Back to the gear head stuff today. I’m doing the lens auto-focus fine-tune on the new 28mm primes lens.
I’ve read the Internet write-ups on this lens and one of the common refrains regarding this particular lens is that it has a phenomena known as “focus breathing.”
Focus breathing on a prime lens is when the camera is focused on a subject and the focus changes when the aperture or focus point is changed. There are other types of focus breathing one can experience with lenses, but they don’t really apply to a prime lens.
With the Nikkor AF-S 28mm f/1.8 G, the reported focus breathing occurs between the aperture of f/1.8 and stopping the lens down to a smaller aperture, say f/5.6 or more. What this means is, if I focus on a subject with the aperture set for f/1.8 and then change the aperture, the lens will drift into a back-focus situation as I stop the lens down more. End result, the focused on subject is no longer in focus. This problem can be bypassed by using live view to focus on the subject, as the focusing method in live view is different from using the viewfinder focus. This focus breathing issue isn’t as severe of a problem as some websites will lead you to believe. It’s a characteristic of this lens and many other lenses on the market.
The real issue here is doing a micro-focus calibration to account for how one intends to use the lens. By the book, one is told to do the micro-focus calibration at maximum aperture, f/1.8 in the case of this lens. But, I don’t intend to use this lens at f/1.8 much. That aperture will produce a very narrow depth of field on close subjects. I intend to use this lens as a landscape lens and will be working at apertures of f/5.6 – f/16 and my intended subject matter will generally be further from me than 50ft or so. I want maximum depth of field on distant objects in otherwords. Closer objects with mountains in the background, everything in focus from front to back. F/1.8 is more of a close in aperture for shooting people and things that are going to be very close to me. I don’t do much of that type of photography.
So, how to calibrate the lens? Since I’m going to use it primarily as a landscape lens, I calibrate the lens to an aperture that I’ll probably be using. In the case of a 28mm lens on a full frame body, at f/5.6 just about everything from 50ft away to infinity will be in focus, therefore I calibrate the lens with the aperture set to f/5.6. That prevents the lens from back focusing because of being calibrated at f/1.8. All I have to do is remember that if I use the lens for a head shot of someone who is 5 feet away, I need to make sure the aperture is stopped down to f/5.6 or more. If I need that really narrow depth of field, I switch to live view and focus using contrast detection at the sensor.
So, how much adjustment difference will I see by calibrating the lens at f/5.6 vs f/1.8. I did the auto-focus micro-adjustment calibration at both apertures and it definitely produced a noticable difference. At aperture f/1.8, the micro-focus calibration on a scale of -20 to + 20 clicks landed at +1. When I performed the same micro-focus calibration at f/5.6, I got a totally different lens error correction at -11 clicks. So, there you have it. The focus breathing is real and if one doesn’t pay attention to how they calibrate their lens, they can go astray quite easily. A 12 click different in focus calibration is enough to cause a problem if you don’t know what you are doing. If you want to get into some really wonky stuff, try figuring out how to calibrate a zoom lens. With zoom lenses, you add the dimension of different focal lengths as well. You can only accurately calibrate your lens to one specific setting (some cameras have wide and telephoto settings) but those are still fixed points in a world full of variation. The idea is to get it to be as close as possible where you intend to use it and hopefully everything else will work out to be acceptable.
So the lesson I’ve learned over the years is this. Calibrating your lenses to your camera body is a good idea. I’ve never seen a lens that is perfect and virtually every lens I’ve calibrated has required a small tweak to get it to focus dead on. But, the thing to understand and remember is that when you do a micro-focus calibration on your camera with any lens, you are calibrating the camera specifically for that lens at those settings. I always calibrate my lenses at the settings I intend to use them. Not for what the book says to use. That way, I’ll have fewer problems in the field with something being front or back focused when using auto-focus.