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Old February 16th, 2006, 09:25 PM   #1
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letus35 flipped light loss

When I'm inside using normal house lights, it seems to cut a lot of light, even when I'm using prime nikon lenses... Does anyone have this issue?

I'm not sure how much I'm losing, but I just know that when I'm on shoots, I have to light it up a lot.
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Old February 23rd, 2006, 09:43 AM   #2
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Any lens adapter will cause a noticeable light loss.
Did you originally have a non-flip unit?

Upon upgrading, I noticed that the pentaprism does cut the light more.
I think Quyen said it would loos an additional .5 or 1 fstop.
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Old February 23rd, 2006, 10:18 PM   #3
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what I've seen reported as light loss of .5 stops I simply cannot reproduce under chart shooting conditions with a conventional GG. I don't see any conceivable way in which light loss of less than 1.5 (with an F1.4 lens) can be observed with a conventional GG adapter. Take the shoot into the real world and expect to see 2 or 3 f/stops loss depending on conditions.

Once a few of the adapters are properly reviewed, I believe you will see the manufacturer reported light losses significantly increased in field testing.

A properly done FS mirror flip/invert solution should not be reducing light anywhere close to an f/stop.
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Old February 24th, 2006, 08:40 AM   #4
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Quote:
Originally Posted by Daniel Rudd
Any lens adapter will cause a noticeable light loss.
Did you originally have a non-flip unit?

Upon upgrading, I noticed that the pentaprism does cut the light more.
I think Quyen said it would loos an additional .5 or 1 fstop.
Well, I could be wrong about the flip device.
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Old February 24th, 2006, 09:23 AM   #5
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Fli mechanism

The Letus Flip does not use a pentaprism but surface coated mirrors which should theoretically account only for a minimal loss of light.
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Old February 24th, 2006, 09:39 AM   #6
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See, that's what i've been *trying* to tell everyone...
It's defnitiely NOT a pentaprism.

:)
I guess I just heard someone refer to it as a pentaprism, and then I thought I saw five mirrors in it, and also... the village in antartica where I was raised (by friendly Saskwatch) called pretty much any kind of mirrror a pentaprism.

Anyway... I clearly have minimal technical expertise, but I have also yet to see anything that clearly documents the actual lightloss of any of these devices. There's a debate on the redrock forum regarding their claim of virtually no light loss in the adapter (but they are saying you loose light in the lens).

To put my experience in practical terms. I use a 3 point light kit (the standard lowell Omni lights and one tota light. I think they are 500 watts but i'm not sure. It's always worked well for my "talking head" shoots. With my letus (and a f1.6 50mm lens, I think it's on the edge. I probably need a little more light.

but that's a something I'm willing to do to get a more narrow dof.
Maybe if we all piched in $10 and loaned one person all the adapters we could do some standardized testing (for all the other issues as well)
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Old February 24th, 2006, 10:06 AM   #7
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Quick and dirty light loss test

Here is the setup for a quick test:

1.Take a white piece of paper. Fix it on the wall
2. Light it evenly from one or two fixed points. Use a strong light
3. Install your 35mm adapter.
3. Using a tripod, move the camera until the piece of paper fills the frame, then stop.
4. Set the zebra of your camera to, say, 80% IRE
5. Use the longest exposure time available 1/50th or 1/25th second
6. Open the 35mm objective to full open.
7. If your light is strong enough, the frame should be filled with zebra patterns.
8. Close slowly the iris of the camera until the zebra pattern disappears totally.
9. Note the f-stop.
10. Take off the 35 mm adapter e.g. Letus 35
11. Using roughly the same focal length angle of view as your 35 mm adapter objective, move the camera until the piece of paper fills the frame.
12. Close the iris until the zebra pattern disappears from the frame.
13. Note the f-stop value of the camera iris.
14. Note the difference in the f-stops measured.
15. You have then a rough overall measurement of the adapter's light loss.

Last edited by Daniel Apollon; February 24th, 2006 at 12:39 PM.
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Old February 24th, 2006, 10:11 AM   #8
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well there you go... my saskwatch friends will be so impressed!

Seriously though, thanks, Daniel:

My letus flip is being replaced with a combo unit, so I can try it both ways at the end of next week.

I know it took some time to document that process so I appreciate it.

Daniel
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Old February 24th, 2006, 11:19 AM   #9
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Hey Daniel,

Off topic but I checked out the stock20 site - really cool!
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Old February 24th, 2006, 11:32 AM   #10
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Thanks Jeremy, we've been working hard on it. We've gotten a lot of help from members of the dvinfo groups.
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Old February 24th, 2006, 01:09 PM   #11
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The concept of the M2 being lossless is simply a joke. The M1 disc tested at 2 f/stops loss in Francois' tests with an F1.4 50mm. I have FS mirros in front of me here, and a design under review by a machine shop for some CNC work. I'll be testing them within a few weeks. Just based on simple math, I'd expect .1 of an F/stop loss...hardly significant.

In similar tests, the POC20 showed .5 to 1 f/stop loss, the Beattie, none. Add 1 or 2 f/stops loss for field work. This is consistent with Steve's reports of -1 f/stops with the MPIC.

I've been using a variation on Daniel's setup tips to test light loss. I would add that opening the aperture until zebras shows up is also a good way to check for light distribution across your GG. If they appear in a round pattern emanating from the image center, you know you've got some hot spot issues.
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Old February 27th, 2006, 09:04 AM   #12
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re POC 20

Dennis, did you perform your light test on the 2x2 square holo diffuser or the 50mm round? I know this sounds like a stupid question, but one's almost 4 times thinner than the other, so I wonder if it might make a difference in light transmission.

Thanks, Jack
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Old February 27th, 2006, 01:02 PM   #13
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I tested the 2x2 sample. It's .5 mm polycarbonate. I just removed my spinning GG and taped the sample over my 36x24 frame guide.
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Old February 27th, 2006, 06:35 PM   #14
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Thanks, that's a shame. Most of the other holo evaluations seemed to come second hand and the firsthand folks didn't state specifics.(And I had such high hopes!) I did notice that POC makes holographic films (bag-like material) but I don't know anywhere to buy it (them). Maybe it's worth a shot?

BTW: is there a correlation btwn size of the taking (video) lens and optimal diffusion angle? When I did the trig on 72mm glass (DVX100) at approx. 3" focal distance, I got something closer to 80 degrees. So maybe it's halved (40 degrees in each direction) and then less than that because we're really only using a portion of the front glass. It's not that I don't believe the 15 degree test results, I only wonder if there's a way to calculate what's optimal for a given camera at a given distance. Something that diffuses as much as possible to eliminate the aerial image bleed-through, but that doesn't diffuse more than we can use so we have optimal light transmission.

Thanks again for the info-jack
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