aperture position on a projector lens

Layer-cake · Joined: 18 Mar 2013 Posts: 560 Location: Cape Town

I have been toying with the idea of building my own 50mm 1.2 lens from a projector lens, I first got a bell and Howell lens but realised it would be too difficult with its registration distance being so short so I managed to find an Eiki 50mm that has a construction and registration distance that I could work with.

don't get me wrong I have very little knowledge of lens construction and I take average at best photos but I really enjoy converting lenses........more than actual photography oddly enough!!

I really want to do a good job on this and i need some help with understanding aperture positioning in a lens, Ian posted a very very nice conversion job he did on a 90mm projector lens a short time back with a built in aperture system but my knowledge of lens construction is dismal so I need to learn.

many thanks in advance
Craig

Layer-cake · Joined: 18 Mar 2013 Posts: 560 Location: Cape Town

Sorry for bringing up an old thread but I'm hoping someone could help me out with the project I'm doing.

So I have already started the project and I'm using a nikon 50mm 1.8 lens to create a helicoid but I want to also add an aperture system to it.

With the positioning of the helix and how close the lens will be to the sensor it leave only one position to place it but I'm a bit worried that it will act more like a choke than a proper aperture system.

Here's a diagram of how the lens is constructed

the first line on the diagram after the first 2 elements is where I could place the aperture system but I suspect the second set of lines after the 3rd element would be the ideal positioning as it is where I would suspect the axis would be.

any help would be appreciated

the photo is from MIR for the canon fl 35mm 3.5 but it shows the same basic layout of the lens groups

CuriousOne · Joined: 31 Dec 2013 Posts: 669 Location: Home

I have also modded an projector lens, but it's design was pancolar-style, so I've placed aperture where pancolar lens have it. You better look for lens designs online, and find similar or close to your design, and adopt aperture location from there.

Layer-cake · Joined: 18 Mar 2013 Posts: 560 Location: Cape Town

agreed, I looked through quite a few designs but most 6 element designed lenses dont have the same layout, the third element is in most cases closer to the first 2 elements and then the aperture system would be placed after the third element.

from what I read about aperture placement is that they normally position the aperture by the axis as it is the smallest area to cover and therefore makes the lens design as small as possible,

Tried to research the type of lens design the eiki is but didnt come up with much

CuriousOne · Joined: 31 Dec 2013 Posts: 669 Location: Home

The designs you mention appear to be double gauss ones.

Regarding the aperture placement, I have Tamron Adaptall 135/2.8 It has aperture at very end, there are no more lens after it, directly goes to film plane (it do has protective glass, but it does nothing to optics, it's flat).

Layer-cake · Joined: 18 Mar 2013 Posts: 560 Location: Cape Town

Thanks for the heads up, after looking I think you are correct in stating that it is a double gauss design, having a look through a Couple of designs and most of them have the aperture system after the 3rd element.

there are a few exceptions such as the UV-Planar 60mm f/4 and Olympus 25mm 2.8 so I think I will go ahead and do the placement after the second element.

I was thinking about just leaving the aperture out but the lens seems to give an interesting petzvalish type effect so I'm more tempted to put the aperture in to see the results stopped down as it also glows quite a bit

CuriousOne · Joined: 31 Dec 2013 Posts: 669 Location: Home

You can try to make aperture of various shapes, will give more interesting effect. And I'll disclose a "secret" - if you make aperture of semitransparent, non-refractive material, and it will be thicker (darker) at edges, than you'll get the famous Smoot Transition Focus effect.

Lightshow · Joined: 04 Nov 2011 Posts: 3669 Location: Calgary

If you use disks you can have interesting shapes like this from a Fuji 85mm soft focus lens.

Layer-cake · Joined: 18 Mar 2013 Posts: 560 Location: Cape Town

I might just do that rather then I can just place it at the rear.

Lightsnow that looks pretty trippy Shocked

i cant really imagine what effect it would give to the images??

Lloydy · Expire: 2022-01-01

I just cut rough discs out of cardboard, black if I've got it - if I haven't I just blacken it with a marker pen. It's all I need to just try a projector lens. Get a cheap protractor, the sort kids use at school with a pencil, and mark out different size discs. then hack away with scissors. Wink

CuriousOne · Joined: 31 Dec 2013 Posts: 669 Location: Home

These perphorated aperture discs, when filming video, produce a set of rolling highlights, which "roll" around the circle outline, formed by distance and curvature. Hard to spell, better check youtube videos.

Layer-cake · Joined: 18 Mar 2013 Posts: 560 Location: Cape Town

DConvert · Joined: 12 Jun 2010 Posts: 902 Location: Essex UK

Lightshow · Joined: 04 Nov 2011 Posts: 3669 Location: Calgary

I'm not an optical designer, I think that the further away from the optical center an iris is, that it's more likely to cause vignetting when you stop down.
The optical center would /generally be at the center of the onion ((|)), though many lenses are not symmetrical these days, but there does seem to be a change in the elements past that point.

DConvert · Joined: 12 Jun 2010 Posts: 902 Location: Essex UK