Leitner's Cinematography Corner

Manual back-focus adjustment on a Sony HVR-Z7U, using a 46in. Samsung LED TV to verify the results. (Click photo to enlarge.)
Photo D.W. Leitner

I began my last column with the question, What is auto back focus?

I compared depth of field to depth of focus and described the lens factors that impact both of them. I defined correct back focus as a function of precise lens mounting, accurate to within microns of spec. And I made the case that for any given angle-of-view, the smaller the sensor, the shorter the lens focal length—and therefore the less depth of focus available at the sensor surface.

Less depth of focus always means tighter tolerances in lens mounting, which is why 1/3in. camcorders are far less forgiving of back-focus errors than 2/3in. camcorders.

Ironic, isn’t it, that smaller, consumerish camcorders require tighter tolerances in lens mounting than larger 2/3in. cameras.

So, what is auto back focus?

Auto front focusing, of course, is the familiar autofocus or AF mode found in countless point-and-shoot cameras, consumer camcorders—and these days, a few adventuresome pro camcorders too. In camcorders it involves continuously tracking the contrast of fine detail at the pixel level—why autofocus falters in murky, low-contrast situations.

Auto back focus, by comparison, is not a continuous focusing mode but a camera-assisted back-focus adjustment. To date, it is limited to a few camcorders, but will grow more familiar if camcorder manufacturers grace us with further 1/3in. and 1/2in. camcorders that bestow the advantages of interchangeable lenses associated with larger cameras.

I’m speaking, of course, about Sony’s HVR-Z7U and PMW-EX3. If you’ve followed my reviews (here and here), you already understand that these camcorders introduced a new type of zoom lens, a “hybrid� with an option of mechanical focusing or electronic focusing. Slipping the focus ring a few millimeters forward or backward toggles between the two. What’s more, in electronic focusing mode, these lenses offer a further choice: manual electronic focus (focus ring spins endlessly) or full autofocus.

What’s not to like? These are impressive camcorders and a joy to explore. However, as with any interchangeable lens system, back-focus errors are a real concern. The risk introduced by using multiple lenses is the possibility they can be seated inconsistently, mechanically speaking. As mentioned above, back focus errors are not easily forgiven in professional cameras with miniature 1/3in. and 1/2in. sensors. The razor-thin depth of focus in these compact camcorders calls for greater vigilance in properly adjusting back focus, not less.

Enter auto back focus, which Sony variously calls FLANGE BACK in the Z7 and Auto FB ADJ in the EX3. Both are menu tree items that provide automated routines for adjusting back focus in Zeiss and Fujinon zooms dedicated to the Z7 or EX3.

(Flange focal distance is the measurement of the precise distance between the mounting surface of the camera’s bayonet mount and the focal plane at the sensor’s surface. FLANGE BACK conflates the terms flange focal distance and back focus.)

Displaying a Siemens star on a Mac laptop. Mine is encapsulated PostScript—i.e. vector-based, not bit-mapped—so it can be enlarged without seeing pixels.
Photo D.W. Leitner

The first time I used the EX3’s little brother, the PMW-EX1, was to film a well-known artist working at a foundry in Walla Walla, Wash. During that shoot I had a concern about softness in the image when the lens was zoomed back to wide angle. (See last week’s column for a discussion of why.) So I located a distant image of sufficient contrast, entered the EX1’s equivalent of the Auto FB ADJ mode (found in a hidden service menu), focused the lens at full telephoto (infinity focus), and let Auto FB ADJ do its thing.

And what thing is that?

In film lenses, back focus is calibrated using an infinity image artificially produced by a device called a collimator. Next, the lens’ mount is mechanically adjusted by inserting a thicker or thinner spacer called a “shimâ€? into the mount itself, thus raising or lowing the height of the lens from the fllm plane.

Any video zoom that offers autofocus, however, cannot be a mechanical lens—a nimble element in the third lens group nearer the camera body must float under electronic control—therefore back focus must be adjusted electronically too. It is the electronic, not mechanical, adjustment of this same rear lens group that comprises back-focus adjustment in an autofocus-capable lens.

Back in Walla Walla, I had made a big mistake. I had forgotten that video lenses, unlike film and photography lenses, are adjusted for back focus using focus charts at about 10ft. instead of at infinity focus, where back focus is actually most critical. (Why do video cameras follow different conventions such as labeling ND as fractions or measuring focus from the front of the lens instead of the focal plane—unlike every other photographic system in the last 100 years? Don’t get me started.)

Yes, I had trusted the Auto FB ADJ mode and as a result, inadvertently misadjusted back focus (with unhappy consequences). Worse, I was working without a field monitor. Using the EX1′s soft LCD viewfinder, it was impossible to detect, no less verify, subtle focus differences—differences that made themselves painfully obvious when viewed later on an HD monitor. (The new PMW-EX1R, thankfully, features a new, noticeably sharper LCOS viewfinder, same as the Z7’s.)

In light of the Z7′s smaller 1/3in. sensor, Sony added an option to its FLANGE BACK menu, a choice between AUTO ADJUST or MANU ADJUST. Why would Sony do that?

FLANGE BACK menu item on a Sony HVR-Z7U.
Photo D.W. Leitner

I admire the Z7 and use it often with a wide-angle Carl Zeiss 3.3-26.4mm zoom, a longer Zeiss 4.4-52.8mm, and a handful of still camera lenses (using an adapter).

(The Z7 can, in fact, store flange back data for up to 10 Zeiss zooms, although only two presently exist for the Z7. For larger-format lenses adapted to the Z7 or EX3—be they Alpha, Nikon, whatever—no electronic flange focus adjustment is possible.)

In a nutshell, here is what I have learned the hard way. Sony’s auto back focus adjustment demands a high-contrast target image at about 10ft. If the background is of equal contrast and brightness, the auto back focus can mistakenly shift to the background—just like conventional autofocus. Which is why I highly recommend, at least in the case of the Z7, choosing MANU ADJUST.

MANU ADJUST of back focus is idiot-proof, but only if you use a large, sharp HD monitor to display your focus test chart.
Photo D.W. Leitner

Since these are HD camcorders, viewfinders and LCD screens can’t be trusted when you’re discriminating among fine degrees of focus. My solution is depicted in the accompanying photos. I display an ultra-sharp Siemens star on a laptop positioned at about 10ft. Using HDMI, I rout the camera’s output to a big 46in. Samsung LED TV, where I can clearly see every pixel. Starting at the telephoto end of a zoom, then checking the wide end, I manually determine the best focus at either end and, using MANU ADJUST, register these choices with the Z7.

Idiot-proof. You can clearly see what is in focus and what is not during zooming. No worries that a bright background object might confuse the camcorder during auto back focus adjustment.

The ability to swap lenses hails from the earliest days of photography. Tiny sensors with ultra-precise back focus requirements is a recent development. Wish to avoid back-focus anxiety altogether? Stick to camcorders without interchangeable lenses such as the HVR-Z5U and PMW-EX1, one-piece Sony counterparts to the Z7 and EX3.