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Equipment Discussions >> Cats & Casses

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Ed Holland
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Reged: 06/16/10

Loc: San Jose, CA and Oxford, UK
Re: CO vs. All That "Other Stuff"! new [Re: DesertRat]
      #5612418 - 01/08/13 06:07 PM

Another excellent thread, with more good theory than I have time to absorb in full at the moment.

Practically speaking, from my experience as a casual observer with MCT, SCT and achromatic refractor, I wish an instrument's intrinsic limitations were the only obstacle to a good view. Usually seeing (high level and local) is the most significant barrier to obtaining a detailed view. I've been careful to accumulate each of my scopes on a modest budget. If one keeps that factor in mind, the views through a sub $300 Maksutov or circa $400 C8 are astounding value, and show me far more than the 6" APO that I don't and won't own. They are also easy to mount for visual use. That these instruments are often considered as a "runner up" is a shame - a well tuned catadioptric scope with good optics can be a powerful tool. Rather like my MGB providing an excellent and fun drive, despite it not being an E Type Jaguar...

Of course, CATs do require attention to detail - for starters, the importance of good collimation is hard to over emphasize. That last "whisker" of adjustment can make all the difference, yet the mechanisms aren't so user-friendly to the newcomer. Furthermore, based on experience with both my CATs I note that mechanical assembly can fall short of what is needed to get the best out of essentially blameless optics. Tilted baffle tubes & primary mirrors, off centre secondarys and the like can really spoil the show. However, many users will be unable to recognise, or be unwilling to investigate & tackle these problems. I'd love to know how many good sets of optics have been compromised by non optimum assembly - could this have contributed to some of the received wisdom regarding CAT performance? As the writer and broadcaster Sir Patrick Moore would have put it "well, we just don't know", but I have my suspicions.

What I do know is that when I got my C8 properly "sorted" to the best of my abilities - ensuring that everything was coaxial, squared-on and collimated - I was amazed by its planetary performance. This was quite different to its "as received" capability, despite seemingly succesful attempts to collimate with the secondary mirror screws. My Orion 127 Maksutov also benefitted from similar treatment, if not to the same degree.

I've rambled on long enough, perhaps too much, but will finish in saying that good execution of a design is as important as a good design.


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Asbytec
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Re: CO vs. All That "Other Stuff"! new [Re: Ed Holland]
      #5612926 - 01/08/13 11:29 PM

Yea, this is an interesting thread, especially the discussion between Glen and Frank. A lot of that is difficult to grasp, at least the importance of it. Yes, all aberrations such as seeing, collimation, cooling, and everything have to be absolutely minimal if not perfect to squeeze that bit of performance from a scope. And any amount of de-focus from best diffraction focus counts, too.

What's interesting to me is the idea an obstructed scope has a smaller Airy disc that is a bit brighter than it would normally be if it's intensity were spread over 1.22 Lambda/D. This, in my understanding, is what differentiates CO diffraction and spherical aberration. This is what causes an obstructed scope to exceed a perfect unobstructed aperture on very small scales (near the radius of the Airy disc.) And it is what allows, under very minimal induced aberrations, to actually exceed Raleigh and Dawes limits by about half an arc second.

The peak intensity is a bit higher for an obstructed scope with good correction than with one with 1/4 wave SA. SO, yes, a scope with an aberrant Strehl of 0.95 can be very good while the CO reduces peak intensity (not aberrant Strehl) further to about 80% (a peak intensity comparable the Raleigh limit) or less depending on the combined affects of both the wavefront error and added diffraction.

The scale of these effects seem to be over rated for general viewing because we're talking scales to about 4x the Raleigh limit. But, for critical viewing, they can be important. On Airy disc scales, this is where obstructed scopes rule. On slightly larger scales is where unobstructed scopes rule with peak intensity (nominal, system Strehl) equal to their aberrant Strehl. CATs do not enjoy this trait.

But, we're focused on the CO and not the "other stuff" such as veiling glare and scatter in CAT optics (including reflectors), which happen to be obstructed.


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siriusandthepup
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Re: CO vs. All That "Other Stuff"! new [Re: Mike Harvey]
      #5613042 - 01/09/13 01:16 AM

I have pretty much the same opinion as Mike on the SCT's.

Optical quality rules and is THE most important factor.

Now I've heard the argument about the 1/4 wave derating for the 30% CO and it's overblown. Let's discuss it. In the case of the 10" SC with 30% obstruction, is it as good as a 10" perfect APO? No. Obstructed scopes will always take a performance hit for the obstruction. Do you really want to compare it to a 10" APO? Get real! A 10" SC with a 30% CO will only perform about as well as a 7" perfect APO. I can live with that...


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freestar8n
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Re: CO vs. All That "Other Stuff"! new [Re: Asbytec]
      #5613087 - 01/09/13 02:21 AM

Hi Norme-

I haven't been talking specifically about CO stuff but more to encourage formal terminology in these discussions, and references to optics textbooks. The nomenclature has been honed over centuries to avoid some of the ambiguities that come up in these discussions. I think there is over confidence in going by a few metrics of a system performance, when things like Strehl and MTF are meant more as guides and figures of merit rather than hard performance metrics that allow systems to be ranked against each other.

When someone looks at an object in an eyepiece, the resulting image involves the object itself, the optics, the perceptual process, and the personal preference for choice of focus. Much of that is not captured in fixed metrics of the optical system alone, but it's all playing a role.

I am always amazed at how different jupiter looks at different powers in the same telescope. If I put a big image of jupiter on the wall and view it from close or far, I don't see much change in detail - but with different eyepieces and magnfications, the colors and spatial frequencies all have different impact - and I always adjust focus to bring out optimal (for me) detail. You could say that at low power I bias the focus so that MTF emphasizes the mid-range, since mid-range is higher frequency at low power; and at high power I bias it toward the high frequency part. But either way - there is a lot going on just in changing powers with a given telescope - let alone comparing two with different CO's. And in terms of the MTF or Strehl of the OTA itself at "best" focus - nothing should be changing at all.

Frank


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Jon Isaacs
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Re: CO vs. All That "Other Stuff"! new [Re: Asbytec]
      #5613159 - 01/09/13 05:35 AM

Quote:

What's interesting to me is the idea an obstructed scope has a smaller Airy disc that is a bit brighter than it would normally be if it's intensity were spread over 1.22 Lambda/D. This, in my understanding, is what differentiates CO diffraction and spherical aberration. This is what causes an obstructed scope to exceed a perfect unobstructed aperture on very small scales (near the radius of the Airy disc.) And it is what allows, under very minimal induced aberrations, to actually exceed Raleigh and Dawes limits by about half an arc second.




It is my understanding that the central disk is slightly smaller than it would appear in scope without a CO because there is more energy in the rings and less energy in the central disk. One thing to remember is that hidden inside every scope without a central obstruction is a scope with any sized central obstruction one desires.

Jon


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Asbytec
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Reged: 08/08/07

Loc: La Union, PI
Re: CO vs. All That "Other Stuff"! new [Re: Jon Isaacs]
      #5613183 - 01/09/13 06:20 AM

Jon, that was my impression, as well, the visible spurious disc being smaller due to evacuated energy. Turns out, the actual Airy disc, itself, is a bit brighter than it would otherwise be. This is because it, indeed, is smaller in an obstructed scope, as I read it (link below.)

There is more energy in the rings, but the added diffraction changes the radius of the first minimum a tiny bit from 1.22 to 1.11 for CO = 0.3D. The resulting Airy disc is both smaller and a bit brighter that it would be at 1.22 Lambda/D. This is what gives that kick to the MTF curve beyond the performance of the unobstructed aperture at very high frequencies: a smaller Airy disc.

"The reason is the effect unique to CO (at least in its extent), namely, the reduction in size of the Airy disc caused by it."

http://www.telescope-optics.net/obstruction.htm


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DesertRat
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Re: CO vs. All That "Other Stuff"! new [Re: Asbytec]
      #5613214 - 01/09/13 07:27 AM Attachment (18 downloads)

Here is an animation of a PSF with obstructions of 0, 0.33 and 0.5. It hints at what you are referring to. Its difficult to display the visual appearance well, I just used a gamma of 2.0 to display the rings a little better. The visual or eyeball response is actually quite complicated being nonlinear as well as color dependent (and several other issues).

Glenn


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Asbytec
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Re: CO vs. All That "Other Stuff"! new [Re: DesertRat]
      #5613365 - 01/09/13 09:19 AM

Thanks, Glenn.

Frank, I reduced the size of my obstruction by 10mm (from 52/140 - effective aperture to 42/150 at full aperture.) The most immediate improvement seen was on the visible rings and reduced diffraction effects on the moon. It took weeks or more to begin to realize Jupiter might actually be better, as well. Planetary improvement was just not instantly obvious.

As you make clear, observing is complicated. I do not know how to bias focus for mid range contrast - maybe smallest blur? If that happens, it does so by accident because it looks like best focus. I just focus at whatever power so the image is crisp or stars show best focus patterns.

Of course, you are correct. Models like MTF, while valid on a scientific level, are never complete. They are meant to illustrate concepts and it is up to us to explore them in all their complexity. They allow us to explore or compare one or two variable against two samples and make general statements. Generally we can say things like, all things held constant or negligible, this should do that. Throw in some seeing and it all goes to pot because we've changed the initial conditions.

Edited by Asbytec (01/09/13 09:41 AM)


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maknewtnut
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Reged: 10/08/06

Re: CO vs. All That "Other Stuff"! new [Re: Mike Harvey]
      #5613501 - 01/09/13 11:00 AM

I believe many who frequently calculate area (often in comparison of obstructed vs unobstructed systems) are often incorrect as well. It's often seen that CO area is subtracted from apeture, and then resolution determined based on the result.

It seems to be a common mistake when attempting to do the same with a Maksutov. For example...since the radius of curvature on a Mak corrector is rather 'steep', we must remember that such a lens diverges incoming light. As a result, even axial rays might not be blocked even when a secondary baffle is larger than the secondary mirror (incuding those that are tapered just for that reason).


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Darren Drake
Carpal Tunnel
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Reged: 10/09/02

Loc: Chicagoland
Re: CO vs. All That "Other Stuff"! new [Re: maknewtnut]
      #5613732 - 01/09/13 01:29 PM

Wow there certainly is a lot of info here in this thread. I would like to see a real world comparison to see if reality matches theory. Is someone out there with a premium apo and an identical aperture mak or good sct able to place an artificial obstruction of the same % in front of the apo lens and do a detailed comparison? This would be an apples to apples comparison and reveal if the cass is optically on par with the apo in every way except obstruction. I may try this myself sometime. I have a good buddy who just had first light last night with a newly acquired 6 inch AP superplanetary and I was there and may go back again tonight.

Edited by Darren Drake (01/09/13 02:54 PM)


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Joe Cepleur
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Reged: 03/18/10

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Re: CO vs. All That "Other Stuff"! new [Re: Ed Holland]
      #5613778 - 01/09/13 01:58 PM

I love reading about this sort of thing, and I try to learn what I can. The subject is so complicated and my budget so limited, that I'm glad Ed's theory is an ultimate truth for me:

Quote:

Practically speaking, from my experience as a casual observer with MCT, SCT and achromatic refractor, I wish an instrument's intrinsic limitations were the only obstacle to a good view. Usually seeing (high level and local) is the most significant barrier to obtaining a detailed view. I've been careful to accumulate each of my scopes on a modest budget. If one keeps that factor in mind, the views through a sub $300 Maksutov or circa $400 C8 are astounding value, and show me far more than the 6" APO that I don't and won't own. They are also easy to mount for visual use. That these instruments are often considered as a "runner up" is a shame - a well tuned catadioptric scope with good optics can be a powerful tool. Rather like my MGB providing an excellent and fun drive, despite it not being an E Type Jaguar...




That said, another thread describes Orion61's refurbishing of my formerly salt-encrusted C8. We know the optics are not perfect, because we mixed one scope's good mirrors with another scope's good corrector. Still, I'm betting that because the parts were made with good precision to begin with and, more importantly, Larry will align all the parts as perfectly as is humanly possible, the scope should work pretty well, at least at lower powers. That's a good deal if one enjoys saving classic scopes, and a clear illustration of the importance of assembling everything precisely. He's not finished yet. Stay tuned! Low budget, 30% obstructed, classic C8 coming back to life!

My sense of obstructions is that, in exchange for a complicated set of problems, one buys also the offsetting larger aperture and smaller tube. Tremendous resolution there. Not quite the same brightness as an unobstructed scope, but it's easy to make that up in increased aperture. Whatever problems with contrast similarly vanish into the larger aperture (if one's taste says they do!), and all this requires a smaller mount to keep it steady. This makes astronomy affordable and portable. A 6" refractor is a wonder, if one can afford one, but is it really radically better than a much larger obstructed scope? I'm not trying to start flame wars. I'm only commenting that I can't afford to answer that question, and am happy with an old C8!


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DesertRat
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Reged: 06/18/06

Loc: Valley of the Sun
Re: CO vs. All That "Other Stuff"! new [Re: Asbytec]
      #5613907 - 01/09/13 03:12 PM Attachment (13 downloads)

Norme wrote:
Quote:

I do not know how to bias focus for mid range contrast - maybe smallest blur?




For a good scope, either sign of defocus drops contrast all along the MTF but especially at the mid range.

For a scope with spherical aberration however, you can defocus a small amount to tease details at the high end. But when that is done the mid level details suffer badly. An example: for a scope with spherical undercorrection a small amount of defocus (a fraction of a wave) inside will enhance details at the limit of resolution. See attached MTF.

Glenn

Edited by DesertRat (01/09/13 03:47 PM)


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GlennLeDrew
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Re: CO vs. All That "Other Stuff"! new [Re: DesertRat]
      #5614476 - 01/09/13 08:41 PM

Frank,
Your noting the difference between looking at a picture of Jupiter on a wall as you vary your viewing distance vs its telescopic appearance while varying the magnification is not surprising. In the first case, your eye's pupil is (more or less) constant, and so both diffractive effects and surface brightness do not vary. But at the eyepiece, changing magnification profoundly affects the visible extent/impact of diffraction/aberrations, as well as the image surface brightness (not to mention atmospheric seeing.)

A better comparison would result if:

- The picture was printed at low resolution so that at the distance from which it subtends an angle of several degrees it reveals the degradation, and,
- At each halving of viewing distance the illumination source intensity is reduced to 1/4.


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Asbytec
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Re: CO vs. All That "Other Stuff"! new [Re: GlennLeDrew]
      #5614655 - 01/09/13 10:52 PM

Glenn, that's interesting. So, -defocus is toward marginal focus? So, your finding best focus. Then +defocus is away from marginal focus toward paraxial focus finding the smallest blur? (or do I have the sign reversed?)

Gotta pay attention to that, I wonder if one does so automatically depending on the magnification or the target.

Edited by Asbytec (01/09/13 10:53 PM)


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DesertRat
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Re: CO vs. All That "Other Stuff"! new [Re: Asbytec]
      #5614786 - 01/10/13 12:52 AM

Norme, thanks!

The signs you have are correct for spherical undercorrection. They would swap places (as do the paraxial and marginal points) in the case of overcorrection.

In a scope with very good correction any defocus is going to degrade contrast transfer all across the board. So if you have a good scope as it seems from your reports, I'd be surprised if you could see this effect. In really good seeing while imaging I can see defocus errors of as little as 1/10 wave. Either way the image starts blurring similarly. Imaging with a ccd does not forgive defocus as it has no accomadation properties as our eyes do.

Glenn


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freestar8n
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Re: CO vs. All That "Other Stuff"! new [Re: GlennLeDrew]
      #5614909 - 01/10/13 04:33 AM

Hi GlennLeDrew-

Yes, I'm aware the brightness has an effect, but I don't think it explains everything since I see such differences even between eyepieces at the same mag. And even if it is related to brightness, it means MTF alone doesn't describe the impact of CO - and it gets to my point about optimally setting defocus to match the current mag. and intentionally alter the MTF.

Overall I'm saying it's a complex system and not easily explained or described - so it's hard to predict the role of CO, and it's hard to explain a preference of refractor over SCT, for example, strictly as resulting from CO. And MTF of the OTA by itself is not a complete description of the experience at the eyepiece.

Frank


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freestar8n
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Re: CO vs. All That "Other Stuff"! new [Re: DesertRat]
      #5614911 - 01/10/13 04:41 AM

Hi Glenn/DesertRat-

I notice in your plot legend that you describe one line as being 1/4 wave SA, and the others being SA plus defocus. How would you label those other lines in terms of the amount of SA they contain? Normally I would say they all have about 1 wave of SA, and varying amounts of defocus. This conveys the point that there is an inherent defect in the OTA that results in 1 wave of SA - always - and you can play with defocus to alter the P-V and RMS - and MTF - to partially correct for it.

If you had 1 wave of coma, you would get no similar benefit from defocus, in terms of reducing the RMS wavefront error. But spherical and astigmatism can be partly cancelled by defocus.

Also - what software did you use to make that plot? I'm interested in what convention is uses.

Thanks,
Frank


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Eddgie
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Re: CO vs. All That "Other Stuff"! new [Re: freestar8n]
      #5615128 - 01/10/13 09:43 AM

Quote:

I'm saying it's a complex system and not easily explained or described.




I agree 100%. It is very complex.

MTF plotting gives an exact result for one specific case, which is sinosodial 100% contrast lines varying in frequency from 1 line pair per millimeter at the focal plane to the maximum linear resolution at the focal plane.

For example, any two f/10 scopes have linear resolution of about 182 line pair per millimeter at the focal plane. It does not matter about the aperture, though while both a 4" f/10 telescope and an 8" f/10 telescope both have 182 line pair resolving power, an extended target will appear twice as large (half the frequency) in the 8" scope.

But the point here is that almost none of the detail we view consists of alternating black and white sinosodial lines.

While a line extends to either side/end of the focal plane, a lot of extended object detail consists of splotchs, curves, and other non-geometric shapes.

While a black line on a white background is narrowed only form the sides by diffraction. Suppose you had a dark "Peanut" shaped feature on Jupiter.

A bigger scope with better contrast transfer might show this "Peanut" to be exactly what it appears to be. But the smaller scope might show only the ends as small ovals because the contrast loss might cause the "Waist" of the peanut to disappear, severing the ends. Two observers using two different scopes with different contrast transfer... one reports seeing a "Peanut" shaped feature, the other reports seeing two teardrop shaped features with the tails pointing to one another!

A recent thread in the Solar System forum dealt with how some observers saw Io as distinctly oval, while I (using at larger scope with better contrast transfer) saw it as distintly round.

It is an excellent thread that is a textbook case for how MTF works. Different observers using scopes with different levels of contrast transfer, all seeing something different.
. And MTF theory explained it exactly. There are even some CAD drawings in the tread that show this. It is a great story with some historical figures playing an interesting role, as some great observers of the past also saw the same thing and reported seeing Io as being oval!

In general, I think that MTF though does a pretty good job of desribing how contrast is transferred, but as you say, it is extremly complex.

And while I think that the 1/4th wave = 30% obsstruction slightly overstates the damage (especially at the high frequencies), I do not at all dispute that a big obstruction lowers contrast.

But a bigger aperture puts it back.. LOL.

And this has been my own message for a long time. From my own experience, given reasonable optical quality, the biggest differentiator on extended object performance has been clear aperture. In just about every scope I have owned or used, the more clear aperture, the better view extened targets I have had.

I pretty much simplify it to that level because as a general rule, it matched my own experience. If two scopes plot similarly in MTF in the low and mid-frequency part of the spectrum, for visual use, the amount of detail visible has been similar.

This does though ignore the brightness/scale advantage that the larger aperture always has, which can offset a little contast loss simply becuase the image for a given magnificaiton is much brighter. I personally find that brightness makes the lowest contrast detail easier to see, especially as the magnification gets higher and the exit pupil in the smaller aperture gets below about 1mm.


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: CO vs. All That "Other Stuff"! new [Re: Eddgie]
      #5615190 - 01/10/13 10:25 AM

Quote:

This does though ignore the brightness/scale advantage that the larger aperture always has, which can offset a little contrast loss simply because the image for a given magnification is much brighter. I personally find that brightness makes the lowest contrast detail easier to see, especially as the magnification gets higher and the exit pupil in the smaller aperture gets below about 1mm.




I thought that "contrast" was simply the difference between the brightest and darkest parts of an image. Lower the contrast on a zebra, and he eventually looks solid gray. If contrast were indeed tied to brightness, then clear aperture would be its most important determinant. A large aperture with a large obstruction might have the same clear aperture as a smaller instrument with a smaller or no obstruction, for equal brightness. I know that, with an obstruction, diffraction effects also blur the image a bit, further reducing contrast as details become too fuzzy to see. Reading all the complexities described in this thread, I have to wonder, is my understanding of the basic relationship between contrast and brightness correct?

I know I'm in over my head here, but that's how I learn. I research topics I don't know. I'm currently working on "Modulation Transfer Function," but fear it may be time to revisit the seemingly basic notion of "contrast!"


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DesertRat
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Re: CO vs. All That "Other Stuff"! new [Re: Joe Cepleur]
      #5615669 - 01/10/13 02:47 PM

Joe,

Contrast is not just the difference between the brightest and darkest part of an image.

I think what people are referring to above has to do with the properties of the human visual system. It is not a linear system, so the mean level of brightness, the size of the exit pupil, magnification and many other things come into the picture.

It is a complicated problem. In your reading get hold of the definition of intensity, contrast (there are several) and gamma, and explore the visual system. That is if you want to.

You might start here:
http://en.wikipedia.org/wiki/Contrast_(vision)

Glenn


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