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Michael C, as well as others, have said when using extension tubes to set your focus ring to infinity (and leave it there), then focus on the subject by moving only the camera (or subject), not the focusing ring. I've some questions: What is the purpose of that? I'm using a Canon 50mm f1.8 STM lens (in conjunction with extension tubes)- the lens has NO infinity indicator - so how do I set it for infinity? As I add/subtract extension tubes, will the "infinity" change?

feetwet
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Mike R
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    I'd like to see a link to were you "quoted" me. Are you sure it was not in the context of a reversed lens? – Michael C Apr 05 '23 at 11:08
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    I'll see if I could locate it. It's possible that I misinterpreted the context. I've not the brightest bulb in the knife drawer. – Mike R Apr 06 '23 at 06:12

2 Answers2

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There are cases where the lens should not be set to infinity. If there is a specific magnification level you want to use, you can set it directly. Otherwise, infinity is a convenient, reproducible setting on the lens that allows the same setup to be used across sessions. When you add/subtract extension tubes, the focal point shifts accordingly. It's more or less the same as operating the focusing ring, but at a greater scale.

Moving the lens shifts the focal plane by a greater amount than the lens is moved. Moving the camera and lens together shifts the focal plane by the same amount that the system is moved, which allows for finer control. This is useful for small subjects at high magnification.

For electronic lenses, some cameras and lenses set the focal point to a specific setting when the camera is turned off. Usually, it's infinity, but you can still use that setting if it isn't. For systems that don't set the focus point when deactivated, infinity is still the easiest setting to set consistently. While the lens is attached to the camera, turn the focus ring until the lens is fully retracted. Then turn off the camera and detach the lens.

Zooms and lenses that focus by moving internal elements may shift the focus point inside the lens when not set to infinity and extension is added. Such lenses wouldn't normally be my first choice for use with extension tubes. This shouldn't be a problem with unit focusing lenses, but the focal point can effectively be on the glass surface, which isn't really usable.

Reversed lenses may have additional factors, but the above ponits should still be relevant.

xiota
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    The Canon EF 50mm STM is a fully focus-by-wire lens, so you can't just turn the focus ring fully to a stop, because there is no stop. – xenoid Apr 05 '23 at 07:52
  • Does it not provide a little extra resistance to turning the wheel at each end? my Nikon focus-by-wires do. No hard stop, because the wheel position bears no relation to the actual focus [& doesn't move at all in auto] but there's just a gentle hint when you reach either end. – Tetsujin Apr 05 '23 at 08:19
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    btw, the other reason to set initially at infinity, especially on a short lens, 50mm or lower, is that the actual focus point can be inside the lens once you get enough extension on it; which makes it too easy to just bang the lens into what you're trying to photograph whilst lining up. – Tetsujin Apr 05 '23 at 08:21
  • @Tetsujin How can that focus ring ever reach "the end". Say you focus to infinity using manual focus. Then you use AF to focus on an object at the MFD, but the ring didn't move. Now, to manually focus anywhere further than MFD you would need to move the focusing ring in the same direction past where your "gentle hint" was before? – Michael C Apr 05 '23 at 11:05
  • @MichaelC - The restriction must be electronic. It is unmarked & doesn't move at all in auto-focus, so it is not related to the ring position directly. However, if it reaches either end of its 'travel' it stiffens up, so you know it's hit 'a stop'. You can actually keep turning it through this stiffness, but nothing happens. It's easy to test empirically. Auto-focus on something far but not 'infinity'; you can move the ring maybe an inch before it hits the stop. Re[auto]focus on the same point… you get another inch. Rinse/repeat ad nauseam. [same for the other end at just under min distance.] – Tetsujin Apr 05 '23 at 11:14
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    @Tetsujin Are you sure that's focus-by-wire and not something like an internal clutch? If it is focus-by-wire, that's an amazing attention to detail by the designers. – xiota Apr 05 '23 at 14:32
  • @xiota - I really don't know. I've three Nikon lenses & all operate pretty much the same way, plus things like Tamron with a scale on the dial that you simply cannot move if it's set in auto-focus. However it works internally, the end result is a 'stop' mechanism that's never in the same place twice, but you can feel when you hit it. – Tetsujin Apr 05 '23 at 15:06
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    @Tetsujin I have Nikon and Canon lenses with the soft stop you're describing, but they're not focus by wire. Focus-by-wire lenses that I've used (Canon STM, some USM, some mirrorless systems) have focus rings that freely rotate with no discernible stop. – xiota Apr 05 '23 at 17:51
  • @xiota - OK… I'm still back to my original question… "Does it not provide a little extra resistance to turning the wheel at each end?" The intention being that 'you know when you got there'. – Tetsujin Apr 05 '23 at 17:52
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    @Tetsujin OP has a Canon STM lens that is focus by wire. The focus ring freely rotates. There is no discernible stop or resistance (as long as the lens is not broken). – xiota Apr 05 '23 at 17:54
  • OK. They missed a trick there then, imo ;)) – Tetsujin Apr 05 '23 at 17:57
  • Please provide specifics for one of your lenses that do this. This sounds like a Silent Wave Motor (AF-S) lens, not focus-by-wire. The only F-mount focus-by-wire lenses are the AF-P ones. As far as I know, all Nikon manufactured lenses in z-mount are FBW. – Michael C Apr 05 '23 at 19:18
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The purpose of adding extension tubes is to move the lens closer to the subject by reducing the minimum focus distance, increasing the subject's relative size. And to simultaneously increase the distance between the lens and the image plane, allowing the image circle to expand more (greater "magnification" at/from both sides of lens).

Setting the lens at infinity is simply a technique that helps make it easier to initially find the subject; because the DOF is extremely thin at the minimum focus distance; and the minimum focus distance may be extremely short, to where the lens blocks most of the available light.

In this case "infinity" really just means "at max focus distance." And all lenses have a max because the focus motor can only move the focusing elements so far. You can judge max by looking through the viewfinder to see where turning the focus ring causes no additional change; but it's really not critical at all.

Note than the maximum magnification only occurs at the minimum focus distance. And with some extended lenses the minimum focus distance may actually be inside the lens.

Steven Kersting
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