Hi guys, as you may be aware, i'm (still) unemployed, and that leads to boredom and too much time on my hands.
So, one thing i've always wondered is what is the maximum magnification using Kenko extension tubes for macro.
Canon quote the numbers for their own 12 and 25mm thick extension tubes, but a set of the Kenkos comes in 12 / 20 / 36, which gives 7 length-combinations available of 12 / 20 / 32 / 36 / 48 / 56 / 68mm. I'm not sure, but I'd guess that the Kenko tubes are a lot more popular than the Canon ones, the Kenko set is about the same price as a single Canon tube.
The only reasons I can think of for wanting the Canon over Kenko ones are that the Kenkos are not available, the customer has never heard about them, they don't trust the strength, or they really like the canon logo.
So I thought i'd do some (rudimentary) experimenting of my own, and I thought I'd share the results for anyone who wishes to know.
Procedure:
7D on tripod, levelled using its gyro (not essential, but helps with re-framing each shot). Lens on Manual Focus, set to Minimum Focus Distance. Triangular-style ruler on table, parallel to sensor (using eyeballs, nothing special).
Using Live-view, slide ruler towards camera until centre is exactly in focus (all lenses i've done so far have had a bit of field-curvature so outer edges may have been slightly oof).
Take a photo, measure width of frame in focus, and divide into the sensor width (22.3mm on 7D) to get Magnification factor. Add an extension tube, and repeat.
Notes on the results table:
I haven't done all the possible lengths of tubing, just each tube individually and then all 3 together (the "+ Tube" column).
The "+ Lens" column is using my 37mm Lensbaby +4 and +10 (and both together in +14) macro lenses as well as the extension tubes.
The "+ Thick" column is how much extra this combination takes away from the working distance, the thickness of the lenses and the step-down rings to 37mm (ie, if you can find a 52mm +10 lens 4.4mm thick to put on your EF 50/1.8 ii, your "working distance" would be 33mm)
For the "Working Distance" measurement, that's the distance from the outermost-point of the lens (including any close-up lens) to the ruler in focus. That's the number that matters to me, because it's how close you can get to the subject in question without scaring it, and also give space for flashes etc.
The "Frame Width" column is how much of the parallel-ruler was in frame width-wise, and the "Magnification" column is the sensor-size of 22.3mm divided by the Frame Width. People who want to correct for pincushion distortion will lose a bit obviously.
The table has been updated for both Min and Max Working distance and Frame Widths, with the focus ring at Minimum FD and on (or past) infinity. Measurements with no extension tubes at infinity focus are meaningless, obviously, so not included.
Min Max Min Max Lens + Tube + Lens + Thick WD WD F. Width F. Width Max Mag Min Mag 50 f/1.8 ii 0 0 0 350 148.5 0.15 0 50 f/1.8 ii 12 0 0 142 230 57.5 95 0.39 0.23 50 f/1.8 ii 20 0 0 112 194 43.5 62 0.51 0.36 50 f/1.8 ii 36 0 0 73 86 26.5 33 0.84 0.68 50 f/1.8 ii 68 0 0 48 50 16.5 17.25 1.35 1.29 50 f/1.8 ii 68 4 10.4 32 35 13.25 15 1.68 1.49 50 f/1.8 ii 68 10 10.4 27 30 11 12 2.03 1.86 50 f/1.8 ii 68 14 17.3 21 23 10.5 11.25 2.12 1.98 * 70-300L @70 0 0 0 876 295 0.08 0 70-300L @70 12 0 0 256 447 86 38.5 0.26 0.58 70-300L @70 20 0 0 188 285 62.75 89 0.36 0.25 70-300L @70 36 0 0 109 148 36.5 46 0.61 0.48 70-300L @70 68 0 0 62 76 20.75 24.25 1.07 0.92 70-300L @70 68 4 19.4 37 45 17.5 19.25 1.27 1.16 70-300L @70 68 10 19.4 30 34 14.25 15 1.56 1.49 70-300L @70 68 14 26.3 23 26 13.25 13.5 1.68 1.65 * 85 f/1.8 0 0 0 720 173 0.13 0 85 f/1.8 12 0 0 365 600 82 154 0.27 0.14 85 f/1.8 20 0 0 295 400 64 101 0.35 0.22 85 f/1.8 36 0 0 207 221 41 52.5 0.54 0.42 85 f/1.8 68 0 0 144 129 25 28 0.89 0.8 85 f/1.8 68 4 12.6 85 79 16 19 1.39 1.17 85 f/1.8 68 10 12.6 56 53 10.5 13 2.12 1.72 85 f/1.8 68 14 19.5 43 41 9 11 2.48 2.03 * Sam 35 f/1.4 0 0 0 181 122 0.18 0 Sam 35 f/1.4 12 0 0 50 79 43.5 65.5 0.51 0.34 Sam 35 f/1.4 20 0 0 32 42 32.75 43 0.68 0.52 Sam 35 f/1.4 36 0 0 12 11 20 23 1.12 0.97
Interestingly, for the 70-300L at 70mm with 68mm of extension tube, and then step-down rings to 37mm, there's no vignetting on APS-C because of the enlarged image-circle. (No vignetting for any of them, actually, but I didn't try on the WideAngle Samyang 35/1.4)
The EF 50/1.8 lines up nicely with the lens specifications from Canon for normal and +12mm (0.15x and 0.39x), the 70-300L is a tiny bit out at +12mm (0.26x vs 0.29x from Canon).
The Samyang is a great sharp normal-wide angle lens. But macro it is not. The working Distance was measured from the front of the filter thread (which stays fixed, the front element moves forward and backwards within it). The working distance was too small to bother trying +macro lenses, even with extension tubes the base of the triangular-ruler was hitting the front of the filter thread.
The 85/1.8 I only got a week or two ago, and it's already my new-favourite macro lens (instead of the 70-300L). No IS, but it's a lot lighter and easier to mange. It lags a bit behind the 70-300L and 50/1.8ii for MM figures, but the working distance is so much bigger too. With all tubes and +macro lenses it beats all others for MM, with much larger WD too.
The 85/1.8 also produced a weird result, with 68mm of extension tubes, with and without +macro lenses, focussing at infinity actually *reduces* the working distance with a zooming-effect. Magnification still goes down though, as expected.
