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Old December 10th, 2006, 06:17 PM   #1
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LetusHD100 and DOF problem

Hello Letus people. I'm hoping to buy a 35mm adapter for my HD100 before the end of the year and have been considering the LetusHD100 as it is made for my camera. However after thinking more about the 1.9x magnification, I have a question about how this would affect the DOF.

Forgive my lack of knowledge on lens optics, but it's my understanding that a prime lens will produce only one range for DOF at a given F-stop. So if I have a 28mm lens and a 50mm lens both at 1.4, the amount of racking each of them can achieve should be different right?

To test this idea I took two photos, both at the same aperture of 5.6 (my digital still lenses aren't very fast). In one of the shots the Focal length is around 28mm. The other is at 55mm. Then in order to simulate the 1.9x magnification of the LetusHD100, I cropped the 28mm photo to get about the same FOV as the 55mm photo (it is my understanding that the LetusHD100 simply crops into the image about this much).

It would appear my theory is correct that using a 28mm lens with a practical FOV of around 53mm would result in a deeper (less shallow) DOF. Since the purpose of the adapter I get is to achieve shallow DOF this is a big strike against the LetusHD100.

Is the DOF shallower than the stock lens? I'm sure it is. Perhaps it looks more like 16mm or a 2/3Ē camera with this setup. But is that enough? For some it may be, especially when you consider the plusses of the LetusHD100/XL - upright image, attaches directly to camera. The adapter appeals to me because it means the smallest change in workflow and shooting style of all the available adapters. And itís cheaper!

But for me, I'm out for a very cinematic, shallow DOF and I do hope Quyen is able to get around the 1.9x magnification issue in the next version. It would not bother me if it in fact did not alter the DOF.

Here are the photos from my test. Taken with a Canon Digital Rebel at 800 ISO:
28mm -cropped to 55mm:
http://www.terpstar.com/test/ChristmasTree-28mm.jpg
55mm straight:
http://www.terpstar.com/test/ChristmasTree-55mm.jpg

I appreciate your comments and corrections. Thanks!
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Old December 11th, 2006, 08:26 AM   #2
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The Letus35, M2 and Mini35 are all believed by myself at least, to aquire an image off a groundglass area of about 21mm to 24mm wide. My own homemade device does likewise.

These are nearly the width of the 4:3 35mm motion picture frame of 24mm x 18mm, so the relayed image is consistent in relation to shallow depth of field ability and field of view, with the motion picture image most people are familiar with.

Shallow depth of field is a creative tool with specific reasons for use and is best not overdone.
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Old December 11th, 2006, 09:27 AM   #3
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Thanks for your reply, Bob. I'd agree about shallow DOF not being overdone. It is a tool and should NOT be seen as a cure-all for the cinematic look (especially when content is always king).

However, it is nice to be able to achieve as shallow a DOF as a 35mm camera can. And according to what I understand of the LetusHD100/XL adapters (not the regular Letus35 or LetusFlipEnhanced), the 1.9x magnification leaves you with almost half the DOF as other adapters. This is because you are forced to use a 28mm lens as your 50mm normal. But a 28mm lens doesn't have the same characteristics as a 50mm. And when you get down to the wider end at 15-18mm (effectively around 28-34mm), you have to take into account barrel distortions (by this I mean that fish-eyed look) that a regular 28-35mm lens wouldn't have.

Like I said, if it weren't for this one limitation, the LetusHD100 would be perfect for me. It's sharp, clear (Jim's footage can attest to that), and upright. I just have to choose something else for my media at this point. But I look forward to the next enhancement by Quyen down the road. Perhaps it will be the next adapter I buy.

Thanks again.
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Old December 11th, 2006, 05:14 PM   #4
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I've found on my Letus 35XL that even a 28mm will produce DOF so shallow that it can be hard to use. Perhaps your tests with the Digital Rebel showed the 28 to be lacking in the desired quality of DOF because of the slow lenses you used. With a 1.8 28mm the bokeh rather closely approximates that in your 50mm photo, at least on my setup: Canon XL2, Letus 35XL Enhanced (1.9x magnification also), f/1.8 28mm Canon lens.

H.
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Old December 11th, 2006, 08:52 PM   #5
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Thanks for chiming in, Henry. I appreciate you sharing your experience. Iíve seen how a shallow DOF can lead to problems in shooting as itís very tricky to always stay in focus. Two things are a must:
1. A good monitor (HD if shooting in HD)
2. A follow focus (or at least a very good 1st AC)

I think you confirmed my findings in your post. You say that your 1.8 lens looks like my 5.6 lens of similar FOV in terms of bouquet. Imagine what a 1.8 lens would look like without the magnification. I don't have a digital one available, but the Nikon 1.8 50mm I have has an even greater bouquet of the tree lights. Ėvery blurry with a short in-focus range.

Once again, itís not totally necessary to get this blurry, but itís nice to know you can if you want (at least thatís how I see it). I am interested in seeing some footage from your setup though. Have you done any tests with and without the adapter of the same shot? I find those to be the most encouraging and useful. Or if you have any other footage available Iíd love to see it.

Itís been said before that itís not what youíve got but how you use it. I donít doubt you could do some great cinema with this adapter. It would be great to get a comparison between the major $1000 range adapters (including the LetsuHD100/XL) because I donít know that the others are as sharp as the LetusÖ So many things to consider.

Anyway, post some footage if you can. Thanks.
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Old December 14th, 2006, 02:08 PM   #6
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Has anyone been able to prove or disprove this theory yet? It'd be great to see some comparissons.
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Old December 14th, 2006, 03:45 PM   #7
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The shootout between three adaptor types by Phil Bloom being discussed in the neighbouring discussion threads here

http://www.dvinfo.net/conf/showthread.php?t=81526

should tell you what you want to know as at least one of those adaptors does a 36mm x 24mm groundglass or near to it.
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Old December 16th, 2006, 10:05 PM   #8
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Quote:
Originally Posted by Bob Hart
The shootout between three adaptor types by Phil Bloom being discussed in the neighbouring discussion threads here

http://www.dvinfo.net/conf/showthread.php?t=81526

should tell you what you want to know as at least one of those adaptors does a 36mm x 24mm groundglass or near to it.
I'm not really sure how that relates to the LetusHD100. None of the adapters compared have any Focal length magnification (1.9x) that I know of.

But that's just what I know. Is this not the case or is there something else I should know or be looking for? Thanks for your help.

I really wish the LetusHD100/XL could have been compared as well as it would further satisfy my curiosity. It's so hard to buy equipment without being able to try it out first. Just a couple of hours with two or three of these adapter would tell me all I need to know... But alas that is not really possible for most people. So I look for footage and wait on other people's reports.

Here's a somewhat new question: Is it the relay that causes the magnification? This is my guess because those adapters that use the on-camera lens and achromats don't seem to have the problem. If so, is it possible that this part of the Letus may be updated and thus current LetusHD100 owners could update their systems to have the normal 35mm FOV?
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Old December 17th, 2006, 01:27 AM   #9
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You could likely get a wider area off the Letus GG with say a 35mm relay lens but these are not conveniently available at the same price as the 50mm lens in use and likely a redesign would be required.

The addition of a flip path sets some practical limits to how large a GG image area can be passed to the camcorder.

Whilst there are real benefits, resolution being one, to relaying the larger still-camera 36mm x 24mm image area, this is not faithful to the "traditional" motion picture look in terms of field of view/depths of field choices of motion film format if "filmlook" is what we are chasing.

The relay of the 36mm x 24mm image legitimately becomes a style and look in its own right and yet another creative option which will be happily and innovatingly pushed by users to its limits and maybe beyond.

Diversity rules and a stage has been reached where the bar set by the "real thing" is now being matched and possibly being exceeded. If P+S Technik can economically sustain another R & D and product generation before large format CMOS makes all GG based image relay redundent, I think we can expect some exciting things as a result of the bar being pushed up by alternative endeavours.

Within their means and some technical limitations, the Letus35 direct relay flip models provide a representation of the traditional 24mm x 18mm motion picture frame image or the widescreen vertically cropped version of the format.
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Old December 20th, 2006, 06:26 PM   #10
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Quote:
Anyway, post some footage if you can. Thanks.
I'd love to, in fact a while back I proposed to do a thorough battery of tests on the Letus & post it all (including footage) on a wiki, but I've run into some absolutely terrible problems with my editing software that makes mush of my footage. I'm trying to find solutions slowly. When I'm able I still intend to do the wiki & I'll post the information here.
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Old December 24th, 2006, 02:20 PM   #11
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Sounds good, Henry. I look forward to it.
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