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Author Topic: The Megapixels are Coming [CR1]  (Read 30908 times)

Tuggem

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Re: The Megapixels are Coming [CR1]
« Reply #75 on: March 15, 2012, 12:04:13 PM »
I believe you have made this argument before. Lets see if we can end the debate once and for all here with a little bit of math and fact.

I was referring to spatial resolution. The spatial resolution of an 18mp APS-C sensor (3456 lines @ 14.9mm high) is 116lp/mm, where as the spatial resolution of a 22mp APS-C sensor (3820 lines @ 14.9mm high) is 128lp/mm.

Sorry, wrong already.  You're assuming a monochrome sensor with no micro lenses and no AA filter.  All three are wrong.  The real-world with Bayer masks, microlenses, and AA filters mean you have to divide those numbers by something like 1.5.
Quote

Real-world MTF tests on Canon lenses show that rather few of them are even capable of resolving 116lp/mm at around f/4-5.6 (where optical aberrations and diffraction tend to normalize in the average lens), and at f/5.6, the maximum spatial resolution physically possible is 123lp/mm, some 5lp/mm less than what a 22mp APS-C sensor would be capable of.

Sure, you can't get exactly correct with bayer sensors, as they have CFA's and low-pass filters. The numbers I've listed are the theoretical maximums for green pixels, but thats largely besides the point. I'm not arguing 99% vs. 100% accuracy here, I'm just arguing about the the way you seem to abuse the conversion of magnification into megapixels, and the much larger inaccuracies of doing so.

Also wrong.  Those tests were either shot on film that can't resolve better than that or shot on digital through an AA filter.  Thus, they aren't lens tests but system tests.

Also, your diffraction-limit calculation is wrong.  See here:

http://en.wikipedia.org/wiki/Spatial_cutoff_frequency

"As an example, a telescope having an f/6 objective and imaging at 0.55 micrometers has a spatial cutoff frequency of 303 cycles/millimeter."

Thats the absolute maximum for diffraction with an MTF at Dawes Criterion. At that level, a sensor would image nothing but smooth, solid, flat gray. Sensors need a greater separation of airy discs for there to be enough contrast in spatial frequencies to be recorded usefully by the sensor. The human eye is, at best, JUST BARELY able to resolve detail with diffraction at 9% contrast...and that is generally too low for a digital sensor to resolve. Usually, an MTF of around 50% contrast is necessary for film or a digital sensor to resolve useful, unmuddied detail. If you have a subject with particularly low contrast, you might get away with slightly less, but as a general rule, MTF 50% is used to determine lens and sensor resolution in a spatial context.

I usually use a table from Luminous Landscape as a quick reference for spatial resolutions at acceptable contrast levels for photography:
http://www.luminous-landscape.com/tutorials/resolution.shtml

The table there is based on Norman Koren's work. If you have issues with his work, you better take it up with him, as he is highly respected when it comes to lenses, film & sensors, resolving power, sharpness, contrast, etc.

http://www.normankoren.com/Tutorials/MTF6.html

Some of our lenses are now diffraction-limited at f/2.8.  That's 650lp/mm.

LOL! Yes, a PERFECT f/2.8 lens is capable of 649lp/mm...at just infinitesimally above 0% contrast! You wouldn't get any useful detail from such a lens at that level of contrast...other than flat, solid, unbroken consistency of a single tone. The human eye can't even resolve any detail if contrast is less than 9%, and at that level (well below what a bayer CMOS or CCD sensor can resolve) your down to 532lp/mm. Detail is still rather close to monotone at that level of contrast. At 50% contrast, which would be necessary for a sensor to resolve USEFUL detail (i.e. detail where all line pairs are resolved with enough clarity to consistently tell them apart) you are down to 247lp/mm. That is the number most people would normally use when talking about spatial resolution.

As for DSLR lenses that are actually diffraction-limited at f/2.8...there are VERY FEW. Zeiss may have a lens or two that are diffraction limited at around f/1.7...however I believe those were cinema lenses, not DSLR lenses...and highly specialized to boot. MTF's provided by Canon are THEORETICAL most of the time (I believe their book on lens technology may have included a few real MTF charts for some lenses), generated by computing optical performance using computer models of their lenses, not actual lenses. Their MTF charts depict reproduction accuracy of 10lp/mm (for contrast) and 30lp/mm (for sharpness) meridional and sagittal (opposing diagonal) line pairs. Even with a relatively low f/2.8 resolution (such as 50-70lp/mm, which many Canon lenses ARE capable of) you can get very high marks when the most detail you are resolving is 30lp/mm.

Real-world tests of Canon lenses at maximum aperture have NEVER demonstrated resolutions much above 70lp/mm, so its highly doubtful were getting 247lp/mm out of any Canon lenses...let alone 532, or in your rather humorous case, 650! :D

Quote
Magnification and spatial resolution are NOT the same thing. Adding on a 1.4x TC changes magnification, it does not increase spatial resolution.

It does increase system spacial resolution if you are undersampling the optics without it.  That's exactly what we're doing.

If you don't believe me, go outside with any 200mm lens you like attached to a Canon 1.6-crop 18MP sensor with no TCs and see if you can get a picture of Jupiter that looks like this:
http://photos.imageevent.com/sipphoto/samplepictures/T2i__3105%20old.jpg
Or a picture of the moon that looks like this:
http://photos.imageevent.com/sipphoto/samplepictures/T2i__3054%20edited.jpg

Your still talking about magnification, not spatial resolution. You can magnify a subject and still project it through a lens at THE SAME spatial resolution. Magnification and resolution are disjoint concepts, and as such, they can vary independently of each other. You can magnify a subject to a greater extent while also reducing spatial resolution..and you will see greater apparent detail of your larger subject...even though your actual resolving power is lower. (This is normally the case to a small degree when tacking on teleconverters...the additional optical elements each have their own optical aberrations that reduce resolving power...simple matter of physics there until you stop down to smaller apertures...where in the longer focal length results in a smaller effective aperture, so diffraction takes a much larger toll than without a TC.) Its like moving closer to a highly detailed 600ppi print. If you view it at 8 feet, it looks nice as a whole (i.e. looking at the moon with a 100mm lens), however walk closer to 4 feet, and you can see finer details (i.e. looking at the moon with a 200mm lens). Just because you walked closer to the print doesn't mean your eyes are magically capable of resolving more detail, either optically or via your retina...both remain exactly the same as they are...the subject is simply larger, so given a CONSTANT spatial resolution, more detail can be observed.

This is all pretty basic physics. I recommend reading Norman Koren's work...solid stuff, should clear things up.

Interesting information
http://forums.dpreview.com/forums/read.asp?forum=1019&message=35719448

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Re: The Megapixels are Coming [CR1]
« Reply #75 on: March 15, 2012, 12:04:13 PM »

jrista

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Re: The Megapixels are Coming [CR1]
« Reply #76 on: March 15, 2012, 12:34:15 PM »
Interesting information
http://forums.dpreview.com/forums/read.asp?forum=1019&message=35719448

If you are referring to the discussion about how the 50/1.4 is "sharp" in the corners of a 5D II, thats not surprising. The 5D II, given its pixel pitch, only resolves about 72lp/mm (not accounting for things like the low-pass filter, which probably push that down to 70lp/mm or so). According to DXO's wide-aperture lens tests, the Sigma 50mm f/1.4 resolves 60lp/mm at f/1.4 on a 5D II (and at most 63lp/mm in their best test case.) It is not surprising that, at that level of resolving power, the Sigma 50/1.4 appears sharp (its resolving just about as much as the sensor can handle.) In the grand scheme of things, 60lp/mm is only 12% the resolution that a perfect f/1.4 lens is capable of resolving...that should give you an idea of how much potential room for improvement there can be for lenses at very wide apertures (and how hard it really is to achieve perfection in the face of overpowering optical aberrations.)

As for the comment about 300mp in FF to show pixel softness center-frame...I'm rather skeptical of that. I'll have to see if I can find a real-world MTF that actually indicates center-frame sharpness is that high for that lens. If it was possible, it would reach that only at the VERY CENTER pinpoint of the lens, and it would likely fall off rapidly from there due to optical aberrations. Its highly unlikely a 286mp APS-C sensor would be sharp at a pixel-level...the pinpoint center lens resolution is only the absolute maximum, and the bulk of a 286mp APS-C photo would...when pixel peeping...appear extremely soft. Measuring resolution at the center area of the lens is more useful...and tends to be between 50 and 70 line pairs/millimeter for most wide fast lenses. When you move into telephoto territory, where the widest apertures are f/2.8 and supertelephoto territory where the widest apertures are f/4 or f/5.6, diffraction limits your resolution from the get-go, even though its easier to get closer to perfection with such lenses. Assuming you did have a "perfect" f/4 lens, you would only realize that much resolution at a single aperture. Stop down to f/5.6 and you can't resolve more than 22mp APS-C worth, and anything beyond that current sensors are already outresolving lenses.

Kahuna

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Re: The Megapixels are Coming [CR1]
« Reply #77 on: March 15, 2012, 12:52:33 PM »
I wonder what life would be like if Steve Jobs was involved with DLSR's?
then we wouldn't be able to save photos, only view them on the icloud at $1 per view

Ouch.  Just came crashing back down to earth. I'll think next time before I post something so stupid next time.
 
You are correct sir.

Tuggem

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Re: The Megapixels are Coming [CR1]
« Reply #78 on: March 15, 2012, 02:04:56 PM »
Stop down to f/5.6 and you can't resolve more than 22mp APS-C worth, and anything beyond that current sensors are already outresolving lenses.

Interesting information
http://forums.dpreview.com/forums/read.asp?forum=1019&message=33770775

KitH

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Re: The Megapixels are Coming [CR1]
« Reply #79 on: March 15, 2012, 02:41:18 PM »
I wonder what life would be like if Steve Jobs was involved with DLSR's?
then we wouldn't be able to save photos, only view them on the icloud at $1 per view

Ouch.  Just came crashing back down to earth. I'll think next time before I post something so stupid next time.
 
You are correct sir.


Don't worry,  I enjoyed that one anyway.   I was thinking he's probably too caught up arguing with St Peter about the Pearly Gates having rounded corners to have much time to take pictures.   He'll have to make the case by himself, all the lawyers are in the other place.



Lee Jay

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Re: The Megapixels are Coming [CR1]
« Reply #80 on: March 15, 2012, 05:47:48 PM »
I'm not arguing 99% vs. 100% accuracy here,

I was talking about more like 67% versus 100%, which is more like a factor of 2 in pixel count.  You're ignoring that.

Quote
The table there is based on Norman Koren's work. If you have issues with his work, you better take it up with him, as he is highly respected when it comes to lenses, film & sensors, resolving power, sharpness, contrast, etc.

I've read, created spreadsheets from, and quoted Norman's work many times.  What I'm saying is entirely consistent with that work.

Quote

Real-world tests of Canon lenses at maximum aperture have NEVER demonstrated resolutions much above 70lp/mm,

Shot through an Optical Low-Pass Filter!  Do you not see the difficulty in that approach?

Quote
Magnification and spatial resolution are NOT the same thing. Adding on a 1.4x TC changes magnification, it does not increase spatial resolution.

It does increase system spacial resolution if you are undersampling the optics without it.  That's exactly what we're doing.

If you don't believe me, go outside with any 200mm lens you like attached to a Canon 1.6-crop 18MP sensor with no TCs and see if you can get a picture of Jupiter that looks like this:
http://photos.imageevent.com/sipphoto/samplepictures/T2i__3105%20old.jpg
Or a picture of the moon that looks like this:
http://photos.imageevent.com/sipphoto/samplepictures/T2i__3054%20edited.jpg
Quote

Your still talking about magnification, not spatial resolution.


Fine - show me the images with the same spacial resolution as those shot the way I said.  I'll help you out - you can't.  I've already done this experiment, and the teleconverters do indeed drastically improve the overall system spacial resolution despite very slightly decreasing the optical resolution.  This is exactly why we need more pixels, and a whole lot more - so we aren't undersampling the optics in the first place.

Have you ever wondered why the best amateur planetary imagers operate pixels that are about the size of those on the 40D through optics set at f/30?  According to you, they're way, way beyond the capability of those optics, yet they increased focal length to that level in an effort to preserve maximum detail.  Why would they use expensive barlows (Televue Powermates) if those small pixels were extracting all the detail from their bare f/11 optics in the first place?  Answer - they don't.  And that's with monochrome sensors with no OLPFs!!!

Have a look.  This was shot at about f/30 with pixels that are about 40D sized:

http://damianpeach.com/barbados10/2010_09_12pic.jpg

wickidwombat

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Re: The Megapixels are Coming [CR1]
« Reply #81 on: March 15, 2012, 07:04:54 PM »
I wonder what life would be like if Steve Jobs was involved with DLSR's?
then we wouldn't be able to save photos, only view them on the icloud at $1 per view

Ouch.  Just came crashing back down to earth. I'll think next time before I post something so stupid next time.
 
You are correct sir.


Don't worry,  I enjoyed that one anyway.   I was thinking he's probably too caught up arguing with St Peter about the Pearly Gates having rounded corners to have much time to take pictures.   He'll have to make the case by himself, all the lawyers are in the other place.

LOL
Actually i think you'll find the pearly gates are now a svelte brushed aluminium with a cool white glowing logo.
and st peter now wears a blue tshirt that says "genius"
« Last Edit: March 15, 2012, 07:06:38 PM by wickidwombat »
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Re: The Megapixels are Coming [CR1]
« Reply #81 on: March 15, 2012, 07:04:54 PM »

jrista

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Re: The Megapixels are Coming [CR1]
« Reply #82 on: March 15, 2012, 09:18:37 PM »
Fine - show me the images with the same spacial resolution as those shot the way I said.  I'll help you out - you can't.  I've already done this experiment, and the teleconverters do indeed drastically improve the overall system spacial resolution despite very slightly decreasing the optical resolution.  This is exactly why we need more pixels, and a whole lot more - so we aren't undersampling the optics in the first place.

Have you ever wondered why the best amateur planetary imagers operate pixels that are about the size of those on the 40D through optics set at f/30?  According to you, they're way, way beyond the capability of those optics, yet they increased focal length to that level in an effort to preserve maximum detail.  Why would they use expensive barlows (Televue Powermates) if those small pixels were extracting all the detail from their bare f/11 optics in the first place?  Answer - they don't.  And that's with monochrome sensors with no OLPFs!!!

Have a look.  This was shot at about f/30 with pixels that are about 40D sized:

http://damianpeach.com/barbados10/2010_09_12pic.jpg

Ok, this is my last attempt. Words are certainly insufficient, so hopefully some visual demonstrations will clear things up. Some facts:

1. Diffraction limits resolution at narrow apertures
2. Optical aberrations limit resolution at wide apertures
3. The more lens elements, the more optical aberrations introduced
4. The longer the focal length for a fixed physical aperture, the smaller the relative aperture (i.e. add TC's)

Lets assume we have a hypothetical 200mm lens capable of producing a 1"x1" image circle. Lets assume lens is capable of1.97lp/mm in terms of spatial frequency of the virtual image at the sensor, would roughly translate into a 50x50 "pixel" area within which our subject is resolved. Lets assume spatial resolution is not impacted by the addition of teleconverters. Lets assume our sensor resolution is infinite in the context of this discussion, so we don't have to factor in its effects on resolution. We are JUST talking lens resolution in this case.

Our subject is a small moon.

At 200mm without TC's, the moon is 14 "pixels" in size in the center of our frame. If we slap on a 2x TC and a 1.4x TC, our subject grows to 44 "pixels" in size, nearly filling the frame. Our SPATIAL RESOLUTION is CONSTANT, however we are suddenly able to observe FAR GREATER detail in our subject. If we reduce our spatial resolution by 50%, the more magnified subject IS STILL MORE DETAILED than the original, unmagnified subject (an exaggerated example of the effect of stacking on multiple TC's...which at the very least are going to increase diffraction and therefor reduce spatial resolution.)

This effect can be seen below in this simple animated gif (frame 2, unmagnified; frame 3, magnified same spatial resolution; frame 4, magnified w/ 50% less spatial resolution). Note, I've purposely kept resolution the same or lower do demonstrate the effect of, say, magnifying Jupiter such that it fills the frame (rather than being a small dot in the center of a largely empty frame) without changing spatial resolution:



Two TC's are added to a lens increasing magnification, spatial resolution remains constant, yet we are capable of "seeing" more detail in our much larger subject, even at a LOWER spatial resolution. Magnification and spatial resolution are not the same. Magnification and spatial resolution are disjoint concepts that can vary independently. Increasing magnification by adding teleconverters, while keeping spatial resolution constant, DOES increase the apparent detail we are capable of observing...because OUR SUBJECT IS LARGER RELATIVE TO THE FRAME FOR A GIVEN RESOLUTION.

Well, thats the best I can do. If a small animated picture isn't worth 4000 words, then no amount of proof in this case will sway your opinion. I do indeed believe science backs up what I've said here.

Fandongo

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Re: The Megapixels are Coming [CR1]
« Reply #83 on: March 16, 2012, 02:54:20 AM »
They need to hurry with the 4K C-DSLR. Sony is kicking Canon's A$$ now in video and a LOT of people have switched to the Sony FS-100, especially now that the Metabones adaptor allows full electronic interface with the FS-100 and Canon lenses. Canon could have owned that market, but they slept on it, so Sony took advantage and came out on top.

The 5D3 is shaping up to be a real disappointment in video since it's still has the soft, up-res'd false HD (at least in the early test models). So yeah, the 4K C-DSLR really needs to come out. And have a FLIP SCREEN for God sake. We need it for video.

If it's not soon, I'm going to have to buy the Sony FS-100. I'm not thrilled about it since I now hate shooting with the video camera form, but I need a better picture.

If the 4K DSLR came out at NAB in April, I would buy it the next day even it it is $6000. I need a better video solution and I cannot wait much longer.

And NO, that is not going to be the high mega-pixel camera from Canon. Video requires low mega-pixels because then the processor has less work to do to down-scale the image into a 2K output (or in this case a 2K to 4K) output.

I can't imagine the 4k DSLR coming out before Winter for one reason:
Summer is the only time you can shoot (without actors whining/dying) in much of the world.

I'm expecting around $6k for 4k as well, but I don't see it happening...
unless they offer $13,000 rebates on the stupid C300.

Don't buy the FS-100.
Buy 10 Gh2s instead.

Nobody can wait much longer for a solution, especially Canon.  The ONLY reason the 5Dii broke every camera sales record ever was video.  "Video cameras" were poised to destroy film, but they forgot to make them anything like film.

DSLRs swooped up everyone looking for the "film look" for not the price of a car ("video" cams) and not the price of a house (film).
But it's only a matter of days before phones are shooting 4k, global shutter.

We'll be rotobrush/gaussian blurring phone footage to look like the DSLR that never was.
Hollywood is a perpetually dazed Raiden.
FINISH HIM!!

=)
"There is no good and evil. There is only power, and those too weak to seek it."

Lee Jay

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Re: The Megapixels are Coming [CR1]
« Reply #84 on: March 16, 2012, 10:00:05 AM »
Fact is that you will get to better result by doubling the number of pixels than using 1.4x converter and 4-doubling the number of pixels will be better than 2x converter. Only if the converters were ideal they could compete with increasing the number of pixels.

Yup...100% right!  Fortunately, TCs are pretty good.

Lee Jay

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Re: The Megapixels are Coming [CR1]
« Reply #85 on: March 16, 2012, 10:10:45 AM »
Well, thats the best I can do. If a small animated picture isn't worth 4000 words, then no amount of proof in this case will sway your opinion. I do indeed believe science backs up what I've said here.

Believe me, I understand the science here quite well.  You just aren't reading what I wrote.

"It (the addition of TCs) does increase system spacial resolution if you are undersampling the optics without it.  That's exactly what we're doing."

Note the word "system", meaning lens + sensor + processing.  Ideal TCs can only preserve optical resolution (as you seem fond of pointing out, regardless of the fact that this point is not in dispute), not increase it, but they can increase system resolution if the sensor is undersampling the optics without them.  Since we can extract more real detail with TCs than without them even on our exiting 1.6-crop 18MP sensors, we are obviously undersampling the optics without the TCs.  The question is, by how much?  And the answer is, A LOT.  I can easily demonstrate that we are undersampling by at least a factor of 4 in pixel count, and others have shown up to a factor of 16 on the best lenses.  So the idea that 18MP (or 20, or 22 or whatever) is all we'll ever need to squeeze everything out of the best optics is just not even close to correct.

jrista

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Re: The Megapixels are Coming [CR1]
« Reply #86 on: March 16, 2012, 11:59:44 AM »
Well, thats the best I can do. If a small animated picture isn't worth 4000 words, then no amount of proof in this case will sway your opinion. I do indeed believe science backs up what I've said here.

Believe me, I understand the science here quite well.  You just aren't reading what I wrote.

"It (the addition of TCs) does increase system spacial resolution if you are undersampling the optics without it.  That's exactly what we're doing."

Note the word "system", meaning lens + sensor + processing.  Ideal TCs can only preserve optical resolution (as you seem fond of pointing out, regardless of the fact that this point is not in dispute), not increase it, but they can increase system resolution if the sensor is undersampling the optics without them.  Since we can extract more real detail with TCs than without them even on our exiting 1.6-crop 18MP sensors, we are obviously undersampling the optics without the TCs.  The question is, by how much?  And the answer is, A LOT.  I can easily demonstrate that we are undersampling by at least a factor of 4 in pixel count, and others have shown up to a factor of 16 on the best lenses.  So the idea that 18MP (or 20, or 22 or whatever) is all we'll ever need to squeeze everything out of the best optics is just not even close to correct.

Here is some reality:

Fact: A Canon 5D III full-frame sensor resolves (at best) 80lp/mm.
Fact: A "perfect" 200mm f/2 lens @ f/8 is physically capable of resolving an absolute maximum of 86lp/mm.
Fact: Two "perfect" 1.4x TCs attached to that perfect 200mm lens reduces f/8 to f/22, limiting the physically possible absolute maximum resolution to 31lp/mm.

Fact: At apertures of f/9 or narrower, even the least-dense sensors of today outresolve lenses.
Fact: Artifacts caused by waveform interference, such as moire, could be minimized or eliminated by increasing sensor resolution up to 2x beyond lens resolution, however sharpness and contrast will not increase (and likely decrease relative to 1x sampling at 100% crop.)
Fact: A sensor with 2x lens resolution will produce photos that, when viewed at 100% crop, appear very soft.
Fact: Increasing sensor resolution beyond 2x lens resolution produces minimal or imperceptible returns at increasingly disproportionate cost.

Fact: An oversampled photo at 80lp/mm would have to be reduced to an image size with equivalent resolution to 31lp/mm in order to restore original sharpness that was lost to diffraction and oversampling.

FACT: A "perfect" 560mm lens achieved via combining a 200mm f/2 with two 1.4x TCs shot at an aperture of f/8 (which is an effective aperture of f/22) can resolve at best 31lp/mm, and a sensor capable of recording 80lp/mm is capable of resolving every last scrap of detail from that lens...and then some. Total system resolution is 31lp/mm of (sharp) actual resolution when the final output is downsampled to the lens native resolution, or 62lp/mm of (soft) oversampled resolution without artifacts when left at native camera resolution.
« Last Edit: March 16, 2012, 12:01:52 PM by jrista »

ebrakus

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Re: The Megapixels are Coming [CR1]
« Reply #87 on: March 16, 2012, 05:52:24 PM »
Lee Jay, you were THIS close to having it right ;D

Lets go back to the question you asked..... Why does the mag seem to make the image come out better.

You guys contest if the lens or sensor is the limiting factor in this system.   For sake of argument and example, lets assume the lens has just a bit of headroom left above the sensor.

In that case the magnification will improve (here is the important part) the resolution of your OBJECT OF INTEREST.
It will do this wile simultaneously reducing your system resolution through additional artifact producing elements in LOS.
The (reduced) system resolution is better applied to your magnified object.

Now if we blow away the starting premise, that the lens has some headroom over the sensor, the mag doesn't do a bloody thing for you, it just gives your more precision of less accuracy.

Regarding sensor or lens in this case?  Sounds to me like both your guys are operating in the magin of error.

Great posts though, plenty of people reading them would have found the thread to be a good mini-tutorial.

cheers
-E



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Re: The Megapixels are Coming [CR1]
« Reply #87 on: March 16, 2012, 05:52:24 PM »

jrista

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Re: The Megapixels are Coming [CR1]
« Reply #88 on: March 16, 2012, 07:04:44 PM »
@ebrakus: Good way to explain it with "resolution of the OBJECT OF INTEREST". ;) Like that.

@those-still-interested:

For reference, here is a little bit of math on "system" resolution. Every element of a system has its own independent resolution that combine to create the final outcome system resolution. System resolution is never as high as the independent resolutions of each component. Converting element resolution into a "blur circle" gives you the finest size "dot" that can be resolved with that element, regardless of the actual size of the source dot that is being resolved. The total resolution of a system is the square root of the sum of the squares of each elements blur circle. In other words:

Code: [Select]
totalBlur = sqrt(blur1^2 + blur2^2 + ... + blurN^2)
Total blur increases as you add elements to the system, based on the formula above. Assuming we have a lens @ f/4 and a sensor that both produce a 5.3 micron blur circle. Total system blur is greater than 5.3 microns:

Code: [Select]
sqrt(5.3um^2 + 5.3um^2) = 7.5um
Add a teleconverter that produces the same 5.3 micron blur, and your total system blur increases:

Code: [Select]
sqrt(5.3um^2 + 5.3um^2 + 5.3um^2) = 9.2um
Given that there are 1000 um/mm, and twice the airy disc diameter produces a line pair for MTF 50% (um/lp), we can calculate spatial resolution in lp/mm as such:

Code: [Select]
lp/mm = 1000/(totalBlur * 2)
With our base lens + sensor system, spatial resolution at f/4 would be:

Code: [Select]
1000/(7.5 * 2) = 1000/15 = 66.665 lp/mm
When we stack on the TC (lets say 2x), we either lose resolution both due to the additional lens element and narrower aperture (since our focal length increased), or we lose resolution both due to the additional lens element and the need to use a wider aperture. Assuming we actually have a perfect lens and used a wider aperture, we would then only lose resolution due to the additional lens element. We can demonstrate the first and last cases (as computing lens aberrations is a lot more complex than diffraction). If we leave the aperture as-is, the blur circle for both elements increase to 10.65um:

Code: [Select]
sqrt(10.65um^2 + 5.3um^2 + 10.65um^2) = 16um (airy disc size)
1000/(16 * 2) = 1000/32 = 31.25 lp/mm

Adding a single 2x TC has cost us over 50% resolution when keeping the aperture setting the same. If we widen the aperture by 2x to compensate for the TC, assuming perfect optics:

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1000/(9.2 * 2) = 1000/18.4 = 54.35 lp/mm
Even with perfect optics, adding on a TC and increasing the aperture has cost over 18% resolution.

Tuggem

  • Guest
Re: The Megapixels are Coming [CR1]
« Reply #89 on: March 16, 2012, 07:33:45 PM »
Actually Lee Jay is the one who is more correct here.
The system resolution of the object will be the same with an ideal 1.4x as with doubling the number of pixels. If you can gain object resolution with TC you can do the same by increasing the number of pixels.

It has also been shown that 7D with 18MP APS-C doesn't fully oversample a lens at f22 but some more MP is needed as according to the link above.

Lens resolution and sensor resolution are just two completely different things.


canon rumors FORUM

Re: The Megapixels are Coming [CR1]
« Reply #89 on: March 16, 2012, 07:33:45 PM »