All lenses and cameras that return to one of our two headquarters are tested and cleaned by our Receiving Team. Sometimes a lens, in particular, will go out on a rental and need to be replaced by another one from our stock because of focusing issues. The majority of these re-tested lenses end up having nothing wrong with them. Here is an explanation for why this happens and how you can dig deeper into the settings of your camera so that you get the most out of not only rental lenses but your own stock of glass as well.
Mass Manufacturing and Range of Accuracy
All cameras and lenses are manufactured within certain tolerances. This means that a camera or lens is considered in spec if it falls within a certain range of accuracy. Every manufacturer is different. Sometimes a lens that is front or back focusing +/-3 is considered within normal operating quality. Other manufacturers are more stringent.
What Is Front/Back Focusing?
Front focusing is when the focus falls in front of your intended subject and back focusing is when the focus falls behind your intended subject. Most of the time this is caused by the user. Barring user error, a lens could be tested at -2 and back focusing slightly or tested at +1 and front focusing slightly. Both are considered within the range of normalcy. Cameras compound the issue. Sometimes, a lens can be back focusing slightly and that is not a problem. But if it is mounted on a camera that is also back focusing slightly then you are now shooting outside the range of spec.
How to Fix a Front/Back Focusing Issue
All of our lenses are shipped out within spec as are all of our cameras. However, we cannot control for every possible combination of lenses and cameras and their slight front/back focusing, in spec quirks. A lens that is slightly front/back focusing on your camera may be perfect on someone else’s. Fortunately, you can compensate for this.
The good thing about making microadjustments on your camera is that it increases the accuracy of your focusing even on borrowed or older lenses. It also puts you in control of the deeper settings on your camera and it is not permanent – you can always change it later or zero it out. Most cameras have microadjustment options, including Canon, Nikon, Sony, Olympus, and Pentax. If your lenses, or a rental, is back or front focusing, you can go into the menu of your camera and make it aware of this lens’ issue, however slight the issue may be.
The following are instructions to access this menu on a variety of cameras:
• Canon AF Microadjustment – Make a global adjustment for all of your lenses or adjust for an individual lens.
• Nikon AF Fine Tune – Save the adjustment values for up to 20 different lenses on some models.
• Sony AF Microadjustment
• Olympus AF Focus Adjust
• Pentax AF Fine Adjustment – Find the manual for your camera and follow instructions from there. Each one is slightly different.
How to Know if Your Lens/Camera Really Is Front/Back Focusing
Auto focus is one of the most complex elements of both lenses and cameras. We ask a lot of it. AF point coverage, phase detection, contrast detection, focus tracking, and lens motors can only do so much for us without some influence from the users. So there is a lot of user error that happens when focusing. That said, checking a lens on your camera to see how it behaves is probably worth doing. We check all lenses on our cameras but, as explained above, an in spec lens on an in spec camera that are both on the edge of the acceptable range can result in the need to microadjust your AF.
The following are resources to test a lens/camera combo for front or back focusing:
• Use a Lensalign, which we rent. See instructions on how to use it.
• Use a homemade align chart or a simple method such as aligning objects in a particular way.
• Explore further about making the most out of your adjustments.
If a lens really is misfocusing we’re always happy to replace it for you. While our Receiving Team is thorough, they are still human. However, understand that it is incredibly rare to find front/back focusing outside the range of manufacturer tolerances and that, if anything, you just need to microadjust the lens to your camera.
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