# What causes lens flare along specific axes?

When photographing bright objects, what causes lens flare along specific axes?

First two images captured from Reds Rhetoric & Vincent Jones's video, last from SpaceX's feed.

In reflecting telescopes, the secondary support can cause a cross-shaped diffraction spikes, but how would a similar thing happen in lenses?

What causes lens flare along specific axes?

This answer assumes the 1st and 3rd images are caused by the same phenomenon, and image 2 is caused by something different.

## Image #2

To my eyes, this flare appears to be caused by a smudged front element. This can easily happen when one attempts to wipe the element clean, but doesn't get all of the (presumably) finger oils. The last swipe with the lens cloth went from upper right to lower left (viewed from behind the camera), thus generating the perpendicular flare.

See the following questions for similar effects (and answers):

## Images #1 and #3

As you noted in your question, diffraction spikes occur when light passes a straight edge, resulting in the light spreading out perpendicular the edge. In the "Diffraction spikes due to non-circular aperture" section of the Wikipedia article on diffraction spikes, you see that a square iris aperture with straight-edged bladed causes a 4-pointed star.

However, a diamond- or lozenge-shaped 4-bladed iris will produce an X-pattern. It's still 4-pointed, but the points aren't all 90° from each other. Let me demonstrate using my rudimentary ImageMagick skills.

First, let's create our aperture, a diamond pattern (Figure 1):

convert -size 1000x1000 xc:black -fill white -draw \
"polygon 500,400 673,500 500,600 327,500" 01-aperture.png


Figure 1: The horizontal diamond-shaped aperture (01-aperture.png)

A point source of light traveling through the aperture creates the X-shaped diffraction pattern in Figure 2. A Gaussian filter is applied to the diamond aperture as a windowing function, then passing through a Fast Fourier Transform (FFT), followed by another Gaussian filter after the transform to smooth out some of the intensity peaks of the star.

convert 01-aperture.png -morphology Convolve Gaussian:1x1 \
-fft +delete -auto-level -evaluate log 100 \
-morphology Convolve Gaussian:3x3 -auto-level -evaluate log 10 \
02-fft.png


Figure 2: 4-pointed X-pattern from a point source light through a diamond aperture (02-fft.png)

A diamond shaped aperture? Really?

Indeed. Several camcorder models had a diamond-shaped aperture. This is desirable because it is mechanically much simpler than a typical N-bladed iris aperture with N rotating iris blades. All that's needed is two V-notched blades opposing each other, and moving linearly towards or away from each other to close or open the aperture.

This arrangement also explains why X-pattern diffraction in camcorders is always perfectly oriented. In a lens with an iris aperture, such as in almost all modern photographic lenses, as the iris blades collectively rotate to close the aperture, the created N-gon aperture shape rotates. Thus the orientation of the diffracted star pattern in photographic lenses is dependent upon the aperture value setting. But in the camcorder diamond aperture, there is no rotation, so the X-pattern is always oriented the same way.

I can't find concrete details about the Canon Vixia HF R400's aperture blades, other than according to CNet's product review, its "Lens Aperture" is f/1.8–4.5. This is probably just the physical aperture range, as it also has built-in ND grad filters to further control light entry.

A review of the prosumer Panasonic AG-HMC40 1/4″ 3-CMOS HD camcorder shows that camera's diamond-shaped aperture in closeup:

Diamond-shaped aperture of the Panasonic AG-HMC40 camcorder. Retrieved from Provideocoalition.com under fair use for educational purposes

• It's a great answer with a Bounty offered (if one could "demonstrably invalidate" the answer) that will expire in 20 mins. The answer is fine for the question. IF it were a photo I tried exploring if a type of Shutter could produce such an effect, instead of it being produced by the aperture blades - but time runs low and it's not a better answer. I did find a duplicate answer over at photo.stackexchange.com/questions/6605/… which is applicable to photos and not this video stillframe. Nothing I could propose would be a better bounty answer.
– Rob
Commented Nov 25, 2017 at 19:06
• @Rob the bounty note was written against my answer as originally written (view the edit history). I had a theory that seemed plausible, I even tried to demonstrate the effect, but later decided didn’t explain what was going on
– scottbb
Commented Nov 25, 2017 at 19:15
• @Rob I was looking for more examples of X-pattern video stills, from different videos or situations, because I know I’ve seen them, but I couldn’t place them, and had a really hard time coming up with examples.
– scottbb
Commented Nov 25, 2017 at 19:17
• I found a few examples: The Componar 75mm f4.5 enlarging lens adapted to a camera: youtu.be/le2_0uQuRzA?t=44s - The Helios 44-2: youtu.be/EJ2uuYg6tuU - The Zenitar ME-1: youtu.be/sD4uee0_MRY -- Try searching for "square bokeh". They produce 'ugly?' square bokeh UNLESS confronted with a bright point of light, then you get a four pointed star. IF you're shooting video on an ILC those lenses (and any old (Russian?) lens) will duplicate the result shown above... But this comment isn't an "answer" to the OP question, thus not bountiful (new use for that word ;) ).
– Rob
Commented Nov 26, 2017 at 5:19
• @Rob I've never seen those lenses, very interesting. What I was looking for in my bounty offer were more examples of video camera stills / footage showing X-patterns (say, at 60° and 120°, or even higher angles). Diffraction spikes in the same shape as OP's, but not created by an elongated light source (such as a rocket plume) would have completely disporven the hypothesis in my original answer. Those were the types of examples I was looking for.
– scottbb
Commented Nov 26, 2017 at 13:12
• Can be due a specific star filter https://www.google.com/search?q=star+filter

• A dirty lens, for example fingertips smudged. This can be very specific, and actually controlled with the direction of the wipe.

• Some internal element of the lens. (difraction spikes as you mentioned)

• I get the star filter (a weak diffraction filter) and wipe (inadvertent diffraction filter from residue in a pattern), but what sorts of internal lens elements are you thinking of? Commented Jul 25, 2016 at 2:43

These flares are a diffraction pattern caused by something in the optical path. Their number and orientation tell you something about what caused them. In a refractive system, it could be a stray hair, a piece of dust, even very mild striae in one of the lenses, when one uses something ultra-bright as a light source.

If they are oriented along a particular row or column of the sensor and the sensor is a CCD, it can also be bloom. In these cases, it is along odd diagonal angles, and most likely not a form of bloom.

• The amateur and SpaceX video have nearly the exact same angles, doesn't seem random, and is symmetric along the vertical and horizontal. Commented Jul 25, 2016 at 13:50
• @NickT There's nothing that says the apertures causing the diffraction pattern cannot be the sun, its geocorona, or the earth's atmosphere. Commented Jul 25, 2016 at 14:22
• @BrandonDube how is the sun or its geocorona involved in a shot of a rocket at night?
– scottbb
Commented Jul 26, 2016 at 13:57
• @scottbb my answer is general. Here, it is probably the stabilizing fins of the rocket. Commented Jul 26, 2016 at 14:51
• I wasn't commenting on your answer. I was commenting on your comment to @NickT comment. Stabilizing fins of the rocket?? How can fins at the top of the rocket, nowhere near the rocket exhaust, be an aperture for light not going through it?
– scottbb
Commented Jul 26, 2016 at 14:56