Why did this happen?

[Paul]

Using my small lense i was photographing plants at night the moon was full so i snapped her too, on a tripod long exposure what's with the starburst halo?

 

[Tom Mann]

Hi Paul -

This effect is due to diffraction of light from each of the blades in the aperture (iris) in the lens of your camera. Diffraction in camera lenses is a relatively weak phenomena so it only becomes obvious when you have one or more very bright small light sources on a very dark background (eg, the moon against a dark sky).

To make the effect easier to see in the photo you posted, I cropped and enhanced it a bit.



There is quite a bit of discussion of this on the web, for example:

http://www.cameratechnica.com/2011/02/24/how-to-create-and-avoid-starburst-highlights/
- - - intro

http://www.photoplaza.nl/lindolfi/starshaped1.jpg
- - - illustrates diffraction perpendicular to one aperture blade

http://www.phototechno.nl/img/lindolfi/Aperture7blades700.jpg
- - - illustrates diffraction perpendicular to each edge in a 7 blade aperture

http://www.slrlounge.com/school/diffraction-aperture-and-starburst-effects
- - - discussion of how the effect varies with f-stop and other factors

http://www.dpreview.com/forums/post/33245730
- - - you can tell how many aperture blades in your camera by the number of radial rays (aka, "spikes").
- - - your lens has 7 blades
- - - the number of arms depends on the number of blades in your camera's aperture (iris)

more relevant info and pix 1

more relevant info and pix 2

HTH,

Tom

 

[Paul]

As always thanks Tom.