7
images.

Why in the world did they chose such a subject, which was lacking in any
significant detail for their "diffraction test" ?

On my screen, the bottom background images were sharpest at f/8.0. and
were progressively worse at f/11, at f/16, at f/22, f/32, and at f/45.
Was the lens properly focussed, or did the lens have a problem
with spherical abberation, such as a focus shift at different f/stops.


Best regards,

DON

Hi Don,

Regarding this link:

http://www.luminous-landscape.com/tutorials/understanding-series/u-diffraction.shtml

On Wed, 15 Nov 2006, Don Lopp wrote:

> Why in the world did they chose such a subject, which was lacking in any
> significant detail for their "diffraction test" ?

I don't know, but it does illustrate a progessive decay as you move to
smaller apertures.

I'd like to point out that just because the diffraction seen in these
sample images becomes unacceptable below a certain f/stop, that does
NOT mean that diffraction will impose the exact same amount of degredation
when using that same f/stop at OTHER enlargement factors.

The author of that page, Michael Reichmann, wrote that those are "100%
crops" from a Phase One P45 back. That means that we are looking at a
very small portion of a huge 39 Megapixel file, with pixel dimensions of
7216 x 5412. The crops have dimensions of only 450 x 417 pixels.

On my display, each crop has a displayed width of 136mm (about 5 and 3/8
inches).

We know the crops are 450 pixels wide. We know that translates to 136mm
as displayed on my monitor at 100% scaling. We know that the original
images from which the crops were made are 7216 pixels wide. We know that
those 7216 pixels reside on a sensor that is 49mm wide. This gives us
everything we need to know to calulate the enlargement factor I'm
experiencing when viewing the crops on my display.

If a 450 pixel crop is displayed as 136mm on-screen, then the original
7216 pixel image would be how wide?

450 pixels / 136mm = 7216 pixels / x

x = 2181 mm = 85.9 inches

If our enlargement is 2181 mm wide and the sensor is only 49 mm wide, so
what is our enlargement factor?

2181 / 49 = 44.5x

That's a really huge enlargement factor - and it explains why most of the
samples look so bad.

Here's the formula for determining the stop (f/N) at which diffraction
will begin to inhbit a desired resolution in the final print:

Maximum N = 1 / enlargement factor / desired resolution / 0.00135383

Let's calculate the smallest apeture that will support a desired
resolution of 5 lp/mm at an enlargement factor of 44.5x...

Maximum N = 1 / 44.5 / 5 / 0.00135383

Maximum N = 3.3

Whoa! f/3.3 is the smallest aperture that can support a resolution of 5
lp/mm at an enlargement factor of 44.5x. No wonder those sample images
look "fishy". With an enlargement factor of 44.5x, diffraction has
prevented the desired 5 lp/mm resolution even in the first sample photo -
taken at f/5.6 (not f/3.3).

Keep in mind that the range of 5 to 8 lp/mm is considered to be at the
limit of what we can resolve from a viewing distance of 10 inches (25 cm).

I'm currently sitting about 20 inches from my monitor, so let's solve for
Maximum N using a desired resolution of only 2.5 lp/mm:

Maximum N = 1 / 44.5 / 2.5 / 0.00135383

Maximum N = 6.6

So f/6.6 is the smallest aperture that can be used to support a resolution
of 2.5 lp/mm (enough for a viewing distance of 20 inches) in a 44.5x
enlargement.

This correlates PERFECTLY with what I SEE when I compare the f/5.6 photo
to the f/8 photo. At f/8, the image is already falling apart due to
diffraction.

The formula works and there's nothing "fishy" about the images at that web
page.

Mike Davis

Hi Mike:

You suggested that: "I do not know,but it illustratesa progressive dwcay
as you move to smaller apertures". My interpretation is that were
same size images. I am not aware that the APO Sironar f/5.6/180mm
lens was optimized for same size photography, therefore a poor choice.

I interpret the term, "these are 100 percent crops", to mean same size
images.

On my screen, these images appear to have been, 'same size', images


Looking back at the URL, IMO, the f/5.6 and f/ 8.0 images are equally
soft, with the serious image degradation beginning at the f/11.

Why would diffraction degradation start showing as early as at f/11 ?



Best regards,

DON

Hi Don,

On Thu, 16 Nov 2006, Don Lopp wrote:

>
> Looking back at the URL, IMO, the f/5.6 and f/ 8.0 images are equally
> soft, with the serious image degradation beginning at the f/11.
>
> Why would diffraction degradation start showing as early as at f/11 ?

The onset of *visible* degredation is dependent on the aperture used, the
enlargement factor, and the viewing distance. As I outlined in a previous
post, the enlargement factor of the sample images, as displayed on my
computer monitor, is huge. You are not looking at the entire 39 Megapixel
frame when you look at one of those crops. That's why even f/11 looks bad
in those samples. Just because f/11 looks bad at the enlargement factor
experienced when we look at the samples on that web page, doesn't mean
that f/11 will look bad at lesser magnifications. The aperture is not the
only variable affecting vulnerability to visible diffraction.

As aperture decreases, vulnerability to visible difraction increases.

As enlargement factor increases, vulnerabiility to visible diffraction
increases.

As viewing distance decreases, vulnerability to visible diffraction
increases.

Mike Davis

The reason that I suspected that the the web page DoF pictures did not
encompass a width of 85.9 inches, as has been suggested, is because the
leaves appear to to me, to be, approximately, same size images.

If this is true, this would indicate that the digital sensor unit on the
Linoff camera must have been about 7 (+/-) feet in width, which, in my
opinion, is very unlikely. The digital sensor was alleged to be
,"only 49mm wide".

Incidentally, the depth of focus, with a same size image, at f/11 is
very shallow, much less than 1/8th of an inch, which is why f/105 is
often used.


Best regards,

DON