Just up graded to the 5DmkIII and trying to find the best lens from 40 mm to 55 mm with a minimum aperture smaller than f/16. I shoot scale model aircraft up close for detail and f/16 just isn't going to give me the greatest depth of field I need to keep the entire model in focus. Was used to my old Nikon 55 mm Macro that had a minimum aperture of f/32. I want a fixed focal length, but only the zoom lens seem to stop to f/32. any ideas appreciated.
As you stop down, depth of field increases however acuity decreases. The sharpest f-number setting is about two f-stops down from wide-open. As you continue to stop down, the twin demons of interference and diffraction degrade. A lens manufacturer restricts how far the lens can be stopped down to protect their reputation.
You can increase depth of field by simply stepping back and zooming into the realm of a moderate telephoto. In other words, increasing subject distance results in a gain in depth of field.
Now this will sound crazy: You can stop down any lens by this method. Cover the entire front of the lens with aluminum foil. Pre-punch a pin-hole and center this tiny aperture over your lens. You can experiment with different size holes. This method will grant you the depth of field you carve but at the cost of acuity. Try it, you might like it
Canon made a EF 50mm f/2.5 Compact Macro lens that sounds like a close match for your old Nikon: the focal length is close, and the smallest aperture is f/32. That lens has been discontinued, but you can easily find it used for around $200.
Macro photography where maximizing depth of field is desired is one of the few areas of photography where a larger sensor can be more of a disadvantage than an advantage:
You'll get narrower depth of field with a larger sensor than with a smaller sensor when shooting at the same reproduction ratio.
Most such macro photography requires total control of the lighting anyway. It's normally much easier to provide more light to overcome the signal-to-noise ratio disadvantage of a smaller sensor than it is to deal with the additional diffraction caused by using a narrower aperture to get the same DoF with a larger sensor.
For more, please see:
How does sensor size impact depth of field and diffraction for macro photography? Note that the answers there assume constant lighting conditions between the two camera/lens combos.
Macro with crop body
How do I choose a camera for macro photography without considering special lens?
How does focal length relate to macro magnification?
Why are macros from my new DSLR no better than those from my P&S?