Amazing.Justwalking wrote:This arithmetic method to count airy disks diameter across sides to find oiptimum pixels is useless for practice where we can't do anything with pixel pitch.rjlittlefield wrote:Justwalking, I'm not sure I understand how you're thinking about this.

Up to now, I have been trying to get you to realize that at same DOF and same FOV, the number of Airy disk diameters across the frame will be the same for all format sizes.

Now, it sounds like you agree that the number of Airy disk diameters across the frame will be the same, but you're thinking that the small sensor will somehow see higher contrast than the larger sensor.

Is that what you're thinking?

If so, then what is your mathematical justification for that idea?

--Rik

It simplified count that can't compute many other factors.

The highest spatial frequency a sensor can resolve is its Nyquist frequency, equal to 0.5/(pixel spacing). When a lens is stopped down so its Rayleigh limit is below the Nyquist frequency, the camera is limited by the lens rather than the sensor

For optimum quality (when extreme depth of field is not required), the aperture should be set at least one stop larger than the aperture where the Rayleigh limit equals the Nyquist frequency:

NR=N = 3.2 * pixel spacing (um).

http://www.normankoren.com/digital_came ... iffraction

For a pixel spacing of 1.4 microns, NR=N = 3.2 * 1.4 = f/4.48, so the aperture should be set at f/2.8-f/3.2 or larger.

With small sensor i can leave this "small" DoF as is and it will be not the extreme Dof for my sensor, so i can choose optimum quality for the Dof, but for the FF it is not so to achieve the same Dof.

Very small formats - for compact digital cameras with 11 mm diagonal or smaller sensors (1/4 the size of 35mm) are severely diffraction-limited at f/8, where DOF is equivalent to f/32 or more in 35mm.

But tiny digital cameras still produce very sharp images at f/4 and f/5.6 because their tiny pixels with no anti-aliasing filters — have far better lp/mm resolution than 35mm.

Once again, Justwalking has found a relevant formula, computed for one sensor, then stated an opinion

*without ever bothering to compute for the other sensor*.

Apparently he has missed the concept that to use the math, you have to actually

**do**the math.

Let us do the math for the FF sensor, as Justwalking did only for the 5.5 crop sensor.

At same MP, the FF pixels are 5.5 times larger, so 5.5 * 1.4 = 7.7 microns pixel spacing.

By normankoren's formula, NR=N = 3.2 * 7.7 = f/24.6, so the FF aperture should be set at f/17 or larger.

Then note that f/17 on FF is just 5.5 times larger than f/3.2 on crop 5.5 .

So, normankoren's formula recommends an aperture that is scaled exactly in proportion to the sensor size.

As we all agree now, that condition is exactly what's needed to make the DOF be the same.

In other words,

**normankoren's rule for optimal aperture on each sensor, also gives the same DOF on both sensors.**

Earlier,

Yes.Justwalking wrote:The Dof at crop 5.5 at m=0.5 at Feff=15 will be same as on FF but FF must be ... F'=82.5(!)

Do you think that they will be looks the same?

Exactly as shown in my image pair at m=0.68 and f/8.5 on crop 5.5, versus m=3.74 and f/47 on FF. And as predicted by diffraction math, and as predicted by normankoren's formula.

Theory and experiment agree completely.

**At same FOV, same MP, and same rule for optimal aperture, the small sensor image and the large sensor image look the same**.

--Rik