Friday, 11 October 2013

Making Big Things Look Small

Any photo normally give you an immediate sense of scale, but what is it about the picture that let's you know how big something is? Take a look at these two photos:

The first photo is clearly of some tiny water droplets on a plant, while the one on the right is clearly of a train running through a city. It isn't just content (train vs. droplet) and context (city vs. leaf) that inform you about the scale; you can trick you eyes into thinking a massive building is actually a miniature. There are other properties of the image that come into play.

The key is blur. In photos of tiny objects the background and foreground of the image are normally very blurred, but with big objects the background and foreground are normally sharply in focus. I talked about this effect in my last blog post; the bigger the ratio of the size of the lens to the distance to the object, then the more blurred the background and foreground look. Microscopes take this to the extreme, when out of focus parts of the image are so blurred that you can't see what is there at all.

This effect is universal, it even happens with your eyes. You can try it out: hold your finger close to your face (about 15cm/6in away) and look at it with one eye closed. Notice how blurred things in the background look. Now hold out your hand at arms length, and look at it with one eye closed. Now the objects in the background look more sharply in focus.

So to make something big looks small you need to make the foreground and background look blurred. Simple! Before digital image processing (Photoshop) effects like this had to be done in camera. It was surprisingly easy; take a camera with a detachable lens, and detach the lens from the camera. If you now set up the lens pointing at something you want to take a photo of, but tilt the camera relative to the lens, then you get an effect which is a bit like blurring the foreground and background of the image. This happens because one side of the image sensor/film is now a bit too close to the lens. This makes that side of the camera long sighted, and that side of the image blurry. The other side of the image sensor/film is also at the wrong distance from the lens, but is a bit too far instead of a bit too close. This makes the other side of the camera a bit short sighted, and makes that side of the image blurry too. The middle of the image sensor/film is still at the correct distance from the lens though, so the middle of the image is still in focus.

In principle this is simple, but in practice detaching the lens from your camera just means lots of stray light can sneak into the photo making glare. In practice you need to use a special lens which can be tilted while still attached to the camera, called a tilt shift lens. These are very specialised and cost a huge amount; it is much cheaper to fake the effect using a computer! There are many pieces of software that let you imitate tilt shift lenses, all that is needed is a gradually increasing blur as you move away from the line across the image that you want to remain in focus. I actually wrote a filter in ImageJ which does this processing. It is amazing how much this simple effect can trick your eye; it makes big things looks small by imitating the limitations of focal depth when using lenses to make an image.

This is the same photo as above, but with a tilt shift effect applied to it. This chunk of London now looks like a miniature, with a tiny toy train running through it.

The Tower of London, looking a lot smaller than usual!

The reverse effect happens too, though is psychologically less strong. This is part of the reason scanning electron microscope images are so compelling; they don't use light or lenses (at least in a conventional sense) to generate the image. This lets everything, from background to foreground, be sharply focused. It makes the microscopic world feel free big and accessible.

This diatom shell is less than a 0.1mm wide, but the sharpness of the foreground and background make it feel larger.

Artificially blurring the foreground and background makes it seem much smaller.

When communicating science it is important to think about the scale things appear; scanning electron microscope images break your intuitive concept of scale, and it is hard to imagine just how small the sample is. The image below, by Donald Bliss and Sriram Subramaniam and awarded Honorable Mention in the National Geographic best science images of 2008, is of the structures inside a single cell, and is a perfect example of why you need to think about scale. This image is a computational reconstruction, so it could be presented with pin sharp focus, but by blurring the background it conveys the sense of scale extremely well. It feels like a single cell.

Software used:
Photomatix Pro: Generating the tonemapped HDR photos.
UFRaw: Camera raw file conversion.
ImageJ: Custom tilt shift filtering.

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