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I'll be going to a place where the night sky is quite nice to photograph. I plan on taking a star trail long exposure shot with a Canon 3000N, an analog camera.

I'm pretty sure it won't be able to autofocus on stars, and since it's an analog camera, there's no live view from which I can perfectly adjust for infinity focus.

I'll be using a Canon 50mm f/1.8 STM lens and from past experience, just setting it manually to the furthest focus possible makes the picture out of focus for stars, at least on my Canon T6.

So my idea is to attach the lens to my T6, use live view to adjust for infinity focus, then detach it from the T6 and without changing focus, put it on the 3000N for the long exposure shot.

Is this idea feasible, or is there some optical difference between the cameras that wouldn't allow for this to happen?

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    Have you considered using an old manual focus lens with adapter? Pentax PK, M42, and Nikon F would work with your camera. – xiota Dec 21 '19 at 22:32
  • @xiota yeah, I'll take a couple of shots with an M42 Helio 44M-7 as well. – Pedro von Hertwig Batista Dec 22 '19 at 1:57
  • @PedrovonHertwigBatista What prevents you from testing this yourself prior to your trip? – Michael C Dec 23 '19 at 2:48
  • @MichaelC it's coming up shortly and I have no time to finish a roll and send the film to a lab to see the results. – Pedro von Hertwig Batista Dec 24 '19 at 9:30
  • @PedrovonHertwigBatista You can still do at least a "proof of concept" test to make sure the lens' focus is not automatically reset when attached to the other camera/when the other camera is turned on. – Michael C Dec 26 '19 at 17:28
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There's no optical difference that would would make what you are suggesting be insurmountable.

The variation in actual registration distance between two Canon EOS bodies will likely be less than the slight variations we experience with film from one frame to the next as it is rolled through a film body after having been stored curled up in a canister for several weeks/months/years.

Only when we started using perfectly flat, non-moving, high resolution digital sensors and "pixel peeping" the results did the miniscule differences between one camera and the next, as well as between one lens and the next, start to matter very much.

The distance covered by one "step" of the lens' STM focusing mechanism is also likely to be a distance comparable to the manufacturing tolerance of the EF mount's registration distance.

There is a non-optical issue, though, that at first glance looks like it could prevent what you suggest from working.

Your STM lens is focus-by-wire. That means that there is no direct mechanical connection between the lens' focusing ring and the lens' AF motor. Instead, the manual focusing ring is an electrical switch that sends its position changes to the camera and the camera sends a signal back to the lens to move a certain amount.

Believe it or not, even a film camera as old as the EOS 3000N is perfectly compatible with Canon's new focus-by-wire STM lenses. So are all other EOS film cameras. When the STM lens is attached to the EOS 3000N with metering active, moving the lens' focus ring will cause the camera to send signals to the lens' AF motor to reposition the lens.

But here's where the issue comes in: Most STM lenses have the following (slightly paraphrased) statement included in the lens' handling instructions:

When the camera is turned on or when the camera is 'awakened' by pressing the shutter button halfway down when the camera is off due to the use of the auto power off function, the lens performs an initial reset of the focus lens.

This could be a fly in the ointment for obvious reasons.

It turns out, though, that the above advisement only seems to apply to STM zoom lenses and a few EF-S STM prime lenses, such as the EF-S 35mm f/2.8 Macro.

The Instructions for the EF 50mm f/1.8 STM include no such warning. Nor do the instructions for any of the other STM lenses I looked at that are prime, rather than zoom, lenses.

You should be fine.

As always, though, there's nothing to keep you from testing this method ahead of time just to be sure. Even if the night sky where you are is not as dark as where you will be going, you can still use the brightest stars in the sky to focus your lens using your Rebel T6/1300D and then place the focused lens on your 3000N and take a few test shots. I'd do it two or three times, with 2-3 test shots each time. Just write down your frame numbers for each test cycle so that you know what you're looking at when your film is developed and printed.

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    Absence of a warning does not mean the lens will not behave that way. I just tested an EF-S 24/2.8 STM prime lens. Focus is reset when the lens is attached and the camera turned on. I also have USM lenses that exhibit the same behavior. Detaching and reattaching lenses to set aperture or focus are hacks. They might work, but they shouldn't be expected to because the lenses were not designed to be used that way. – xiota Dec 22 '19 at 8:34
  • My Canon EOS RP has "C.Fn III-8 Retract lens on power off" custom function for retracting gear-type STM lenses (such as EF40mm F/2.8 STM). When enable is set, this function takes effect regardless of the lens's focus mode switch setting (AF or MF). My understanding is that the EF50mm F/1.8 STM is a gear-type lens. – juhist Dec 22 '19 at 9:26
  • Since no one commenting has tried it with an EF 50mm f/1.8 STM, then it's up to the OP to try it with OP's copy. – Michael C Dec 23 '19 at 2:46
  • In tens of thousands of examples, I've never had any Canon USM lens reset focus position when the Canon camera is powered up or when the lens is attached to a powered up Canon camera. Is your experience using USM lenses with Canon cameras, or when using Canon USM lenses adapted to other, non-Canon cameras? – Michael C Dec 23 '19 at 23:10
  • Information about the caution applying only to stm zoom lenses is incorrect. The manual for some stm primes contains the same warning. – xiota Dec 28 '19 at 10:19
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Ideally it should be possible as the distance from the lens mount to the sensor or the film plane should be the same. But in real life I doubt it. After all, if there is just a tiny difference between the distance on the two cameras, your focus will be off.

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    Yes, they are compatible, but that doesn’t mean that the distance from lens mount to sensor/film plane is 100% identical. – John Sørensen Dec 21 '19 at 21:31
  • Which is why is wrote "ideally". The flange distance for Canon EF mount is 44.14mm (+/-0.03) the part in the parenthesis is what MIGHT cause problems. – John Sørensen Dec 21 '19 at 21:44
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    Depending on where the manual focus ring is, it's certainly possible to accidentally turn the focus ring while dismounting the lens, too. – Davidw Dec 22 '19 at 6:03
  • @Davidw That's what gaffer tape is for. – Michael C Dec 22 '19 at 6:07
  • I, personally, don't have any, and there's no guarantee that the OP has any. – Davidw Dec 22 '19 at 6:08

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