Hefin
With a manual tube you either have to focus at the stopped down
aperture, or change the aperture after focussing. With an automatic tube
you focus with the lens wide-open irrespective of the aperture which you
set for the exposure.
Can make quite a big difference in relatively low light levels if
you are prepared to use a longish exposure to get depth of field. Also
awkward if you are doing a hand held close-up and moving the camera to get
the focus rather than adjusting the lens.
I have automatic 7 and 20mm OM extension tubes and a manual 14mm
OM extension tube. Manual/automatic seems to make surprisingly little
difference to the price asked and sellers do not always advertise which
type they are selling unless you ask.
Brian
UK
brian.gray@xxxxxxxxxxx
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