Marlin's Law
A handy procedure for accurately measuring subjects in macro photography.
By Bruce Marlin
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Suppose you are a macro photographer
with a scientific mindset or are simply curious. You might often find yourself wondering, "How big is that thing?" Perhaps your proclivity lies in macro photographing living subjects that won't hold still. Good luck holding a scale (ruler) next to that crazy ant or butterfly flitting from flower to flower, much less photographing it at the same time.

Perhaps you are simply curious and have heard people give all sorts of assertions as to the size of small objects, from the ludicrous, "That spider was as big as a grapefruit!" to the eminently plausible, "That spider was at least 3 inches across." Who's zooming who? Most people (un)intentionally overestimate the size of things, (small or otherwise) especially if they are afraid of them. Many are simply unfamiliar with measuring things at all.

Who Cares?
Plenty of people care. Especially in the art and science of photographing insects or spiders or other small living creatures, issues of size come to the fore when reporting your observations to other scientists, or when you hope to identify the organism in question or its species. My website attracts upwards of 500,000 pageviews a month during spring, summer and autumn, yet I am under no illusion that many would visit if I was unable to tell them what, exactly, they are looking at.

How many millions of Flickr account photos yield such captions as "big spider" or "bug" yet offer no other information? Sure, there are some magnificent images there, (many better than what I produce) but they get lost in the shuffle of a thousand other photogs posting equally compelling shots. "Eye-candy" comes to mind. I don't mean that derogatorily: it's simply not my bag. I want my photographs to get noticed.

Insects and spiders (arthropods) do not give up their secrets easily; it is a rare photograph that yields a decisive verdict on the organism's identity to species: most identities are revealed only with capture and dissection, or at least close inspection with a hand lens and careful reference to a species key. However, many well-known or prominently marked arthropods can be frequently identified to species or genus, especially if an accurate measurement of its size can be counted upon. A minor detail, perhaps, but I believe dedication to the scientific art of insect photography requires attention to details.

If the object of your object is moving, or skittish, or tiny, or dangerous, (or simply disgusting) you might think yourself out of luck. But if you're interested, there is an easy, reliable method for obtaining very accurate measurements of living, unposed, uncaptured, unfrozen and not-squashed-in-a-scanner creatures, directly from the photographs themselves. All it takes is a bit of foresight and diligence when out in the field, and a firm grasp of Marlin's Law, which states:

Every in-focus object in a macro photograph can be measured against any in-focus object in a different photograph providing both images were taken at the same focal length.

Of course, this is nothing more than a bit of simple logic and physics. But I have had many photogs tell me "It won't work" without being able to explain why. Well, it does work and I'll show you why.

In fact, the smaller the object and the higher the magnification, the more accurate the measurement will be. Let's think about this for a moment. If you have done any macro work at all, you realize and often curse the fact that as magnification increases, your depth of field (objects in focus) decreases. Obtaining good macro shots of creatures that might only be say, 10mm long and 4mm deep requires a camera be held very steadily within a very small range of distance from the subject. Your depth of field might only be 3mm or less and objects outside of this parameter will not be in focus; perhaps your fly's legs are but its eyes are not? Shit-can that one!

All this means is that as long as you don't touch your focus or zoom between shots, your camera (i.e., the plane of its CCD) will be exactly the same distance from objects within the depth of field in each shot. If you take an in-focus picture of a quarter-dollar filling your frame and then take another, (say, using a different shutter speed) can you not use one image to measure the other? Of course you can. The size of the quarter did not change. Your camera's optics did not change. The distance to the subject did not change, else it would not be in focus. The focal length is identical, and the size of the image cast onto the CCD is the same. Q.E.D.

If you take a decent shot of a bug with its body closely parallel to you camera's focal plane, capturing the full length of the critter in the frame and aligned with the x axis, you can use an image of a scale taken in the same orientation to measure the bug in the preceding shot. It's as simple as that.

The quality of the image is unimportant, as long as the preponderance of the subject is in focus or nearly so. I'm not a mathematician, but I believe errors of parallax, when minimized, would affect the measurement only a fraction of a millimeter. Making sure your bug's body is not tilted away from or toward you (an essential bug-macro practice anyway) and making sure your picture of the scale is sharply in focus across the frame are adequate for this method.

Requirements: Patience, a camera with macro capability, and a ruler (scale) calibrated in centimeters and millimeters. Please do not fool around with inch measurements; they are frowned upon by scientists, and the rest of the world will never understand your archaic system. Remember, it was a an English-metric foul-up that sent the Mars Climate Orbiter crashing into the Martian surface instead of obtaining orbit! Inch-worms will quickly get a handle on the millimeter/centimeter scale.

First, TURN OFF YOUR CAMERA'S AUTOFOCUS. You must obtain a horizontal, full-length shot of the creature you wish to measure. Use your zoom and/or move your camera toward or away from your subject to focus. The bigger the better - fill the frame if possible. The quality of this shot is relatively unimportant. As long as the subject is in reasonable focus and not blurred by movement, it will do. Try to maneuver your camera lens barrel as perpendicular to the subject as possible to minimize errors of parallax.

Now - WITHOUT CHANGING YOUR FOCUS OR ZOOM - immediately take a picture of your scale, in focus, in the same orientation you used in the bug photo. Move the scale to fill the frame and achieve focus while splitting the field of view in half horizontally. When in focus, the scale will be at the same focal distance as the bug, and this photograph can now be used to very accurately measure the creature in the previous photograph.

I am always prepared to take such shots while out shooting bugs. I almost never use autofocus anyway, instead moving my camera to achieve focus, so it's easy to quickly take a scale photo when I get a good full-length shot. What's difficult is remembering to do so - I try to make this shot a top priority when encountering a new bug. When these shots are accomplished, I can easily set my sights on getting the diagnostic shots I desire - providing my subject has not decamped in the meantime. In any event, I will always know how big that specimen was and can rest secure in that knowledge.

Now, when you get your pictures unloaded, remember to do your measuring before you cull any photos. Having a shot of the scale without the immediately preceding photo is a little like having a leash but no puppy-dog.

Without resizing, crop each image horizontally across the full image, retaining only the pertinent portion. That is, crop as close to the subject as possible, and discard the waste frame. I generally end up with the bug photo cropped to the top of the bug, and a scale photo cropped to the bottom of the scale (see illustration) or vice-versa. Display both photos (be sure not to let your editing program resize the images while moving them) next to each other. Et voilà! You now have a scale next to your object of affection and can read the pertinent datum. Take a screen shot to document your measurement.

You can see the above shots did not line up exactly. You can visually make adjustments, or if you are as anal-retentive as I am, take a straight edge (a business card or the like) and place it parallel to the long axis of the fly's body with the end lined up with the fly's westernmost extremity. Using the hypothetical intersection of the "2" index and the fly's head as the center of rotation, swing the card up to the scale and read it directly. (I use a push-pin to hold the card against the monitor.)   I measured this flesh fly at 12.5mm +/- .5mm.  That's a big fly!

You'll not do much better even if you manage to lay a scale next to the creature (highly doubtful) or capture and kill the beast (onerous and out of the question, in my world).

Karma works at small scales too. Take only pictures, leave only footprints!


Wondering how to get that bug identified? Please see the kind folks at Bugguide.net. (North America)
North American Insects & Spiders is dedicated to macro photography of live, wild organisms in situ.

Disagree? Have questions? Wish to rain scorn? Email WEBMASTER.

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Class Arachnida / Order Araneae: Spiders are the largest group of arachnids.  They are easily recognized by their eight legs, and there are few creatures great or small that elicit such irrational fear in mankind. The vast majority of spiders are completely harmless and offer beneficial services, chief of which is keeping the burgeoning insect population in check.
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