70mm DOF Adapter Test By Marcel Van Someren On ExposureRoom

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70mm DOF Adapter Test	1votes
	Views:249 Favorited:0 Reviews:0 Comments: 16

This is the first test of a DIY 70mm DOF adapter. It uses a spinning GG cut from a large glass focus screen. Except for the Mamiya 80mm lens used in this test, the total cost was about $60.

You'll notice that the CA (Chromatic Aberration) is pretty bad. This is probably attributed to the cheap $5 macro lens I used for an achromat. A quality achromat or perhaps adding a PCX condenser lens would solve the problem. A higher quality GG would also help with edge-to-edge sharpness.

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All Production Elements: Marcel Van Someren Rating: 5.00

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Comments

Henry Peplow

August 10, 2008 05:51 PM

That was really interesting. I do like these sort of projects. The adapter must be huge! Please keep posting. I was wondering about fixing a Mamiya 6x7 prism to the LCD on an EX1. It'd be cool, but a little too small to cover.

Shiv Kumar

August 10, 2008 06:24 PM

Marcel,

The picture looks pretty good! What aperture was the lens set to?

Why have you chosen to go the 70mm route? Aren't the lenses for these medium formats more expensive? Granted they are probably a better quality and such.

Would love to see photos of your adapter and parts. What are your plans for this "project"?

Marcel Van Someren

August 10, 2008 06:30 PM

Shiv,

I'm not quite satisfied with the picture just yet, I'm adjusting the flang focus distance. I noticed that at infinity, the background was slightly out of focus. I will post new footage after I make the adjustments and have an opportunity to shoot some more footage.

Actually, I found the Mamiya lenses to be somewhat less expensive than Nikon 35mm, for example. I paid $45 for the 80mm lens. The reason I chose that format is because the image projected on the GG is larger so you don't have to zoom in as far which helps with the sharpness of the picture. If you send me your email address, i'll send pics.

Shiv Kumar

August 10, 2008 06:43 PM

Marcel,

Ok, I see the reasoning behind the 70mm versus 35mm. It doesn't quite make sense to me, as in is that confirmed (less zooming in helps with sharpness).

I'll send you an email so you can send me photos.

Marcel Van Someren

August 10, 2008 06:51 PM

Pretty much all I've read in various forums leads me to believe that you lose a little sharpness when you zoom in fully or almost fully.

Shiv Kumar

August 10, 2008 06:55 PM

I see, I wasn't aware that zooming in hampers sharpness.

Bob Hart

August 11, 2008 09:47 AM

Large Format Adaptors

This is the first I have encountered for 70mm.

Another member here, Ted Ramasola, has built a hybrid medium format/35mm film format disk based adaptor and it works very well. A commercial he shot on it with a JVC GY-HD--- to promote a resort on Bohol Island as a tourist destination is posted here.

You will find colour rendition improves noticeably along with apparent sharpness when you use a proper achromatic dioptre.

Ted I think got his from SGPro - Wayne Kinney in the UK.

A PCX might only be helpful for areas of brightness falloff in corners or edges. If your adaptor works without one, best leave it off. PCX elements are generally used in pairs, one either side of a groundglass as in the Movietube.

If there is a distortion introduced by the front PCX in the projected image on the groundglass, this is corrected by the opposite element of the pair. This does not work for edge softness or chromatic abberation which a mismatched PCX might cause.

Marcel Van Someren

August 11, 2008 07:03 PM

I think I've seen that commercial on another forum and recognize the names including yours from other forums. He also mentions that he had CA when using the Mamiya lens and felt it was due to the wrong achromat.

I've been researching the PCX equation heavily and there are still some that think (and have apparently proven in their own DIY project) that a PCX (just one) resolves not only brightness but CA as well. I don't know which is true. What I do know is that a quality Achromat are expensive and PCX lenses are relatively inexpensive and might be worth experimenting with. I wish there was a difinitive answer as to which is the correct solution.

Apparently both the SGPro and the Letus Extreme use Condensers (PCX) in their design.

I would love any solid information...

Bob Hart

August 12, 2008 03:49 AM

Large Format Adaptors

The commercial was linked from dvinfo.net. Ted and myself also have posted on the subject there.

Ted did experience some CA in earlier evolutions of his adaptor. He has largely overcome this with an improved achromat and careful positioning of the various lenses.

The clip here I think may have been shot with his hybrid adaptor but with normal 35mm stills Nikons. Ted would have to verify this himself as he also built a separate 35mm stills lens adaptor.

Marcel Van Someren

August 13, 2008 11:42 AM

I contacted Ted directly and he was very helpful. As you mentioned, it seems the achromat used for the SGpro is the solution to the CA problem so I will order one and see what happens.

Shiv Kumar

August 13, 2008 12:38 PM

Marcel,

Thanks for the pictures. The adapter looks awesome!

Marcel Van Someren

August 13, 2008 05:10 PM

Thanks Shiv. If the achromat for the SGpro solves the CA problem, then I will build a more compact version. This adapter was more a proof of concept than anything else. Should be fun!

Shiv Kumar

August 13, 2008 05:27 PM

Make a few more test videos and pictures of the adapter (interior, exterior etc.) before you start with building the next one :). Unless you're going to keep this and build another from scratch?

Marcel Van Someren

August 13, 2008 07:06 PM

I will build the next from scratch, but I will get some stills of the interior of this adapter the next time I open it up, which probably will be sometime next week or so. Before I build the next one, I will post a split screen video showing the difference between the cheap macro lens and the SGpro achromat.

Bob Hart

August 14, 2008 12:12 AM

An undiscplined stills archive.

Back over at dvinfo.net there is a stills archive which is not catalogued or systematically arranged where in my learning curve, I posted some of my gains and setbacks over a few years.

www.dvinfo.net/media/hart

There is also another member's images at

www.dvinfo.net/media/mellor

With apologies to webmaster Shiv however I am sure he will not be grieved by linkage to another site in this instance.

Marcel Van Someren

September 02, 2008 12:28 AM

Well, I recieved the achromat from the Shoot35 (SGpro) folks. Wow, it sure made a big difference!

You can see the updated video and more info posted here:

www.exposureroom.com/70mmtest