Homemade 35mm Adapter

[Bob Hart]

Imagine two pairs of mirrors, each mirror of a pair arranged around a 90degree angle. One half of a mirror pair is placed for centre of lens axis for a beam running along the lens centre axis to hit it dead centre at a 45degree angle. It comes off at 45degrees relative to the mirror and 90 degrees relative to the centre axis onto the centre of the other half of that mirror pair. From that mirror, it comes off a full 180degrees from the direction it went into the first mirror. The first mirror pair is perhaps sitting horizontal.
The second mirror pair sits vertical. The lower mirror of that pair faces mirror two of the first pair but is inclined vertically at 45degrees. So the centreline beam off mirror two is deflected by mirror three upwards into the centre of mirror four. Mirror four sends it back in a rearwards direction, the original direction when it hit mirror one.

If those two mirror pairs are arranged so that both pairs lay horizontal and mirror two and three are aligned to each other, each with a 45degree offset, the recovered image out of mirror four is the right way up but apparently a little furthur away due to the detour through the mirror path. But, when the second mirror pair is rotated 90degrees relative to the first around the centre axis of mirrors two and three, then the recovered image rotates through 180 degrees.

That's how the two prisms per eyepiece in binoculars (10) erect the inverted image and (2) conveniently shorten the length of the appliance by turning the path back and forth between the two prisms (same as the four mirror in two pairs).

This prism arrangement is simple, basic and proven. There are other newer single prism or mirror plus prism arrangements (like reflex viewfinder on cameras). The single prisms such as roof prisms according to info I have read so far render an inferior image quality unless they get enhancements like better coatings. This is offset by the more compact and convenient arrangement they enable.

I went down to an optics factory in Canning Vale today to get some finer powder. While there I discussed the Agus. Their rep suggested that for my application of the device, that mirrors would be the better option in terms of experimenting and prototyping. It was also suggested that the glass CD if it arrives in rough form (groundglass on both surfaces that machine polishing would be superior to the hand method I proposed ( and a lot less fatiguing. When I asked if there were larger diameter common industry lenses like the telescope eyepiece or achomatic lenses, he advised me to look at any junked video projectors I might find. The optics in the earlier generations of those which might be dying now were apparently good.

I received the lens blank from Ohara in Japan today so will be trying the aluminium oxide on that. If that is okay, the next step is to ask them how much their glass CDs will cost.

As my my qualificiations to comment. I have none, = jack-of-all-trades so regard my suggestions with some caution.

[Brett Erskine]

Got it. Thanks so much for the good explaination. Yeah originally I was only thinking of two mirrors all together and thats not physically possible. Wow four mirrors. Ever stood between two mirrors facing each other and get that infinity effect? The images in the mirrors quickly start to become dark (and sometimes green). Looks like it would be nice if the mirrors are of high clarity and perhaps optically coated. Hmm...may or may not bother optically fliping the image in my adapter now.

[Filip Kovcin]

<<<-- Originally posted by Bob Hart : Imagine two pairs of mirrors, each mirror of a pair arranged around a 90degree angle....etc...-->>>

bob, can you anyhow draw this, and send somewhere with a link? (i mean all mirrors). my english is not perfect, so i'm not sure that i understood this in a proper way.

thank you


filip

[Bob Hart]

The mirrors work in front of the Agus but to get the field of view, they have to be so big and you end up with something the size of a shoe box standing on end fastened on the front.

Two mirrors 90degrees edge to edge resting like a gutter with the camcorder or Agus lens pointing down at the centre of the gutter from one end can be arranged to give an upright image but the field of view available is extremely narrow and the image has a join in the middle. Depending on how well the mirrors are aligned the join is invisible. To provide a usable image I guess they would have to be be about three feet long and nearly as wide assembled, again not a useful accessory to hang on the front.

I tried dressing the Ohara glass sample with the aluminium oxide and it seems to be fine so have asked for a price per 10 pieces for the disks and am waiting for this.

Filip. I'll see if I can make a .jpg of this and ask Chris to post it here on www.dvinfo.net/media/hart

I dont think the mirrors on front will be much use but smaller mirrors between the CD and the camcorder and a lower powered macro say about 7x might work. I haven't tried that yet.

[Chris Hurd]

Would be glad to host it as always, Bob! Hope this helps,

[Bob Hart]

I have sent some images along with a request for Chris to post them on www.dvinfo.net/media/hart

The mirror arrangement at its crudest will be illustrated in “agusmir1.jpg”. This view is from above. You will observe one “V” pair of mirrors sits horizontal. the other “V” pair of mirrors sits vertical with the front edges of mirror 3 bottom and mirror 4 top on the same vertical line. From the right side of the image the mirror pair 3+4 looks like >. This is a replication in mirror glass of a prism pair from one side of a pair of prismatic binoculars.

The inverted and erected image are set together in “agusmir4.jpg“. The erected image quality is very poor for several reasons. Simple wardrobe mirror glass was used. The mirrors were not perfectly clean. They were not the appropriate size. They were not in a lightproof enclosure. The two mirror groups were hand-held in position.

My imagining on using prisms or optically pure mirrors if I could get my greasy hands on them, would be to split the usage of them between the two stages in the Agus35. One small prism or mirror pair to reverse the image and project it forward onto the CD disk within the limited space permitted by the SLR prime lens flange to focal plane distance. The other larger prism to pick up the disk image amd return it around the disk to the camcorder via the macro lens and the Camcorders own zoom lens, again enabling the prism or mirror pair to fit within the available space between camcorder + macro and the CD.

It might be necessary either to make a much Smaller disk or to have a clear centre to the disk and return the image through that if outer edge of the disk crops the return image to the camcorder. This arrangement might permit the whole added assembly to be contained within the existing Agus55 boxes that some constructors have built but CD position would have to be re-arranged. This arrangement with a 7x to 10x macro lens achromatic if it can be had) also might permit the close coupling desirable for portable or handheld camera operation which is what I was chasing with the the telescope lens set I have used.

[Chris Hurd]

These are up now: see www.dvinfo.net/media/hart. Cheers,

[Filip Kovcin]

cris and bob -i can't find mentioned earlier .jpg file:

<<The inverted and erected image are set together in “agusmir4.jpg“. >>

thanks

filip

[Bob Hart]

Filip.

Keeping looking furthur down the list. I made a mistake and its got a small "a" not a large "A" on front of the filename. Its there furthur down after the "a"'s start again.

As for mirrors, for rough and ready miniature prototyping, all those blank CD-Rs everybody bought for the clear disks might provide stock for people to cut up with small fretsaws to make mirrors out of. They won't be good enough for real videography but to set up paths and alignments they should work.

[Bob Hart]

If anyoen is going to try the split arrangement of mirror groups either side of the groundglass CD, don't go there. It's a dead-end. For a mirror group or prism to be large enough to relay a 24mm x 18mm size image frame, it won't fit between the SLR prime lens and the disk. Both mirror groups have to be in the groundglass to camcorder+macro relay stage.

Inital tests indicate a mild macro lens of 3+ or 4+ might be all that is needed as the 7+ stack I was using is too close for the mirror groups to fit in. The bonus with this arrangement is there is virtually no barrel or pincushion distortion because the target image sits a lot furthur away from the camcorder but is brought closer in real distance by the bent mirror path. The Brett Erskine test pattern can be framed correctly.

The image is still washed out. Good optical quality mirrors or one mirror pair and one prism might be the way to go. Any clues as to sources anybody?

I have sent to Chris for posting on www.dvinfo.net/media/hart a .pdf file titled AGUS ERECTOR. This is a vague conceptual diagram only and to approximate half scale. Don't try to build from it.

[Brett Erskine]

What camera are you using with your adapter because for the DVX100 owners we will need around a +16 diopter to fill the frame with my 24mm x 18mm grid chart.

I'll be using two Hoya +10 achromat macros for the job.

[Bob Hart]

Brett. Its a PD150P.

If the mirrors or prism/mirror combination becomes viable, you won't want to use a strong close-up lens as its usable focal range will fall too short in the path through the mirrors and/or prisms which add considerable distance to the journey between the CD and the camcorder.

To more adequately demonstrate my adventures with mirrors I have sent to Chris three more images titled "agusmir5.jpg" "agusmir6.jpg" and "agusmir7.jpg" of a test with the Erskine chart as a reference. The erected image was shot in very poor lighting = 1 x 60watt overhead ceiling lamp.

The arrangement of the mirrors relative to the chart is different to how it would be set up in th Agus. The small mirrors were cut with the intention of the academy aperture being scanned onto the CD as close to the edge as possible and being lengthwise to the circumference not radius of the disk. In the demo image, there is a bit of edge cropping, which is the back of the testcard getting in the way of the return image on one side and the mirror edge on the other.

If anyone with engineering acumen feels the unremitting urge to do some R&D and contend my assumptions please feel free to offer some serious competition to my efforts as parcel tape, cereal packet corners, broken wardrobe mirrors leave a lot to be desired as far as the scientific method is concerned. I'd like to be headed off from any dead-end before I invest to much time and effort.

[Bob Hart]

Furthur to previous post I have sent to Chris with a request for posting a conceptual diagram in three file types, two as drawing files for Turbocad and Autocad and one as a .pdf for Acrobat. The Acrobat file is 90 dgrees out of whack for viewing on the monitor however use the "view" >>> "rotate 90 degrees right" and all will be well. All files are titled "AGUS35 MIRROR ARRAY CONCEPTUAL".

With a PD150 via a HOYA 4+ CLOSE-UP lens a full frame of the Erskine test chart can be had. How well it looks is up to the quality of the mirrors.

The web or side-wall between the two small mirrors has to be cut down to the minimum sufficient to support them at 90degrees to each other as the image is otherwise cropped. If a prism is used instead of the small mirror pair it will likely have to be broader across the cross-section than the 30mm of the mirrors for the same reason.

Where the small mirror group (1) is positioned against the lower mirror of Group (2) is not written in stone provided it remains with each small mirror horizontally oriented at 45degrees to the image plane and vertically oriented to the image plane. One small mirror has to remain outboard of the large mirror group to pick up the image from the CD.

The diagram is a guide only and variation between lenses and camcorders may require other positions.

The drawing has been set up for US letter size paper and should print at 1:1.

I'm going to have a rest from this for a while as there are productive things I have to attend to.

[Bob Hart]

Further to the preceding post, I did some more masking tests on the mirror erector and found that the mirrors three and four in Group 2 can be reduced in size to 45mm x 43mm for mirror three and 45mm x 54mm for mirror four, with full frame yield into the PD150P via HOYA 4+ Close-up lens preserved.

The apex of the 90 degree angle between the mirrors has to be positioned much closer to the level of the top edge of Group 1, and the bottom edge of mirror three and the level of the bottom edge of Group 1 are at the same level.

I have sent another image to Chris from this latest revision. It is titled agusmir8.jpg

Vertical smear on the lower edge of the image will be observed. Better alignment may assist but only up to a point. The mirrors were cut from wardrobe mirror glass about 2mm thick and internal reflection is an issue. Optical quality mirror material should improve things or one or more prisms in place of the mirrors.

The image is dull. Lighting conditions were poor with most light coming from behind the test pattern through the paper due to the card facing the mirror array.

The intended permanent mounting method of all the mirrors will be to make an integrated mount out of a single piece of light sheet metal - thin galvanised or jam tin which solders really easily. Catfood tins are no good because of corrugations in the metal. Once the folds are in place, side-reinforcing panels will be soldered in to stabilise the angles and black painted to absorb light scatter. Jam tin should be strong enough but light enough hopefully to still enable short handheld shots. Having a single module in this way will make fitting to the existing Agus35 versions a little easier but expect problems with the CD case and other round Agus versions.

This erector version will only just include a 4:3 academy motion picture image frame of 24mm x 18mm. Larger mirrors in Group 1 would be needed for the 35mm still image frame.

For small single chip camcorders of the 37mm lens size, a prism pair from binoculars might be sufficient if a little cropping of the academy aperture frame size can be accepted.

[Chris Hurd]

These will be online in a few hours from now (Bob's latest images).