# How to measure distance to subject in Depth of Field calculator?

I used Depth of field calculator here.

I should specify subject distance.

How to measure distance to subject?

In theory this is a distance to front principal plane or point. What is in practice?

I tried to calculate expected image size and then made shot with camera. In calculation I measured the subject distance from front lens of photo lens. This result that "theory image" about 1.5 times larger then "practical image".

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Can you clarify what you mean by 1.5x larger? What are you measuring, and how? – mattdm May 26 '12 at 12:23
I mean distance between fixed specific points (feature points) on the scene. I measure distance in pixels on camera shot. – sergtk May 29 '12 at 11:13

According to the FAQ for the particular calculator you're using, the calculations are performed for a thin lens, and hence the front nodal point of the lens should be used. The author recommends using the front surface of the lens, on the assumption that the front nodal point is somewhere inside the lens, and this assumption will yield a conservative estimate of the DoF.

By the way, note that in lens specifications, focusing distances are specified from the film/sensor plane. This location is often marked with a special symbol on the camera body. So these measurements are not exactly comparable to what the DoF calculator uses.

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This is in the FAQ for the calculator site you refer to.

In short, the equations used assume an abstract mathematical lens, not a real-world one. Except for macro or other extreme-closeup photography, this is perfectly fine — it doesn't really make much difference where you measure from. Assuming the measurement is from the front of the lens gives a conservative estimate of depth of field.

In general, because perception of depth of field relies on many factors, these calculators are really only for guidance — you shouldn't be basing scientific measurements on them.

For modern digital photography, getting into the ballpark and then doing some experiments seems to be the best approach for figuring out the real-world results for your lens, aperture choice, and subject distance.

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