David J. Littleboy wrote:

>
wrote in message

> news:(E-Mail Removed)...

>

>>"Roger N. Clark (change username to rnclark)"
wrote:

>>

>>>(E-Mail Removed) wrote:

>>

>>>>Any signal with components less than the Nyquist frequency can be

>>>>reconstructed by using this formula:

>>>>

>>>> sin(pi(t-k)/T)

>>>> x(t) = sum (-inf, inf) x[k] --------------

>>>> pi(t-k)/T

>>>>

>>>>and this is true regardless of the phase of the signal.

>>>>

>>>>(x[k] are the samples, k is the time of each sample, and T is the

>>>>sampling interval.)

>>

>>>This is fine if your do infinite sampling. But imaging sensors

>>>are discrete and finite sampling of a spatial image.

>>

>>Yes, indeed. However, in practice you can get close to the sampling

>>frequency with a window of a fairly small number of samples, say 64 or

>>128. And you can reconstruct the signal from this, regardless of its

>>phase.
Andrew,

How would you reconstruct the image on this page:

http://www.clarkvision.com/imagedetail/sampling1.html
e.g. the "Nyquist sampling" or "1.5x Nyquist sampling?"

> But the only thing you can reconstruct is a sine wave, not a square wave.

> People want images of black and white stripes at frequencies close to the

> Nyquist, and that's completely unreasonable.
But even a sine wave at the Nyquist frequency will be grossly distorted

when sampled at or close to the Nyquist frequency.

> By the way, I checked a portfolio of 4x5 landscape shots in a recent

> Japanese magazine, and fully half of them were shot at f/45. That means that

> the MTF50 from diffraction is a whopping 18 lp/mm. Not a lot of MP there.

> (Well, actually, a lot of MP if you figure 3 pixels per line pair, and 120 x

> 95mm => 33MP or so. But nowhere near the 100MP and over figures people have

> been bandying about.)
David,

You pick one MTF for film, but as you know, that is not the whole

story. Film has higher resolution at lower MTF, and it is that

higher resolution that contributes to image detail. The above

web page illustrates this with a 4x5 image at f/45. At f/45,

the Rayleigh limit is ~9% MTF and detail like grass blades in the

distance are still recorded as lines, and sampling higher than the

Rayleigh limit still brings out a little more detail that

improves the perception of sharpness.

Roger