Michael Heald

5-Oct-2006, 16:16

Hello! I read Robert B. Hallock's article on what makes a print sharp in the current issue of View Camera magazine. My initial impression was that the larger film size always makes better prints because of less enlargement required and less restrictions on the CoC. However, looking at the graphs, if one takes into account the depth of focus, the advantages (based solely on CoC) are minimized.

However, this appears to depend on the depth of field that a person accepts at the larger format and longer lens.

For example, a 4x5 negtive with a 150mm lens and a 10mm spread between lens/GG distance gives an effective focus range of 8 feet to infinity. According to the figure, with an optimum fstop of 60 and CoC=0.12mm.

For a 305mm lens, for a 10mm spread, the depth of focus is 30 feet to infinity, again yielding an optimum f-stop of 60 and CoC=0.12

In this case, if the photographer accepts less depth of focus, then the enlargement will be sharper from the CoC consideration.

However, to obtain the same depth of field, the GG spread goes up to 43mm. Extraplating the curves out suggest that the CoC is double that of the 150mm lens. So to obtain the same depth of focus, the high f-step is needed and the higher CoC results. Hence, no increased sharpness based solely on CoC.

I'm not sure I am interpreting this correclty, but it appears that 8x10 is superior to 4x5 (purely form a CoC standpoint) in those situations where less depth of focus is needed. Is this true? Best regards.

Mike

However, this appears to depend on the depth of field that a person accepts at the larger format and longer lens.

For example, a 4x5 negtive with a 150mm lens and a 10mm spread between lens/GG distance gives an effective focus range of 8 feet to infinity. According to the figure, with an optimum fstop of 60 and CoC=0.12mm.

For a 305mm lens, for a 10mm spread, the depth of focus is 30 feet to infinity, again yielding an optimum f-stop of 60 and CoC=0.12

In this case, if the photographer accepts less depth of focus, then the enlargement will be sharper from the CoC consideration.

However, to obtain the same depth of field, the GG spread goes up to 43mm. Extraplating the curves out suggest that the CoC is double that of the 150mm lens. So to obtain the same depth of focus, the high f-step is needed and the higher CoC results. Hence, no increased sharpness based solely on CoC.

I'm not sure I am interpreting this correclty, but it appears that 8x10 is superior to 4x5 (purely form a CoC standpoint) in those situations where less depth of focus is needed. Is this true? Best regards.

Mike