Do You Need To Refocus After Zooming On Fuji-X Lenses?

Pond, reeds and rocks, Acadia National Park, Maine (click to enlarge)
X-T2, 50-140mm f/2.8 lens @ 180.9mm; 1/17th sec. @ f/16; ISO 200

The question I wanted to answer was to determine if it is or is not necessary to refocus a zoom lens after you change focal lengths. In other words, once you focus your zoom lens, do you need to refocus it every time you zoom it in or out or can you trust the original focus to remain accurate no matter at what focal length you eventually choose when you press your shutter button?

In my last post, I took a close look at my Olympus 12-100mm f/4 PRO zoom lens in regards to this and was able to successfully answer the question.  You can read that post here.  It is an interesting read.

In this post, I wanted to conduct the same tests for my Fuji-X zoom lenses.  Currently, I own four Fuji-X zoom lenses; the 10-24mm f/4, the 16-55mm f/2.8, the 50-140mm f/2.8 and the 100-400mm f/4.5-5.6.  I tested each lens in an identical manner.


For those of you who have read my post on the Olympus 12-100mm f/4 lens, you can skip this next couple of paragraphs as they recap of some of the background information about different types of lenses we call zoom lenses.

There are prime lenses, consisting of only one focal length and then there are zoom lenses, which allow you to change and use many focal lengths without changing lenses. (Leica makes some multi-focal length lenses that allow you to switch between three predetermined focal lengths but nothing in between, but that is another story)  Zoom lenses go back to about 1902 when the first was patented, according to Wikipedia.

In the past, zoom lenses did not have a good reputation due to the many compromises in image quality that had to be made in order to facilitate the ability to offer a variety of focal lengths in one lens.  However, that has changed dramatically as with the advent of computer lens design and modern manufacturing of aspherical lens elements, zoom lenses have largely (not exactly, but pretty close in most cases) equaled most prime lenses.

However, there still is an issue with zoom lenses other than image quality that most don't realize or to which some don't pay much attention.  The question arises, "Do you need to refocus your lens after you zoom?"  The answer is "depends," of course, as does most things concerning photography.

There are two types of zoom lenses.  A parfocal lens theoretically maintains accurate focus while changing focal lengths, in other words, when you focus at one focal length then change to another focal length, the focus stays accurate.  No need to refocus the lens.  The other type of multi-focal length lens is called a vari-focal.  This type of lens has to be refocused each time after changing focal lengths as it is not designed to maintain accurate focus.  Zoom lens, in today's culture, is the name often times given to both.

This had me thinking that I have never tested any of my current zoom lenses, either for my Fuji system or my Olympus system.  Are they parfocal or vari-focal?  If I don't set out to test them, I really won't know.  However, my habit has always been to refocus after zooming.  I am almost obsessive about this due to using zooms since the 1970s and how they almost never maintained accurate focus after zooming.

In this post, I tested all four of my excellent Fuji zoom lenses.  I tried to set up a testing protocol to remove any variables other than focus accuracy after zooming.  Here is how I set it up:

(Disclaimer:  this test may or may not be scientific in nature.  I did my best to take into account all variables and eliminate any variable except focus accuracy in the testing procedure.  Your results with your lenses may or may not vary from my results.)

Before I placed my Fuji X-T2 on a very large and sturdy tripod, and due to the difference in temperature and humidity indoors to outdoors this time of year where I reside, before I began I acclimated the camera and lenses to the outdoors for about 30 minutes. The camera was leveled and squared to the target.  Only the center of the image, where the focus point landed, was considered when gauging accurate focus.

The test was basically set up to cause focus to be more critical rather than less critical to attain a sharp image. In other words, widest aperture, relatively close distance.  Less depth of field influence.

The camera was attached to the tripod via a secure L-plate.
The camera was set in Aperture Priority at each lens' widest aperture.
The camera was set on single frame advance using the electronic shutter feature to eliminate any potential shutter shock or camera vibration due to shutter movement
The camera was set for a 2-second delay also to eliminate any potential camera vibration
S-AF was used
Single focus point selected—mid sized
Facial Recognition turned off
Daylight white balance
Base ISO—ISO 200
Image stabilization in the lenses that had it was turned off

Here is how I made the exposures.

For the first test set with each lens, I set the lens' focal length at its widest focal length, then focused the camera on a flat brick wall with good detail and contrast (except the 100-400mm lens which I needed more distance and focused on a different subject). Remember, I'm only going to consider focus accuracy from the small area under which the focusing spot highlighted in the very center of the frame.  Corners and edges were not considered. I then zoomed to the lens' longest focal length and exposed a frame.  For direct comparison with that exposure, keeping the lens at it longest focal length, I then refocused and made a second exposure. I repeated this, and all test subsequent exposures, 5 times to remove any anomalies or sample variation.

(Note: any slight tonality differences in comparison images is due to slight variation in sun/cloud cover.  Other than that, the images were made in an identical manner)

Below is a 100% crop of one of the exposures focused at 10mm with the 10-24mm zoom, 
then the lens zoomed to 24mm, then the shutter pressed.  Each image can be seen with a greater magnification if you click on the image.

Below is a 100% crop of one of the exposures focused directly at 24mm with the 10-24mm zoom, then the shutter pressed.

Below is a 100% crop of one of the exposures focused at 16mm with the 16-55mm zoom, then the lens zoomed to 55mm, then the shutter pressed.

Below is a 100% crop of one of the exposures focused directly at 55mm with the 16-55mm zoom, then the shutter pressed.

Below is a 100% crop of one of the exposures focused at 50mm with the 50-140mm zoom, then the lens zoomed to 140mm, then the shutter pressed.

Below is a 100% crop of one of the exposures focused directly at 140mm with the 50-140mm zoom, then the shutter pressed.

Below is a 100% crop of one of the exposures focused at 100mm with the 100-400mm zoom, the lens zoomed to 400mm, then the shutter pressed.

Below is a 100% crop of one of the exposures focused directly at 400mm with the 100-400mm zoom, then the shutter pressed.

As you can see, the lenses held accurate focus even during extreme focal length change. That is very beneficial as it give the photographer a level of confidence that if minor or even larger adjustments to focal length and/or composition occur, the focus stays accurate.

(Note: to be absolutely accurate, I did see a very minute difference in the 100% crops of the tests for the 16-55mm f/2.8 lens.  However, that being said, the difference was so small as to almost be unrecognizable unless one was specifically looking for any perceptible differences as I was. For all practical matters, there was no difference.)

For the next set, I wanted to see the difference when zooming the other way, from the longest focal length to the shortest focal length or widest angle.  I set the focal length at each lens' longest, focused, then zoomed to its shortest and made an exposure without further focus.   I then focused at the shortest focal length and made a second exposure. Again, I repeated this 5 times to remove any sample variation.  For space and time's sake, I'm not going to post those images so I can get this out today.

The results of the second set of tests was identical to the first.  All four lenses remained in sharp focus when zooming from full telephoto to full wide angle.  Of course, there may be some influence of the increased depth of field at the wide end of the zoom that could certainly compensate for any small errors.  But I suspect the focus remained accurate as it did in my first set of tests.

For the third set, specifically with the 100-400mm f/4.5-5.6 lens, I wanted to find out if I could see any difference between using the smallest focusing square and the intermediate sized focusing square.  The reason I wanted to test this is that in the past, I found it more difficult to acquire sharp focus on distant small objects (song birds) with the smallest square and had more success with a larger square.  Same procedure was used as in the first focus test set.

In this test, I found no difference in focus accuracy.  However, this test may not have been the best to access the issue at hand.  The subject matter I used as a focus target has different characteristics from the problems others and I have had acquiring accurate focus on small, distant objects with the smallest focusing square. The problems in the past occurred with trying to acquire accurate focus on distant small birds.  The birds in the area wouldn't cooperate and stay still long enough to conduct a test. (Just kidding) So, it may not be a valid test for that issue.  Focus was just as accurate with either square in this case.

To sum it up.  Good news.  All four lenses maintained sharp focus no matter which way I zoomed and pressed the shutter after initial focus.  This is in sharp contrast to my last post when I quickly discovered the excellent Olympus 12-100mm f/4 lens badly lost focus when zooming from wide to telephoto.  It did fine zooming from telephoto to wide, but some of that may be attributable to compensation of increased depth of field at wide angle focal lengths.

Are these lenses "parfocal lenses?"  To me the answer is a resounding yes.  However, and of course there seems to always be a however, I will continue to refocus after zooming due to my obsessive nature, past history with lessor zoom lenses and a certain amount of paranoia with which I suffer in doing everything I can to ensure I've covered all bases to achieve success.

You may want to test your own lenses of any manufacturer to better understand their characteristics.  Again, another example of better knowing your gear for more success.

Thanks for looking. Enjoy! 

Dennis A. Mook 

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