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Thread: Front/Back Focus

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    Senior Member ecliffordsmith's Avatar
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    Front/Back Focus

    Hi All,

    Do you know if a front or back focus issue should be consistent throughout the entire focus range? By this is mean can a lens be bang on at 0.7 and not at 3m for example?

    My eyes are far from perfect and hence I wear glasses and I have noticed that at about 3m, consistent across apertures things seem to be a bit sharper a little bit behind the intended focus point. I have not done any real testing yet, I was just wondering whether the phenomenom could be lens related or eye related.

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    Re: Front/Back Focus

    my eyes are not so hot either, but any focusing issues i've had with a lens is not consistent at all ranges. i mean, if will still focus behind or in front, and will do so consistently throughout the range -- i.e., back or front. the distance of the travel, however, will travel through the range. not that bad at 1 meter, horrible at infinity or vice versa.

    keep in mind, though, all my lenses are a little long in the tooth except the C-Sonnar.

    (btw, what aperture? i'm talking about f/2 and faster)

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    Senior Member ecliffordsmith's Avatar
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    Re: Front/Back Focus

    Hi Cam,

    Thanks for the response. I am using a 35 Summicron ASPH at F2 here. I see the same thing at 2.8, 4 and 5.6 too though. This is why I dont think there is a focus shift as such.

    I will take a few more detailed looks I think and check it out. The focus distance I am talking about means that the visible difference between the intended subject and the section that appears to be in better focus is very very close in the focus patch of the camera.

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    Re: Front/Back Focus

    and only on this lens? (certainly sharp enough to *really* notice the difference!)

    do you wear glasses?

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    Senior Member ecliffordsmith's Avatar
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    Re: Front/Back Focus

    Hi Cam,

    This is the only lens that I tend to use (in the last 6 months or so) but I should try with my 75 and see if I get the same issue. I do wear glasses and to be honest I suspect my eyes are the issue.

    Thanks for your opinions!

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    Re: Front/Back Focus

    definitely try it out with your other lens. if the result is the same, then it's either your eyes or your camera... if not, it's the lens.

    I asked about glasses because I'm pretty blind myself (i wear contacts most of the time) and definitely need to update my prescription. that said, i found my wearing glasses very much exaggerated a problem I may or may not have on my Nocti -- which seems to have an ever so slight front focus. (i get so confused with the terminology here. it seems to do the opposite of yours -- focuses slightly in front of the object.) i've also found different type of glasses easier to use with the VF than others....

    regardless, let us know!

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    Senior Member ecliffordsmith's Avatar
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    Re: Front/Back Focus

    Hi All,

    Well, I tried some more tests last night and it seems that if I have the rangefinder image focus spot about 2mm (visibly) in the viewfinder I get a bang on focus image so it seems that the 35 SUmmicron ASPH is back focussing a little. It has actually been back to Leica in the past for a infinity allignment issue when the M8 needed repair so I dont think I will do that again. It has little real world impact as I use the lens almost exclusively and very regularly. Typically the DOF of the lens at 4 or 5.6 where I usually have it in good light and hyperfocalish focus combine to give tack sharp images from front to back.

    I will play around with it a bit more though. The 75 Summicron ASPH was dead on focus so it seems my eyes are off the hook!

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