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Equivalent Depth of Field Parameters?

ZoneV

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ZoneV Silver Member Nikonian since 08th Jan 2005
Thu 15-Sep-11 06:35 PM | edited Thu 15-Sep-11 06:37 PM by ZoneV

Depth of fields is affected by image magnification and f/stop. For example, if I come close to an object and photograph it, depth of field will be shallower than if I back away. Likewise, switching to a longer lens to enlarge the object again restores the depth of field to the previous (shallower) range.

There is a calculator here:
http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

Now, I want to figure out the exact paramters needed to maintain the visual appearance of a consistent constant depth of field, but with while systematically alterting three out of the following three variables between each frame: lens focal length, f/stop, and shooting distance.

According to the calculator:

For a 50mm lens, aperture of f/1.2, and distance of 2m (assuming 1.5x sensor factor), the DOF range will be: 0.079m, with a near limit of 1.961 and a far limit of 2.04m.

The above parameters describe a recent shooting situation I encountered.

I would like to now use the calculator to extrapolate to a longer shooting distance, a smaller f/stop, and a longer lens focal length (while maintaining roughly the same DOF range). Reason is, I'm curious whether or not I could have made a similar image (speaking strictly in terms of DOF) with two other lenses: an 85/1.8 and a 135/2.8.

I will now use guess-and-check while inputting differing parameters and attempt to maintain the 0.079m DOF:

135mm @ f/2.8: 3.56m shooting dist.
85mm @ f/1.8: 2.245m shooting dist.

Now here is a question: would the distribution and degree of blur in the background actually look the same for all of these combinations, or do laws of optics predict subtle differences even though the DOF supposedly remains the same?

Also: the calculator has no way to lock image magnification. Therefore, it's not clear whether or not these combinations would yield equivent image sizes.

I'm still struggling. If someone could help by providing equations and guidance, this would be helpful. Again, I want to keep image size and DOF range constant and vary the shooting distance, f/stop, and focal length to achieve the same DOF range. I've had math up to basic calculus.

Thanks.

Nikon user since 2000

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