Rail Focus or Lens Focus

[cbphoto]

Greetings all,

First post here. Great work all around!

Question:
When should one use a rail to focus on an object/scene, and when should someone use the lens to focus on an object/scene?

After some web research, it seems that true macro work (1:1 and closer) needs to use a rail system such as the Cognisys StackShot. And when one is shooting a scene such as a room (camera-to-subject distance ≥ 2 meters to 10 meters) lens focusing should be done. Is my assumption correct?

At what enlargement ratio does the use of a rail system become valuable? 1:1? or maybe 1:5?

Any info is appreciated.

[rjlittlefield]

For starters, see the article at https://zerenesystems.com/cms/stacker/docs/troubleshooting/ringversusrail .

Then feel free to discuss further here. I wrote that article, so I'll be happy to expand or clarify as needed.

--Rik

[cbphoto]

Thanks!

[blekenbleu]

For starters, see the article at https://zerenesystems.com/cms/stacker/docs/troubleshooting/ringversusrail .

In that table, what is meant by bellows front vs rear?

Some lenses, e.g. my Canon EF 50mm f/1.4, change focus by moving their front element in and out,
while others do not. When changing focus by ring, should one be preferred over the other,
or is minimizing focus "breathing" (change in magnification) more important?

[rjlittlefield]

For starters, see the article at https://zerenesystems.com/cms/stacker/docs/troubleshooting/ringversusrail .

In that table, what is meant by bellows front vs rear?

"Bellows front" means to move the front standard on which the lens is mounted, while keeping the camera in a fixed location.
"Bellows rear" means to move the rear standard on which the camera is mounted, while keeping the lens in a fixed location.

Some lenses, e.g. my Canon EF 50mm f/1.4, change focus by moving their front element in and out,
while others do not. When changing focus by ring, should one be preferred over the other,
or is minimizing focus "breathing" (change in magnification) more important?

In general, the troublesome differences in perspective are caused by movement of the entrance pupil. Simple changes in magnification, without movement of the entrance pupil, can be compensated in the stacking software. That's why "bellows rear" is so good -- it introduces a lot of change in magnification, but the entrance pupil does not move at all.

In the few lenses that I have studied, internal focus lenses have had less perspective change than lenses that extend. But I do not know of any reason why that should be true for all lenses, so for this question I think the bottom line has to be "I don't know. You'll have to try it and see."

--Rik

[dougsmit]

I'm a beginner so I may be missing the point here but my practical use makes the choice obvious in many cases. I have a WeMacro rail which has a track only about 100mm long after you subtract the length of the part on which the camera is mounted. That makes it useless for shooting anything with a near to far difference greater than that and do remember you can damage the thing by running it all the way to either extreme. Landscapes and thick flowers, therefore, need to be shot by adjusting the lens focus either by hand or using the focus bracketing feature of many recent bodies. However, I have found it harder to control very small movements of the focus ring (WeMacro claims one micron but manually adjusting a whole millimeter requires a bit of practice especially with some lenses with steep threads). That means rail focus is best for close, close work of 1:1 or more. Also remember that the focus bracketing routines built into bodies only works with auto focus lenses so lens focus will depend on your ability to move that ring ever so slightly. My most recent stacks this week used manual lens ring advance on a manual focus lens but that lens had a long focus throw which helps a lot. They were posted elsewhere on this site so can not be repeated here.

Short answer: I prefer rail focus unless the depth front to back is approaching the practical limit if the rail being used in which the only answer will be lens focus. Zerene stacker handles both to my satisfaction. A beginner like me might do better starting with subjects easier to handle (less overlapping, non-wilting, inside out of the wind) and plan to push the limits later after having done a few thousand easy ones.

[Lou Jost]

It's easy to check whether the entrance pupil of a lens moves when focusing. Set it to a focus on a needle stuck vertically in a piece of styrofoam, near the edge of the viewfinder frame. Stop the lens way down and stick another needle somewhere exactly behind the first needle, according to the image in the viewfinder. So basically you'll only see a single needle, the first one, which blocks the one behind it. Then, without moving the camera, open the aperture fully and use the focusing ring until the second needle is in focus. Then stop down again so both needles are in focus again. If the entrance pupil has not moved during the focusing, the second needle will still be exactly behind the first needle. If the entrance pupil has changed, the second needle will be off to one side of the first needle.

Of course any lens that focuses purely by extension will fail this test. Some point-and-shoot cameras pass this test.