PorkyB Posted April 13, 2013 Share Posted April 13, 2013 I bought a Sigma 70-300mm Apo lens for my Canon 1100d. I have been getting some nice results for a relatively inexpensive lens, but i have noticed the brighter stars are a bit spiky. I've had a careful look and it's a star shape. I'm guessing this is from the aperture blades in the lens. I believe one solution is to make a circular mask and put it on the end of the lens. So, does anyone know:will this work?how do I calculate the size of opening?how to cut perfectly circular holes in card or plastic sheet?are there any alternatives?an example of the sort of images I am getting - have a look at the bright star near the bottom Link to comment Share on other sites More sharing options...
DaveS Posted April 13, 2013 Share Posted April 13, 2013 Did you have the lens wide open?Those spikes do look like aperture blade diffraction, and AFAIK adding an aperture mask in front of the lens won't make any difference as the aperture blades are deep inside.If you do want to have a go at making a mask I would try a hole saw, preferably on a pillar drill, though you might get away with a hand held electric with care.I'm not sure about the calculations, though you could start with the basic equation of Focal Length / F No. Link to comment Share on other sites More sharing options...
steppenwolf Posted April 13, 2013 Share Posted April 13, 2013 The stars you have shown here do indeed look as though they have been affected by diffraction artefacts from the lens iris. This is quite common and you either like 'em or hate 'em!! A circular aperture mask can help although I have found that this can simply lead to vignetting. A hole cut out of black card using a compass cutter like this one. I have also used step down rings for this purpose but I am not yet convinced that it works for all circumstances! Link to comment Share on other sites More sharing options...
Superdavo Posted April 13, 2013 Share Posted April 13, 2013 Is it a problem that really needs solving? I think the spiky effect it pretty nice and makes the image look brighter, even hubble photos have it Link to comment Share on other sites More sharing options...
PorkyB Posted April 14, 2013 Author Share Posted April 14, 2013 As the tools and materials needed are inexpensive, it's worth a little experiment. I will have a go on a cloudy night, using the lights on radio masts as targets, after all, you can't afford to waste time on a clear night. As the lens covers a large focal length range it looks like I will have to make a numer of masks of different sizes. Link to comment Share on other sites More sharing options...
ollypenrice Posted April 14, 2013 Share Posted April 14, 2013 It did work for me and was easy to do. This should explain most of it. I'm sure Steve's right though. Lots will depend on the lens and I think I heard someone say it didn't work at short FL. I've never tried it.Draft excluder strip, black matt spray paint and a compass cutter from a graphics/art shop or the web.The focal length is known, you decide on the F ratio you want and by dividing the focal length by the F ratio you get the aperture whch you need to cut. I'm sure that for complex optical reasons this will only be an approximation.I don't like diff artefacts, really. I feel they are most obtrusive on widefield images because there are SO many stars in the field. Forget the Pleiades with aperture blade artefacts!!Olly Link to comment Share on other sites More sharing options...
melsky Posted April 15, 2013 Share Posted April 15, 2013 I use stepping rings on my telephoto lenses with no problem (wide angles will vignette), the 200mm L lens pictured below gives six point diffraction patterns if you stop it down but clean dots with the rings. I use a 72mm - 62mm ring to give me F3.2 and a 72mm - 55mm ring for F3.6, if you search on ebay you can buy these for £1.99 inc postage.Mel Link to comment Share on other sites More sharing options...
mindburner Posted September 21, 2014 Share Posted September 21, 2014 I use stepping rings on my telephoto lenses with no problem (wide angles will vignette), the 200mm L lens pictured below gives six point diffraction patterns if you stop it down but clean dots with the rings. I use a 72mm - 62mm ring to give me F3.2 and a 72mm - 55mm ring for F3.6, if you search on ebay you can buy these for £1.99 inc postage.Melhi a bit of a post revival, and this may sound like a daft question, but did you also then leave the F stop fully open. I tried a step down ring last night on my 200mm lens but forgot I had left the lens on at f3.6. It's coupled with an Atik 428ex OSC.The step down ring I used was a 72 to 55mm but I still got some spiking, so I am thinking using the Fstop down and the ring caused the problem.One from last night, although mot death from aperture blades it's heading in the wrong direction:)cheers Link to comment Share on other sites More sharing options...
Stargazer33 Posted September 21, 2014 Share Posted September 21, 2014 Set the camera on its widest setting.BTW: these look like they would cover any possibility! Step-up/down ring set. Link to comment Share on other sites More sharing options...
melsky Posted September 22, 2014 Share Posted September 22, 2014 Yes you have to leave the lens on F2.8, still a nice image Mel Link to comment Share on other sites More sharing options...
mindburner Posted September 24, 2014 Share Posted September 24, 2014 Set the camera on its widest setting.BTW: these look like they would cover any possibility! Step-up/down ring set.wow:) a ring for every occasion, could use the spares to make biscuits or something Link to comment Share on other sites More sharing options...
mindburner Posted September 24, 2014 Share Posted September 24, 2014 Yes you have to leave the lens on F2.8, still a nice image Melthanks v much and I'll have another try with the lens wide open and the step down ring Link to comment Share on other sites More sharing options...
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