[wim]

So does the lens act as a low pass filter?

 

No, but but system resolution can in simple form be calculated with the old formula for this:

 

1/(system resolution) = 1/(lens resultion at specific f-stop) + 1/(sensor resolution)

 

For ease of use we are assuming that we are talking base iso for a sensor, and in that case sensor resolution is in principle equal to the nyquist frequency, plus or minus 5 to 15 % depending on de AA-filter stack being used.although fro ease of use we can ignore this.

 

Generally speaking, in the olden days, we worked all of this back to lp/mm, and for a 16 MP MFT sensor you then end up at approximately 133 lp/mm, for a 20 MP MFT sensor at base iso that is 150 lp/mm. If we assume a 10% reduction due to the AA-fiter, this then becomes approximtely 120 lp/mm and 135 lp/mm.

 

For diffraction based maximum lens resolution, we used to use the so-called Rayleigh criterion, and that amounts to using a wavelength of 512 Angstrom and an MTF of approximately 9%, which for normal shooting purposes is average. Note that MTF-50s are therefore typically a lot lower. Anyway, at F/1 the Rayleigh limit is about 1600 lp/mm, based on the Airy disk size, the diffraction circle size of a point, one gets at these parameters.

 

Because of diffraction, we then roughly get the following diffraction limits for perfect lenses at different f-stops, all based on the Rayleigh criterion:

 

F/1: 1600 lp/mm

F/1.4: 1143 lp/mm

F/2: 800 lp/mm

F/2.8: 571 lp/mm

F/4: 400 lp/mm

F/5.6: 286 lp/mm

F/8: 200 lp/mm

F/11: 145 lp/mm

F/16: 100 lp/mm

etc.

 

Now, there is no such thing as a perfect lens, and off-axis performance of a lens deteriorates as well, and at large apertures a lens is generally not at its best. However, many of the high quality lenses do rather well when stopped down a few stops, and some even get close if not equal diffraction limits at those f-stops, at least in the optical centre.

 

Because lenses tend to be best when stopped down a bit, generally speaking, and because diffraction gets worse with stopping down, what often happens when you measure lens resolution, is that the resolution starts at a specific point, gets higher, and when diffraction becomes teh limiting factor, lens resolution goes down again. You get a parabolic or elliptical type curve, with the top lying often at the f-stops a few stops away from maximum aperture.

 

The so-called diffraction limit of sensors, essentially is the aperture at which point a sensor cannot resolve beyond teh diffraction limit, purely caused by the fact that lights bend. It is a bit more complex than that, because sensor wells are indeed wells, and create shadows depending on the angle of the incoming light, but roughly this is correct. For MFT sensors at 16 MP and 20 MP these sensor diffraction limits are F/12 and F/11 respectively, but to get the most out of this you generally need to stop down  a full aperture less, so F/9 and F/8. it doesn't mean you can't take a picture, or can't get a usable image, but this is about making optimal use of what is available; the resolution of the sensor does not alter regardless.

 

As many testers use MFT-50 for their resolution findings, I have also calculated a set of MTF-50 diffraction limits. Here it is;

 

F/1: 760 lp/mm

F/1.4: 543 lp/mm

F/2: 380 lp/mm

F/2.8: 271 lp/mm

F/4: 190 lp/mm

F/5.6: 136 lp/mm

F/8: 95 lp/mm

F/11: 69 lp/mm

F/16: 48 lp/mm

etc.

 

I have created a few tables in which I also calculated the system resolution for different sensors, and perfect lenses, based on teh two diffraction limits I mentioned here (rayleigh criterion and MTF-50).

 

First, Panasonic Lumix GF2, a 12 MP MFT camera:

[ATTACHMENT NOT FOUND]

 

Olympus O-MD E-M10 Mark II (16 MP, same as O-MD E-M5, etc.):

[ATTACHMENT NOT FOUND]

 

Olympus Pen F, 20 MP (same as Panasonix GX8):

[ATTACHMENT NOT FOUND]

 

In short, as you can see, at F/4, 12 MP and MTF-50 maximum resolution is 72 lp/mm, at F/4, 16 MP and MTF-50 it is 78 lp/mm and at F/4, 20 MP and MTF-50 it is 84 lp/mm. Similarly, at F/5.6 it is 62 lp/mm, 67 lp/mm, and 71 lp/mm respectively. At apertures larger than F/4 lenses tend to have a lot of residual errors and resolve less, and as you can see at F/5.6 diffraction already starts to become signifcant (for any lens and any system).

 

Even so, resolution is well above what we used to have with film, as I mentioned before, in other posts.

 

HTH, kind regards, Wim