on-axis front tilt focusing-technique

[daniel dumitru]

Hello. I have just acquired an acra swiss camera with the micrometric orbix or on-axis tilt ability to the front standard. I have only previously used base tilts to focus and was wondering if those of you with on-axis tilt ability to the front standard would be kind enough to share your favorite technique of focusing landscapes. I have read the various books (stone, stroedel, etc.) and the articles in this forum's beginners sections but I could not really find where they address an actual field technique for focusing with the use of an on-axis front tilt method as opposed to the base tilt which is covered in nice detail.

thank you,

dan dumitru

[Ed Richards]

I use the successive approximation (messing around) method. I first focus about 1/3 of the way into the scene. I then tilt a little, and refocus, then I tilt a little more or less, and refocus, then repeat as necessary, then stop down. Since I tend to be using short lenses, a little tilt goes a long way. I have done this with a long telephoto, which has a crazy node position, making tilting really confusing.

[Ole Tjugen]

With a little trial and error I can set the tilt axis on my camera to coincide with the lenses node. That makes it very easy: Focus so that the image is sharp on the axis, then tilt until the rest is sharp.

[Jorge Gasteazoro]

I have always used axis tilt and what I do is focus on the far, mark the position of the standard, focus on the near mark the postion of the standard, then place the standard in between both marks. I then tilt until the near is as sharp as possible and then close the aperture to get the rest in focus. Of course it helps that both my cameras have a scale on the beds so I can see where the focusing falls.

[Bruce Watson]

I use the successive approximation (messing around) method. I first focus about 1/3 of the way into the scene. I then tilt a little, and refocus, then I tilt a little more or less, and refocus, then repeat as necessary, then stop down.

I do something similar. I pick a near and a far point to establish the plane of focus I want to use. Then I focus on the far, tilt for the near, repeat, repeat. Three iterations is usually sufficient. Then I may refocus to move this plane to where I want it to be. Then I stop down and do a final check, especially all four corners.

Everybody has their own method - there's a surprising variety.

[Brian Ellis]

I don't think there is a major difference in technique between using the base tilts you've been using and the axis tilts on your new camera. Some people suggest that with base tilt you should focus on the far and tilt for the near, whereas with axis tilt you should focus on the near and tilt for the far. I've read the technical reason for this suggestion but don't offhand remember it.

[Leonard Evens]

Brian,

I don't have axial tilt, so I can't speak from experience. But on the basis of theory, provided the principal points are close to each other and the tilt axis, it would seem that there is a real difference. In that case, you can tilt while keeping part of the desired subject plane in focus. Thus, the visual consequences of the seesaw effect on the plane of exact focus should be apparent on the gg as you tilt, and you could position that plane directly. As you change the tilt, the near and distant parts of the desired subjtect plane should come into or go out of focus simultaneously. No refocusing would be necessary.

Of course, in paractice, this theoretical advantage may not be too useful. It may be difficult, for example, to identify a point in the 'fulcrum' of the seesaw on which to focus in the first place. But Ole's response above says that is exactly what he does.

Another interesting point is that in principle one could use a modified version of this method even with base tilt. You would have to refocus on the 'fulcrum' after tilting. Also, you wouldn't be able to see the changes as you tilted, but perhaps there is some way to proceed anyway. I will have to play with this method the next time I need to tilt.

[Ole Tjugen]

It may be difficult, for example, to identify a point in the 'fulcrum' of the seesaw on which to focus in the first place. But Ole's response above says that is exactly what he does.

There's a "trick" to it:

First of all, most LF lenses are more or less symmetrical, but with slightly more power to the rear group. That means the node will be just a litle bit behind the aperture, and so only a few mm in front of the lens board. As an example, the 180mm Symmar has the node 4mm in front of the lensboard. The 180mm Symmar-S has it 3mm behind.

Telephoto lenses have the node so far off that it's useless to worry about it - you can't tune it in anyway.

Some LF lenses are more asymmetric, like the classic Tessar type lenses. Sticking to 180mm, a 180mm Xenar has the node 7mm in front of the lens board. Many wide-angle lenses (like the classic Super-Angulons) have the node a bit behind the lens board. A 90mm f:8 SA has a 8mm difference.

With a fair idea of what to expect from the lenses in question, it only takes a few seconds to adjust the mounting point on my camera.

And of course, having a Carbon Infinity camera makes all this a LOT easier.

[Jack Flesher]

Here's what I do with my Arca's Orbix:

If the near and far points of desired focus are relatively close to one-another in a relative fashion, such as 5 feet and 7 feet or 200 feet and 500 feet then I simply pick a point approximately half-way between them that is on-axis*, focus there and add tilt until both pop into focus. This rarely requires more than one touch-up tweak to tilt and focus.

If however the near and far focus points are further apart relatively, such as 5 feet and 500 feet, then I pick a point about 1/3rd of the way into the frame on-axis*, focus on it and add tilt until they both pop. Usually this is also close, but depending on the image may require a few focus/tilt tweaks to get it where you want it.

Tweaks: Often, one will pop into focus before the other and if this is the case you know you have to add more tilt and more or less focus extension depending on if the first in-focus point is near or far. If the far -- usually below the axis on the GG -- pops first, then you'll need more extension and more tilt, if the near -- usually above the axis on the GG -- then you need less extension and more tilt.

*Note: Adding rise or fall before the tilt affects the tilt-axis and you must compensate accordingly where you initially focus on your GG.

In the end, you develop a feel for it just like you do for primary focus when using a slow lens.

Cheers,

[Leonard Evens]

*Note: Adding rise or fall before the tilt affects the tilt-axis and you must compensate accordingly where you initially focus on your GG.

I suppose this depends on the camera. If the tilt axis moves with the standard as you rise or fall, there should be no problem.

If not, the situation is more complicated. You should tilt with the front in its default position, to determine the exact subject plane . Doing otherwise would negate most of the advantage of axial tilt since you would no longer be tilting about an axis through a principal point. In this case, if you used back movements for rise/fall you could do them either before or after tilting the front standard. Again, in this case, if you need to use front rise or fall, you should do so after tilting. In most circumstances there would be a neglible change in the position of the lens plane from such motions after tilting, so the subject plane shouldn't be materially affected. Minor adjustments after that should suffice.