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Thread: FF or not FF?

  1. #1
    Flyfisher's Avatar
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    FF or not FF?

    Hello all,
    Yesterday I took all my lenses and made the focus chart test and for the 35mm lens the results look a bit different than the other lenses.
    Considering the attached image, can I consider it as front focus?

    Thanks in advance!

    FF or not FF?

  2. #2

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    Re: FF or not FF?

    It does seem to be focusing beyond the point you focused on ... if the thick line is that distance.

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    Re: FF or not FF?

    Have you repeated the tests?

    These focus charts give an indication, but they aren't totally accurate.

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    Flyfisher's Avatar
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    Yes, I did, all tests look the same as the image I uploaded

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    Re: FF or not FF?

    Hi Iosif

    I'm no expert on this sort of thing but I did try the test once with one of my lenses. I came to the conclusion that the test couldn't really be relied upon to give a precise result. Having said that however your result does seem to indicate that the sharpest focus is around the 6mm point behind the "focus here" point - at least to my eyes. Wouldn't this be back focus ?

    Another test you might like to try is to attach some sort of test chart vertically to a wall, set your camera up on a tripod so that it is level with the centre of the chart and is facing at right angles to the chart. Then do comparison shots using autofocus (centre point) and either manual focus in magnified Live View and/or Live View mode focus (contrast detection). The latter two modes use the main sensor as the focus detector and should be spot on whereas the autofocus uses a different light path to the phase detect sensors which is where a calibration problem might occur.

    Dave

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    Re: FF or not FF?

    Quote Originally Posted by dje View Post
    Hi Iosif

    I'm no expert on this sort of thing but I did try the test once with one of my lenses. I came to the conclusion that the test couldn't really be relied upon to give a precise result. Having said that however your result does seem to indicate that the sharpest focus is around the 6mm point behind the "focus here" point - at least to my eyes. Wouldn't this be back focus ?

    Another test you might like to try is to attach some sort of test chart vertically to a wall, set your camera up on a tripod so that it is level with the centre of the chart and is facing at right angles to the chart. Then do comparison shots using autofocus (centre point) and either manual focus in magnified Live View and/or Live View mode focus (contrast detection). The latter two modes use the main sensor as the focus detector and should be spot on whereas the autofocus uses a different light path to the phase detect sensors which is where a calibration problem might occur.
    Dave
    Try this test here and see what happens.


    http://photographylife.com/how-to-qu...tofocus-issues

  7. #7
    dje's Avatar
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    Re: FF or not FF?

    Quote Originally Posted by Melkus View Post
    Try this test here and see what happens.


    http://photographylife.com/how-to-qu...tofocus-issues
    Yes that's the one.

  8. #8
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    Re: FF or not FF?

    Another indication of the focus point is where the spherochromatism color shift goes from purple to green which looks like it is at the -6mm point.

    John

  9. #9
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    Re: FF or not FF?

    The problem here is if you only have one lens and use one of these test and it dose show a problem then you won't know if it's the lens or camera body.

  10. #10
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    Re: FF or not FF?

    It looks back focused.

    The problem with these tests is that the focus point is much larger than you think. So the camera may have focussed behind the line anyway. You need a planar target.

    Find a flat target with some fine details, e.g. A poster on a wall. Setup a camera on a tripod and lens perpendicular to the wall (so the sensor is parallel). Focus in live view and take a photo. Then switch to phase detect mode and focus. Repeat the test a few times defocusing to infinity each time to get some data.

    Then compare the phase detect images to the contrast detect images. If the phase detect images are worse then you have a front/back focus and can try and figure out which using an angled ruler next to a planar target. If both sets are equally blurred then you may have an issue with the lens. If both are equally sharp then the calibration is fine.

    Alex

  11. #11
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    Re: FF or not FF?

    I am CERTAINLY no expert. But the focus on the right hand side looks to my eyes a lot sharper than on the left. What does this show?

    I have been trying the same focus chart with my Sigma 17-70, which has a reputation for front focusing, but I've struggled to decide just what it is telling me

  12. #12
    William W's Avatar
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    Re: FF or not FF?

    Quote Originally Posted by Flyfisher View Post
    Yesterday I took all my lenses and made the focus chart test and for the 35mm lens the results look a bit different than the other lenses. Considering the attached image, can I consider it as front focus?
    No.
    You could conclude that for this test (and considering that ALL your other tests appeared similar) the lens and camera combination might be making a slight back-focus.

    ***

    My conclusions are:

    The Lens is at the point of sharpest focus on then Left and Right Scales at the positions indicated by the “<” and “>”.
    FF or not FF?

    ***

    Quote Originally Posted by davidedric View Post
    . . .the focus on the right hand side looks to my eyes a lot sharper than on the left. What does this show?
    I concur.
    The left scale, overall, is less sharp than the right scale.
    I would first suspect that the lens is not at 90 in the Horizontal Plane to the test cart and / or the chart is slightly bowed.
    However if the lens was set absolutely perpendicular to the target, in the Horizontal Plane (unlikely) and the test chart was indeed flat: this result would mostly indicate a misaligned Element or Group in the Lens: but the deviation is very slight in this sample.

    ***

    Consideration of the SD (Subject Distance) should factor in how much relevance this bench test actually has on real world results.
    In that regard I ask – what was the Subject Distance?
    If the SD is short – then although a fun test to do, it is very most likely of very little practical purpose.

    WW

  13. #13
    Flyfisher's Avatar
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    Re: FF or not FF?

    Quote Originally Posted by Melkus View Post
    Try this test here and see what happens.
    http://photographylife.com/how-to-qu...tofocus-issues
    Thank you for the link. I will try this when I'm at home. With all my lenses.

    Also thank you all for pointing me to the right directions (not FF but BF ) and also for the advices.

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