[Leica] Why sharpen?

[sonc.hegr at gmail.com (Sonny Carter)]

I  guess that about wraps it up, huh?

On Sun, Jun 7, 2009 at 6:37 PM, Jean Louchet<jean.louchet at gmail.com> 
wrote:
>> Date: Tue, 2 Jun 2009 16:22:27 -0700
>> From: "Frank Filippone" <red735i at earthlink.net>
>> Subject: [Leica] Why sharpen?
> [...]
>
> Hi,
>
> my (late) answer to this thread. A bit mathematically biased though.
>
>> Why sharpen a digital shot? ?What do you gain over the basic " negative" (
>> which could be RAW, Jpeg, TIFF or something else....) ?????
>>
>> I understand correcting some color qualities, and overall "blend" of 
>> colors,
>> and exposure.....
>>
>> But the idea of taking an image that was captured digitally and putting it
>> through a sharpening filter ( all the time) , somehow escapes me.......
>>
> Digital does not mean perfect.
>
> The first factor to take into account is the "impulse response" of the
> lens when it is on focus. An ideal lens should project a point of the
> scene into a point on the film (or electronic sensor), but real lenses
> make a (small or large) spot on the sensor. If the spot has a diameter
> about equal to the interpixel distance, then this will have an
> antialiasing effect. If it is bigger, there will be a loss of
> resolution. In some cases it is possible to make calculations in order
> to compensate for this blurring effect and more or less rebuild an
> image closer to what an "ideal" lens would have given. This spot is
> due to geometrical aberrations _and_ to diffraction. More technical
> details about this later.
>
> The second factor is light diffusion inside the sensor itself. In
> solid-state sensors, this diffusion is much lower in the absence of an
> antialiasing filter on the sensor, but it still exists. On film, there
> is some diffusion of light through the emulsion. Years ago, an
> "anti'halo layer" was added on the back face of the emulsion in order
> to reduce this diffusion (otherwise the light would get to the film
> base and get a double reflexion).
>
> The third factor is the impulse response of the lens when it is _not_
> on focus. The so-called "Setala formula" gives the diameter of the
> bokeh spot (i.e. the impulse response) in function of the focusing
> error (formula given below). In reality, the bokeh spot has the
> diameter predicted by Setala's formula but the illumination inside is
> not really uniform (this is the well-know bokeh problem), depending on
> lens design.
>
> The fourth factor is all the other factors contributing to image
> blurring, in particular motion blur (or camera shake, which is
> equivalent).
>
> Thus it appears a good idea, when it is possible, to try to
> mathematically reverse there four factors to get as close as possible
> to "ideal" images.
>
> Geometrical aberrations give impulse responses with strange shapes,
> extremely awkward to reverse mathematically. The very best lenses
> reduce the size of these strange shapes to a minimal size, smaller
> than interpixel distance (7 microns on the M8) ?that does not require
> any correction. This may be one of the best reasons to buy Leica
> lenses... however some other lenses are already very fine in this
> respect (among them, my own CV 12mm, 75mm and M-Rokkor 40mm look
> excellent).
>
> Diffraction is a major factor of blur in small format cameras. On the
> M8, the diffraction spot is small enough to have no practical effect
> as soon as the diaphragm is opened at least at f/8. Closing the
> diaphragm further than f/8 (e.g. at f/11 or f/16) is a bad idea in
> this respect.
>
> In first approximation, the 2nd (diffusion) and third (focusing)
> factors result in impulse responses approx. in the shape of
> homogeneous circles. The math-inclined may remember the 2-D Fourier
> transform of a circle is a function looking like (sin(d)/d) where d is
> the distance to the origin (physicists know this as the "Airy spot").
> The basis of deblurring algorithms is closely related to this - out of
> the scope here to give all the math. The important thing is that,
> before applying a de-blurring (or "sharpening") filter to compensate
> for this, one must adjust the deblurring filter depending on the size
> of this impulse response. The bad news is that usually we don't have
> any clue on this!
>
> The consequence is that using a pre-tuned deblurring/sharpening filter
> without knowing how it has to be adjusted can be a real problem. In
> practice, it may be good to use a "soft" sharpener in order to
> compensate for diffusion; this will be obviously more necessary on
> cameras using anti-aliasing filters and even more on microformat
> cameras where diffraction may be terrible; on the M8 with a very good
> lens it is not really necessary I think. Then, using a "medium" or
> "large" sharpening filter may be useful to try to compensate for other
> factors contributing to blur, but this is highly problematic unless
> one knows very exactly ?the lens characteristics, the diaphragm used,
> and the focusing error. So this is essentially a trial and error
> process which is better to apply in post-processing rather than before
> saving the "master" image.
>
> Hope this helps.
>
> By the way, here is the Setala formula:
> ? ? ? ? ? ? ? ? Delta = d *f *abs (1/z - 1/d)
> where Delta is the diameter of the bokeh spot, d the absolute aperture
> (diaphr. diameter), z focusing distance, d object distance. "abs" is
> the absolute value.
>
> Setala, a Finn (amateur?) photographer, was a Leica user who found
> this formula and engraved it himself on his Elmar, thus inventing the
> "depth of focus graduations"; Leitz discovered this when Setala sent
> them his Leica for a repair, and copied this into production lenses.
>
> Jean
>
>> Frank Filippone, digital-less.
>> red735i at earthlink.net
>>
>>
>>
> --
> Jean Louchet
> INRIA-APIS 4 rue Jacques Monod 91893 ORSAY Cedex France
> jean.louchet at inria.fr ? http://jean.louchet.free.fr/
> "I will permit no man to narrow and degrade my soul by making me hate
> him" (Booker T. Washington)
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>
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Sonny
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