Size Matters

Osprey taking flight with fish in talons. (click to enlarge)
Fuji X-T2, 100-400mm f/4.5-5.6 lens + Fuji 1.4x tele-converter @ 560mm (840mm-e); 1/1000th sec. @ f/8; ISO 500

Late last year, after a long weekend wildlife photography trip with some friends, I discovered many or my images of small wildlife subjects, such as birds, were not in sharp focus.  That was unexpected and disheartening as the focus on my Fuji X-T2 was clearly locked on these creatures.  There was one in particular bird, a Belted Kingfisher, that was perched on a limb, would leave and dive into a small canal below, then return with a fish to the same spot.  I photographed the bird over and over again sitting, taking off and landing upon the same branch.  Many were not sharp.  As I had things set up, they all should have been.  You can read that post here.  

All this was baffling to me as my Fuji X-T2 and Fuji 100-400mm f/4.5-5.6 lens was firmly mounted on a huge and very sturdy Induro carbon fiber tripod with the equally huge and sturdy Really Right Stuff B55 ball head.  

Shutter speed was fast—1/2000th second to stop any movement.  Aperture was stopped down one stop to f/8 (however with this particular lens wide open is still excellent) and the image stabilization on the lens was also turned off.  I was using single shot AF and sequential shooting of at least 8 fps.  Completely stopping the bird's motion was not the issue.  Camera movement was not the issue.  The issue was that even though the focus locked onto the bird (which was relatively far away and very small in the viewfinder), focus varied.  Focus was inconsistent.  Some were in sharp focus, others missed sharp focus by just a little.

I normally keep my "focus box" on the middle setting.  As I photographed this bird over two days and saw after the first day that many of my images were not dead on focused, on the second day I made the focus box as small as possible.  Logic told me that the smaller the focusing area, the more accurate.  That night, looking at my images on my computer screen, I found the same thing.  Many images that should have been in crisp focus were not.  I had no idea why my excellent camera and lens didn't want to hit focus every time.

After I posted this issue to my blog, I received two comments, both suggesting the same thing.  Both "Doug" and Ton van Schalk suggested that enlarging the focusing box gave them a higher hit rate than medium or small.  Helpful suggestion.  Since I didn't do any more wildlife photography last winter, I filed it away, or thought so, and would remember it the next time I photographed wildlife.

Example of focus just missing using a "smaller" focusing box. (click to enlarge)
Look at the head and wing feathers and you will see it is a bit soft
Fuji X-T2, 100-400mm f/4.5-5.6 lens + Fuji 1.4x tele-converter @ 469mm (703mm-e); 1/1000th sec. @ f/8; ISO 500

Same series of shots, however, the focusing box was enlarged to its largest size (click to enlarge)
Again, look at the eye and wing feathers.  Focus is dead on.
Fuji X-T2, 100-400mm f/4.5-5.6 lens + Fuji 1.4x tele-converter @ 469mm (703mm-e); 1/1000th sec. @ f/8; ISO 500
(In my opinion, this is really superb sharpness for an APS-C zoom lens, wide open with a 1.4x tele-converter attached!)

On Tuesday of this week, I spotted an osprey in a nearby dead tree eating a fish.  I mounted my Fuji 100-400mm lens with the Fuji 1.4x tele-converter to my X-T2 and moved slowly closer to try to capture a few images of it having breakfast.  I made several images of the bird picking apart the fish then decided I better take a look to see if focus was sharp.  Well, for some of the images it wasn't!  I then thought of Doug's and Ton's suggestion and enlarged my focus box from the middle position to its largest position and made some additional images.  All were focused crisply and accurately.  Their suggestion was right on.

In the illustration above of the osprey eating the fish in the dead tree, the only difference in the two images is the size of the focusing box.  In the top one, it was sized second from the smallest.  Again, logic tells me that it would be more accurate being smaller and not including any extraneous subject matter to confuse the focus module.  In fact, when first buying my X-T1, I remember reading that the smaller the box the more accurate the focus but the slower to acquire accurate focus.  In the lower image, the box is sized at its largest size.  I was under the impression that the opposite would be true—larger sized box, less accurate focus, faster focus acquisition.  However, I have now proved to myself that the larger box seems to be more accurate when placed over a very small object at a great distance.

I post this so you have the benefit of the knowledge I've gained from my own experimentation as well as from those knowledgeable readers who send in suggestions from their photographic experience.

I'm not sure why focus is better when plain blue sky is included within the focus box as opposed to just the bird's head, but it was.  Evidently, this is kind of a quirk in Fuji's focusing system but it is a quirk that seems to work and is good to know.

Thanks for looking. Enjoy!

Dennis A. Mook 

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