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08/08/2006 01:54:29 PM · #1 |
If a lens (28-70) says it has a minimum focusing distance of around 19" "throughout the zoom range", does that mean that I can get within about 19" and then use the zoom to get in closer and it will still be able to focus? Does that also mean at any aperature or will the minimum focusing distance change with the change in aperature? (not sure I spelled that right...)
Judy |
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08/08/2006 02:34:11 PM · #2 |
Originally posted by jpochard:
If a lens (28-70) says it has a minimum focusing distance of around 19" "throughout the zoom range", does that mean that I can get within about 19" and then use the zoom to get in closer and it will still be able to focus? Does that also mean at any aperature or will the minimum focusing distance change with the change in aperature? (not sure I spelled that right...)
Judy |
Minimum focusing distance tells you just that; how close you can be to your subject and still focus on it. It's measured from the sensor plane to the subject, btw, not from the front of the lens. You are correct that you can set up at 19 inches and then vary the "magnification" by zooming to see more or less of the subject.
Aperture does not affect minimum focusing distance.
R.
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08/08/2006 03:33:10 PM · #3 |
Only thing I can add to Robert's excellent response is that at small aperture (like f/16, f/22) your DoF will include things closer than 19 inches, whereas at f/2.8, DoF will be limited (of course) to just a very short distance in front and behind the subject. Example:
@ 70mm, f/2.8, DoF = 0.17 inches (!), 50% in front, 50% behind.
@ 70mm, f/22, DoF = 10.1 inches, 38% in front, 62% behind.
For the second case, everything from about 15.2 inches to 25.3 inches will be in focus, though things may look just slightly soft at the limits of that range. |
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08/08/2006 03:43:28 PM · #4 |
| just to add to what Kirbic said, here's an excellent DoF calculator with just about every camera known to man on it. |
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08/08/2006 04:06:27 PM · #5 |
Great question and good answers! Learned some new stuff myself that I think will help me when setting up shots.
I'm not the original poster but thanks for the link as well, very helpful. :)
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08/08/2006 04:22:34 PM · #6 |
Originally posted by Megatherian:
just to add to what Kirbic said, here's an excellent DoF calculator with just about every camera known to man on it. |
Excellent it is. A little known trick with that online calculator is this:
Below all the cameras is a list of CoC (Circle of Confusion) values. You can choose one of those directly, picking the one that's closest to twice your pixel pitch.
Example:
Canon 10D, Pixel pitch = 7.4 microns or 0.0074mm, so choose 0.015mm as CoC.
This choice gives a more conservative result (narrower DoF) than when choosing the 10D from the list, and when tested you should see that the far ends of the given DoF are very nearly as sharp as the focus plane itself.
For the techno-dweebs, the value of 2 times the pixel pitch is just the Nyquist resolution limit for the sensor (not including the AA filter). |
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