Another Canon extenders question

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I tried out my 500mm f4.5 at the weekend with the 1.4x TC and using both the 12mm and 25mm extension tubes. The shutter speed was very slow but I did get a few reasonable images in the poor light. I was photographing small garden birds on the feeders. What I noticed was how shallow the depth of field seems to be - even though the eye is sharp, in most pictures there simply is not enough DoF to get the whole bird sharp. Does the use of the extension tubes impact on DoF? What did amaze me was how much difference the 37mm of extension meant in image size - much greater than I imagined. I need to practice with them a lot more to make sure my manual focussing is spot on but I'm sure going to put them in my bag for Galapagos in November.

-- Andrew Hardacre (andrew@jhardacre.freeserve.co.uk), September 19, 2001

Answers

Magnification has an impact on DOF. If you use an extension tube to get greater magnification by getting closer, you lose DOF. If you add a TC to give greater magnification without moving closer, you lose DOF. There is no way around this. At any given magnification and f-stop, you have the same DOF, no matter how you get that magnification

-- Bob Atkins (bobatkins@hotmail.com), September 19, 2001.

For a given aperture, depth of field depends on magnification (image size on the film). When you add extension, you permit closer focus and, thus, greater magnification. The greater the magnification, the shallower the depth of field for a given aperture. In fact, if you were to use a shorter focal length lens and move closer to obtain the same magnification as you have farther back with a 500mm, the depth of field will be the same if the apertures are the same.

-- AC Gordon (cliffdeb@ciris.net), September 19, 2001.

Bob, you say "If you add a TC to give greater magnification without moving closer, you lose DOF". However my aperture has reduced from f4.5 to an effective f6.3 [IIRC]. This led me to believe that I would actually have more DoF despite the greater magnification. Clearly I don't understand the physics (no surprise to my old science teachers, I suspect) but I had always thought that reducing the aperture increased DoF.

-- Andrew Hardacre (andrew@jhardacre.freeserve.co.uk), September 20, 2001.

37mm of EXTENSION as well as the TC? Well...there's your problem...This set-up is surely not giving you focus far away (ie infinity). With a given aperture and magnification, the other thing that alters depth of field is closeness of focus. EG, I used to use a Sigma 70-300mm APO. Using it at 300mm, F5.6, when focused to less than a metre away, the depth of field was about 1/3rd of an inch. Looking at Canon's DOF data for this lens on it's own, 500mm F4.5, focused to 5m away, give a near distance of 4.99m and far of 5.01m. Thus your DOF is 2cm, or 20mm (ie less than 1"). Even if you stop down to F8, you now only have 40mm of DOF.

See for yourself: http://www.canon.com/camera- museum/camera/lens/ef/data/ef_500_45l_usm_dof.html

-- Isaac Sibson (isibson@hotmail.com), September 20, 2001.


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