Possibly a silly question, but....

[sonicgroove]

......When you use an adapter, does it act like an extension tube and allow closer focussing? I'm sure it's been covered before a zillion times, but please indulge me.

[mo]

If you are talking about an adapter that allows a different mount to be say mounted to a M4/3 or nex camera, no it does not act like an extension tube. I think some adapters may not allow you to reach infinity but I am unsure of which ones they are.

[duckrider]

Depends on the distance between mount and sensor: most EVR cameras have a very near space, SLR lenses need of course wider space to allow the mirror to swing upwards. So a Nikkor e.g. will work on EVR camera, but adapting a M42 lens to a Nikon SLR will not focus to infinty.
There are adapters with a lens to compensate this, but optical performance is horrible.

[duckrider]

Depends on the distance between mount and sensor: most EVR cameras have a very near space, SLR lenses need of course wider space to allow the mirror to swing upwards. So a Nikkor e.g. will work on EVR camera, but adapting a M42 lens to a Nikon SLR will not focus to infinty.
There are adapters with a lens to compensate this, but optical performance is horrible.

[guardian]

If you're thinking of adapters for the popular (or once popular) types of interchangeable mount lens systems (e.g., Adaptall, Adaptall 2, Adapt-A-Matic, T-mount, YS mount, Automount, etc.) the answer would be no. The adapters for those various lens series are designed to, in each individual instance, correctly adjust the registration distance such that it is right for the camera body/lens combo served by the adapter.

Even in the instance of adapters designed in a more ad hoc manner, the designers always attempt to be respectful of the lens/camera body's nominal registration distance. Though in these instances I think the outcome is more subject to question. (ad hoc) Adapters for Petri breech lock lenses, for example, can be problematic, i.e., they can move the lens too far forward, causing the closer focusing about which you inquired, together with possible loss of infinity focus. But well-designed adapters do not act as extension tubes.

[jun]

Using some packing tape, I invert the lens & tape it to the adapter. Or use 2 adapters taped end to end & you will have a very close marcro lens. Just set your fstop to f4 or higher & you will get a very sharp image.

[DConvert]

As above most adapters are designed to make up the differnce in rear flange distances, so will not affect focus.
Some cheap chinese examples of M42 adapters will not quite reach infinity, others are made fractionally short which will ensure infinity focus is maintained but will loose a little of the close focus capability of the lens. There may be batch to batch variations here with poor QC not controlling things properly. I've heard reports of brand X 'doesn't reach infinity' followed by 'mine does with all my lenses' Of course any diferences will show more with short focal length lenses.

You can get some mirrorless M42 adapters with a helicoid built in specifically to allow for closer focusing. My µ4/3 to M42 helicoid adapter is anoying in that its ~0.1mm too long (according to my calipers) at it's shortest to reach infinity, otherwise it would have been an ideal M42 mount, drastically increasing the focus range of M42 lenses without missing anything. At some point I may try to lap a little off...

C mount lenses require a short adapter - for micro four thirds its recessed into the camera and can require the lens itself to be adjusted (something like a grub screw to alter a stop on the rear thread). CS mount lenses have identical apearance to C mount but 5mm less RFL, so would almost certainly be only be for macro. I know µ4/3 doesn't have the shortest RFL of all mirrorless mounts but I don't think any are more than a couple of mm shorter, & I doubt adapters would want to go far enough into the camera to get CS to work. On E-bay CS lenses are sometimes sold as C mount, which MAY have given rise to reports of problems.