As much as I try to minimize it, I can’t teach the Nuts & Bolts of Digital Photography class without a little math cropping up. Some adult learners just don’t want to revisit the painful experiences of school-age math tests. But if you refresh your mind on the concept of ratios, it’s easy to avoid a common pitfall where an image is too constrained to crop to fit a standard frame. Or an unhappy surprise when your photo lab’s computer crops your enlargement to fit the paper ratio. Yep, it’s a just computer and the ‘puter doesn’t know that it just chopped off Aunt Martha head.
First, know your sensor size ratio. It’s not that you have 12 kabillion gigapixels, it’s the ratio of the length and width of the rectangle array that counts. Most DSLR pixel arrays are organized in a rectangle that is proportioned like an old 35mm slide: two-thirds as many pixels on the short side as the long, or a 2×3 rectangle*. Multiple each side’s ratio by two and a 2×3 rectangle makes a uncropped 4×6 print, straight out of the camera. But if you multiple by four, you get an 8×12 print. These days we are starting to see some picture frames and print services that make 8×12″ prints, but an 8×10″ frame is a lot easier to find.
The colored boxes in the image of Ray’s Boathouse represent the cropped sizes of the image if I keep the full width of the image the same and cropped to a 4×6 (yellow), 5×7 (lime green) or 8×10 (aqua) size.
Hard-won experience when laying out my Highway 89 book taught me to consider the future uses of an image. Today I would compose this picture of an antler seller in Jackson so that I had more cropping options. As it was, the only proportion that eliminated distractions, but kept the essentials of MY story (from skull to arch) is the one shown here. It’s even narrower than a 4×6, and you can see from the aqua lines what a disaster result with an 8×10 crop. The result was that the no-compromises crop dictated the entire spread layout and forced design decisions that weren’t 100% consistent with my other pages. You might not notice it unless I pointed it out, but it was an unnecessary constraint. Keep the options open.
So back to ratios. An 8×10 crop is equal to a 4×5 or 2×2.5 proportion or ratio. To go from a 4×6 to a 4×5, you need to throw away 1/6th of the image on the long side. But what if you filled the image from edge to edge along the longer edge with essential visual information (like a skull and an arch)? Then you don’t have 1/6th of the image length to crop away. That limits your options.
Now there’s a lot to be thinking about in the heat of battle, and your first concern is to get the shot. But if you are previsualizing (thinking about future uses–I want this photo of my kid in my rogue’s gallery in at least a 5×7 frame), you can make it easier on yourself if you find a mental guide in your viewfinder that’s about 1/6th of the way to the left or right of a short frame edge. Maybe you can use one of the focus points as a measuring stick, or one of the data elements at the bottom of the viewfinder. In this example of a generic viewfinder, you could use the indicator of the number of remaining frames a reference point. If you keep all the essential image the the left of the imaginary line, you know you can crop to an 8×10 size later. That’s not to say that the other 1/6th of the frame can be ignored. You might want to use the image at other proportions, like 5×7, or uncropped at all. Your goal for this area is interesting but croppable, if you want an image that works at several sizes.
“Fill the frame” is still good advice-always in the shorter dimension. But think about what you want to do with the picture before you accidentally commit yourself to the 2×3 rectangle. Try to shoot some variations in composition that allow for a squatter crop size, and it just might save the design day.
* Some point and shoot cameras use a 3×4 proportion sensor. If you aren’t sure, you can look it up in your manual, or calculate it yourself. Divide the width of the sensor pixel count by the length: 3000 pixels wide divided by 4000 pixels long is 3×4 or .75. The same calculation for a 2×3 ratio sensor would have a result of about .66.