Experimenting with macro photography

Macro lens coupling

Several places suggest getting a macro lens coupler (male-male ring) in order to mount, say, a Canon 50mm f/1.8 lens (which is a lens any Canon SLR owner should have, very sharp, lightweight, and fast for hardly any money) in reverse on a normal lens. Sounded like an interesting idea, but no macro lens couplers were available. The photo guy at Viby Torv gave me the right clue, though, when he suggested using two normal rings, with or without glass (possibly punching out the glass of a cheap filter), gluing them together. This becomes a cheap-ass macro coupler for any lens combination easier than finding it online. Any photoshop worth its salt, and some second-hand stores too, will have a box of old step rings or filters, and surely some of them can be used to mix the two sizes you'll want.

In my case, I have for my Canon 350D in addition to the 50mm a Sigma 18-125 f/3.5-5.6, a nice lens if you can get one that focuses correctly. It has 62mm threads, so with a 62mm skylight filter and a 52mm ring of some kind. They weren't ideal for the purpose, as the 52mm was wide enough to just rest on the edge of the 62mm, making glueing tricky. Slightly smaller would have been better, then they would have fit inside each other, and glueing would have been a snap. As it is, I applied some contact cement and for safety let them dry under pressure for several hours.

The combined rings (pushed two f-stops over ISO 1600):

The mounted lens, front view and full setup:

(These two pictures taken with my old Nikon Coolpix 995)

After putting the coupler on, I reverse-mounted my 50mm and cast about for something to snap that would be recognizable, easy, and measurable. The result is below: One of the knobs on a standard 2x4 lego brick, which measures exactly 5 mm across. On the picture, it came out to 1100 pixels out of 2314, or .475 of the picture. Thus, the whole picture is 5 * 1 / .475 mm = 10.5 mm high. On a sensor 14.8 mm high, this equals a magnification of 1.4:1, slightly better than what you can get in a dedicated lens.

Lego brick at 1.4:1 (100% crop)

Next test, for a little more accuracy, a measuring stick. This one showed clearly a curious detail: When moving the camera closer to the subject, the magnification drops. At the maximum distance (3.2 cm from the front of the lens) with the focus at infinity, the magnification is 1842/3474 pixels (wide) to 5 mm = 2.3:1, but moving closer and focusing closer (down to 2.5 cm) drops the magnification to 852/3474 pixels to 5 mm = 1.09:1. Changing the focus of the reversed lens does not appear to have any effect whatsoever. Thus, the longest working distance, a whopping 3.2 cm, gives an extreme magnification of over twice lifesize, while shorter distance gives something more like a normal macro lens.

Measuring rod at maximum magnification (100% crop)

Measuring rod at minimum magnification (100% crop)

Also quite obvious, however, is the extreme light dropoff, especially at the higher magnification. At shortest focus, almost 1500x1500 pixels are usable in the middle, with a fairly sharp drop-off to black. At maximum magnification, dropoff is much softer, with some detail visible almost all the way to the corners, but vignetting starts at not much higher than the 1500x1500 for the short focus. This is of course due to the inner lens having a fairly large opening (62 mm threads), on a smaller lens it should be not nearly so pronounced.

As a final treat, here's a closeup of the label of a scented candle can. Yes, those are the dots that the offset printing uses. The drop-off in sharpness is due to the can being rounded, though the measuring stick photos also show some sharpness lost out from the center, most of course in the extreme magnification, but still noticable in the 1.09:1 end. The fast drop-off in sharpness clearly illustrates the very small depth of field.

Label of scented candle (100% crop)

None of these pictures are sharpened in any way, but exposure of the RAW photo was adjusted using UFRaw.

Further experiments: Testing depth-of-field

It may not be obvious from the previous pictures, but the depth of field is really tiny with this setup, on the order of a millimeter. To test a bit, I took a minature dragon of my wife's as an example, and took pictures with apertures from f/5.6 up to f/36, the maximum for the camera.

Close-up add-on lenses

Don't have these yet. According to Julian's Lens Calculator, it should work well with my 18-125, if I use a full set of three.

Extension tubes

Don't have these yet. According to Julian's Lens Calculator, it should work well with my 50mm, if I use a full set of three.

Macro lens

The best in the macro world, but at about ten times the price of close-up lenses and extension tubes, and easily 100 times the cose of a couple step rings.

-- LarsClausen - 05 Apr 2006
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