FINE-TUNING YOUR AF AIN'T LIKE DUSTIN' CROPS, BOY!

Time to reactivate the blog. It's something that scratches an itch but sometimes infrequently, so apologies for the terrible lack of posts in recent months. Having been added to several lists of Bloggers by Tweeters recently, I guess it's best if I do something to put that right...


A technical post, then, first off.

Something that I've recently worked on, with very positive results, is making sure that the Autofocus on my pair of D3s bodies and my D4 is working as well as it should.

The D4 AF Fine Tune, within the Set Up menu. I found with all my lenses that this body likes to back-focus (i.e. sharp focus is actually behind the subject) quite a bit, so all lenses have individual adjustment to make them
focus in front of where the AF system wants at its default, or they use the body default I set up of -4.

ALL mechanical devices are built to tolerances. That is, "10mm" may be between 9.99mm and 10.01mm (for example) and the machinery should still work. The more expensive a thing, normally the tighter the tolerances, because you are paying for tighter quality control as much as anything.

More after the break...




This works with photographic equipment, too. The mounts on interchangeable lenses may vary fractionally in depth, the glass elements in lenses may fit into the lens fractionally differently. Even the AF module in the body will have a tolerance to its fitting. Note that we are talking Phase Detection AF here, the type of AF that gives DSLRs their focussing speed and ability to track moving subjects well.

All these tolerances add up and with today's higher resolution camera bodies, it's become more important to have some way of tweaking the focus system of DSLRs to allow for all these fractions and get best use of glass. Especially if it's long, fast glass, with very narrow depth of field allied to a high-res body.

One of the advantages of most modern Nikons (and Canon etc, too) is that they have Live View, the ability to use the sensor like a compact camera. Why does this help here? Well, Live View uses the other common AF system, Contrast Detection, which works directly from the sensor plane. It's slower and less able to track movement but being based on the sensor plane - where the image is actually made - immediately irons out all those mount and module tolerances.

Put the two together and it's possible to adjust the AF of modern DSLRs. Nikon call it AF Fine Tune and it's found in the Set-Up menu.

Of course, Nikon then promptly do what they do when it comes to Sensor Cleaning, too. Which is to say "NO! Don't do this!"

Erm. But it's there and it's straightforward, if a little time consuming to do. And easily undoable, so I'm slightly puzzled by Nikon's stance.

However...

Nikon publish their own link to the process https://nikoneurope-en.custhelp.com/app/answers/detail/a_id/51633 which is useful.

Additionally, I recently came across another method - "Dot Tune", linked here http://www.fredmiranda.com/forum/topic/1187638 which is intriguing and potentially time saving, since it requires no image capture to make any adjustments.

Dot Tune works by first focusing on a decent target using Live View (which, remember, gets focus off the sensor plane, thus ignoring all the mount tolerances etc). Then you adjust the AF Fine Tune using the "in-focus" dot that appears in the viewfinder until it sits in the middle of its tolerance for confirming focus. It makes more sense in the link.

The bad news; you'll need to do it for every lens and lens/converter combination you use - in my case 300mm f2.8 alone, then plus 1.4, 1.7 AND 2.0x converters. And the 70-200 alone, then plus 1.4 and 1.7x converters if you use those on the zoom, too. Ideally you do the 70-200 at either end of the zoom range also, averaging any adjustment if necessary...

Eek.

But the good news is, and it's most noticeable on the 16mp D4, that the 300+1.4x combination, used wide open (a very narrow Depth of Field and so margin for error!) now visibly performs much better.

In the D4 AF Fine Tune menu - this is the "bare" 70-200 f2.8 VR2 and that's a fairly
big adjustment. But it does make things very much sharper, especially wide open.

I did find Dot -Tune less useful with the 12mp D3S bodies. It seemed to me that with these cameras focused in Live View, the "in-focus" dot was lit across most of the adjustment range, so these were adjusted using the Nikon method and sharp focus judged by images displayed on computer screen. It may well work for you.

As an aside, I also found that both the D3s bodies were much closer to a base 0 in all their adjustments. Read into that what you will about relative QC moving from D3s to D4... 

Be warned as well, that the focus distance recommended for AF Fine Tune work uses a target that's 50 x focal length away. That's 20m for the 300/1.4x (420mm equiv) so you might struggle to back up far enough with big glass!

It takes a while to do thoroughly. Everything needs to be repeatable, a heavy tripod is essential and you will probably have to perform the Fine Tune regularly as the camera wears in normal use.

The reward, as I've noticed, is visibly crisper images, particularly with long lenses.