Are square lenses better?

December 3, 2007 8:45 PM

Optics geeks: Why do some cameras matte off a rectangle in front of the lens like this?

I see this a lot with (cheaper) point and shoot cameras, film cameras, but not SLRs. Does masking this way cut down on flare? Would the square aperture reduce light to the sensor. Would it make for funny diffraction or bokeh?

Just curious why the two approaches.

posted by Popular Ethics to science & nature (12 comments total)

The light that would be blocked by this structure would fall off of the sensor/film anyway, and in many cameras this mask is part of the builtin lens cap mechanism.
posted by spasm at 8:50 PM on December 3, 2007

What you see there is called an aperture. There are several reasons why apertures are used.

Light spreads from a point such as your object of interest spherically, so apertures are used in lenses to prevent light from the object from hitting too far off-center of the lens. Generally, aberrations such as colour differences (chromatic aberrations) or image distorsions are more profound when edges of lenses are used.

Also, many apertures are used to decrease scatter inside of your camera. Imagine light hitting an edge of the objective - it will then bounce around the camera and create "noise" which will register randomly on the detector or film. If an aperture is used, one can eliminate those nasty, noise creating photons from getting to the lens in the first place.
posted by Yavsy at 8:52 PM on December 3, 2007 [1 favorite]

The light that would be blocked by this structure would fall off of the sensor/film anyway,

Really? I had the (wrong?) idea that all parts of the image circle received light from all parts of the lens.
posted by Popular Ethics at 8:54 PM on December 3, 2007

It's a flare cutting mechanism, to reduce the chance of light bouncing around inside the camera and causing "regular reflections" which are a problem with digital sensors (because they are flat and shiny).

There are a number of SLR lenses that have such a feature as well, like this one.
posted by Sukiari at 9:12 PM on December 3, 2007

Really? I had the (wrong?) idea that all parts of the image circle received light from all parts of the lens.

Yes, but the sensor/film isn't a circle - it's a rectangle within that circle, so parts of the circle are "wasted."
posted by spasm at 9:16 PM on December 3, 2007

Many SLR lenses come with a Lens Hood (for example), which I believe serves a similar purpose. To quote an Amazon reviewer:

"Hoods offer several benefits to the photographer. Primarily the hood shades the lens from stray light that may result in an unwanted discoloration or hazing of the image, or the appearance of lens flare. Shading your lens helps improve contrast and image quality."

Here's a photo I took with the kind of "stray light" a hood would prevent.
posted by erikgrande at 9:47 PM on December 3, 2007

Like others have said, this is basically a lens hood. Hoods don't frame the shot, they just prevent stray light from bouncing around at odd angles in the lens messing up an exposure. We would have lens hoods built into everything if: 1) all focus mechanisms moved only in-out and not twist-in-twist-out, 2) all zooms functioned the same way, 3) the hood wasn't visible at the widest angle of zoom. Otherwise, the hood would get perpendicular to the sensor and/or the hood would crop the light hitting the sensor/film.

This only matters at the wide end of the spectrum. Longer lenses just have circular hoods.
posted by cowbellemoo at 10:20 PM on December 3, 2007

spasm: that's not the point, and Popular Ethics is right. Each point on the film receives light from the whole lens. However, the obstruction here isn't at the lens but slightly in front of it, which would change the situation somewhat.
posted by edd at 1:38 AM on December 4, 2007

That's not an aperture, that's a very crappy lens hood. It reduces flare due to light hitting the front of the lens and scattering, resulting in bright spots (obvious flare) or a generalised veiling glare (poor contrast, like fog). The aperture is buried inside the lens and has a variable diameter; light from all of the aperture does in general contribute to all of the frame. Note that because the aperture is NOT the front element, there are parts of the front element that do not contribute to the image.

SLR lenses have lens hoods too but they tend to extend forwards, which makes them more effective because they can (unless the lens is very wide) shade the whole front element from the sun when it's a bit off-axis. The wider the field of view, the less effective the hood can be (it must admit a wider field of view, therefore greater chance of getting direct sunlight on some part of the lens) and therefore the more flare-prone the lens. On longer telephoto lenses, the hood can sometimes be as long as the lens.

Next time you're taking a picture with a point & shoot camera while facing the sun but without the sun in the frame, shade the lens with your spare hand and see the contrast improve dramatically. Hands make excellent lens hoods when the front element is so small; you're helped by the fact that P&S lenses are generally not very wide so it's pretty easy not to get your hand in the frame.
posted by polyglot at 1:48 AM on December 4, 2007 [2 favorites]

Thanks everyone. I get the "stray light" explanation, and cowbellemoo's reason SLR lenses don't have rectangular mattes (they rotate!)

I'm still a bit confused that the front lens isn't the "aperture", and that it passes wasted light. Certainly it's not an iris, but doesn't the diameter of the lens determine the F-stop, regardless of film size (or shape)? That would suggest (to me anyway) that all parts of the front lens pass light to the sensor, and that this kind of mask (as opposed to a round lens hood) would reduce the exposure.

*shrug*
posted by Popular Ethics at 4:34 AM on December 4, 2007

Searching "rectangular lens hood" was much more successful than "rectangular lens matte" I found this interesting piece:

"So far the discussion involved circular lens hoods. Indeed, a circular lens hood has the same rotational symmetry as the lens and aesthetically matches the round image formed by a photographic lens. However, the round image is not fully used as the presence of a field stop, a 36x24 mm mask in case of a 35-mm camera, crops the image to a rectangular section. This has important consequences for lens hood design and the optimum lens hood is not round. The accepted light cone that is used to illuminate the frame is pyramidal. At full aperture, going from the lens towards infinity, the cone starts out circular at the front element and converts to a rectangular cross section at some distance. At small apertures, and depending on the design, the cross section of the cone is already rectangular at the position of the front element. The pyramidal cone is illustrated in figure 8."

source. I wonder how hard it is to calculate / test how rectangular the light cone is at the front element of any given lens.
posted by Popular Ethics at 4:47 AM on December 4, 2007

If you look inside largeish lens, you can see the aperture; it's made of a number of matte-black metal blades that slide past each other in an iris action. The point where this is placed is the point in the lens where light arriving at each point on the film/sensor is uniformly distributed across the lens' cross section.

At other points (like at the front element), the uniform distribution doesn't hold. Imagine a macro lens capable of focusing on something only 1cm in front of the front element; light from a particular part of the subject is clearly not arriving on the film from all parts of the front element.

The diameter of the front element forms an upper bound on the aperture of the lens; if you have e.g. a 50mm f/2 lens, the front element must be no smaller than 25mm diameter but typically it will be larger to reduce vignetting and other effects. If you look into a lens and can see the aperture, look in from some distance to the side; if the aperture is partially obscured by the lens body (i.e. the edge of the front element) then the lens will vignette because some parts of the scene (the angle you're looking from) cannot illuminate the whole aperture via the front element. Similar effects can occur at the rear of the lens but they're more difficult to see.

If you want to map out the region of your front element that useful light passes through, try obstructing parts of it with a piece of black card.

If the card is making the whole scene dimmer, then light from the whole scene is passing through that point on the lens and being recorded (or it would if the card were not there). If it makes part of the scene dim, it shows that only light from that part of the scene goes through that part of the front element. If there's no effect, that piece of glass is doing nothing.... though beware that the effect may just be too small for you to notice; you don't want to go taping up the front of your lens.

The smaller the aperture you use, the smaller the region that each part of the front element is used for. Think of something on the front element as being really out of focus; if you use a really small aperture, you might still be able to make out its form. You may also be able to make out the form of any obstruction on the front element in the bokeh of the image, i.e. the shape of the out-of-focus highlights in the image. That is why reflex lenses have donut-shaped bokeh.

Googling for bokeh can be a very interesting read.
posted by polyglot at 9:30 PM on December 4, 2007