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All the cameras that I'm aware of move lens elements to obtain focus, although I'm not familiar with the workings of in-phone cameras. It occurred to me this morning that you could also focus by moving the sensor in or out. Just curious if a) it's really possible, b) it has any advantages, c) it's been done?

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Wouldn't it allow for "macro mode" with any mounted lens if the sensor could be moved further apart from lens? I mean this as a hypothetical advantage, as the camera would need a deep well for the sensor to creep into. – Jahaziel Mar 22 at 20:12
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Do spider eyes count? – Jan Dvorak Mar 22 at 20:24
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It doesn't make much economic advantage, at least with SLRs. The lens is the expensive part with lifetime measured in decades, while body with sensor is the cheap and disposable part meant to be replaced in 2-3 years. So packing expensive moving mechanism for focus with part that already has another expensive moving mechanism for zoom has bit more sense. – Agent_L Mar 24 at 10:29

Not a digital camera/sensor, but the Contax AX film SLR was able to move its film plane forward and back via a ceramic rail and ultrasonic motor in order to be able to autofocus the normally-manual-focus-only C/Y mount Zeiss lenses. Indeed, Hasselblad V system lenses could be mounted to the Contax AX via an adapter, and the moving film plane allowed for autofocus with these lenses too.

You can find a sales brochure for the AX with information on the feature here: http://www.forums.camera-info.com/contaxinfo/broshures/AX/FrameSet.htm

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+1 Also, that link says that moving the film plane is common for large format cameras. – Sparhawk Mar 23 at 0:59
    
The AX was only semi-AF—it only had 10ish mm of movement—so for longer lenses you had to manually set focus to something ballpark and let the camera AF from there. – Roger Krueger Mar 23 at 9:28
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Some old rangefinders work this way as well (though manually). I have an early Mamiya 6 that focuses by moving the film plane. camerapedia.wikia.com/wiki/Mamiya_Six – moorej Mar 23 at 23:23

AFAIK, it has not been done yet but it is possible. Canon recently patented the idea. There would be some latitude for determining focus but not as much as moving lens elements. Many modern lenses move multiple elements in middle groups to be able to focus throughout their entire zoom and focus ranges. The advantage though would be that you would not change the relative position of optical elements which could help preserve aberration corrections.

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View cameras are, essentially, two flat panels (normally squarish) with a bag or bellows in between. One panel holds the sensor (or ground glass/film holder) and the other panel holds the lens (which normally doesn't itself have movable elements). The panels can be moved independently, so you could focus by moving the rear panel.

A potential advantage of rear focusing is that for close-in photography keeping the lens fixed also keeps the magnification fixed (otherwise you might end up wanting to move the whole camera to get in focus). [See comments]

See Wikipedia's page on this camera type for pictures: https://en.wikipedia.org/wiki/View_camera

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Moving the film/sensor back away from the lens also increases magnification. That's how extension tubes work: they move the entire camera further back from the lens. – Michael Clark Mar 23 at 2:40
    
Right, doesn't really matter how you increase the distance between the lens and the sensor, not sure why I worded it that way (it has been a while since I've used a view camera). I guess from a workflow perspective (when using a view camera) with close-in shots you could put the lens where you wanted it and have it and the camera's position fixed and then adjust the focus using the rear standard. Not sure if that is a huge advantage. – David Rouse Mar 23 at 14:56
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Keeping the lens fixed keeps the viewpoint fixed, not the magnification. Keeping the viewpoint fixed is important. In my experience, it is a huge advantage when taking pictures of relatively small objects with a 4x5. In fact, if you move the camera and then move the lens to focus, it might be outright impossible to focus only by moving the lens. For example, with a 150mm lens, it is impossible focus unless the film plane is at least 600mm away from the focal plane. – Dietrich Epp Mar 23 at 18:18

a) it's really possible

Sure. Whether you move the lens or the sensor, the important thing is that you're changing the distance between the two. I have a EF 50mm f/1.4, and when I turn the focus ring the entire internal barrel seems to move as one unit. If I were to clamp the front of the lens to something stationary and turn the focus ring, it'd be the camera body, including the sensor, that moves, right?

b) it has any advantages

Simplified lens construction (and associated cost reduction and improved reliability) would be one advantage. A disadvantage is that zoom lenses might be difficult or impossible to build. Also, some lenses move the optics a significant distance while focusing, so the camera might need to be fairly deep to cover the necessary range.

c) it's been done?

I don't know if it's been done at all with respect to focus, but some cameras move the sensor instead of optical elements to implement image stabilization. Two obvious advantages are that you get stabilization with every lens, and you don't have to pay for stabilization for every lens.

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And the not-so-obvious but oh-so-very-real disadvantage of body based image stabilization is that it provides the least amount of stabilization when one needs it the most: at longer focal lengths. – Michael Clark Mar 23 at 2:42
    
I'm also still waiting to see those non-stabilized lenses that are actually cheaper than their stabilized counterparts when the optical formula is the same. When comparing third party lenses in various mounts those made for systems with body based IS are rarely any cheaper than those made for systems with lens based IS. And I've yet to see a IS-less Sony lens with the same focal lengths, apertures, and image quality that is cheaper than its Canon/Nikon counterparts with IS. – Michael Clark Mar 23 at 2:47

A standard camera for use with a telescope is focused this way. In a typical optical telescope (I'm not sure if this is also true for the fancy new scientific research-oriented telescopes), you simply slide the the section that holds the camera (the same section that can hold an eyepiece) in or out to focus the image on the camera's CCD. The camera (again, this is just in my personal telescope photography experience, but I imagine it would hold for any telescope camera) has nothing to do with the focusing. It just records the photons that strike it.

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This occurred to me as well; but I'm not sure if it's valid. The focuser is part of the telescope (lens equivalent); and it's only the camera moving not the telescope because in most cases its the scope attached to the tripod with the camera hanging off the back. (IIRC similar weight/balance reasons cause a lot of telescope sized telephoto lenses to use similar mounting mechanisms.) The position of the imaging sensor within the camera remains fixed. – Dan Neely Mar 23 at 15:49
    
I might be misunderstanding what you're saying, but let me address what I think you're saying. I'm not an expert on telescope construction, but from what I have read, telescopes are more or less unadjustable (sure, you can change the shape of the mirrors in a reflector, but that doesn't seem to be what the OP was asking). According to Wikipedia, both refractors and reflectors manipulate light in a way that the user can't significantly alter.... – Ben Sandeen Apr 4 at 3:47
    
What the user can do is insert an eyepiece, which helps determines magnify and focus the light for an eye, or a camera, which consists of a sensor, filters, and possibly (I don't know) some small focusing lens. In this scenario (and the eyepiece one as well), focusing the image involves moving the camera (or eyepiece) in or out until the image is clear. If there is indeed a small focusing lens in the camera, then I leave it up to you and others to decide whether that makes this relevant to the original question. If there is not a lens, then this answers the OP satisfactorily, I believe. – Ben Sandeen Apr 4 at 3:49

To follow up on all the other answers, it seems that is has only really been done with

1) Image stabilization on the image sensor.

I imagine this is done as it requires just minor, rapid focus (along with field-of-view) adjustments. Perhaps they use piezo style actuators for this.

2) Push-pull style focusing (View) cameras

Here there is no rotation during focusing, so there does not need to be any rotational symmetry involved.

In a normal SLR, the lens can use a rotational motor to turn the lenses without worrying about the sensor turning. If you wanted to move the sensor, you would need a linear motor or a way to translate a spinning motor to a linear motion so as not to turn the sensor. Caleb's answer addresses this by noting what happens if you clamp the lens such that the camera body rotates (you focus, but your image may be upside down on a now rotated sensor)

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"Perhaps they use piezo style actuators for this." - actually a good old spring suspension works wonders by itself. – Jan Dvorak Mar 24 at 2:44

a) it's really possible

Sure.

b) it has any advantages

No specific requirements for lenses for AF to work. Every objective may be focused this way.

c) it's been done?

I recall that Contax G or AX had similar system but I cannot find confirmation of this ATM.

Other than that, all old enough objectives had focusing done only with single helicoid moving lens back and forth inevitably causing breathing - quite similar to what you are asking about.

There are certain disadvantages though: 1) focus breathing

2) macro not achievable without sensor being able to move few mm in and out

3) objectives will behave differently from when being used with internal focusing system.

4) variable speed (brightness drops at macro distances)

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Contax G did not have this feature. Maybe you are thinking of the Contax AX? See my answer. – osullic Mar 22 at 16:30

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