Using my lens cap to guage magnification ratios

May 27, 2013
Seems like when I try to take macros out in the field, I'm spending WAY too much time fiddling with extension tubes. One combination is too much, the next is not enough. So I decided to remedy that.

I did some bench testing of my 105mm Micro lens, set at minimum focus, to determine the rough magnification ratios. With that, I can measure the size of an object, and use that measurement to determine which tube, or combination of tubes, I will need to 'fill the viewfinder'.

Also, with those measurements, I could make a simple-to-reference guide to carry with me. The idea was to simply hold the guide next to a small subject, and it would tell me which tube/combination I will need. I ended up with four 'primary' measurements: 1½", 1", ¾" and ½". These figures correlate to my Micro lens at minimum focus (sans any tubes), with ~32mm of extension, with all three extensions stacked together (68mm), as well as the Micro with my 50/1.8D reversed.

Then after all this, I tried several varieties of hand-held guides to carry with me. First was a paper copy... it didn't last long, and was too easily lost or destroyed if gotten wet. Laminating it prevented water damage, but it was still too small to make it handy. Next, a precisely-scaled .pdf file on my smart phone. Eh; takes too long, and is hard to see in bright light.

So......... my thoughts were focused the past few weeks on some sort of durable, yet readily accessible medium onto which to place these four simple measurements. Now, granted, they're easy to remember, but I find myself just strictly guessing how 'big' 1½ or 1" is when a ruler isn't handy. Hmmmm. what to use..........

Then I had an epiphany. What do I ALWAYS carry with me (besides the camera & lens)??? Why, the lens cap! So I pulled off the 52mm Nikon 'center-squeeze' lens cap and grabbed a ruler. Crap, that was WAY TOO EASY! Two of the measurements are built right into it! All I needed was to mark the other two. So I just pulled out my pocket knife and marked ALL my 52mm lens caps with two simple little scratches.

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Now, when I want to shoot a macro in the field, I just pull of the lens cap, hold it next to the subject, and I can INSTANTLY tell whether the Micro lens will be enough by itself, or how many extension tubes I need to add.......... or if I need to get REAL close and throw the 50mm on the front of it.

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Granted, I realize this is not going to work out so well for everyone else, but it may get you thinking about how to make your own custom macro-shooting guide!


Nov 10, 2008
That should work. or thumb tip to little finger tip is x. I would get a i/8 plywood from hobby store, score , fill with paint. Aluminum yardstick, mark with black tape and packaging tape over that. Good for lifetime.

For a symmetrical lens, 1.5 inches long is 2 stops, 3" is 1.5 stops, 6 " is 1 stop, 12" is .5 stop. Chart your with exposure meter.
Oct 15, 2011
Cool gauge.

I haven't gone this far as I don't really care that much about magnification ratios, but I do like the Canon 65mm MP-E because I don't need extension tubes or anything as it can go 5:1 with the built in bellows. I really really wish Nikon or a 3rd party (Sigma or Tamron) made a 65mm MP-E equivilent so I could sell my Canon gear and just have my D7000 and maybe a 2nd Nikon body... like a D800.


Cafe Ambassador
Apr 8, 2008
Rutledge, Tennessee
Real Name
Well, I think this is very clever.... I've done ZERO macro...but have the 105 micro and a set of extensions tubes. I wouldn't have a clue which tubes to use!
Dec 30, 2012
Beauty of this art- trial and error made easy/to understand. Its owning the right equipment that's usually the show-stopper.
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