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Related Photoshop specific Issue, Middle Input Box in the Levels dialog is highly inaccurate - Adobe Photoshop 7, CS, CS2 & CS3

I just read this at the AIM site here: <http://www.aim-dtp.net/aim/evaluation/gamma_error/index.htm> The author says the following: "The middle input box in the Levels dialog of Photoshop is not an accurate gamma function it has a feature called "slope limiting" that is a deliberately embedded bug, it seriously damages the shadows if it is used for gamma compensation." Comments?...

  1. #1

    Default Related Photoshop specific Issue, Middle Input Box in the Levels dialog is highly inaccurate

    I just read this at the AIM site here:

    <http://www.aim-dtp.net/aim/evaluation/gamma_error/index.htm>

    The author says the following:

    "The middle input box in the Levels dialog of Photoshop is not an accurate gamma function it has a feature called "slope limiting" that is a deliberately embedded bug, it seriously damages the shadows if it is used for gamma compensation."

    Comments?
    YrbkMgr Guest

  2. #2

    Default Re: Related Photoshop specific Issue, Middle Input Box in the Levels dialog is highly inaccurate

    Tony,

    The middle slider is a pure gamma (power) function between levels 26-255. It transitions from there down to level 16 and finishes as a linear function from level 16 to 0.

    This is done to avoid an infinite slope of the input/outout curve at zero input, which a pure power function produces.

    To say that the modification "damages" anything implies that the system/monitor response in the deep shadows is well behaved and itself follows a pure power curve---and nobody knows that to be true.

    For the levels middle slider (or any of the other tools, for that matter) to correct system/monitor non-linearities, the non-linear behavior being corrected needs to be known. In the deep shadows it is not known. So one extrapolation is as good as another.

    George
    George Austin Guest

  3. #3

    Default Re: Related Photoshop specific Issue, Middle Input Box in the Levels dialog is highly inaccurate

    Hello George

    There are other gamma utilities that do not do slope limiting. The result is a much more open shadow than that provided by Photoshop. This openness also shows the shadow noise in scanned images. Perhaps Adobe have done the limiting because the effect not noticable on the final print.

    It is this clipping in Photoshop that I do not like. I would rather not have it. I can always do the clipping myself.

    I feel that a more useful tool would be to do all gamma in the curves dialogue with a middle input box and a histogram.

    Mike Engles
    mike engles Guest

  4. #4

    Default Re: Related Photoshop specific Issue, Middle Input Box in the Levels dialog is highly inaccurate

    Hi Mike,

    I hear you. I am thinking that it doesn't make sense to fuss over slope details in the deep shadows for an 8-bit system which can't track refinements anyway.

    George
    George Austin Guest

  5. #5

    Default Re: Related Photoshop specific Issue, Middle Input Box in the Levels dialog is highly inaccurate

    I agree. Since installing the Epson 3200 and using the Epson software, their gamma slider does a much better job on open shadows. In PS, I always check between curves and gamma slider to see which works best. Now, I don't need to generally export to PS from the scanner in 16 bit.

    Mike, to what gamma utilities are you referring? I don't believe any gamma slope correction can be implemented without normalization.
    Lawrence Hudetz Guest

  6. #6

    Default Re: Related Photoshop specific Issue, Middle Input Box in the Levels dialog is highly inaccurate

    Hello

    I suppose that I was being misleading about utilities. I meant the Gamma curve function in Picture Publisher as well as the AIM plugin for Photoshop and Nikonscan twain. They work without the limiting.

    Mike Engles
    mike engles Guest

  7. #7

    Default Re: Related Photoshop specific Issue, Middle Input Box in the Levels dialog is highly inaccurate

    I haven't run into problems but then I seldom touch the middle slider in LEVELS. I simply adjust the end points by minimizing the part off the toe of the histogram. I let the middle slider ride. The rest of the adjusting I FINALLY am able to do with CURVES.

    I scan in Nikon LS-40 (or that for the Epson 2450 depending) color space and edit in those too. With my Epson 2200 printer, 16 bit color and those color spaces; files do print better than the same file in Adobe RGB.

    I think the next test will be interesting. The Epsons (current and the now retired 1270) give a slightly better print at 360 dpi file resolution. With the 2200 having twice the resolution of the 1270, I'm going to spend the time and ink to see just what happens at a file resolution of 720 dpi at full printer resolution. I'm already planning to get my hair cut while it prints that monster. <grin>

    As for Nikonscan twain, try VueScan, [url]www.hamrick.com[/url]. I found it to be FAR better than Nikonscan. It does have a learning curve and you will have to adjust the curves, but there is more picture information in a VueScan file than a Nikonscan file. I did several different scans of the same negative with each, worked them up in Photoshop, and in every case, the best I could do with a Nikonscan source image wasn't close to meeting the quality of the worked up VueScan one.

    Bill
    Bill Lamp Guest

  8. #8

    Default Re: Related Photoshop specific Issue, Middle Input Box in the Levels dialog is highly inaccurate

    So far as I know, the 2450 exports to PS on Adobe RGB. Are you using the Epson Twain or the SilverFast?

    I still don't understand how a non-linear function can operate without some limiting. Otherwise it "s up" to infinity.
    Lawrence Hudetz Guest

  9. #9

    Default Re: Related Photoshop specific Issue, Middle Input Box in the Levels dialog is highly inaccurate

    I just bought Photo Elements 2 and I find there is no toolbar for making gama correction. Feedback as to how to do this would be appreciated

    Thanks

    Mak Mahesh
    K. Mahesh Guest

  10. #10

    Default Re: Related Photoshop specific Issue, Middle Input Box in the Levels dialog is highly inaccurate

    Hello

    I have used the demo of Vuescan since it first came out. Initially I found it very good and was pleased with the result, so I nearly bought it. I then noticed all sorts of strange colours and artifacts. I came to the conclusion that NikonScan was 'truer'. I have tried every version since, but alwys prefer NikonScan. I have never actually bought it.

    Mike Engles
    mike engles Guest

  11. #11

    Default Re: Related Photoshop specific Issue, Middle Input Box in the Levels dialog is highly inaccurate

    I was not too thrilled with Vuescan either.

    Native software better than third party. What is this world comming to?
    Lawrence Hudetz Guest

  12. #12

    Default Re: Related Photoshop specific Issue, Middle Input Box in the Levels dialog is highly inaccurate

    Mak,

    There is a separate forum for Elements:

    [url]http://www.adobeforums.com/cgi-bin/webx?1325.YL51atdS0zn.0.eeb4f8b[/url]


    LenHewitt Guest

  13. #13

    Default Re: Related Photoshop specific Issue, Middle Input Box in the Levels dialog is highly inaccurate

    Tony:

    Any mention of Timo's site on this forum will result in a good tongue-lashing from Chris Cox. Surprised Len didn't jump up and down.
    ID. Awe Guest

  14. #14

    Default Re: Related Photoshop specific Issue, Middle Input Box in the Levels dialog is highly inaccurate

    There is one valuable aspect to the site though. How to set your monitor up correctly by setting the black point, that is correct.

    The rest of the site deals with a difference of opinion, the only problem being Timo's ongoing attacks of other individuals (Charles Poynton, Bruce Fraser) of impeccable qualifications in the field who can explain their point of view.

    Timo thinks gamma should be linear to be perfectly correct (I actually scan without a profile and use a linear gamma), the other camp says no, there has to be compensation with hardware colour profiles and non-linear gamma settings to accurately represent what the human eye sees. The argument is more complex than that, but that is the simple jest of it.
    ID. Awe Guest

  15. #15

    Default Re: Related Photoshop specific Issue, Middle Input Box in the Levels dialog is highly inaccurate

    How did we get from Gamma slider to system gamma? These arguments have been hashed and re-hashed over and over.

    All this stuff about system gamma should not be even discussed here. It is the privy of the HW designers to deal with it. Crt's are governed by power laws. The designer of that system is responsible to provide necessary corrections so that a linear input provides an approximate linear output to the eye, likre audio volume controls. 0 to 100 IRE (NTSC) are linear steps from total black to total white, or nearly so. That nearly so is what the designer has to reconcile.

    I don't want to even know about it. Just tell me what voltages it needs.

    Norman Koren does some of the best prints I have seen. I spent a couple of days last fall with him when he visited his son in Portland.

    He doesn't use Photoshop, but he didn't bad mouth it to me either.

    Power laws can get out of hand, especially when they are applied sequentially.
    Lawrence Hudetz Guest

  16. #16

    Default Re: Related Photoshop specific Issue, Middle Input Box in the Levels dialog is highly inaccurate



    How did we get from Gamma slider to system gamma?




    As I read it, Timo says the gamma slider is inaccurate, and his argument is that all monitors gamma should be set to 1. I think.<grin>
    YrbkMgr Guest

  17. #17

    Default Re: Related Photoshop specific Issue, Middle Input Box in the Levels dialog is highly inaccurate

    I was asking rhetorically Tony. Wasn't aimed at you. Sometimes I get too caught up. Must curb those tendencies.

    Peace,
    Larry
    Lawrence Hudetz Guest

  18. #18

    Default Re: Related Photoshop specific Issue, Middle Input Box in the Levels dialog is highly inaccurate



    Sometimes I get too caught up. Must curb those tendencies.




    Me too Lawrence. If I'd had my wits about me, I probably would have realized that it was rhetorical. Unfortunately I don't <grin>

    Peace,
    Tony
    YrbkMgr Guest

  19. #19

    Default Re: Related Photoshop specific Issue, Middle Input Box in the Levels dialog is highly inaccurate

    Let's throw more fuel into the bonfire:

    <http://chriscox.org/gamma/>

    <http://www.bberger.net/gamma.html>

    <http://www.cgsd.com/papers/gamma_intro.html> and <http://www.cgsd.com/papers/gamma_definitions.html>

    <http://www.normankoren.com/makingfineprints1A.html>

    And now I run as fast as I can...
    Gustavo Sanchez Guest

  20. #20

    Default Re: Related Photoshop specific Issue, Middle Input Box in the Levels dialog is highly inaccurate

    You will like this one too:

    <http://www.brucelindbloom.com/index.html?LevelsCalculator.html>
    ID. Awe Guest

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