External aperture mask on camera lenses?

[ollypenrice]

I'm going to try making some external aperture masks for stopping down my camera lenses, starting with the EF200L. I want to avoid the diffraction artefacts created by the internal diaphragm which is not, of course, round.

So is there anything wrong with this thinking? FL is 200mm and the lens is good at F3.5 so 200 over 3.5 gives an aperture of 57.1 mm. I'll probably try one at F3.3 as well, so 60.6mm aperture.

Anyone think I'm missing something? They may not work but they cost nothing to make so why not try?

Olly

[steppenwolf]

I have done this with success, Olly for the same reason, the blades on the 200L do cause some hefty diffraction spikes. I aimed for F4

[ollypenrice]

I have done this with success, Olly for the same reason, the blades on the 200L do cause some hefty diffraction spikes. I aimed for F4

Ah, that's great to know, Steve. Thanks. I'll try a few apertures. It's heartbreaking to lose the aperture but needs must! I'll go and make an improvised compass for the Stanley blades and find some matt black paint...

Olly

[darkside]

very intresting idear, olly could post some photos when you have made them would love to have a go myself. thanks ady

[davew]

As stated, they do work. The only problems I can see are that the circle must be circular ( No ragged bits ! ) and the larger the mask opening, the closer to centralised it needs to be.

The second one means that the mask must be equidistant from all parts of the outer lens edge. Otherwise you get great stars in one part and duff in another.

One method used is step down rings. That's going to be expensive. On the other hand they can be used to hold the mask in place.

This and internal masks used to be used to alter Bokeh in the old days.

Dave.

[ollypenrice]

Good points, Dave. I have had an initial go using an improvised compass to guide a Stanley knife but I really want to get hold of one of those compasses which holds a blade. That and some thin hard plastic, as used for modern margarine tubs etc. But it looks perfectly do-able. Hey great, I can reshoot my 55 hour Orion without diffraction effects!!

Olly

[Psychobilly]

Cutting compasses are cheap enough and make life a lot easier... Plastic document wallets work well for this sort of thing and cut easily...

Here's a link to the one like I use...

COMPASS CUTTER for cutting cirles BRAND NEW TOOL | eBay

Also handy for making thin shim washers for tweaking spacings as long as you remember to cut the OD first

Aperture masks are also handy for getting rid of diffraction artefacts on refractors if the lens spacers protrude into the light path...

Peter...

[ollypenrice]

ONE NINETY NINE???!!!!

Peter thinks I'm made of bleedin' money! If I can find one a bit cheaper I think I might just invest... Heh heh, a done deed. Thanks Peter.

Olly

[davew]

Nice one Peter,

I've been looking for a way to cut some plastic rings out of an old ice cream lid !

Just the job. Scissors don't do internal circles.

Dave.

There's always someone with the answers round here

[ollypenrice]

Exactly what I was thinking but can I find any ice cream tubs? Or margerine? No, too much home cooking around here. However, there are dustbins!!

Olly

[MjrTom]

Hi Olly,

I have been researching into aperture masks myself.

I was thinking along the lines of filter step down rings.

Stacking a 77mm to 67mm step down ring onto an 67mm to 52mm

Fotodiox 7 Metal Step Down Ring Set, Anodized Black: Amazon.co.uk: Electronics

http://www.amazon.co.uk/67mm-52mm-Step-Stepping-Adapter/dp/B0040YOXT6/ref=sr_1_18?s=electronics&ie=UTF8&qid=1315773996&sr=1-18

If my calculations are correct that should give you approx f/3.8

[ollypenrice]

Hi Olly,

I have been researching into aperture masks myself.

I was thinking along the lines of filter step down rings.

Stacking a 77mm to 67mm step down ring onto an 67mm to 52mm

Fotodiox 7 Metal Step Down Ring Set, Anodized Black: Amazon.co.uk: Electronics

67mm to 52mm Step Down Stepping Adapter Ring 67 - 52: Amazon.co.uk: Electronics

If my calculations are correct that should give you approx f/3.8

That's very expensive, though, no? I tried an improvised cutter on some stiff card and it works fine, so with the proper compass-cutter and a sheet of margarine tub lid it shoud be a cynch. Matt black to finish. It can fit snugly inside the dewsheild.

Olly

[steppenwolf]

Hi Olly, I've just looked out my compass cutter and was going to offer to send it to you but having looked at Peter's Ebay link, it would cost you more in postage to send the darn thing back to me than it would to buy a new one and keep it!!

How do they make things so cheaply these days?

[Merlin66]

Peter, et al,

The ebay supplier won't ship outside the UK....bumma!!

[MjrTom]

Ok fair enough, for less than a tenner you could have a more secure attachment but if you have had success with a more DIY option I am not one to argue

[ollypenrice]

Hi Olly, I've just looked out my compass cutter and was going to offer to send it to you but having looked at Peter's Ebay link, it would cost you more in postage to send the darn thing back to me than it would to buy a new one and keep it!!

How do they make things so cheaply these days?

Thanks Steve, I'll get one in an art shop here easily enough. Monique knows them all since she's a painter by profession.

Tom, sorry, I misread the advert. I thought the rings were £13 each, I didn't see it was sold as a set. You have a point. However, with a DIY I can experiment with getting the widest distortion free aperture possible on my chip, I suppose. I'm hoping F3.5 will do it, maybe even 3.3 since the chip is only 15mm square.

Olly

[Psychobilly]

Peter, et al,

The ebay supplier won't ship outside the UK....bumma!!

I guess Olly could find them on the "french" equivalent... The search engine on ebay.fr doesn't like my franglias i supose i could try frelsh...

[Freff]

Model shops sell these Compass Cutters, I use them for cutting balsa sheets. The French are very big on their model planes so it should be easy to find one locally. They also come with spare blades in the end.

[Psychobilly]

I got mine for cutting wheel arches out of RC car shells...

[ollypenrice]

Good tip for widening the search, thanks! I'll have a look today. My next megaproject will only be possible if I speed things up by collecting colour in the 200 lens and I don't want blade spikes.

I wonder if I can get away with coma in the rgb layer if the luminance is pristine? Speed is everything.

Olly

[Andrew*]

If you struggle to find the compass cutter, try using one of those basic compasses which clamp a pencil, and instead clamp a "pen" knife (as in a knife with a thin round handle). Trouble is I don't know if it would be harder to find the knife....

But I still think there's weight in the step-down ring idea to be certain your mask is completely secure and central...

FWIW, I like lens spikes

Andrew

[ollypenrice]

FWIW, I like lens spikes

Andrew

Thanks Andrew and, yes, I sometimes like them as well, much more than Newt diff spikes or small square stars. However, I want to use the camera lens to collect large areas of colour data and apply longer FL luminance and Ha over the top of it on a forthcoming project and you can't mix spikes with non spikes, so to speak!

Olly