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Old Mar 6, 2007, 10:14 AM   #1
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I have read that DOF significantly narrows when ext. tube is used for macro work in order to focus at closer distance.

My idea is to use it for portraits to get advantage of greater lens sharpnes at smaller apertures (although I know that it isn't always desired) and still have narow DOF.

Would that work for those who don't have fast primes and still want the narrow DOF effect?

I don't have any experience with any of these issues so forgive me if I'm asking something stupid.

Thank You in advance!

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Old Mar 6, 2007, 10:28 AM   #2
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I posted the question here because I guessed that at this forum people have experience with portrait work.

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Old Mar 6, 2007, 11:12 AM   #3
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Old Mar 6, 2007, 10:45 PM   #4
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Using an extension tube probably means the lens will no longer focus to infinity or even to portrait range. You would have to test this to make sure.
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Old Mar 7, 2007, 4:29 AM   #5
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I suspect it wouldn't do much for depth of field.

A big part of depth of field is distance from subject. When you are already up close, maybe the extension tube has a significant effect there. But if the subject is already 10 feet away, I doubt it has any effect.

I don't really know much about extension tubes. But I believe the extension tube is only designed to increase magnification by the stated amount at the closest focus distance. As you get farther away from that, I espect the increase in magnification would become minimal (as would the effect on depth of field).

Also, as an aside, sharpness will peak on APS-C sensor cameras by about f8. After that, diffraction will start to limit sharpness. So don't stop down too far for sharpness, unless you want more depth of field.

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Old Mar 7, 2007, 9:55 PM   #6
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Your really looking to complicate your life. An extension tube does nothing more than move the lens further away from the film/sensor plane. The built in focus mount does the same thing as we focus closer than infinity. An extension tube simply allows the lens to focus closer than the built in focusing mechanism permits. This has two affects: 1) The image size (magnification) increase as we move the camera closer to the subject, and 2) the effective aperture gets smaller. As the magnification (the area with the picture more closely approachesthe size of the sensor) become greater the depth of field becomes narrower even though the aperture is also becoming smaller. This also has the effect of causing the lens to become diffraction limited at a lower indicated value than when it is used at "normal" distances.

Practical apps. If you had the hypothetical short tele portrait lens focused at 6' and you inserted a tube such that the lens, without refocusing now focused at 3' your depth of field would be decreased because the camera is now at 3', the effective aperture would be smaller than indicated and the size of the image would be increased. On the other hand, if the lens focusing mount permitted you to leave the camera at 6' and your refocused your image size, aperture and DOFwould remain the same.

If the lens is truly optimized for the size of the sensor then peak sharpness/contrast will occur in the region of 2 to 3 stops down from wide open and then should fall off as diffraction overrides residual aberration correction resulting from reducing the aperture. If the lens is actually designed for a larger sensor the optimum apertureMAY occur earlier because only the central part of the lens is utilized. I'm saying in properly designed lenses the optimum aperture is irrelevant to sensor size.
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Old Mar 11, 2007, 4:43 PM   #7
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Thank You for Your responses.

I have read some about DOF theory, but I'm still curious about this idea in the real world. As stated above, I don't have a tube to test it, so I wondered if someone is interested to try it out and post some reply or images even better. We can only profit in experimenting with our ideas and as a result be more creative.

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Old Mar 11, 2007, 10:35 PM   #8
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You my choose to call this theory but we dealing with laws of physics. This is not where you're going to find a crease in the time space continuum.
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Old Mar 12, 2007, 2:49 PM   #9
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I've read that with ext tubes one can not focus at infinity but only up to 10m (the number is just for for an example). I wondered if other parameters would also change.
Anyway, thanks again for the response.
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Old Mar 12, 2007, 6:54 PM   #10
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A lens is its shortest physical length when focused at infinity. To focus at a closer distance the lens focusing mount (a helical, which means a screw thread, assembly) moves the optical part of the lens further away from from the plane of the sensor. That is to say that the lens gets physically longer to focus on an objects closer than infinity.

If, on some hypothetical lens, the optical elements must be moved 10mm further away from the sensor relative to infinity focus location to focus 10' it's irrelevant we use the internal helical mount to move it 10mm or leave the helical mount at infinity and inserted a 10mm extension tube it will still focus at 10'. The physical, optical and photographic results are identical.

If our hypothetical lens will only focus to 10' and that results in a photo of the subject head then the addition of 10mm extension tube may allow the lens to be focused at 5'. This would result in a narrower DOF and smaller effective aperture (with the lens set on f4 it might really be f5.6) but instead of taking a picture of someones face we are now taking a picture of their nose. Most subjects willobject to having a picture of their nose called a portrait.
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