Prism for use with the 35mm adaptors

[Richard Mellor]

the next stage will be with the following parts.

http://www.thorlabs.com/ProductDetai...oduct_ID=11138

ball bearings and shafts ,pulley system ,dc motor

and the last part will be the prism system with mirrors.

the image will come through the cage and mirror system will either reflect up or down and come out on a cage plate.

http://www.thorlabs.com/ProductDetai...oduct_ID=11714

this is a link to a part notice the tolerence of the part.

http://www.thorlabs.com/ProductDetai...Product_ID=294

[Valeriu Campan]

Just noted that the new DSLR Olympus E-300 camera (to be released in Dec 2004) is using instead of a pentaprism a combination of 1+3 mirrors (porro arrangement).
I still want to know what is the loss of light from using the prisms? 60-70%?

[Anders Floe]

The e-mail I sent to Thorlabs was answered today. They said that the best way for me to find out if it fitted the purpose was to buy one and return it again if it didn't!

So I could go ahead and order the 60 mm (the two sqaresides of the prism are 60mm * 60mm) or would the 40mm * 40mm do better???

Can the prism be to big???

[Bob Hart]

Anders.

I am not an optical engineer so you should test anything I suggest with other opinions.

If you only want to relay the 24mm x 18mm 4:3 movie frame, then 40mm x 40mm x 56mm 40mm common thickness will be enough.

If you want the larger still camera frame, then you will need the larger prism.

Too large is not so much the issue as too heavy or too expensive.

If your 60mm prisms are 60mm x 60mm x 84mm and 60mm thick across all faces, there may be a space problem for a 7+ close-up lens path though I think you might just squeeze it all in. However it may not allow you much space to add a condenser lens if it becomes needed, but a lower power close-up lens could be chosen which would lengthen the path. That in turn puts the heavy bits furthur outboard of the front-end of your camcorder but if you are using a studio style set-up with rod mounts, filter box hood etc to support the appliance, then this is not an issue anyway.

There is another issue which has slipped by. The Aldu35 versions mostly use an image tube with the unused area of the groundglass being spread around the outside. The 4:3 frame sits in center which means the rear prism had to be set back from the GG for the front prism to clear the gg.

With the Agus35, it is possible to place both prisms within a safe clearance of the groundglass up to half a millimetre off if you are confident your disk isn't going to move and hit the prisms. This is because you can choose where the image sits on the groundglass and ideally it should be as close to the outer edge as possible for best linear speed for a given disk rpm.

The alternative for the Aldu is to groundglass one half of the hypotenuse face of the rear prism but this burns a lot of bridges :-

Scratch that prism and there is no going back.

You eliminate the option of adding a condenser lens in the part of the relat path that matters.

[Anders Floe]

Hi Bob

I've bought the canon XL-1 relay lens which requires a 12 cm distance to the GG. (P+S technik reduces this distance by using a prism) Do you have any idea of the prism size in the P+S adaptor??? Thanks

[Bob Hart]

Anders.

The relay lens sounds about 7+ at that distance and should frame the movie frame at about 40%-60% zoom. The PD150 will autofocus on the groundglass while the zoom is adjusted but once it finds focus you need to lock it off with the manual switch.

I think P+S Technik also relay an image smaller than the 24mm x 18mm movie frame. I saw reference somewhere a while ago to a 21mm image and I think it may have been 21mm diagonal corner to corner. This may have been how they avoided the hotspot problem by simply shooting the center out of it. Their image frame is certainly much smaller than a 35mm still-camera frame.

I believe they use a mix of prism and mirror in their relay path. There is a site with a breakdown diagram of their system but it is a stylised graphic and not a design drawing.

[Anders Floe]

I think that you are talking about this pdf file:

http://www.pstechnik.de/sheets/downloads/pp-dec12.pdf

I don't get how the image travels from the GG to the camera?? I want to replicate the PRO design.

Since I have already bought the P+S relaylens it is already to late for me to use a bigger GG so I might as well get the 40mm - right???

[Bob Hart]

Anders.

I don't get it either. That diagram might be disinfo to protect the actual design.

If you are going for the small frame size, 24mm x 18mm, the 40mm x 40mm x 56mm by thickness 40mm prisms should work.

If your P+S Technik relay lens crops corners of the groundglass 24mm x 18mm frame and you have to go smaller, then you might even get away with using a pair of smaller prisms.

If you can run this enquiry past others before you commit it would be a good thing to do as I am no engineer. All I can recommend is what has worked for me so far which may not be the best or most effective methods.

[Anders Floe]

So could any optical-expert confirm what Bob is saying???

[Bob Hart]

Anders.

I can't see any hands up in the class so far.

My method with the prisms was to take the rear prism, the vertical one in my arrangement, as close as I could get to the groundglass image.

This gave me less of a problem with lining up the position of the usable area (across a half of the long side) with the part of the groundglass image I wanted, the 4:3 movie frame.

With the spinner version, I have the front prism, the horizontal one in my arrangement sitting above the disk with about 1/2 to 1mm clearance. I used a whiteboard marker pen to draw a 24mm x 18mm target on a spare groundglass which I fixed in temporarily.

I then positioned my prism path which is the two prisms mounted up together in a wooden block. I adjusted the block until I had the frame in the prism image without it being cropped on the edges of the path.

These edges are the sides of the three holes you cut in your prism mounting block. Alternatively you cut a single letter "L" shaped hole which covers the area of all three holes. The prisms each have one half face covering the corner of the "L".

I chose a close-up lens which enabled me to keep the camcorder as close as I could get it to the prism path. With the 40mm prisms it ended up being a +7. Distance from gg to rear prism face which faces frontwards is about 10mm. Distance from front prism face which faces rearwards to the camcorder close-up lens is about 45mm.

With a fixed ALDU35 arrangement inside a tube or stack of filter rings, fixing the gg close to the front face of the rear prism is not going to possible unless a smaller GG with a flat upper edge is made to provide clearance for the prism. Otherwise another 45mm of so has to be allowed for which means a +5 or maybe +6 close-up lens.

The current ALDU arrangements won't conveniently work with prisms unless the tube is made at least 130mm to accommodate them plus the offset SLR lens mount and Camcorder adaptor.

[Anders Floe]

Thanks Bob!

Before I buy the right angle prism could you explain the differerence between the right angle prism and the pentaprism??

Does the pentaprism rotate the image 90 degrees or what (thorlabs also has a pentaprism)?

http://www.thorlabs.com/ProductDetail.cfm?&DID=6&ObjectGroup_ID=143&Product_ID=12205

If so I would rather get the pentaprism and avoid any mirrors!

[Bob Hart]

Anders.

I am not wise relating to pentaprisms. My understanding is that for a relay path into a camcorder the exit face of the prism can not be large enough to be practical. The pupil of the eye is a much smaller apeture and for a SLR viewfinder the pentaprism is fine.

I understand also that the pentaprism requires an added mirror (the shutter mirror in a SLR camera) for the related image to be completely erected.

If one uses image manipulation software speak to describe the Agus35 application, right angled prisms have to be a pair before you can relay and erected image. The first rear (forward facing and upright) prism, flips the image vertically and it exits forward in the direction it came from. The front (rearward facing and horizontal - resting on its side) prism, flips the image horizontally and it exits backward towards the camcorder. Each prism works like a pair of mirrors set around an included angle of 90 degrees.

If the two 90 degree mirror pairs a set 180 degrees opposed, you get a periscope. The image is the same as your eye sees.

If the two 90 degree mirror pairs are set 90 degree opposed, then the image your eye sees has been rotated through 180 degrees.

There was a lot of discussion on pentaprisms, roof prisms and some others. It proved much of a dead-end though one visitor here claimed success with a small domestic camcorder and SLR camera pentaprism viewfinder optics.

For the Agus35 application, a roof prism which would provide an adequate relay path for a camcorder would likely be too long to be of practial use. It is unlikely to be economically available in the size required if available at all.

[Valeriu Campan]

Find a pentaprism here or look for one from stills cameras like PentaconSix, or russian medium format SLRs that can give you a cheap option too.

[Anders Floe]

So would that pentaprism work??? Or would it be easier to get a right angle prism and 2 mirrors??

I'm getting more and more confused by the day!

[Bob Hart]

Anders.

The pentaprism on that website seems to be large enough. As you can see from the diagram, the image comes into the prism in a vertical direction and exits rearwards in a direction 90 degrees to the entry direction.

With this I think you have to have a mirror where the camera shutter mirror would normally be in a still-camera. It would sit under the bottom of the pentaprism, be tilted rearward at an angle of 45 degrees so that the image coming from front is reflected up into the bottom face of the pentaprism.

The mirror would have to be a surface-coated type. You could use a broken scrap of ordinary mirror to test with before going to the cost of a surface coated mirror.

With the right-angled prism design, I went with two prisms, not one prism and a pair of mirrors.

I did this to keep inaccuracy out of my build as much as I could. One prism is near to perfect in alignment when used as a pair of mirrors. Two prisms = four mirrors.

If you want to test the principle quick and easy, go to a glass shop and ask them to cut four small pieces each 40mm x 40mm, or the larger 60mm x 60mm you may prefer.

Make two 90 degree mirror pairs by glueing or taping each mirror in a pair to the corner cut out of a breakfast cereal carton. Trim the carton down to the outer edges and ends of the mirror but leave a 45 degree web of carton between the two sides to support them. This should form the long side of a triangle. It should be cut to the edge runs from one mirror end to the end of its pair partner.

You end up with two corners out of a carton, each with two mirrors in it.

Next you need to find a piece of plywood panel to tape your mirror pairs onto.

Rest one corner piece flat on its side so that the "V" point faces towards you. Hold the other corner upright so that the "V" point faces away from you.

Then move it so that the bottom end of one of its mirrors, is against the the edge of the web of carton which connects the two mirrors in the piece which is resting on its side.

The web edge of the upright pair should be vertical. The lower mirror in the upright pair should point toward only one mirror in the horizontal pair. It should not overlap the other mirror because it will block some of the image off.

You can use parcel tape and a matchbox to hold the upright mirror pair in place. Because they are at 45 degrees, the carton corner will not stand up.

You should now have two mirror pairs, one resting on its side, one upright. Each pair is the same as a right angled prism.

Your assembly on the plywood panel should be positioned so your camcorder looks at the center of one of the mirrors of the horizontal pair which is not masked off by the vertical pair. The mirror will be at 45 degrees to the camcorder. With your zoom backed off it will seem you are looking down a tunnel. Line up the tunnel so the camcorder sees centered down it, then zoom in until the edge reflections just run out of the frame.

With furniture mirrors your image will have lots of straight lines running up and across, but it is good enough to test the idea with.

My next step was to use a close-up lens on the camcorder and attempt to focus on the 4:3 24mm x 18mm size movie frame. To make it easier I taped a barcode from a grocery wrapper along one web edge and one mirror edge of the vertical mirror pair and set the camcorder back from the mirror assembly. With 7+ it sits back about 50mm.

I'm sorry if this is confusing but it is the best I can do without demonstrating.

There were two images on www.dvinfo.net/media/hart titled "mirror array" but they seem to have gone faulty as I could not download them. I still have the second mirror assembly I made with pieces of sheetmetal instead of carton corners. I'll make another .jpg image and email that if it is likely to be of any help.