Question about DOF

[ericf]

If you desire to have very shallow DOF the lens should be opened up as wide as possible (1.4-1.8). let's assume that at 2 meters from the lens the DOF of your lens is 6 inches, then at 4 meters with the same settings will the DOF be 12 inches, 6 meters 18 inches?? In other words is it a linear progression the further you are from your subject to infinity??
Is there a way to accurately pedict the DOF or is it just experience with that lens?
Is it safe to assume that each lens is going to behave differently even though the same settings are used?
Thanks from a newbie that's thinking too deep this morning over coffee.

Actually I'm like my children on Christmas morning. Todd suppose to get my 70-200 lens in today ,, And my new Nikkor/2.8 should be arriving here via fed ex tomorrow. Double the fun.

[srwatters]

http://www.dofmaster.com/dofjs.html

You're going to love that 28-70 The bokah is sweeeeeeet!

[ericf]

Scott, I think that picture is great. Not only is it a great picture but the subject is so cute. Kinda brings back memories of my daughter when she was that young and I was using my Maxxum 9000 for family pics. I liked it so much I set it as "wallpaper" on the laptop.

[Nevyn]

Originally Posted by ericf

If you desire to have very shallow DOF the lens should be opened up as wide as possible (1.4-1.8). let's assume that at 2 meters from the lens the DOF of your lens is 6 inches, then at 4 meters with the same settings will the DOF be 12 inches, 6 meters 18 inches?? In other words is it a linear progression the further you are from your subject to infinity??
Is there a way to accurately pedict the DOF or is it just experience with that lens?
Is it safe to assume that each lens is going to behave differently even though the same settings are used?
Thanks from a newbie that's thinking too deep this morning over coffee.

Actually I'm like my children on Christmas morning. Todd suppose to get my 70-200 lens in today ,, And my new Nikkor/2.8 should be arriving here via fed ex tomorrow. Double the fun.

From what I understand, it's not a linear progression like that, but then DoF is a bit of a subjective call to be honest - what may be just out of focus to you may be just okay to me. What you can say is that for a fixed focal length and aperture, the DoF will increase with increase in focal distance.
Along the same lines, longer lenses (eg 300mm) will have a smaller DoF than a shorter lens (eg 50mm) at the same aperture and focal distance. This is why long lenses at short focal distances need to be closed down quite a bit to give useable DoF (the number of pictures I have from the zoo where there's an eyeball in focus while the cheeks, nose etc are fuzzy.............)

Similarly, different lenses with the same focal length should give the pretty much the same DoF when they are at the same aperture and focal distance. The out of focus elements (bokeh) may or make not look the same and you may prefer the look of the bokeh from one lens over another, but that is something else entirely.

Bob Atkins has a DOF calculator on his site and a downloadable version that runs on the desktop. If anybody is listening, this would be a really good thing to get running on a Palm pilot..........

http://bobatkins.com/photography/technical/dofcalc.html

A quick hack at it confirms that it is non-linear.

He has a pretty good site overall, actually - I like his lens reviews and comparisons.

PS, Scott, nice picture. I like the framing.

[engstrom]

DOF is related to the square of the distance to the subject. In other words if you double the distance to the subject (holding your aperture and focal length constant) then you have 4 times the depth of field. DOF is an inverse square relationship to the focal length of the lens. A 400mm lens will have one quarter the DOF as a 200mm lens (holding the aperture and distance to the subject constant). If shoot a picture at a given aperture with a 200mm lens then you walk twice as far away and shoot the same subject with a 400mm lens at the same aperture you will have the same depth of field in the 2 shots (as well as the same coverage of subject in the frame) because the increase in DOF due to the distance is cancelled out by the decrease in DOF due to the longer focal length.

DOF has a linear relationship to the sensor/film size: the smaller the sensor the less DOF. However, a picture with the same subject coverage from a APS sensor and a full frame sensor will yield a smaller DOF to the full frame sensor because it has to get closer or use a longer focal length to get the same subject coverage. The square relationship of the distance to subject/focal length more than overcomes the linear relationship of sensor size.

[jheiliger]

Yep... What John said. There is a definite mathematical relationship...