Lens testing - checking for front or back focus

[bmm]

<p>Hi guys</p>

<p>In my time on this forum people have mentioned tests for lens front/back focus using rulers etc.</p>

<p>These have just been quick mentions and I'd be very grateful if someone can desribe the setup and process in detail.</p>

<p>Thanks!</p>

[iwong]

<p>Lay a ruler flat, longitudinally, on a table. Stand up, or set your camera on a tripod, and shoot, with the axis of the lens making a roughly 45-degree angle to horizontal. Pick single focus area, and single servo (not dynamic, not AF-C). Ideally the focus area is the middle one. Keep the ruler in the center of the frame and shoot, then check to see if the sharpest part of the ruler is the area you focused on.<br>

Let's say you used a 12" ruler with the 0" mark closest to you, and you focused on the 6" mark. If the 6" mark is not clear in the image but 5" is, you have front focus. Similarly if 7" is sharper, you have back focus.</p>

 

[wade_roth]

<p>http://www.lensalign.com sells a product that helps calibrate your lens and camera. I've never tried it but they had some videos that helped me understand what I needed to do for a home made version.<br>

Good luck.<br>

--Wade</p>

[pisq]

<p>what f-stop would be best for the test?</p>

<p>How far from focus point to camera lens?</p>

[kevin_delson]

 

<blockquote>

<p>what f-stop would be best for the test?<br>

How far from focus point to camera lens?</p>

 

</blockquote>

<p>These two questions carry more weight than the original question.</p>

<p>W/o a solid understanding of each lens being tested, you may find yourself chasing your tail.</p>

<p>Example 1: Wide excursion zoom lenses are notoriously poor candidates for this type of test as they work great at some F/stop at some focal length. When you (zoom in) the test seems to indicate a problem.</p>

<p>Example 2: The Nikkor 80-200 is not intended to work in the macro arena, but many people try to check BF/FF at 8 feet! Invalid test.</p>

<p>Each lens has to be tested within it's operational characteristics.</p>

 

[tedms gallery]

<p>for front/back focus, there is a test pdf you can find on the web that is graduated with high contrast bars, with a thick one for the target.<br>

I'd recommend getting a protractor, or consider verifying focus by shooting objects straight on with known objects in front of, and behind the target, at known distance intervals.<br>

You'll need to pick an aperture to start at, or index with, and for me, that is usually wide open.<br>

If you notice focus changing with the focus locked at the on-focus point for wide open, when stopping down, then you are observing focus shift, where your single point of focus may drift, and your whole window of DOF may move.<br>

I wrote up about this on my blog about a week ago, with some manual lens - tedmphoto.blogspot.com</p>

 

[anthony_bez]

<p>I agree with Kevin the ruler or target tests can give misleading results. I use my lenses for there intended use and evaluate the images.</p>

[francisco_disilvestro]

<p>One way is to use <a href="http://www.openphotographyforums.com/forums/showthread.php?t=4708">this</a> procedure described in a tread in the Open Photography Forums.<br>

This will work if:<br>

You have a camera with Liveview<br>

You have a flat LCD monitor</p>

 

[oskar_ojala]

<p>I prefer to use something like a newspaper taped to a wall or similar, as rulers are often smaller than the typical subjects. The subject should be flat, easy to focus on (i.e. no repeating pattern but a pattern still) and large enough to correspond to a typical subject. Then just do shots at each aperture. To test focus shift when stopping down, focus once and take shots at each aperture. To check focus accuracy, focus at f2.8 and check if the focus hit the right spot. Do this a few times to see if it is consistent. Use a tripod. Start with the center focus point. Light the subject properly.<br>

The test is ruined if your movement changes the distance between camera and subject (and interpreting the results is harder with motion blur), the camera misses because of darkness or lack of contrast, the camera misses because the subject is too hard to focus (no detail or pattern that messes up the phase detect AF), so employ correct methodology. And remember that there will always be a small inaccuracy, so you have to correctly judge if it is significant or not.</p>

[Rene11664880918]

<p>Here you'll find the PDF that Ted is talking about. I tried it once but I think I was doing something wrong. All my lenses failed.... and i haven't noticed anything wrong with them while using them.... since then I don't care about testing lenses unless there is something that bothers me about them.<br>

Link: http://FocusTestChart.com</p>