Invisible Pencils

Okay, take a look at the photo above. Notice anything strange about it? Probably not, but in fact there’s something quite different about it, or at least about the camera setup used to take the photo. Here, let me show you:

Yes, that’s right, the camera has a pencil rubberbanded right across the middle of the lens. Look back at the original photo… no pencil! So what gives?

This subject came up on the This Week in Photography podcast a few weeks ago where someone asked why the scratches on the front of his lens weren’t showing up in his photos. And the reason has to do with the fact that shooting with a large aperture will do an amazing job of hiding dirt or scratches on the front of a lens (or on a filter in front of the lens).

Without getting too deep into the optics behind this phenomenon, it primarily has to do with the fact that a wide open aperture means that you’re gathering light from the entire surface of the lens and focusing it on each element in your sensor. When you ‘stop down’, i.e. set the lens to a smaller aperture, every pixel in your photo is based on light from a much smaller area of the lens. The net result is that with a lens set to a large aperture, about all you’ll notice when you’ve got something blocking a portion of the lens is a decrease in the brightness of the image. Here’s the original image side-by-side with a shot taken without the pencil, using exactly the same f-stop, shutter speed, iso and lighting conditions. The image on the right is brighter, due to fact that light isn’t being blocked by a pencil…

So how is this information useful to the average photographer? Well, I can think of at least two times I’ve used it to my benefit. The first was when I wanted to get a photo of something that was behind a chain-link fence. Shooting at some small aperture like f16 would put an ugly bit of the fence across part of my image but dropping down to a wide aperture would make the fence disappear.

Another more common example is when shooting in conditions where water drops can end up on the lens – either if it’s raining or if you’re standing next to a waterfall, perhaps. Shooting at a wider aperture gives the photographer the ability to ignore a little bit of spray without fear of ruined photos.

Here’s a little animated gif where I shot a sequence of frames, decreasing the aperture each time. The range is from f1.4 down to f22.

Now, to be fair, there are subtle degradations that can happen to the image, including a loss of sharpness and various diffraction artifacts. But it’s a nice trick to have in your repertoire of techniques.

Anybody else have good examples of using a wide aperture to make something invisible?

UPDATE:  A couple of good notes from the comments.  My buddy Joseph Linaschke notes that this trick is also useful if you’ve got a bit of dirt on the actual image sensor of your camera, and Hugh makes the great point that this is effectively changing the shape of the aperture, which means you can use it to modify the characteristics of the bokeh in the out-of-focus areas.  Thanks guys!
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