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Chromatic abberation

Discussion in 'Pentax Chat' started by Monobod, Sep 11, 2014.

  1. Monobod

    Monobod Phantom of the forum

    Hi,

    Much is made these days of the performance of modern digital lenses in this respect. I am interested to hear what you consider to be a limiting value in terms of pixel width.

    Are you bothered by it or do you just accept that it is there and ignore it?

    Do you worry if it can only be seen when pixel peeping?

    I have a few legacy film lenses, which I consider to be of acceptable quality and it has always been my assumption that, because this flaw cannot be corrected with film, film lenses were more carefully designed to correct this than some lenses designed for dslr's. Am I wrong, or is it again a function of price?

    How good is software at dealing with it?
     
    Last edited: Sep 11, 2014
  2. AlexMonro

    AlexMonro Old Grand Part Deux

    I don't worry about it excessively, but when it's obvious in an image at normal viewing, I try to reduce it. I have a plugin for GIMP called Darla which is quite effective, especially for sensor blooming ("purple fringing"), as opposed to true CA.

    I note that the K3 has built in CA reduction for JPEGs when used with Pentax lenses, but despite my 16-50 DA* suffering from this a bit, I haven't actually tried it yet.
     
  3. Monobod

    Monobod Phantom of the forum

    TBH, I have never used the jpeg option. For my sins I shoot everything in RAW (DNG) and convert them in ACR. It is a laborious process, but a good discipline to get into perhaps.

    On occasions I have noticed green and purple fringing in corners, perhaps where tree foliage is against a grey or blue sky. There doesn't seem to be an option in ACR for correcting this aberration, which isn't surprising really.

    So I have to deal with it in other ways, such as dumping the image! :(

    Perhaps when the K-3 finally arrives, I will try the jpeg option just to see what comes from it.
     
  4. El_Sid

    El_Sid Well-Known Member

    Must admit it's never bothered me that much. In the days of film there was nothing I could really do about it and at normal viewing distances I can't say it ever really stood out - certainly club judges never seemed to make a big song and dance about it.

    I suspect that the trend for concern about abberation is that pixel peeping makes it more visible. The problem is that pixel peepers often forget that they aren't looking at a real world scenario, particularly in relation to prints where the screen resolution at 100% is way lower than you would ever rationally print and that you are effectively viewing a very large print from far too close. I'm sure that if I were to project a slide onto a 6ft+ screen and inspect it closely from 6 inches I might well see chromatic abberation but from far enough away to see the big picture then I wouldn't...

    Were film lenses designed more specifically to corrrect this?... In the pre colour days then probably not - at least in the days before panchromatic emulsions came out - but as panchromatic and then colour film became more popular a desire to better render the image without unwanted fringing probably became more urgent, especially as smaller formats with shorter focal lenses started to dominate. Certainly glasses were made with unusual formulations were made to improve matters - IIRC Leica made lenses with Lanthanum glass - though some of these early attempts turned out to have other issues of their own...

    I'm not sure when the anomalous dispersion glasses now in common use first appeard but I remember Nikon making a lot of fuss about their ED lens elements back in the early eighties and Canon talking a lot about their Fluorite lense elements around the same time but both tended to be limited to lenses that seemed very much in the professional end of the market and therefore not cheap. Today many if not most lenses feature at least one anomalous dispersion element since development inevitably meant that these glasses became easier and cheaper to produce in large quantities. This means that even at the cheap end of the market it's possible to buy lenses that have far less abberation than might once have been the case - particularly where ultra wide and zoom lenses are concerned; two categories which have traditionally had issues at either the extremes of their range and/or image area.

    As far as software goes my experience is that how effectively it can be removed does seem to depend on how bad it is. I normally shoot RAW and process in ACR. With lenses that produce minimal abberation, generally primes and more recent glass, it is sometimes possible to eliminate it completely. With poorer lenses, especially older zooms, I often find that if you eliminate it in one part of the image it can reappear or get worse elsewhere in which case I have to compromise on a setting that seems to reduce it to a consistent, ideally minimum, level across the image. Note that although I do the adjusting at 100% view I assess the effect at 50% and a sensible viewing distance - if it looks acceptable then I find it will look okay in the final result.
     
  5. Catriona

    Catriona Well-Known Member

    Excuse me butting in, but I had to say that I wasn't bothered by it - until I got a really good lens, with wonderful CA reduction. Then I realized what I had been missing.
     
  6. AlanW

    AlanW Well-Known Member

    When I'm uploading to Alamy I always check for CA and use the PTLens plugin for Photoshop Elements to get rid of it - I'm sure there's tools in Elements to deal with it but I'd been using a standalone version of PTLens for years so stayed with it when I started using Elements a while ago.

    Recently I've begun to use a Nikkor 50mm prime lens and I have to say CA is near enough non existant to the extent that when I've used the 50mm lens I don't bother checking!

    Alan.
     
  7. RogerMac

    RogerMac Well-Known Member

    I can honestly say that since I bought my first DSLR I have never noticed any CA. I do remember seeing some on my Canon G2. Perhaps I have not looked hard enough - but then it does not seem worth hunting down something that is not apparent and would only annoy me if I found it.
     
  8. Monobod

    Monobod Phantom of the forum

    I must admit that with most lenses on my K-5, it is only apparent when pixel peeping at 100% or more. Even then it is only about two to three pixels wide, so disappears at normal viewing size.
     

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