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02-22-2011, 06:26 AM   #1
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Lenses outresolving sensor

Well this might be a bit stupid question, but sometimes i see the statements from others that such and such lens outresolves the K--d sensor. That keeps me wondering - how actually do you find out that your lens outresolves the sensor? How outresolving demonstrates itself when you lookin at the normal photos?

02-22-2011, 06:40 AM   #2
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I take it to mean that when you zoom to 100% (just as you begin to see the actual pixels) you still get new detail instead of a blurry mash. But this is not precise of course.
02-22-2011, 07:01 AM   #3
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QuoteOriginally posted by vrrattko Quote
Well this might be a bit stupid question, but sometimes i see the statements from others that such and such lens outresolves the K--d sensor. That keeps me wondering - how actually do you find out that your lens outresolves the sensor? How outresolving demonstrates itself when you lookin at the normal photos?
There is nothing like lens over-resolving - it's just that sensor is capable or not and I guess all k-* sensor are just fine.
02-22-2011, 09:34 AM   #4
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What does it mean exactly if a sensor out-resolves a lens? The real question is "what are the practical consequences?"

02-22-2011, 12:05 PM   #5
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QuoteOriginally posted by newarts Quote
What does it mean exactly if a sensor out-resolves a lens? The real question is "what are the practical consequences?"
I'd say a sensor out-resolves a lens when you cannot get sharp images at the lens' optimum aperture (viewed at 100%). And the other way around, if you still get sharp images at a lot of different apertures that would point to the fact that the lens still has more to give. As in the case with people reporting that they get better images with the same lenses with the K-5 as opposed to older bodies as the K10D/K20D/K-7. Even though the images surely looks very good on those bodies also.
02-22-2011, 01:28 PM   #6
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Original Poster
RE:

Thanks guys, things you wrote make sense....but practically is it noticeable? Without pixelpeeping or printing huge enlargements - would you anyhow notice that lens resolving power is not enough for sensor? Would the photos appear softer as a result? In other way - is that a reason to worry at all?
I used some pretty old budget lenses, cheap zooms as well as FA limiteds and FA35 - and all of them managed to produce sharp images on K20d sensor at 100% magnification (when focused properly).
02-22-2011, 08:25 PM   #7
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I saw it very well in cheap old Sigma UC zooms. Even on my first dSLR, Sigma SD9, I could see a blurry mush after zooming in. Once I bought the excellent Sigma 70-200/2.8, the mush disappeared to be replaced by actual detail.

But even premium lenses vary in microcontrast, sharpness, transition from focus to out of focus, especially wide open, in the frame borders and at their extreme focal lenths (or one of them).

You may not see it, but it does give a different feel to the images.

Of course most people wouldn't really notice the difference between an image taken by a $4000 Leica Summilux-R 50mm E60 and our humble $100 Asahi Super-Takumar 50/1.4, instead concentrating on the image itself. But the differences are there. Whether they are worth the price difference is another matter.

I posted two images of a rose one taken by a Leica 35/1.4 and another by Vivitar macro 105/2.5. You can't really compare these lenses, they do different things and are both capable of producing nice images. But the pixie dust of Leica (the micro-contrast, the transition to OOF, colour rendition and sharpness across the frame at all apertures are hard to beat and in the hands of a real pro (not me!) these things can make a real difference.

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/128552-leica-z...xperience.html

Last edited by DanielT74; 02-22-2011 at 08:37 PM.
02-23-2011, 06:23 AM   #8
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Lets look at a case with an ideal lens and sensor. Here's an MTF (contrast vs resolution) graph for a couple perfect lenses published by Bob Atkins with ideal APS-C sensor limits added by me:


The ideal resolution limits of the sensors correspond to a print at 100%. For a perfect f:8 lens, the 6mp sensor's resolution is about 62 lp/mm while the 12mp sensor's resolution is somewhat better at about 87 lp/mm. But the 6mp's fine detail has a contrast of about 65% while the fine detail in the 12mp image is only 50%. Which is better?

Maybe it is unfair to compare the two sensors at 100%, instead let's compare image quality at various print sizes. The SQF image quality is roughly proportional to the MTF at 2lp/mm on the display. The graph shows the display's 2lp/mm translated to the sensor by the enlargement factor.


I had to stretch the graph sideways to fit in normal display sizes. Notice there is no difference in predicted image quality until the 100% limit for the 6mp sensor is exceeded - until then the display is limiting the image quality, not the sensor or lens.

This is an idealized, schematic, simplified representation of what's going on. In real life, the sensor's MTF must be taken into account, which has the effect of bending the resolution curves so the total MTF goes to zero at the sensor's limit. Depending on details of sensor MTF shape, this could decrease the higher enlargement display image quality of the 6mp sensor compared to the 12mp sensor.

I hope this helps and isn't too confusing but it is a complex topic because there's so many ways to look at it.

Dave


Last edited by newarts; 02-23-2011 at 06:30 AM.
02-23-2011, 07:22 AM - 1 Like   #9
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SIMPLE EXPERIMENT:

Maybe someone with Pentax camera bodies of differing resolution will do this simple experiment to demonstrate the effects of a sensor out-resolving a lens.

Use the same good lens on bodies with differing sensor resolutions and use the lens f-stop to make the lens out-resolve and under-resolve the sensors by changing the size of the diffraction spot.

The relationship between a perfect lens' diffraction limit, f.diffraction, and the pixel sensor's pixel spacing is close to:

f.diffraction* ~ 3 pixel.pixel.pitch.micrometers

f.diffraction limit...Camera
f:23................... K100D, 200D
f:19 ...................K10D
f:16....................K-x, r
f:15................... K20D
f:14 ...................K-5

Say you have a K100D, a K-x, and a good lens; around f:8 the lens is likely better than either sensor (the diffraction spot is smaller than either lens' pixel pitch) while above f:16 the K100D's 100% crop at should look better than the K-x' 100% crop (at f:20 the diffraction spot is bigger than the K-x's resolution but smaller than the K100D's).

Is this experimentally true? Which then is the better sensor? Which is the "better lens"?

What if the images are displayed at 10", 20", 30", etc widths?

Enquiring minds want to know,

Dave

*EDIT m42man observed that the diffraction limit I used was different from that at Cambridge in Colour's web site. That's because I used the Rayleigh Criteria which is about when two points can no longer be distinguished. The diffraction effect's onset should become observable at about 2 times the Rayleigh Criteria separation. Here's the table using this looser criteria:

f.diffraction.onset......Camera
f:12......................... K100D, 200D
f:9 ...........................K10D
f:8............................K-x, r
f:7.5........................ K20D
f:7 ...........................K-5

Last edited by newarts; 02-24-2011 at 04:24 AM.
02-24-2011, 02:20 AM   #10
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QuoteOriginally posted by newarts Quote

...The relationship between a perfect lens' diffraction limit, f.diffraction, and the pixel sensor's pixel spacing is close to:

f.diffraction ~ 3 pixel.pixel.pitch.micrometers

f.diffraction limit...Camera
f:23................... K100D, 200D
f:19 ...................K10D
f:16....................K-x, r
f:15................... K20D
f:14 ...................K-5
No offence, but I was surprised to see those diffraction limits.

If you use the calculator here, you'll see that their diffraction limit for the Kx, for example, is about f11:

Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks

Possibly the formula you're using is a little optimistic (I'm assuming we're pixel-peeping here)?
02-24-2011, 04:05 AM   #11
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QuoteOriginally posted by m42man Quote
No offence, but I was surprised to see those diffraction limits.

If you use the calculator here, you'll see that their diffraction limit for the Kx, for example, is about f11:

Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks

Possibly the formula you're using is a little optimistic (I'm assuming we're pixel-peeping here)?
I used the so-called "Rayleigh Criteria" for resolution which is when 2 points overlap with a contrast of 9% between them; beyond this limit, adjacent points appear as one. This is a more stringent criteria than used by Cambridge in Colour, which is closer to when diffraction becomes observable (a single point's diffraction disk "slops over" onto adjacent pixels substantially)

For an estimate of when diffraction becomes observable, divide my diffraction limited f stop by 2.

Thanks for pointing this out.

Dave

PS I used the Rayleigh Criteria which is about when two points can no longer be distinguished. The diffraction effect's onset should become observable at about 2 times the Rayleigh Criteria separation. Here's the table using this looser criteria (actual pixel spacing used):

f.diffraction.onset......Camera
f:12......................... K100D, 200D
f:9 ...........................K10D
f:8............................K-x, r
f:7.5........................ K20D
f:7 ...........................K-5

Last edited by newarts; 02-24-2011 at 04:25 AM.
02-25-2011, 06:25 AM   #12
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QuoteOriginally posted by newarts Quote
This is an idealized, schematic, simplified representation of what's going on.
Dave
Two further reductions:
a) if pattern is 60lp/mm for a 6mp sensor is shifted exactly left by one pixel, then you get grey mush -> factor of 2 lower -> resolution limit for a "black and white" pixel is 30lp/mm
b) pixels are rgb, so a further fudge factor of something like 1/sqrt(2), depending on whether the pixels are diagonally packed etc etc
c) colour: blue light offers twice the resolution of red, due to diffraction limit being proprtional to 1/wavelength (hence blue ray discs)

On the flip side, digital SLR's are incredibly good value and very practical when all is considered, and the lenses will fit onto next year's improved model anyway.
02-25-2011, 09:37 AM   #13
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QuoteOriginally posted by hoojammyflip Quote
Two further reductions:
a) if pattern is 60lp/mm for a 6mp sensor is shifted exactly left by one pixel, then you get grey mush -> factor of 2 lower -> resolution limit for a "black and white" pixel is 30lp/mm
b) pixels are rgb, so a further fudge factor of something like 1/sqrt(2), depending on whether the pixels are diagonally packed etc etc
c) colour: blue light offers twice the resolution of red, due to diffraction limit being proprtional to 1/wavelength (hence blue ray discs)

On the flip side, digital SLR's are incredibly good value and very practical when all is considered, and the lenses will fit onto next year's improved model anyway.
I agree.

I used the Nyquist limit as the "sensor resolution". Perhaps it would have been better to use assume a sensor MTF=.5 at half the Nyquist limit, but that wouldn't change the illustration of how good lenses are compared to sensors.
02-25-2011, 12:13 PM   #14
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yeah, its incredible how good lenses are

cant remember where I saw it, but apparently the 10lp/mm resolution (on full frame) is the one which affects how sharp we think a picture is anyway (for a 5x print, so its 2lp/mm), and the limit of our visual powers is 8lp/mm at normal viewing distance, which equates to 60lp/mm on a cropped sensor
02-25-2011, 02:46 PM   #15
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QuoteOriginally posted by hoojammyflip Quote
yeah, its incredible how good lenses are

cant remember where I saw it, but apparently the 10lp/mm resolution (on full frame) is the one which affects how sharp we think a picture is anyway (for a 5x print, so its 2lp/mm), and the limit of our visual powers is 8lp/mm at normal viewing distance, which equates to 60lp/mm on a cropped sensor

That's consistent with the Subjective Quality Factor - SQL - which is roughly the average MTF in the 1/2 to 2 lp/mm range on a print viewed at normal distances. For an un-cropped 8x10" enlargement of a APS-C sensor that correspond to about 5-20lp/mm on the sensor.

Dave
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