Zeiss ZF 25 - massive backfocus at a distance

Started Sep 15, 2007 | Discussions thread
Joseph S Wisniewski Forum Pro • Posts: 35,734
It's called "spherical aberration"...

digislr wrote:

I was wonderting if anyone else has a Zeiss ZF 25? I have that and a
50 and a 100 coming but the 25 is massively backfocusing on my Fuji
S5, but only at a distance. Up close I can get razor sharp, even at
the closest focus distance. If the camera is between 2.8 and 5 and I
confirm focus with the dot in camera, the resulting shot is way
backfocused. if I am in close up distance I get can super sharp
images, even at 2.8.

It's perfectly normal, for many lenses.

There's an annoying optical phenomenon called "spherical aberration", and typically shortened to "SA". What it means, basically, is that you can view the lens as a series of concentric rings, like the rings on a cut tree. The outermost ring is just light that comes from the outer edge of the lens, the "wide open" part (we call these the "marginal rays"). The very center of the circle is the fully stopped down part (the "chief rays"). There's an infinite number of rings, an f2.8 ring, f2.8001, 2.8002, etc. but we're going to treat your Distagon as if there were only 5 rings, an f2.8 ring, f4, f5.6, f8, and f11, and an f16 circle in the center of it all.

SA is when light from these different rings focuses at different planes. So when you've got the f5.6 ring focused perfectly, the f2.8 ring is focused in front of the sensor (and light from behind your subject is actually in focus) and the f11 ring is focused behind the sensor (light from in front of the subject is actually in focus on the sensor). This effect varies with focusing distance. It's normally only possible to optimize a lens for good SA at one particular distance. The Distagon is optimized for close up performance, as you've already learned. (then why did they call it a "Distagon"? I wish I knew). Nikon was among the first (with their "close range correction system" lenses) to concoct lenses with a system where the entire lens didn't move as one unit to focus, instead different groups of elements had different gearing and moved at different rates in relation to each other, to fight SA at different focusing distances. I don't think your Cosina/Zeiss has this.

I didn't just pick f5.6 at random: it's very important to the functioning of the camera. Both the AF sensors and the MF focusing screen (and split image prism, if you have one) concentrate on the f5.6 ring. More on that in a minute.

SA causes two annoying effects. First, it makes the lens softer at wide apertures. At f2.8, you've got the total of six different rings, each focused at a different plane. Put them all together, and you get the characteristic Zeiss softness (just kidding). But you do get softness, and on just about any lens.

Second, there is a "focus shift" with aperture. Since the rings in this example are 1 stop wide, opening up one more ring worth doubles the light. That means that the f2.8 ring contains as much light as all the other rings (f4, 5.6, 8, 11, and 16) put together. So, an f2.8 shot is 50% f2.8 ring, 25% f4 ring (and the f8, 11, and 16 rings, being 12.5%, 6%, and 3% of the light, respectively, hardly count, except for reducing the so-called "microcontrast"). The overall effect is a shift in focus backward relative to the f5.6 ring that you used for focus. So, if you shoot an f2.8, f4, f5.6, f8, f11, and f16 picture, the plane of focus will appear to shift as you stop down.

Now, I said the f5.6 ring was "special". The autofocus sensors in most cameras, including the S5, are aimed at diametrically opposite (think "north pole" and "south pole", except east and west) points on the f5.6 circle. (the sensor angles are + - 5 degrees, for those interested). That insures there will always be something to focus on with an f5.6 or faster lens. Typical split image screens like the Katz Eye are also set up for f5.6, + - 5 degrees. (Nikon used to make multiple split image screens, you could change to an f11 screen for macro work or long teles and teleconverters, or an f2.8 screen for fast lenses. But those days are gone...) And even the ground glass part of the viewfinder is set up for about f5.6 or f4. It scatters light in a + - 5 degree cone, so you get to use most of the light from the slow zooms that started getting so popular in the late 80s, and you get the brightest possible viewfinder and the best focusing "bite" with those lenses. (earlier cameras used to have a + - 10 degree cone for focusing well with the "classic" normal lens like a 50mm f1.8 or 50mm f1.4). The big difference is that both a prism and the AF sensor only see the f5.6 ring, while the ground glass gets the sum total of the f5.6, 8, 11, and 16 rings.

So, for a perfectly aligned camera, the AF sensor and the split image will always agree with each other, but a lens with an SA problem will only produce a picture that agrees with the AF system for a shot at some particular "magic aperture" between f4 and f5.6, when the back focus outside the f5.6 ring and the front focus inside the f5.6 ring cancel each other out. The magic aperture will vary from lens to lens.

SA will also cause the ground glass to disagree with the split image, the AF sensors, and the image that the sensor sees. The "magic aperture" for a modern bright screen is about f5.6, for an old style screen f2.8-f4, and it's around f8 for one of those Katz Eye with "opti-brite" screens.

Unfortunately, although it's possible give the camera a table of SA corrections for many lenses, each at several different apertures and different focusing distances, no camera maker to date has actually done this. Even the "tuning" functions on the new Nikon and Canon cameras only lets you adjust the lens to a different "magic aperture". You might tune the D3 so you Distagon is great at f2.8, then you'll get outrageous front focusing at f8.


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Ciao! Joe


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