Photo of the Rising Sun.

March 30, 2008 10:43 PM

What caused the light to do the weird radial effect in my picture? I'm guessing it's the UV HAZE filter I leave over my lens? Does the effect have a name? Like some weird form of "blooming"? Shot with a Canon Rebel XTi with a 28-135mm USM IS, 6 second exposure.
posted by disillusioned to media & arts (14 comments total)

That's a pretty standard flare. It happens when you point a lens at a light source.

The exact shape of the flare depends on the glass you have in front of your sensor. Taking off the UV filter may change the flare a little, but it won't go away.
posted by b1tr0t at 10:47 PM on March 30

It's called "lens flare".
posted by mr_roboto at 10:47 PM on March 30

Right, I know about lens flare in general; I should've been more specific: I was just more wondering what caused that shape since it seems strangely symmetrical, and wondering if that sort of "rising sun" form of it happens often. Thanks!
posted by disillusioned at 10:54 PM on March 30

It's the aperture blades that cause it. Someone might be able to explain better, but there's a relationship between the number of blades in your lens's aperture, and the number of points on the "star" flare pattern. For example, your flare has 12 spokes, I would suspect your lens has a 6-bladed aperture.
posted by knave at 11:19 PM on March 30

What Knave said, and I'll add that it's exacerbated by the sodium-vapor light source. That type of streetlamp puts out a single wavelength of light, so it makes the effect more dramatic. If it were the sun, the different wavelengths that compose full-spectrum light would diffract at different angles, and you would end up with a softer flare shape.
posted by scose at 11:55 PM on March 30

So is there any way to avoid these flares?

I take it they're a result of the smaller aperture creating a "less circular" opening, so is opening the aperture up and useing an ND filter the only work-around? This would cut down your depth of field, though.
posted by SAC at 12:24 AM on March 31

I don't think changing the aperture will have any effect. Here's some tips for eliminating lens flare. In the case of the poster's photo, I think the UV filter may have something to do with it, especially if it's an inexpensive filter without a good coating. Experiment with removing the filter to see if it goes away. Something else that article doesn't mention is that lens flare is less common with prime lenses due to fewer lens elements (and thus fewer internal reflections).

A ND filter would just darken the image, not prevent lens flare. A polarizing filter however would help cut down the flare by filtering out certain angles of light.
posted by junesix at 2:11 AM on March 31

I'm not totally sure if this would change, but I've found UV filters to be particularly problematic in low light situations. For a while I was wondering why there would be random light spots in my night shots. It was street lights reflecting off the lens into the filter and then back through the lens.

I have long since stopped using filters, and have had no more problems. I'd say most pros do not use UV filters, just lens caps and lens hoods. You have to think...if you had a problem that was likely to destroy your UV filter, it's probably going to take out your lens too, so why walk around with a piece of crap filter over your really good lens?
posted by sully75 at 3:45 AM on March 31

Yep, it's diffraction around the sharp edges of the aperture blades. Increasing the aperture (lower f/number) will usually make the flare shrink, though you will lose depth of field.

While UV filters etc can definitely cause flare, their usual effect is reduced contrast (veiling glare across the whole frame, especially if they're not perfectly clean) and ghosts, i.e. copies of bright things appearing in the wrong part of the image.
posted by polyglot at 5:39 AM on March 31

and a polarising filter will do nothing because the light source is not polarised!
posted by polyglot at 5:42 AM on March 31

There are two things going on. The symmetrical blades of light are caused by the aperture blades. Stopping down the aperture will make the blades even more obvious (which I deliberately tried to do in this picture). The second thing going in is the lens flare, which is the circular splotch of light that's overlapping the upper left part of the street light. That's probably being caused by a UV filter.
posted by alidarbac at 7:58 AM on March 31 [1 favorite]

Technically, the pattern is the "Fourier transform" of the aperture blades. As someone noted, there will be twice as many light spokes as aperture blades, unless there's an even number of aperture blades. More clearly: there will be one double-sided flare across the center of the photo for each aperture blade, and if the aperture is symmetrical, half of the flares will superimpose over the other half.

Opening up the aperture as far as possible will reduce the effect. That's all you can do. Of course, the visibility is dependent on how high the contrast is in the photo, so you only notice it around point sources like the streetlamp, but in principal, it happens to every point of light in the entire photo (just millions of time less obvious, and therefore effectively below the noise threshhold.
posted by IAmBroom at 10:57 AM on March 31

as an aside, you can get filters that do similar things
posted by Large Marge at 7:10 PM on March 31

Whoa, alidarbac, that is a COOL picture.

Thanks all for the answers. I'm definitely having fun getting started with a "real" camera.
posted by disillusioned at 10:44 PM on March 31