Depth of field for 35-105mm/f3.5?

I just got a 35-105mm/f3.5.

I see four redlines where the depth of field scale usually is. The vendor told me that actually it's for infrared film focus.

Can I use these somehow to guess depth of field? Or otherwise how can I find out the depth of field other than stopping down?

Those lines are to adjust focus for IR film, the vendor is right.

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For DOF, you either use DOF preview, or you can look at a table of DOF

and remember some basic numbers. And in critical situations, use a

table or DOF calculator.

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Remember that the DOF only depends on the focal length and the

aperture. All lenses of the same focal length wll have the same DOF

at the same apertures.