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Front and Back focusing.

Discussion in 'Pentax Chat' started by Monobod, Jul 8, 2014.

  1. Monobod

    Monobod Phantom of the forum


    I have long been meaning to check this with some of my most often used lenses, but something has made me think it is now necessary rather than desirable (perhaps).

    The photos I have been taking with my Sigma 10-20 (original version) on the 12.3Mp GXR are considerably sharper than those taken on the K5. The results from this lens on the K-5 have always impressed me, but they have needed some sharpening from NIK before looking spot on. On the GXR, the results are even better, which suggests that the lens is not focusing spot on on the Pentax perhaps.

    On the other hand, of course, the A12 Mount (Leica M with Novoflex adapter) on the GXR has no anti aliasing filter and I wonder if that is the reason.

    Interesting, I must have a closer look at this.

    More fuel to suggest getting a K-3 is moving up the 'I want one now' list. :) How do i convince my Financial Controller though? A weekend away? A Bunch of roses? A box of chocs? All three? :confused:
  2. AlexMonro

    AlexMonro Old Grand Part Deux

    Whe you take a photo with the 10-20 (I have one too!) on the K5 of a scene that covers a range of distances, are there any parts that look sharper than the point you focussed on than are closer or farther away? If so, it's possible that front / back focussing could be responsible, or it could be that the lens is slightly decentred. If not, then it could well be that the GXR module's lack of AA filter gives it that extra sharpness.

    I guess other factors could also account for it - default sharpening if JPEGs, raw conversion demosaicing algorithms if raw files etc...

    Sorry to be somewhat inconclusive. It's probably not a bad idea to do a focus check anyway, if only for peace of mind. I've been meaning to do that for may lenses for several years now, but never quite got around to it. But I don't see any signs of obvious problems in my shots - I usually shoot fairly well stopped down though.
  3. Done_rundleCams

    Done_rundleCams AP Forum Ambassador to Canada

    Blackmail pictures ;) or, and I hesitate to use this rationale, honesty :eek:


  4. Monobod

    Monobod Phantom of the forum

    Thanks Jack,

    Honesty is the best policy, but it does invite an ear bashing sometimes. TBH there are other more important things we need, like new furniture and a decent holiday. But work is very buoyant so there is still hope. :)
  5. Monobod

    Monobod Phantom of the forum

    I always use RAW (DNG) and post process in ACR and Adobe CS6, with sharpening turned off, then tweak in NIK and finally sharpen as the last action. I will have to do some controlled testing, but its finding the time as always...

  6. El_Sid

    El_Sid Well-Known Member

    Sharper is not the same as better focused. TBH with the huge depth of field inherent with the 10-20 I honestly doubt you'd even notice a bit of mis-focus unless it was really way out.

    I don't think anti-alias filters are equal either. My Nikon D50 produces inherently and noticably sharper images than any of my Canons so I'm pretty sure the differences you see are down to the lack of AA filter rather than any fault with focusing.

    If you weren't getting 'spot on' results from the K5 after sharpening then that would be the time to worry.
  7. Monobod

    Monobod Phantom of the forum

    I noticed this taking close up Photos of tree bark at a distance of about 400mm on a tripod, but I need to look into this more scientifically I think. Also some new glasses wouldn't go amiss, had these for far too long. :(

    Acceptable depth of field for a 20mm at f/8 is 130mm, from 340mm to 470mm, so it should have been pretty spot on.

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