Will the Epic 645 hava a OLPF?

[Daniel Browning]

with 6micron photo sites and stops above f11 is the fuzziness due to diffraction enough?

For most people it would be enough, IMHO.

Is it possible to make a OLPF big enough for the 645...

Yes. It's only money. (Cost rises exponentially with area for the good crystal ones.)

At f/11 the diameter of Airy disk (the smallest point on which a beam of light can be focused) is 17 microns.
[...]
It's more than enough. The smallest detail the lens can focus at f/11 would be 3x3 pixels large.

I think MTF is the best way to measure the effect of diffraction. The higher the frequency, the lower the MTF (i.e., smaller pixels have lower MTF at Nyquist due to diffraction).

To avoid aliasing, an MTF of 0% would be the theoretical ideal, but in practical situations I don't think it's necessary to go that low. I think 10% MTF is sufficient to prevent aliasing. Using the formula for MTF of diffraction[*], I calculate that MTF drops to 10% for green light (550nm) at f/11 at 3.7 micron pixels. Using red light (750nm) I get 5 micron pixels. Both of these are slightly smaller than the 6 micron pixels in the EPICs.

Again, it's a question of what MTF you consider aliasing taken care of.
[*] 2/pi*(acos(f)-f.*sqrt(1-f.^2)) where f is 1/(w*f-number).

[Pawel Achtel]

Thanks Daniel,

I thought the MTF would reach 0% at the size of Airy disk (17 microns) corresponding to f/11. The reason the MTF would reach 0% at this frequency is because the size of Airy disk is the smallest detail the lens can produce. Thus, the frequency limit would be around 17 microns.

Are you sure that at f/11 the MTF would be 10% at frequency of 3.7 microns? Even from practical experience, lenses on most DSLRs (6 micron pixels for argument's sake) start to be diffraction limited passed f/5.6.

[Daniel Browning]

I thought the MTF would reach 0% at the size of Airy disk (17 microns) corresponding to f/11. The reason the MTF would reach 0% at this frequency is because the size of Airy disk is the smallest detail the lens can produce.

Have you considered the fact that the intensity of the Airy disk varies from the center to the edge? Or the effect of alignment on a grid of square pixels for sampling? That to resolve two lines requires a minimum of three samples: one for each line and a third to show a change in intensity (half the Rayleigh criteria spacing)?

Are you sure that at f/11 the MTF would be 10% at frequency of 3.7 microns?

I think I got my math right. Here's an online source for the diffraction formula:

http://www.mellesgriot.com/products/optics/os_2_2.htm

Even from practical experience, lenses on most DSLRs (6 micron pixels for argument's sake) start to be diffraction limited passed f/5.6.

Agreed. Most of us have all of our practical experience with diffraction in similar situations: noticing when diffraction starts to affect the image, and trying to avoid it. But "starting" to be diffraction limited is not enough to eliminate aliasing. For example, diffraction can cause contrast to drop from 80% to 40%, but that's still enough to cause aliasing.

Few have practical experience in the area covered by the thread: finding out how small pixels can be before MTF drops below 10%.

[Pawel Achtel]

Thanks Daniel,

Here is the source I was referring to:

http://www.schneideroptics.com/indus...hingLenses.htm

which, I think, is exactly what we are discussing here.

[Daniel Browning]

Here is the source I was referring to:

http://www.schneideroptics.com/indus...hingLenses.htm

As far as I can tell, they're only talking about the typical situation, which is avoiding diffraction:

What is required is a lens system with a fairly low f/# to even theoretically achieve the sensor limited resolution

In this thread we're talking about the extreme opposite end. First diffraction starts. Then it gets annoying. Then it gets bad. Then it gets really bad. Then it finally drops all the way down 10% MTF. Only then is aliasing finally taken care of.

They are talking about trying to be "sensor limited", which means avoiding the "start" of diffraction, or at least avoiding the "annoying" phase of diffraction, but certainly not going anywhere near 10% MTF. HTH.

[Pawel Achtel]

Ahhh, OK, it makes sense now.

[Stefan Christou]

it wouldn't matter, f number is only affected by the focal length of the lens and the apertures diameter. so any 50mm lens at f11 will ALWAYS have an aperture diameter of 4.5mm.

Tim.

I stand corrected. Very educating posts BTW

[Tim Griffith]

What is required is a lens system with a fairly low f/# to even theoretically achieve the sensor limited resolution[/I]

In this thread we're talking about the extreme opposite end. First diffraction starts. Then it gets annoying. Then it gets bad. Then it gets really bad. Then it finally drops all the way down 10% MTF. Only then is aliasing finally taken care of.

They are talking about trying to be "sensor limited", which means avoiding the "start" of diffraction, or at least avoiding the "annoying" phase of diffraction, but certainly not going anywhere near 10% MTF. HTH.

Great information, thats going to give me something to chew on for a while, thanks.

Tim.

[Dan Hudgins]

The circle of confusion is not just a fuzzy blob with a bell curve, depending on the lens formula it can have some structure, and different structure at different wave lengths, more so off center because of comma.

Also aliasing is not the only issue, you also need to reduce chroma moire.

A good OLPF will cut divide the light over four pixels and help kill those issues, more than just stopping the lens down because the sharpening used in the de-mosaic can be over 400%, which counteracts some of the diffraction making the center of the bell curve brighter than it would be at 100% sharpen (i.e. no sharpen). Also the sharpen needed brings out other shapes in the circle of confusion, which is not a circle off center but looks at bit like a bird on some lenses with the comma as its tale.

I've been stopping the lens down in our scanner to reduce the grain in film scans, but when you sharpen most of the smaller details come back, so it is not an on or off solution. Also you get smaller details in the UV, so if you try it you might want to use a UV+Violet cut filter.

[Rodney James]

The sensor size is irrelevant.

Wrong.