Crop Circles: Part One

…from a topic suggested by Mark Sirota

The Crop You Forgot You Made

Every picture is a crop but likely not in the way you might be thinking. We all make viewfinder framing decisions where ever we decide to point our lens.  We slice out the rest of the world for that rectangular vision we desire. What we likely knew, but have forgotten, is that our camera, specifically our sensor is performing a crop of all the light gathered by our lens.

Lenses are round. They cast circular images. But our digital sensors, like sheets and rolls of film before them, only capture rectangular images. Shutters (talking in-lens shutters) could be round or rectangular but ultimately, modern cameras take a rectangular image to the emulsion or the memory card.  Our digital files, slides and negatives all reside in a rectangular format.

Math Time

Lenses: Focusable area of a standard 35MM format lens is roughly 1520 MM² at minimum to cover the shutter/sensor. The image circle of lenses varies slightly so this is an inexact number.

Sensors: Full Frame are 864 MM²

Cropping: The cropping which occurs in taking the rectangular ‘meat’ out of the lens’ image circle is about 44%. Only 56% of the focused image by the lens actually falls through the shutter and onto the sensor.

In the example I’ve mocked up above, you can see how the sensor effectively crops off much of the image circle. And, in this case, most of that lopped off detail is meaningless.  Nonetheless, we have lost part of the image rendered by our optics. There isn’t much we can do to get it back either.

“What is the point of all this, Rikk?” you might be asking yourself. We are, after all, stuck in a raster world where image formats are defined by an array of pixels that is always rectangular. It turns out, thanks to Mark Sirota’s pointing me to an interesting web community there is a small but determined band of circular croppers out there. Originally I dismissed Mark’s suggestion as being simply a group of matte-ers – sticking a rectangle with a hole on top of every image. But, as I thought about the origin of all our images being circular, it became less peripheral to me and I decided to take this topic on.

In Part 2 of this Crop Circles Series, we are going to dive deeper into this rogue group of croppers. Part 3 is going to talk about compositional considerations for the ‘circular crop’.

Stay tuned.

Rikk Flohr © 2012