I have a set of Mitutoyo digital calipers which run about $180, but you don’t need anything that accurate. This method is not only more accurate than the equal space dividers but is a cheaper method as well, just not as fast at first. Not bad, well within the accuracy of most applications. I came up with an 8″ width, which when I checked the casing with an actual measuring tape, found it to be in reality 7 7/8″ to 7 15/16″. You can mark the distance at the middle point and reduce the spacing of the dividers to equal 1/4″ in scale and so forth. Now we have a scale to measure the spacing between each of the line extensions above the top nomen line. Most dividers are manufactured with fairly ‘tight’ joints but you can easily bump them while you’re working and throw off the setting. For accuracy you’ll want to continuously check the spacing of the dividers against this ‘master’ to be sure you haven’t changed the setting. As each space represents 1/2″, the distance over the width of the dividers is a scale 5″ along the nomen line. Once you have these marks set, carefully move down to the bottom of the page and mark the distance at the first and last point. This means that 7 1/2 spaces will equal 3 3/4″ scale inches along the new datum. They can represent any distance you want them to, but 1/2″ works best for this example. With the dividers, open them along this datum line allowing the distance between each point to equal 1/2″. Now transfer these two points to the new datum line at the top of the photo.
Then mark a known distance on the original centerline, in this case it’s the distance from the tip of the middle finger to the end crease which is 3 3/4″. Now continue the lines perpendicular to this new datum line so that they are parallel which eliminates the perspective/foreshortening effect of the photo. Then draw a line parallel to this at the top of the picture crossing the outlines. The first thing you’ll want to do is draw lines outlining the sides and edges of the moulding details, then you’ll draw a centerline through your scale, whether it’s a hand or tape measure. In this photo of a 1840’s Greek revival casing, we’ll scale the actual size using the hand in the photo as a reference nomen. If you don’t have a set of equal dividers, also called 10 point dividers, you can buy a new pair for between $250 and $300 from various sites such as this one, or this one . They sometimes show up on Ebay but plan to pay around $75 to $100 for a used set. Once you have had some practice, this is the fastest method of the three, even faster that the digital method and you can use them right off a photo from a book or even a smart table screen. Here are three methods, two analog and one digital, that you can use to figure out hard dimensions from objects in photos. In February I posted an article about using your hand in photographs as a scale reference but didn’t go into how you extract that information once you have a copy of your photos.
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