front and back focus

<p>It's my understanding that front and back focusing lenses didn't matter as much in the manual focus days. One focused until the image in the viewfinder was in focus, and took the picture. If the distance scale on the lens was off a little bit, who cared (unless you were relying on the distance scale of course.)</p>

<p>With modern autofocus cameras it's different because the lens is supposed to be focused at a specific distance according to the camera computer and if the lens is off then all hell breaks loose.</p>

<p>Am I wrong? If the viewfinder image is in focus then the picture is in focus, no matter if lens is "backfocusing" or not?</p>

<p>I am a bit puzzled about it as well. As far as I understand, AF cameras move the lens elements until the sensor in the camera detects the highest contrast (meaning sharpest image; or some other way of detecting when the lens is actually in focus). The distance scale has nothing to do with it, like it didn't in manual focusing. The problem comes from the AF sensor not being correctly aligned with the sensor plane. Same can happen with focusing screen if it is not in the exact same distance as the film, then focusing errors happen. But if the sensor is off, then it should be off for all lenses. Why only some lenses are affected? I can understand that fast long lenses are more sensitive to correct focusing and with these lenses the problem is more obvious, but it seems to me that backfocusing can happen with one fast lens but not another on the same camera, at least not to the same degree.</p>

<p>With a decently-calibrated manual SLR, "what you see is what you get." The viewfinders in autofocus cameras are not optimised for manual focussing,but they should also be "what you see is what you get."</p>

<p>The autofocus cameras do the focusing for you, so if the system is off you will get out of focus photos. It can be challenging to see that things are just a bit off in an autofocus viewfinder, especially for a DSLR with a small sensor.</p>

<p>The computer just tells the lens to move the focus in or out and when the camera detects focus it tells the lens to stop. I believe the following can result in back focusing.</p>

<ul>

<li>The lens has mass and therefore inertia. If the focus motor is moving quickly (we all want fast focusing lenses, right?) and the computer suddenly says to stop, power to the focus motor will be cut. However inertia in the focus motor may carry the lenses just past the focus point. Combine this with a narrow depth of field and the lens is back focused.</li>

<li>If one or more of the lenses in the lens assembly are slightly out of position, it may cause the depth of field to shift causing a front or back focus. In other cases it might cause the entire image to be out of focus or only one side of the image to be out of focus.</li>

<li>The focus sensor is located under the mirror. The mirror is only partially reflective. Some of the light passes through and then reflects off of the secondary mirror and down to the auto focus sensor. If there is something on the mirror (finger print, oil, moisture or anything that bends light) it could effect the auto focus system. </li>

<li>When you see the focus points in the viewfinder you assume the focus point is the same size of the auto focus sensor. Its not. In my 5D is significantly larger than the focus point and only slightly smaller than the spot meter circle. So if you put the center focus point right on the image focus point it might actually focus on something else just outside of the view finder focus point with more contrast. I have seen pictures labeled as back focused when in reality the camera focused on something other than the what the photographer wanted. The camera cannot read you mind. </li>

</ul>

<p>Based on all the "focusing problems" posts I have seen only a hand full were actually traced to the camera or lens. In my opinion most "focusing problems" posts are due to user error or the user expecting the camera to "know" what he or she wants in focus. I believe all new photographers would benefit by simply turning off auto focus for a day or more while taking a lot of pictures. In fact it would be even better if they limited all the shots to F1.4. That way they would be more likely to notice situations which could be difficult for the auto focus system. They then could then switch to manual to get the shot rather than asking on photo.net "what's wrong with my camera?". </p>

With manual scale the scale had to be accurate also because if it was off there wa a good chance that the rangefinder wa out of adjustment. The camera was built with the scale correct for focussimg and if it was off nothing else coud be correct.

<p>Don't forget to properly set the diopter, if so equipped, on your DSLR's viewfinder to ensure a match in focus by observation of AF, or if using manual focus. Which is why I prefer to use lenses that have full time manual focusing. Meaning if AF misses, I can manually correct and then take the shot.<br>

Most issues stem from user error, and increase with shallow DOF. However, some misalignments are genuine. Most cameras are mass produced, and it's likely a few can be out of tolerance. There is a tool called LensAlign that can be used to check the problem.<br>

Some newer pro digital cameras (Canon 1Ds Mark III for example) now have sensors that are user adjustable. <br>

Generally I don't use or trust AF for mission critical. I have only used it for sports, and split second shots I would have missed otherwise if I were hesitant. However, looking though the viewfinder on Canon T70 film camera, and then back to my 40D I can tell you that is one area the technology has actually regressed.</p>

<p>I find that auto focus on digital cameras is iffy and doesn't work on images that lack contrast. Auto focus is also near useless with closeups and macros. My practice is to manually focus whenever in doubt. When I focus manually I am more confident about sharpness. Also, to achieve DOF remember to shoot with small lens openings. On a tripod, I often shoot between f/22 and f/32.</p>

<p>Lets get our definitions straight. "Backfocus" is the distance from the rear element of the lens (when focused at infinity) to the focal plane!</p>