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Venus And HIP18508
CREDIT: Alan Hodgson ASIS HonFRPS

A project on Aperture

Illustrating stuff with overexposure

Ever been criticised by an assessor for blown highlights? This is the project for you - the creative use of overexposure. And not just by a little - 12 stops overexposure!

The control and use of aperture is taught as one of the fundamental skills in photography. Resources on this are abundant but there are some good stuff available from the Analogue Group. A common use of this is for depth of field control - example here as a hint for the Potato Photographer of the Year competition.

But there are some less extensively explored use of aperture control and one of these is the production of features from gross overexposure. If you take pictures of points of light your camera will record starburst patterns produced by diffraction effects inside your camera lens. I had previously used this effect in a photograph of the planet Venus where I emphasised the brightness of the planet through a starburst effect.

Here is an example that better illustrates the technique. 2 days after the previous image Venus passed close to a star. Not all stars have proper names like Sirius - this one rejoices under the label of HIP18508 and is just about visible to the naked eye in dark skies. You will see it as a point of light towards the bottom right hand corner of this image.

The key point here is that the planet Venus was that evening about 12 stops brighter than HIP18508. So while the exposure conditions were about right for the star, Venus was a long way overexposed. As a result the camera records the diffraction patterns from the blades of the iris diaphragm used to control the aperture.

There are 18 symmetrical elements to this starburst pattern. The image was shot through my Nikkor 300mm f/4.5 manual lens which has 9 blades in the diaphragm - each blade usually produces 2 opposing elements in the diffraction pattern. Look up the specification of your lens; it will tell you the number of blades in the diaphragm and hence you will know the form of your starburst.

Venus is currently visible in the evening twilight to the West. I used a Nikon D750, 300mm lens at f/11 to produce the diffraction effect with a 5 second exposure at ISO500. The image above was cropped out of the centre to a 450 pixels width - these patterns are fairly small. Go blow some highlights!