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  1. #1
    Join Date
    Feb 2006
    Location
    Des Plaines, IL
    Posts
    9,554

    Lightbulb Címon now, try to focus Ö

    I suggest that you print this, if you have not done it before ...

    THE LENS "BACK" or "FRONT" FOCUS TEST

    Some call it the infamous "battery test", but it really has nothing to do with batteries, per se ... other than they are usually handy ... you use them as symmetrical alignment objects and you probably have four of them, right now, in your external flash.

    You basically do this:
    1. take three batteries and place them in front of you, on a table top, at a 45-degree diagonal, spaced roughly 1-inch apart, front-to-back and side-to-side. (Note: I am assuming you just bought a 17-50mm f/2.8 or the 28-75mm f/2.8 lens and are looking to replace that awful "kit" lens. If you are looking at a f/4 lens, you may need to space the batteries further apart, say 2-inches front-to-back).
    2. Put the DSLR in Manual-mode “M” … we need some control.
    3. Lift built-in flash up to use.
    4. Set your Aperture setting to f/2.8
    5. Set Shutter Speed to 1/125th
    6. Set ISO to 400
    7. Put the dslr on the table top, also, or as close as possible to horizontal.
    8. Back up from the object to at least the lens’ M.F.D. (minimum focus distance)*
    9. Zoom the lens to its longest focal length (or whatever comfortably frames the three batteries), e.g., 17-50mm -> 50mm; 28-75mm -> 75mm
    10. Site up the "center" battery in spot focus mode, with the viewfinder's centerspot on the middle battery.
    11. Autofocus and take the shot.
    12. Inspect where the sharp focus actually occurred.


    Evaluating Results

    If the center battery is sharp AND the other two batteries are out-of-focus ... do the test one more time, just to be sure. The lens should be okay and close enough to use. Consider yourself born under a lucky star.

    If, on the other hand, either the front or rear battery is the sharp image, then I suggest a re-take the shot, focusing on the middle battery once again. Confirm these results. They should be repeatable if there is an error, also. If the results are the same, the lens is misaligned and will require service. Consider yourself a victim of mass production.

    If, by chance, focus cannot be achieved on ANY of the three batteries, add a few more batteries to either end the test, in an ever increasing diagonal and repeat from Step 7, to see where the sharp focus occurs. This would be an incredibly poor performance and I would consider returning the lens to the store you purchased it from for a different one AND REPEAT THE TEST on the new one.

    You really should do this with every lens you have, to be sure they are working correctly. There is also the possibility that your cameras focusing system could be suspect, but 99% of the time, it is the new lens that requires corrective calibration.

    * One thing to be cognizant of is the M.F.D. (look this up in the acronym list (<- click here)), it is the Minimum Focus Distance. Every lens has a limit as to how close you can get before the lens can no longer resolve the focus. With the newer 75mm or less zoom lenses, it is usually about 18-inches. Any closer and the focus is blocked from adjusting further and the shot looks soft.

    The average M.F.D. for the 70-300mm f/4-5.6 lens is about 5-feet. If the subject is any closer to the camera than that ... you screw up the focus on it. Every lens should have this measurement stipulated in the instructions (specifications) or even printed on the lens itself.

    Now … focus!
    Last edited by DonSchap; 04-26-2011 at 12:27 PM.
    Don Schap - BFA, Digital Photography
    A Photographer Is Forever
    Look, I did not create the optical laws of the Universe ... I simply learned to deal with them.
    Remember: It is usually the GLASS, not the camera (except for moving to Full Frame), that gives you the most improvement in your photography.

    flickrģ & Sdi

  2. #2
    Join Date
    Nov 2010
    Location
    Apopka, FL
    Posts
    447
    Ok so I did this test with my Tamron 28-75 f/2.8
    Camera set to "M", ISO: 400, Aperature f/2.8, Shutter 1/125th, On Tripod.

    First is at 28mm


    Second is at 50mm


    Third is at 75mm

  3. #3
    Join Date
    Feb 2006
    Location
    Des Plaines, IL
    Posts
    9,554

    Thumbs up Acceptable result

    This is an "acceptable" result ... go forth and capture your world.
    Don Schap - BFA, Digital Photography
    A Photographer Is Forever
    Look, I did not create the optical laws of the Universe ... I simply learned to deal with them.
    Remember: It is usually the GLASS, not the camera (except for moving to Full Frame), that gives you the most improvement in your photography.

    flickrģ & Sdi

  4. #4
    Join Date
    Nov 2010
    Location
    Apopka, FL
    Posts
    447
    Sweet!

    What would make it better?

  5. #5
    Join Date
    Feb 2006
    Location
    Des Plaines, IL
    Posts
    9,554
    Perhaps going to 1/160 sec, for a little more contrast. It is slightly over exposed. Alternatively, since this is a flash shot ... you can use the camera flash exposure setting to drop it a notch or two. It's a little too bright. TTL has some limitations and you sometimes need to "monkey around" with it. The camera, I believe, will provide you with +/- 3 stops of flash control. Try it out. Go from one extreme to the other to see the effect.

    Also, overall, be cognizant of having a "level" horizon. The table top is slightly skewed and since this is a tripod shot, you may want to be sure your first shot is level ... the others will follow suit.

    A warning about flash. Try to avoid using direct "on-camera" flash around infants. If possible, learn how to use the "wireless" flash feature and light accordingly.
    Last edited by DonSchap; 04-26-2011 at 02:13 PM.
    Don Schap - BFA, Digital Photography
    A Photographer Is Forever
    Look, I did not create the optical laws of the Universe ... I simply learned to deal with them.
    Remember: It is usually the GLASS, not the camera (except for moving to Full Frame), that gives you the most improvement in your photography.

    flickrģ & Sdi

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