DoF may not exist...

Started 3 months ago | Discussions thread
Erik Kaffehr
OP Erik Kaffehr Veteran Member • Posts: 6,177
Re: Another approach...

jonas ar wrote:

Erik Kaffehr wrote:

We can see that mst good lenses reach optimum sharpness at f/4. The only factor reducing sharpness when stopping down is diffraction.

The airy radius is 1.22 * N * Lambda. Assuming Lambda being 0.55 micron, we would get the airy radius where diffraction cames into role to be 1.22 * 4 * 0.55 -> 2.7 microns.

The corresponding diameter is 5.4 microns.

So, it may make some sense to use around 5-6 microns as CoC good lenses at optimal aperture.

But, that would yield a pretty short DoF.

Best regards

Erik

How does color sampling affect your choice?

I don't think it comes into play...

The question is how small CoC we need to achieve maximum sharpness.

We can just look at MTF. MTF is the absolute part of the MTF of the PSF. We know from Jim's data and other sources that most really good lenses peak around f/4 or f/5.6.
Now, why do they peak at those apertures? Because diffraction reduces sharpness when stopping down more. The shape of the diffraction blur is well know.

How comes demosaic into this?

Each element of the imaging chain causes some blur, which we could describe with a Point Spread Function (PSF). It would be said that the original image is convolved with the PSF.
Now, in the frequency plane, convolution is just a multiplication. Therefore we can break down blur into components and just multiply them.

So, lens aberrations have an MTF, that increases when stopping down. All lenses are also affected by diffraction. The MTF of diffraction reduces when stopping down.

Further down the line, the pixels also blur the image, as they have finite resolution, so the pixel has an MTF. Bayer deconvolution also add blur so it has an MTF.

But, the nice thing that in the frequency plane all MTFs multiply. So all PSF have an effekt, except when one component goes to zero, in that case MTF for the system is zero.

So, demosaic does affect system MTF, but it is just a component of the MTF.

Now, once we stop down, diffraction will add more blur. So it probably makes sense to base CoC in the DoF calculation on diffraction.
One of the reasons I look into this is that I use Lumariver DoF calculator. Lumariver uses max of  (2X pixel pitch, Airy) as CoC as default. That will give larger DoF than setting say pixel pitch as CoC.

That approach makes sense. We could use a CoC of say 5 microns, but stopping down would yield much more blur than 5 microns.

Sorry for not being able to provide a better answer.

Best regards

Erik

-- hide signature --

Erik Kaffehr
Website: http://echophoto.dnsalias.net
Magic uses to disappear in controlled experiments…
Gallery: http://echophoto.smugmug.com
Articles: http://echophoto.dnsalias.net/ekr/index.php/photoarticles

Post (hide subjects) Posted by
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
MOD JimKasson
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow