NikonGear'23

Gear Talk => Camera Talk => Topic started by: Eric Borgström on April 22, 2017, 15:31:30

Title: Out-resolving issues
Post by: Eric Borgström on April 22, 2017, 15:31:30

Could someone please explain what lens out-resolving the sensor does to the image.
With new mega pixel sensors we might have the opposite, sensor out-resolves the lens. That probably gives different issues with the image.
And perhaps in the center of the image the lens is out-resolving but in the periphery the sensor might win out.
Who knows about this?

Title: Re: Out-resolving issues
Post by: Bjørn Rørslett on April 22, 2017, 15:43:01

Basically you get increased moiré or spurious resolution. Or both.

Spurious resolution makes strange artifacts when you shoot say a ruler. Some of the division tics will appear dislocated.

Title: Re: Out-resolving issues
Post by: Eric Borgström on April 22, 2017, 17:27:09

Thank you Bjørn.

Title: Re: Out-resolving issues
Post by: Les Olson on April 23, 2017, 12:57:50

Spurious resolution is due to image defocus, not sampling frequency.  There is a good example and lucid explanation at http://toothwalker.org/optics/spurious.html 

Title: Re: Out-resolving issues
Post by: Bjørn Rørslett on April 23, 2017, 13:13:23

Aliasing generates spurious resolution.

Title: Re: Out-resolving issues
Post by: Les Olson on April 23, 2017, 16:22:15

Quote from: Bjørn Rørslett on April 23, 2017, 13:13:23

Aliasing generates spurious resolution.

No, aliasing is when high frequencies are reconstructed as lower frequencies because of under-sampling - "moire".  There are elegant simulations at http://micro.magnet.fsu.edu/primer/java/digitalimaging/processing/samplefrequency/index.html and http://micro.magnet.fsu.edu/primer/java/digitalimaging/processing/undersampling/index.html

Spurious resolution is quite different.  It was observed long before digital imaging, in aerial reconnaissance where variations in altitude cause de-focus, and explained in the early 1980s.  There is a simple demonstration in Figure 1 of https://www.researchgate.net/publication/253579540_Correcting_spurious_resolution_in_defocused_images_-_art_no_64920O

Title: Re: Out-resolving issues
Post by: Bjørn Rørslett on April 23, 2017, 16:31:51

The phenomena are clear, the terminology perhaps not. If you wish to reserve 'spurious' for one particular kind of false resolution it's fine with me. As I'm not a native English speaker I bow to the better informed.

I tried to avoid 'moiré' as that concept so often is associated to colour artifacts while the aliasing concerns any [spatial or other domain] frequency not sufficiently sampled.

Title: Re: Out-resolving issues
Post by: David H. Hartman on April 29, 2017, 08:22:31

Spurious is English for Latin spurius meaning false.

Not being what it purports to be; false or fake.

Hope this helps,

Dave