Keying with 35mm Adapters
 

I am involved with a project that we will do a lot of green screen work in. I currently shoot with modified version of the Letus 35a, and the HV20.

I've noted keying is a bigger problem due to the vignetting issue associated with these adapters.

I am looking for techniques and advice on attaining best keying results using adapters. Anybody ?

Chris.

I'm dabbling in that which I do not know with this comment so whatever comes along afterward has to be better, hopefully guided not misguided in the enquiry direction by my comment.

The Pro35 handbook recommends creating a special "lens file" for cameras like the Sony F950. I understand this to be a feature which adjusts for brightness anomalies of particular lens types.

Some time back, someone published here a method of dealing with corner and edge falloff, also dust and fixed pattern artifacts. This involved shooting a reference plate ( maybe not correct word ), then applying this inversely with a layer in post to counteract the artifact.

This works only with fixed artifacts. If there is a varying brightness anomaly which is caused by a zoom lens like with the PD150, then it would not work.

I realise the HV20 will not have the "lens file" feature but there may be some backyard tricky cheat someone might know.

The creating of a reference plate and applying this to correct a brightness falloff issue before making the matte might work.

This might also be valid for creating a correcting reference plate by lighting a white background before hanging the greenscreen, to allow for subtle variations in the light levels across the screen. Might also be a lot of pointless work as well.

I know too little on this subject so please ignore at will any foolishness on my part. This is an area I have much learn so looking forward to the results of your enquiries here.

Hey Bob:

Thanks for the input. I had seen some references to such plates too in another thread.... hmmm, you got me thinking there......

Big thing I probably need to do is do something so I can soom the camera in a little bit more. I can tell you one thing, Keying really emphasizes the vignetting or brightness fall off issues of 35mm adapters....

When we started talking about shooting with a lot of green screen, I assumed it wouldn't be a big deal.

I will update on what progress we make...

My approach to this might be to examine the camcorder to adaptor path including the close-up lens if there is one. I would be inclined to select the most powerful lens that would still allow me to view the widest useful area of groundglass yet require the least amount of zoom-in on the camcorder.

There will be three sources of brightness falloff :-

dark corners in the camcorder's own lens when zoomed in - maybe - if the HV20 shares this trait with the PD150.

dark corners created by the interaction of the close-up lens and the camcorder's zoom lens.

dark corners created by the corner brightness falloff inherent to some 35mm groundglass adaptors.

You will have to find the compromise balance between the amount of zoom-in and the area of the groundglass scanned. The cost may be a little sharpness or resolution if the target on the groundglass has to be a little smaller.

I find the long setback arrangement with the 4+ dioptre yields for me the best results for sharpness on my home-made but I also pick up edge fall-off from the confined prism path and some wide lenses which are a little tight.

I have to zoom in all the way and this brings in the slightly darker corners from the camcorder's own optics.

With the closer coupled 7+ dioptre, I can back off to about 40mm on the zoom which makes the camcorder's own darker areas less apparent. I however pick up the edges of the prism path more, a sort of a contradiction in terms which I don't actually understand.

The Letus has a more generous flip path but the folded optical axis is also longer than my arrangement by virtue of having those larger mirrors.

Chris, the typical adapter setup does not work well with the HV20...and edge falloff is the typical complaint. In our case, we maintain a line of imaging elements (CF1, 2, 3) that we recommend specifically for the small cams. Using these, our waveform response at 50 IRE with a 50mm f1.4 is nearly dead flat...in other words, very even light distribution. What works typically very well on an HVX will fail miserably with the HV20 so the setup cannot remain the same.

The bad news is that your current setup will never work well with the HV20 unless you start swapping optics :-(

if i understan this properly you will use your adapter on green screen scenes.

i know that my solution is not maybe the best one, but it may work for you.

since you anyhow shoot in front of green screen - this means that you can do whatever you want with your background, and foreground separatelly - yes?

well, if this is not too far from your artistic vision - i will shoot all greenscreen shots normally - WITHOUT ADAPTER!

during compositing process you will make any kind of softness you want, you can simulate DOF etc.

similar, as in normal 35mm or HD shooting you will ANYHOW need everything in focus - especially outside "lines" of someone's body, sharp edges when shooting real objects etc.
usually - working with out of focus elements in front of green screen is kind of nightmare process for postproduction, but you should judge for yourself.

personally - i will try something simple in front of green screen before normal shooting starts, and then make decision.
so, if you think this post make sens for you - try this:

1. shoot without any adapter on your camera, play with different setups in post (unfocused areas etc, to simulate needed DOF)
2. shoot with adapter and compare with 1.
3. decide what to do :)

all the best,

filip kovcin

Hey Dennis:

I am finding out a lot about that. I have managed to make some headway by mounting your 72mm Cinevate achromat piggy back on the 35a achromat, and I am able to achieve a better zoom. I have done this in past because the Letus35a setup itself does not give that much zoom capability with the HV20. I am now trying with green screen set up, but Of course, that is another set of optics in the path. This helps me get inside the vignetting and a more evenly lit frame.

I did not realize how big an issue this was with green screen, until I built my own green screen set up, and started testing. With the straight HV20 Letus35a combo, I had terrible fall off, especially with the slower lenses. At first, I thought it was because green screen was unevenly lit, so I added lights. No change, really. Then I did a control test with the HV20 by itself, and difference was night and day. Using just Vegas I was able to key shots with very little problem throughout the entire zoom range. I am now working with the director to see if we can get acceptable footage with the two achromat combo- but I would love to take the next step to flipped set up.

Is there any issue with your flip set up and HV20 that would cause problems with keying ?

Do you guys have any footage with the Brevis and HV20 with green screen ?

Bob:

As I was telling Dennis above, on the HV20, by adding a second achromat, I have been able to overcome the zoom/vignette issue that is a problem with this HV20 Letus35a combo. I am guessing that if I actually extended the 35a tube a couple of inches, and just used the stock achromat, I might be able to achieve better zoom results too, but I haven't quite figured way to go about it. For testing, I could fashion an extension using black sewer pipe, I suppose.... so much to do, so little time to do it.

Filip:

I have actually completed three different sets of tests similar to what you indicated, and will be talking with director about it tomorrow.

Chris, none, but we have done extensive testing with the HV20 attached to the flipped Brevis using CF1. Edge falloff on an evenly lit screen is nearly zero using our test 50mm f1.4 lens. We use a waveform monitor to assess.

What you do have to be careful with is the amount of zoom used on the HV20. As you move to 100% zoom, the edge falloff from the bare HV20 lens is very pronounced. This is one reason why we've worked so hard to make sure the Brevis/flip can be properly aligned on these cams. Most HV20s have offset imagers, so having a properly aligned adapter means less zoom-in, therefore less edge fall off from the bare HV20 lens.

[QUOTE=Filip Kovcin;814597...since you anyhow shoot in front of green screen - this means that you can do whatever you want with your background, and foreground separatelly - yes?

well, if this is not too far from your artistic vision - i will shoot all greenscreen shots normally - WITHOUT ADAPTER!

during compositing process you will make any kind of softness you want, you can simulate DOF etc.

...
filip kovcin[/QUOTE]

I was thinking the same thing. I've got a Letus Extreme, but if I'm shooting in front of a green screen, I have found no reason to use it. However, if I were to use it, I would simply position my subject(s) in the center of the viewfinder, avoiding the edges. Then, you can reposition in post, using Ultra or whatever.