Macro Tubes

If you are using a 50mm manual lens with macro tubes how do you calculate the focal length to dial into the camera?

I set mine to the focal length of the lens.

So does it not change if you use different lengths of tube? typo

Agreed.

Mine are auto tubes so I just leave them at the focal length set by the camera, which is that of the lens, not had any problems doing this.

You beat me to the post Barrie, no my auto tubes set the focal length of the lens regardless of the number of tubes fitted.

No just give them the focal length since tubes won't change them.
I believe when you use a TC then you should change it though.

I can't get my head round this, because are are totally altering the optical relationship between the lens and the sensor. auto as in auto focus or auto as in "PKA". If it's auto focus then would the camera not just detect the lens without knowing whats going on in between, as happens with auto focus teleconverters? If it's PKA are you not asked to dial in the focal length?

A 50mm lens is a 50mm lens no matter how far away you hold it from the sensor. The focal length doesn't change just because you're using it for macro work...

It's academic anyway if using a tripod. You length you dial into the camera is for the Shake Reduction, but you'll have SR turned off.

Correct It's the engineer in me needing to know how things work.

Yep focal length stays the same but you alter the focus distance.

Don't know about the TC but there is a "chip" in the lens with his data and that gets transferred through the "info-contact"




Focussing just goes through the AF sensor so no need for feedback from the lens about that, however it does give the camera some info about focusing but nothing exciting.

I believe that hardly any TC do give the right focal length automaticly only the Pentax ones does give it through correct so focal length + TC magnification.

My tubes are auto and if I am using an A lens or newer then the camera chooses the focal length (which I presume is the lens). If using a M42 or M/K mount then I dial in the lens myself. Never use a tripod and have never had a problem doing it that way. But if it's wrong and I will get sharper shots any other way then all advice gratefully received. (apart from use a tripod).

Mike, if your shots get any sharper we'll all be cutting our fingers on them...