F Number at infinity focus for optimum Depth of Field for Common roll film Formats
Magnification For 35mm Film For 6x4.5cm Film
    For 6x7cm Film
3:1 7.8 13 15.3
2:1 10.4 17.4 20.5
1:1 15.6 26.1 30.7
1:2 20.8 34.8 40.9
1:3 23.5 39.1 46
1:4 25 41.7 49.1
1:5 26.1 43.5 51.1
1:10 28.4 47.4 55.8
Circle Of Confusion Used 0.03mm 0.05mm 0.0588mm
Lines Per mm Used 33 20 17
What this means is simple -- keep your F number lower than these values or diffraction will limit the sharpness of your photos. For landscapes open up at least one or two stops from the F number used at 1:10 lifesize. For Macro these numbers are pretty much the upper limits as to what you can hope to achieve in terms of depth of field, bracket a couple of stops down in F number (smaller number!).

Note:

All F numbers are for infinity focus (ie.: Are as marked on the lens!)
 
 

 
 
 
 
 
 
 
 
 
 
 
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