medium format dof

I enjoy shooting wildflowers and scenics with my 35mm system.

Typically I use a 24-70mm zoom at 50-70mm with extension tubes

for my close-ups. I am thinking seriously of moving to a Pentax 67, using the 135 macro with extension tubes for close-ups. My questions

revolve around dof. Am I going to notice a significant loss of dof because of the longer lenses in the 67 format? Is this why I see

so few macro shots with larger formats? Would I be better served by

using a shorter lens? I have thought about buying the 75mm as my

start-up lens. Would this work with extension? Any suggestions

appreciated.

Thanks,

Tom

For a given magnification ratio and F-stop, lens length and format size do not affect DOF.

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For instance, at 1:1 mag, 100mm lens and F11, going from 35mm to 6x7 just allows you to photograph larger objects (56x69mm vs 24x36mm), DOF does not change. Switching from a 100mm to a 50mm lens only changes the working distance from 400mm to 200mm (subject to film plane). So the 135mm pentax will give you more working distance than the 75mm.

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OTOH, for a given size object the 6x7 will allow you to use a greater magnification than the 35mm and still fit on the frame, that's why DOF seems smaller with medium format.