Makro:Depth of Field calculations?

[Colwyn Griffith]

Well I just want to know what kind of Depth of Field I can get with a 4x5/5x7 ma kro 120mm lens; either Rodenstock or Schneider. Is there a sytem of calculation I can do to figure this out? If there isn`t, could you tell me what type of lens to use for a close up covering the aproximate dimensions 15 to 60cm wide by 30 to 70cm deep? This 3 dimentional space also has a height that ranges from 5 to 3 5cm. If at all possible I would like to keep most of these dimentions in focus. Is there some lens or leses that can pull it off? Cheers.

[Alan Gibson]

There are a number of on-line calculators, and formula are in a static page somewhere in photo.net. Any decent photo book will also have this information. See also some other posts over the last few days.

I'm not sure that I correctly understand your question. Say, your subject is 20x25cm. 4x5 film is about 10x12.5cm, so your magnigication is m=0.5. Conventional formulae for Depth of Field for a 120mm lens at f/45, CofC 0.1mm, give about 12.5cm (focus is from 30.8 to 43.3cm), and you seem to want 30cm. You get almost this much at f/90, if you can stop down that far.

For macro work, for a given magnification, changing the focal length has very little effect on the DofF. There are some very specialist techniques that might be usable, essentially by lighting a 'thin plane' of the subject, at the focused distance, and moving the subject down during a long exposure.

Before you ask, 35mm format would give you even less DofF.

[Alan Gibson]

My apologies for the statement about 35mm, it isn't correct, because we have small magnifications here. For 35mm format, magnification is m=0.144. For f=55mm, c=0.03, at f/45 (if you can get there), DoF = 20.5cm (which still isn't enough).

If anyone cares, and my apologies for going off topic, the 'cut-over' point where, for the same subject size and same aperture number, the DoF is equal in 35mm and 5x4 formats is about m=0.25 (35mm) and m=0.75 (5x4). The subject size is about 96mm by 144mm. For greater magnifications, 5x4 wins. For lesser magnifications, 35mm wins. This makes the usual assumptions inherent in the conventional DoF formulae.