How to avoid soft focus when using a 35mm adapter on the JVC100/200?... at DVinfo.net

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Old May 18th, 2008, 07:07 PM   #1
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How to avoid soft focus when using a 35mm adapter on the JVC100/200?...

We recently shot a music video in HDV (60p) using the SGPro 35mm adapter on our JVC201 camera, using a set of Minolta primes -28mm, 50mm and 135mm. Overall, the cinematic images we are getting are absolutely fantastic, but every now and then we end up with a shot with very soft focus. We used a focus chart on all shots, also measured others, used a SD monitor and/or a laptop running Adobe OnLocation whenever possible (there were times when no power sources prevented this) and had a professional focus-puller onboard (who has worked on feature films such as Notting Hill). Of course, occasionally actors and focus-pullers don't hit their marks, but this was more -in one instance an entire scene had soft focus.

Here are what I think are the possible reasons/solutions, in order of likelihood:

1. Stock lens focus on the ground glass changing.... to avoid vignetting we need to zoom in slightly on the GC before adjusting the back focus. This is normally around 20mm on the zoom and we mark the spot and gaffer tape the lens with the hope that it will not move. But my instinct is that it possibly does alter slightly when the camera is moved around -or even the zoom stick being knocked etc. Is there any way of locking the zoom barrel down to avoid any slight movement...? Or maybe a better designed achromat in the SGPro to avoid the vignetting in the first place -am curious how users of other 35mm relay devices deal with this.

2. HD monitoring. While we do run Adobe OnLocation on a laptop with a HD resolution screen, it is not always practical to have it running all the time. Maybe we should invest in a proper HD monitor such as the 17" JVC LCD field portable monitor DT-V17L2D?... and check the back focus on every setup?

3. Maybe 20mm is not particularly a 'sweet spot' on the stock lens for focusing?

4. Minolta glass is possibly not good enough. Maybe we should invest in some faster Nikons?

We intend running further tests on this to ensure we can avoid this in future productions, but would like to hear what you guys on the forum think before running such tests.

Many thanks,
Dan
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Old May 19th, 2008, 04:29 AM   #2
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Image sofness

Hi Dan,
Your post is very interesting and although I don't have an answer myself I have had the same issues/remarks on a music video I shot.

Here the link to the thread:
http://www.dvinfo.net/conf/showthread.php?t=119113

While you had a dedicated FF puller we didn't have one but we go the same problem. I don't have the official SGPro but I use the same material.

If anyone had an answer it would be great to have so more advices!

Thanks
Simon
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Old May 19th, 2008, 08:36 AM   #3
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ok. mind you that this all based on my limited experience with a Brevis35. The important elements to successful focus are to ensure that the backfocus is correct on the stock lens, and that the stock lens is also set to somewhere in the range of F5.6. You want to find the sweet spot of every element- for example your minoltas might have their sweet spot at F2 or F3, possibly even higher. This will change your lighting ratios and you will have to adjust accordingly. I've also noticed that certain lens are just plain bad and can be soft no matter what you've set the aperture at. This is of course, assuming everything is properly set up with your adapter. Be sure to tape down the focus ring as it WILL move even with lots of tape so check and re-check after each shot. My two cents good luck!
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Old May 20th, 2008, 03:51 AM   #4
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Thanks for your replies Simon and Alan.

Simon - That's very interesting that you have had similar issues. I've watched your music video and followed the posts regarding it. You achieved a great look and I love the tracking and jib shots -I am wondering with the full SGPro-like set-up whether you had some weight issues using a jib, as the full set-up does get quite heavy. Also, did you have issues with setting the JVC stock lens zoom and backfocus and it possibly moving during the shoot?

Alan -Thanks for the tips regarding have the JVC stock lens at 5.6 and taping down the focus ring. This was a discussion the camera team had several times -what to set the stock lens at, and maybe this is at the heart of the issue. Most times the decision was to leave the stock lens wide open (because we were shooting 60p at 1/100th for slow motion and needed all the light we could get) and control the aperture solely on the primes. In our tests we will try 5.6 or thereabouts to find that 'sweet spot'.

Many thanks
Dan
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Old May 20th, 2008, 04:35 AM   #5
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Hi Dan,
Thanks for your comments! Indeed one of the "issue" of this set-up is that it gets very heavy. We have the sgpro like set-up + IDX batteries. Also the camera is very unbalanced and is very front heavy. However w/ the crane/jib (it;s a kessler crane and we love it :)) we didn't have any issues using it when there was no tilting involved. So basically when the camera needs to stay horizontal it's very easy but when we need to tilt the camera it's very difficult and no so smooth as we would like too.

I think that I will try to balance the camera evenly to smooth the tilting.

As for the stock lens and backfocus we tried to check it before every shot and indeed as Alan mentionned we taped the zoom and stock lens focus ring. It's very easy to move them.

We tend to keep the aperture wide open too for a lot of shots and never really worked as high as 5.6. It's good to know, I will do some tests to check this out! I understood that it's the SLR lens that needs to be closed more than the stock lens aperture just like you Dan. However it might not always be possible to close the lens as far as 5.6 specially when working w/ no extra lights-> need to check this!
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Old May 20th, 2008, 08:41 AM   #6
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each lens will be a little different so you'll have to find the sweet spot yourself, but i feel ya'll on the light issue- things get real dark real quick. the idea is to optimize and try to get as close as you can. i had issues with focus and sharpness until i realized there were so many elements to deal with in the optical pathway, but of course there will always be compromises. how much light loss does the SG pro have? ive always loved their bokeh.
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Old May 20th, 2008, 03:50 PM   #7
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Alan -not sure exactly how much light is lost but I don't think it is more than a stop. However, in our case the higher frame rate and shutter speed didn't help!

Simon - sounds like you have a very similar set-up to us, with the IDX battery, SGPro-like relay and editing using Premiere/AE... in addition to having just shot a music video... so I think we must have a lot in common...at least a lot of the same questions!

I am wondering if I can find a pic of our JVCGYHD201 with SGPro to show how we set it up. Wayne of SGPro kindly arranged the 15mm rods to be level with the quick release plate allowing for good balancing -the IDX batteries help balance it, but we also have the DR-100 harddrive and a mattebox, so all in all it becomes quite bulky. We actually mounted all of this on a Miller jib... probably not recommended....the kessler crane sounds a better option!

Regards
Dan
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Last edited by Dan Parkes; May 20th, 2008 at 04:18 PM. Reason: spelling
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