DSLR lens step down ring confusion

[kirkster501]

Hi folks, Trying to understand this to eliminate the starburst effect on bright stars in my DSLR pictures.

When you set a F number on our camera and press the shutter button for a, say, 1 second exposure, you can see the Iris of the lens move to the selected ratio when the shutter is open.  It then goes back to wide open again when it closes.

So does the step down ring have to be smaller than the smallest aperture that you'd shoot at?  For instance, At F4.5 we are already down from fully open to less than a cm on a Canon 50mm.  There are no step downs that are that small..... the smallest I have is 26mm wide inside aperture. OR, Is it a case that the step down ring blocks stray light from refracting around the inside of the camera and the Iris as the Iris closes to the selected aperture?

[Cornelius Varley]

19 minutes ago, kirkster501 said:

Hi folks, Trying to understand this to eliminate the starburst effect on bright stars in my DSLR pictures.

When you set a F number on our camera and press the shutter button for a, say, 1 second exposure, you can see the Iris of the lens move to the selected ratio when the shutter is open.  It then goes back to wide open again when it closes.

So does the step down ring have to be smaller than the smallest aperture that you'd shoot at?  For instance, At F4.5 we are already down from fully open to less than a cm on a Canon 50mm.  There are no step downs that are that small..... the smallest I have is 26mm wide inside aperture. OR, Is it a case that the step down ring blocks stray light from refracting around the inside of the camera and the Iris as the Iris closes to the selected aperture?

I'm not sure I fully understand your question, but the purpose of a step-down ring is to allow a camera to utilise a smaller sized filter than the lens filter ring size. ie Your camera lens has a filter ring 56mm dia and you have a polarising filter, for example, that is 52mm dia. Either you buy another filter or a 56-52 step-down ring. There are limitations to using a step-down ring though. The step down diameter might be so great that you will get serious vignetting.

[tooth_dr]

Steve

The step down ring effectively reduces the aperture of the lens.  This is the same job at the aperture blades do, but because the step down ring is round it doesn't produce flares like the blades do.  It doesn't need to be the same size at the blades, rather it just needs to cover a proportion of the lens at the front.  I think of it the same way as you use an aperture mask for a telescope.

[wxsatuser]

The ring needs to be as big as possible that allows good stars.

What f ratio produces the best stars for this lens?