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08-12-2010, 11:21 PM   #1
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Lens resolution

Long ago, in a galaxy far, far away... I owned a Contarex and a Leica M3. Back then, popular photo magazines used to do lens resolution tests for their reviews using all sorts of expensive resolution test charts, and then provide the numerical values in terms of lines per millimeter. Many photographers - myself included - purchased lenses partially based on their sheer resolving power.

Since returning to photography in the past couple of years, I note that no one seems to provide such information. It seems to me that a lines per mm rating on a lens would be a very clear indicator of how that lens would relate to a specific sensor size and number of megapixels. It is simple to determine the maximum lines per mm that a sensor can resolve. It's a function - directly - of pixel physical spacing. If I knew that my sensor maxed out at x lines per mm, and lens Y had better resolution, I'd know that I could extract all the image resolution possible. But placing that same lens on a body with a different sensor resolution (say only 10MPx, vs 14MPx) would be a waste as sensor Z could not make use of the IQ of lens X.

I hope I'm making myself clear. If I have a collection of lenses that pretty well match the resolving capability of my current sensor, there would appear to be no advantage in purchasing a body with say, 20% or even 50% greater resolution in the same physical sensor size.

Anyone have any thoughts on this?

Tim

08-12-2010, 11:32 PM   #2
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08-13-2010, 12:12 AM   #3
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For an 25 x 15mm sensor

My logic was that if most lens max out at 2000 lines per inch, and that you need 2 pixels minimum per line for resolution then a 5000 x 3000 would be the minimum needed. So that is why I bought a k20d; as for my use, getting any better resolution would be wasted. I also thought that I would need a compelling argument to upgrade further.
08-13-2010, 12:40 AM   #4
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What? You want to know my resolution????

08-13-2010, 01:48 AM - 1 Like   #5
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I have been in Japan so long that it took me a few seconds to realize that there was something wrong with "Rens Resolution".
08-13-2010, 05:06 AM   #6
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Photozone and SLR gear do these tests so there you are.

However, I find toroughly reading the evaluations of the lenses here in the database is MUCH more informative. Lens resolution charts are all well and nice, but they do not tell the whole story. Bokeh, contrast, colours rendering, sharpness, resistance to flares, distorsion and aberrations, all play a role. Then there's the "pixie dust" some lenses have (like many Limiteds).
08-13-2010, 06:22 AM   #7
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QuoteOriginally posted by bdery Quote
Photozone and SLR gear do these tests so there you are.

However, I find toroughly reading the evaluations of the lenses here in the database is MUCH more informative. Lens resolution charts are all well and nice, but they do not tell the whole story. Bokeh, contrast, colours rendering, sharpness, resistance to flares, distorsion and aberrations, all play a role. Then there's the "pixie dust" some lenses have (like many Limiteds).
Yes, all those things are indeed important, but they are also very subjective. Reading the comments in the lens review section shows that opinions are very varied on most lenses. I was looking for a more objective starting point for making purchasing decisions.

Thanks to all for the links and comments.

Tim
08-13-2010, 06:32 AM   #8
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QuoteOriginally posted by Tim_R Quote
Yes, all those things are indeed important, but they are also very subjective. Reading the comments in the lens review section shows that opinions are very varied on most lenses. I was looking for a more objective starting point for making purchasing decisions.

Thanks to all for the links and comments.

Tim
Opinions vary to some extent on any item, but usually there is a consensus among most of the users.

Photozone, Lenstips, Photodo, Digital Photography Review and a number of other sites on the net provide MTF and other forms of resolution testing (such as shooting targets as Modern Photography used to do). However, I've found that there is almost as much variabllity in these "objective" reviews as there are in the opinions in our review section.

08-13-2010, 07:38 PM   #9
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QuoteOriginally posted by GeneV Quote
However, I've found that there is almost as much variabllity in these "objective" reviews as there are in the opinions in our review section.
There is also the application of Murphy's Law, 3rd Engineering Corollary:
Anything that tests perfectly in the lab will fail in the field.
When I did my research and analysis prior to buying my K20D and lenses, I very carefully charted DPR ratings on bodies and glass -- both the tech reviews, and the user ratings. Any number of products that received very favorable tech ratings were slaughtered by user complaints. Many Canikon complaints were about how quickly the owner wanted to upgrade their system. Discontent was much lower for Pentax stuff, which is why I'm here. But as is said, tech reviews aren't enough -- pay close attention to what owners actually find useful and survivable.
08-18-2010, 02:04 AM   #10
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lens resolution makes a difference, even when its higher than the sensor

QuoteOriginally posted by Tim_R Quote
It is simple to determine the maximum lines per mm that a sensor can resolve. It's a function - directly - of pixel physical spacing. If I knew that my sensor maxed out at x lines per mm, and lens Y had better resolution, I'd know that I could extract all the image resolution possible. But placing that same lens on a body with a different sensor resolution (say only 10MPx, vs 14MPx) would be a waste as sensor Z could not make use of the IQ of lens X.

I hope I'm making myself clear. If I have a collection of lenses that pretty well match the resolving capability of my current sensor, there would appear to be no advantage in purchasing a body with say, 20% or even 50% greater resolution in the same physical sensor size.
As I understand it, the resolution of the system overall is a function of the resolution of each component. Only if the lens and print are "perfect" would you just see the performance of the sensor affecting IQ. The resolution of the system is the square root of the sum of the squares of the other component resolutions.

As an example, for a sensor with 3000 pixels across 25mm, the line pair equivalent is 60lp/mm (half what you might think, due to Nyquist sampling, you need two pixels at least to resolve the difference between a black and white line, but 60 is still pretty high as there are no losses when the image is viewed on the screen, unlike film scanning). This equates to spacing of 0.017mm between line pairs. Now the interesting bit, if you take a lens which can resolve 100lp/mm, its spacing between lines is limited at 0.01mm, more accurate than the sensor. However when you sum the two together and take the inverse square root, you get (0.017+0.01)^-0.5=51lp/mm. The net resolution of the system is LOWER than the sensor, even though the lens resolution of 100lp/mm was HIGHER than the sensor.

Effectively, the errors of both lens and sensor have compounded each other, so it pays to mitigate lack of resolution in each imaging component.

So the short answer is lens resolution makes a difference, even when the lens is "better" than the sensor.

[Prints need only 5-10lp/mm, so if you are enlarging to only 5x7, an enlargement of 7x on digital, you need about 35-70lp/mm.]

Last edited by whojammyflip; 08-18-2010 at 02:14 AM.
08-25-2010, 01:04 PM   #11
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QuoteOriginally posted by hoojammyflip Quote
As I understand it, the resolution of the system overall is a function of the resolution of each component. Only if the lens and print are "perfect" would you just see the performance of the sensor affecting IQ. The resolution of the system is the square root of the sum of the squares of the other component resolutions.

As an example, for a sensor with 3000 pixels across 25mm, the line pair equivalent is 60lp/mm (half what you might think, due to Nyquist sampling, you need two pixels at least to resolve the difference between a black and white line, but 60 is still pretty high as there are no losses when the image is viewed on the screen, unlike film scanning). This equates to spacing of 0.017mm between line pairs. Now the interesting bit, if you take a lens which can resolve 100lp/mm, its spacing between lines is limited at 0.01mm, more accurate than the sensor. However when you sum the two together and take the inverse square root, you get (0.017+0.01)^-0.5=51lp/mm. The net resolution of the system is LOWER than the sensor, even though the lens resolution of 100lp/mm was HIGHER than the sensor.

Effectively, the errors of both lens and sensor have compounded each other, so it pays to mitigate lack of resolution in each imaging component.

So the short answer is lens resolution makes a difference, even when the lens is "better" than the sensor.

[Prints need only 5-10lp/mm, so if you are enlarging to only 5x7, an enlargement of 7x on digital, you need about 35-70lp/mm.]
Not to argue with the overall statement, but the two "formulas" marked in red above, do not say the same thing. Which is accurate? And do you have a link to a source for it? Thanks.

Tim
08-25-2010, 02:44 PM   #12
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Its actually summing the errors squared and taking their square root, from statistics. The error in this case is the inverse of the resolution, so if a resolution of 100lp/mm, the error is 0.01mm. Its related to Pythagorus, in that two orthogonal deviations create the third side of the triangle a^2+b^2=c^2. There could also be cross-correlations between the errors, I guess which would make the thing more complicated. A key parameter in the discussion of resolution is the level of contrast which defines the threshold at which a line pair is satisfactorily resolved. Its a bit arbitrary, I think typically a modulation level of 50%.

In a way, the arithmetic distracts from the key idea, particularly given the definition of satisfactory resolution is a bit arbitrary. The main idea was that degradation of the image quality occurs at every step of the imaging process.

For the source of this idea, I think I saw a reference in an article on the web to a book which is out of print but available second hand on Amazon called Image Clarity by John Williams.
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