We sat down with Alex Munoz this week to discuss his thoughts on camera lens calibration.
Here is an interview of Alex from Alex Munoz Visual Arts in Los Angeles, CA.
I am sure you have had that experience before: you took a great picture with either a new lens, or one you’ve had for a while, and later you notice how the image is out of focus. The higher the actual resolution of your camera sensor, and the lower the maximum aperture, the more likely this is going to happen. Without going too much into the technical details, let’s just say that the increased resolution on your camera, combined with the shallow dept of field at lower apertures, contributes to actually noticing an out of focus image. Incorrect focusing would have ruined he image below.
Hit the spot
There are many reasons for having out of focus images, such as the photographer not focusing properly, camera shake, subject movement, lens flare, or incorrect focus adjustment. While all these issues are very common, this post will detail how to correct a lens through micro adjustments, if it is focusing improperly. Sometimes you can read online reviews about lenses, and how the reviewer claims that a lens is not sharp at its largest aperture (open widest), but how it becomes sharper as it is being stepped down (aperture is becoming smaller). Often this is the result of a focus misalignment, and not a design flaw or defect of the lens.
Now keep in mind, and you may know this already, the wider the aperture, the shallower the depth of field. Stepping down a lens then will increase its depth of field, and observing a sharper image is often the result of the lens having more parts of the image in focus. Yes, there are other factors at play that may reduce clarity and sharpness on wide open lenses, but don’t be surprised when I tell you that very often a micro focus adjustment can significantly improve the sharpness of these lenses. So what is micro focus adjustment and how can it be done?
Many modern DSLR’s and mirrorless cameras offer micro focus adjustment. What this basically does is applying a minimal correction to the focus acquired by the camera in combination with the lens. Commonly these micro adjustments range from +20 to -20 steps. Because micro adjustment is made at the highest possible aperture, doing this reliably can be a challenge. There are however tools out there that make it very easy. I am personally using the SpyderLensCal, but there are other solutions out there as well.
All we have to do is take an image and look at the ruler on the right side. If the sharpest point on the image is on the 0, we are properly calibrated, if it is at any number below that, our lens is front focusing, and if it is above that, we are acquiring focus behind our target. All we now have to do is either increase, or decrease our micro focus adjustment values inside the camera. The picture below shows how that looks on a Nikon D800 before adjustment and where the lens is focusing in front of the automatically determined focus point:
After our adjustments we take another image and get the following result:
As you can see, the lens is now calibrated and focuses as good as it can (in this case it’s actually tack sharp wide open), meaning that images will be as sharp and detailed as possible with the optics inside that particular lens. Please make sure that you repeat this procedure for all your lenses, as each lens has to be adjusted individually. You may find that some lenses focus perfectly, whereas others cannot be adjusted properly at all. In such a case, you have to send your lens to an authorized service center, as they are able to calibrate or fix these problematic lenses internally. Newer lenses from Sigma also allow you to use a USB dock (sold separately), which then can be used to calibrate the lens internally as well, without the need to send it to a service center.
Can you see me now?
Next time you think your lens is not sharp enough, consider checking whether you are not spot focusing, and whether your lens needs a micro focus adjustment. Out of all the lenses I use professionally, I had only one lens that I was unable to calibrate myself, and with my other lenses, micro focus adjustment has made a huge difference whenever the lens was not calibrated properly.