• Ed Dozier

Lens Focus Fine-Tune: Things Can Change

You probably think that it’s a done deal after you get your lens focus calibrated for your camera. Maybe, maybe not.


Depending on things like how long you’ve had that lens or how many bumps have accumulated in your travels, your lens may have changed a little bit. Little changes can have a bigger impact than you might expect, however.



My middle-aged Sigma 150-600 lens


I have had my Sigma 150-600 lens for over 5 years now, and I started having suspicions that some of my shots were consistently just a little bit off from nailing focus. Unsurprisingly, the most focus misses were at 600mm, while shorter focal lengths still seemed calibrated. The same focus misses would happen on more than one camera body, so I knew it was the lens.


I noticed that the infinity focus calibration still seemed correct, but shots in the 30-to-60 foot range were off by about an inch or two when zoomed out beyond 400mm. Since this is a Sigma lens, I have the option to calibrate focus for 150mm, 250mm, 400mm, and 600mm. I can also calibrate at 9.2 feet, 20 feet, 48 feet, and infinity for each of those four zoom settings.



The focus target (from the MTFMapper site)



I use a focus target that is comprised of many little trapezoids with a big trapezoid in the middle. When the left side of this target is rotated away from the camera, the trapezoids give the illusion of being rectangles. The big middle rectangle gives your camera an easy target to guarantee focus on a known edge.


I set up my focus chart at 48 feet (14.6m) and shot it at 600mm. I use the MTFMapper program to analyze the chart shots, so that I get very precise answers about what’s in focus and what’s not. This particular distance and focal length wasn’t arbitrary; it’s one of the 16 Sigma fine-tune settings in their Optimization Pro program for this lens. I found out that yes, indeed the focus calibration was off. Not by a huge amount, but it was generally focusing too far by about an inch (25mm). By the way, the MTFMapper site offers multiple focus-chart files with different amounts of perspective distortion to use with different lens focal lengths, such as wide-angle lenses.


So, am I just way too picky? The focus error resulted in a loss of resolution of several lp/mm. If your own lenses could get improved resolution, wouldn’t you jump at that chance? I think it’s well worth the effort to do this recalibration.



Focus target detail with MTF50 edge measurements


The (cropped) shot above shows the center of my focus chart, shot at 600mm from 48 feet away. The numbers on the little black trapezoids are resolution measurements in units of MTF50 lp/mm. I always shoot in ‘raw’ format; measurements using jpeg files are vastly different. The left side of this chart is farther from the camera than the right side (it’s rotated by 45 degrees). I used phase-detect focus and aimed my focus sensor at the right-hand side of the large black rectangle (where it’s showing the yellow measurement of “43.8”). The highest resolution edges are generally about 3 “little” rectangles to the left of the big rectangle right edge. This focus error is only about an inch along the lens axis. In this shot, the measurements that are displayed in yellow indicate that those edges are perfectly vertical or horizontal, versus the cyan-color measurements on edges having a slight slant to them.


Don’t put a lot of stock in the MTF numbers in this shot, since this isn’t a proper resolution-measuring plot, the target is only a small part of the image frame, and the target (on purpose) isn’t parallel to the image sensor. It’s fair to compare these numbers to other shots taken in the same shooting conditions, however.


I have larger focus targets than the one shown above, but focus targets with smaller rectangles (trapezoids, actually) give me a better idea of exactly where the best focus is located. The measurements show that best focus is further away from the vertical edge I aimed at, and I need to use a “-” focus calibration shift to get the lens to focus closer to the camera. The “large” rectangle is only 2 inches (50mm) tall, and I remind you that I’m shooting this from 48 feet away!



Profile plot of target from MTFMapper


The profile plot shown above makes it easier to see where the focus lands (at the green line peak). The blue line indicates where the right-hand side of the target rectangle edge is (where I pointed the camera focus point). Clearly, the lens is back-focusing. The little red dots represent each measured tiny rectangle edge resolution, and the green line is a smoothed plot of these measurement averages.


Note in the plot above that the intersection of the blue line and the green line has a resolution around 44, versus the green line peak of around 54, showing resolution loss of about 10 lp/mm at the desired focus plane. As I mentioned, these resolution numbers aren’t strictly reliable for this style of chart, but they’re at least representative of the loss in resolution. Who could have imagined that a focus miss of about an inch would result in such a big resolution loss on the intended target? This is typical long-lens behavior, however (don’t miss focusing on that animal’s nearest eye, or you can really tell).


It’s worth noting that some lenses (usually the super-fast lenses) can also shift focus with an aperture change. On these lenses, you need to set the desired aperture for calibration. On most lenses, you just need to set the widest aperture for testing.


I know from experience with my Sigma lens calibration using their USB dock and their “Sigma Optimization Pro” program that I need a calibration change of about 2 units. I saw that I had previously saved a calibration setting at 600mm and 48 feet of “-4”. I reprogrammed the calibration fine-tune value to be “-6” instead, to pull focus a little closer to the camera by about an inch at 48 feet.


The Sigma lens focus-fine-tune firmware is smart enough to interpolate all of the in-between calibration settings for the lens focal length and the focus distance. The 500mm calibration would be affected by both the 400mm and 600mm calibration values.


While I had everything set up, I also took shots at the 400mm zoom setting. At the 48-foot distance, it seemed to focus accurately. I set up the target at 20 feet from my camera and repeated the tests at both 600mm and 400mm. At this distance, the 600mm setting focused perfectly, but the 400mm setting focused a little too close. I changed the 400mm calibration at 20 feet from “-10” to “-9”, to make it focus a little farther away. I didn’t see any calibration shifts at shorter focal lengths, so I left those values alone.



Sigma Optimization Pro calibration settings


As seen above, I now have an updated focus calibration for the 600mm setting at 48 feet (-6) and a new setting for the 400mm setting at 20 feet (-9). These two tweaks got my Sigma perfectly focus-calibrated again at all focal lengths and distances.


I sound like a broken record, but I wish all lens makers had this multiple-calibration feature. It makes all the difference in how a lens performs. It’s so elegant that you’d think all lens makers would adopt this scheme, but so far only Tamron has followed Sigma’s lead.



Improved focus accuracy with new fine-tune calibration


The shot above shows an analysis of the focus target using the new calibration settings. The focus (and highest resolution measurements) is shifted back to the correct distance. You always need to take several shots and re-focus between each shot. There is a small shot-to-shot variation, and you need to determine where the “typical” focus lands.



Profile plot with new fine-tune calibration


You can see in the plot above how focus has shifted back to where it should be. The expected focus point (in blue) now coincides with the smoothed actual best-focus green-line peak.


Conclusion


The focus change isn’t huge, but I’m now getting measurably improved sharpness where I aim my camera’s focus point. There’s an overall improvement of about 20 percent in resolution where I pointed the focus sensor when shooting at 600mm. A small focus error gives you a big resolution penalty. Long lenses need to be calibrated incredibly well, or the resolution will pay a terrible price. I think it’s unreasonable to expect that your gear doesn’t change a little bit with age (and abuse). It’s slightly irritating to go through this exercise more than once, but I’m glad that I did. I have to chide myself periodically to not get lazy.


Everybody ends up losing a step or two with age, and camera gear is no different. I’d recommend you take a second look at your own cameras and lenses just to verify that everything is operating at optimal efficiency. Also, do your testing at the same kind of temperatures that you photograph at, since thermal changes can also throw off focus. This particular lens is made out of “thermally stable composite” to minimize focus changes with temperature.


I’m keeping an eye on any more changes in this lens, but I haven’t noticed any more calibration shifting yet. There’s no way of knowing why the calibration shifted, but the important thing is to note that it hasn’t shifted any more since I did these tests. If I were to find yet more focus shifting, then it would probably mean that this lens has reached retirement age.


As an aside, I also pointed a flashlight into the front of this lens, and it still shows NO visible dust has gotten sucked into it after over 5 years of pretty heavy use! Sigma really did a great job of sealing this lens. I’d still recommend that you stay away from those “colored-dust-throwing” festivals, however. Let’s not press our luck.


As an additional aside, I’d again like to thank Frans van den Bergh for providing his excellent MTFMapper software and test chart files. His program can help you take your camera gear to the next level.

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