How can lens cause consistent front or back focus?

Question

I can understand body focusing problems, but I can't imagine why lens might cause front or back focus. I have two third party manufacturer lenses for my Nikon: one is front-focusing, the other is back-focusing. The body works fine on five other lenses, 4 of those from Nikon.

Correct me if I'm wrong: the phase detection focus sensor gathers light from different angles and steers the motor back or forth, so that images on focus point (be it stripe or cross) align. Given that everything is right with the body, the system of mirrors creates the same image on the autofocus sensor as it does on the film or digital sensor.

Now, assuming body mirrors are calibrated, either the camera algorithm didn't move the motor, when its focus sensor still detected misalignment or the lens moved between mirror going up and shutter firing.

I can understand that some motors might not be precise, but that would give inconsistent focus depending on the direction of the last correction. I can understand poor quality lens, that would blur parts of the image, give chromatic abberations etc. I can understand non-planar focus shape, but it still doesn't explain why the image on the focus point would not be in focus. I can understand moire effects, but that is not the case I observed.

I see no way that the lens can create a consistent focus shift other than body doing this on purpose. How can lens cause front or back focus?

Answer 1

In your case one of two things is probably happening:

• The lens in question is an older design that does not include a position sensor to report back to the camera how far the focus elements actually moved when the camera has sent an instruction to move a specified amount.
• The lens does have such a focus position sensor, but it is in need of calibration.

In either case, AF Fine Tune (Nikon) or AF MicroAdjustment (Canon) can help the problem if the camera in question has AFFT/AFMA capability. AFFT/AFMA will more effectively correct the second case than the first case listed above. If the first case applies, then the first issue identified in HamishKL's answer can still create difficulties if the lens also has that problem.

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A more complete explanation and background for the above answer

Your question appears to make an incorrect assumption about the way Phase Detection Auto Focus (PDAF) works in most cameras. It seems you believe the PDAF system in DSLRs use the optical AF sensor to confirm AF has been achieved before taking a picture. This is not the case. The vast majority of PDAF systems do not take a second optical reading using the PDAF sensor in the camera to confirm AF has been achieved before allowing the mirror to begin swinging up out of the way so that a photo may be taken.

The answer by @HarnishKL correctly identifies ways that a lens may still be soft when it is focused as well as it can be, but it misses the most likely reason for why a lens will consistently miss focus in the same direction: because the lens isn't moving the same amount as the camera has instructed it to move.

When PDAF systems were first developed back in the film days, the emphasis was placed on speed. To be attractive enough to buyers considering upgrading from their manually focused cameras and lenses the new AF systems needed to minimally be at least as fast as a moderately skilled user could focus and also at least as accurate as that user could focus. If the AF system could be either faster while being just as accurate or more accurate while just as fast, it made the cameras with AF more attractive. To do both noticeably better was a little outside the technological capabilities at the time. The computing power of chips small enough to fit in an SLR then was a lot less than the computing power of today's chips.

In the late film era when PDAF systems were born a very low percentage of photos were ever printed or viewed at more than about an 8x10 display size. The vast majority of them were viewed at 4x6 inches. The standard for good enough in terms of focus accuracy was a lot lower then than it is now in the current 36MP, 100% pixel-peeping, 96ppi large HD monitor era. So the emphasis back then was placed on focusing speed.

Because the prime consideration for PDAF systems was speed, until fairly recently most PDAF systems were what can be described as open loop. The camera used the optical PDAF sensor to measure how far and in what direction the lens was out of focus, the camera sent instructions to the lens regarding how far in which direction to move focus, and then the camera took the picture. The camera did not use time taking a second reading via the AF sensor to confirm focus had actually been achieved. It would have taken too long to make PDAF systems usable.

Even more recently, cameras that do attempt some type of confirmation usually use a position sensor on the lens to confirm it actually moved the instructed amount. They still don't normally take another focus reading using the AF sensor. It would still take too long for many applications.

Here's why: In order to make AF and frame rates as fast as possible, as soon as the AF sensor has measured focus and computed the time it expects the lens to take to move the instructed amount, if the shutter button is pressed all of the way down the mirror begins to move out of the way. Once the mirror is moving the PDAF optical system is disabled. But a position sensor in the lens that measures how far the focus has been moved can measure and confirm a specific amount of movement and communicate it to the camera during the time the mirror is swinging up. If any additional movement is needed the camera can send another instruction to the lens to move the amount, as reported by the position sensor in the lens, that is still needed. It can only confirm this movement via the sensor in the lens, though, because the PDAF optical sensor is blind at this point!

As Roger Cicala of lensrentals.com discovered and pointed out in part 3B of his "Autofocus Reality" series for his blog, it takes both a camera and a lens with this position confirmation capability to get the increased accuracy of the more refined system. If the lens has the position sensor but the camera doesn't pay any attention to it, it does no good. If the camera has the capability but the lens has no position sensor, it does no good. It takes both a lens with a position sensor and a camera that utilizes the information it provides to get any of the benefit. But even then, if the position sensor is a little off when it measures the position of the lens' focusing elements, the camera's AF system need to be instructed to correctly offset the inaccuracy.

With the new Sigma Global Lens series of lenses, an optional USB dock can be used to actually calibrate the lens to correct for incorrect focus element positions, rather than have the camera compensate for the expected error.

Answer 2

I can think of a few reasons, assuming no faults with the body, and that you are exclusively referring to AF lenses. Also assuming you are photographing in normal visible light:

1. Mechanical wear of the lens focusing helicoid. This might mean that although the camera does indeed achieve perfect focus, the lens may 'relax' back out of focus after the AF routine has completed, as the shot is being taken. You can usually see this though and often appears as focus inconsistency from shot to shot.

2. The lens' optical groups are misaligned or damaged. Despite what you might think, and contrary to what you correctly stated about phase detect focus, it seems that a camera can indeed think that it has achieved perfect autofocus even though the lens is clearly out of focus. I've only ever seen this with obviously damaged lenses (both dSLR and point & shoot), but it does happen. The fault is often entirely with the lens, as other lenses on the same bodies can still work fine.

3. The last reason is one of the lens' focal plane being too far forward/backward due to manufacturing error. In this case you need to know exactly what the problem is before you can safely recommend compensating with front/back focus adjustments in the camera; these adjustments only work satisfactorily if the lens groups are all in correct alignment but the focal plane is still too far back/forward. Sometimes one or more groups/elements is out of alignment, negatively affecting other aspects of optical performance - not just gross focus.

All of the above problems are much less common with sturdy metal lenses, and even less of a problem with tough manual focus masterpieces of days gone by.

Answer 3

Answer written for the following question that was closed by the OP as a duplicate of this question. There are some other issues in play that answer that other question beyond the issues identified in this question:

Why is fine focus adjustment sometimes necessary on DSLRs? [duplicate]

Many DSLRs have a "fine focus adjustment" option, stored in the camera per lens.

Why is it that some lenses need this?

Why do some lenses not project a focused image onto the sensor when the camera body has adjusted it to a point where it believes focus is correct?

I assume this is a side effect of NOT using the sensor itself to confirm focus, using the phase detection array instead...

It's a side effect of a LOT of things that all conspire to make minor focusing issues more noticeable than they once were.

• For most PDAF systems, neither the PDAF sensor nor the imaging sensor confirm focus. Rather, the PDAF measures the amount and direction of defocus and instructs the lens to move by that amount. If there is any confirmation, it is usually based on a position sensor in the lens confirming that the lens physically moved the instructed amount. This confirmation is often done while the mirror is swinging up and the PDAF system is already 'blind'. With PDAF there is no confirmation that the desired object is in focus but only confirmation that the lens moved as instructed. Only the newest cameras and lenses are capable of even that type of confirmation.
• Digital sensors are a lot flatter than most film was at the time it was exposed. Errors in focusing that would have been hidden by the imprecise shape of the film (and, in the case of color film, the varying depth of the three separate dye layers) are no longer hidden by much flatter digital sensors.
• Digital sensors continue to increase in resolution. Errors that were once within the tolerances of larger pixels are now large enough to be detected by smaller pixels. If a blur circle is smaller than the sensor's pixels it is no more blurry than a theoretically perfect point of light. If that same sized blur circle is larger than the much smaller pixels in another camera, it is detectable as blur compared to a smaller blur circle or point of light that is still smaller than the smaller pixels of the second camera.
• Lenses available on the consumer market continue to increase in resolution. Focusing errors that were masked by blurrier lenses are no longer masked by much sharper lenses. If the focus error created blur smaller than the maximum resolution capability of the lens, it wouldn't be noticeable. If the same amount of blur is larger than the maximum resolution capability of a different lens, it can be detected. In the past some of the sharpest consumer lenses were manual focus only (many of them still are). Thus variation in PDAF systems have no effect on images taken using manual focus only lenses.
• The current resolution of camera sensors and lenses exceeds the tolerances to which we can manufacture them at reasonable cost. Not only are tolerances as small as 50 microns too large to go undetected with regard to the 'squareness' of the camera's lens mounting flange to the camera's sensor, but there are also tolerances that are larger than the minimal variations we can see the effect of with regard to PDAF array positioning, lens focus element movements, lens focus position sensor, etc.

Roger Cicala, chief lens guru at lensrentals.com, has written several series of blog entries regarding both manufacturing tolerances and Autofocus Reality.

Regarding manufacturing tolerances, please see:
This lens is soft and other myths
This lens is soft and other facts
Optical Quality Assurance

For why sensor resolution matters with regard to how well we can see where a lens is less than perfect (and all real lenses are less than perfect, not just in their implementation but also in their design if they are intended to image more than a single wavelength of light), please see:
Why We’re Going to Start Testing Cinema Lenses. And Why We Haven’t Before.

For further reading on AF systems, Roger Cicala's Autofocus Reality series is very insightful: Part 1, Part 2, Part 3A, Part 3B, and Part 4. And: How Auto Focus (Often) Works