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-   -   Knee Point for white backgrounds? (http://www.dvinfo.net/forum/sony-hvr-v1-hdr-fx7/149248-knee-point-white-backgrounds.html)

Ken Campbell April 1st, 2009 02:56 AM

Knee Point for white backgrounds?
 
I like shooting against a well lit white background (2+ stops over the subject) and then in post blow out the background to have pure white for web oriented stuff. Logic tells me I should adjust the Knee Point to high so that white blows out faster. What do you suggest?

Thanks!

Hugh Mobley April 4th, 2009 08:50 AM

this is a good time to test your settings and try all the knee settings, I have spent hours and hours trying different settings, most are very subtle.

Ken Campbell April 9th, 2009 03:40 AM

Thanks Hugh, I did some shooting in the studio against a pure white background and tried all three knee settings. High is better for getting pure white, although as you said, there is not a great amount of difference.

Ron Little April 9th, 2009 08:58 AM

Ken I am interested in your technique. Can you tell me how you go about setting up your camera for such a shoot? And if it is not too much trouble can you explain how you are measuring the lighting are you using your camera or a light meter?

Ken Campbell April 9th, 2009 11:40 PM

Hi Ron, here's a basic rundown of how I shoot for a "web white" background.

Startind with a standard white paper roll, I usually put 4 500w Ianiro photofloods to blast the white background. This is necessary because as you light the subject the white background will go grey in the camera. Getting a perfect white balance is also important. Then I use the minimum light necessary to light the subject and leave the subject slightly underexposed. I always work with the histogram on and the zebra set to 100%. I then go about getting the final aperature setting that keeps the background filled with the zebra and on the histogram you see a space between the white of the backgound and the highlights of the subject. The bigger the space, the easier it is to get pure white. At this point I adjust the amount of lighting on the foreground and background to have that separation. Usually I want to see the background blow out around 5.6 on the aperature and the subject is lit properly around 4.

I recently learned, thanks to this forum, that adjusting the knee point to high in the picture profile helps blow out the whites faster. If set to medium of low the camera tries to preserve highlights.

The last step is done in the NLE where I have to adjust the levels to get to pure white. This is why I usually slightly underexpose the subject.

Here is my last video that I shot against a pure white background

YouTube - Tommy Dell'Olio playing "d.h.s." with a custom Masotti X100M Modern in HD Video

Have fun!

Leslie Wand April 10th, 2009 12:21 AM

very nice work!

leslie

Ron Little April 10th, 2009 05:15 AM

Ken, Thank you. That will help me so much on my next shoot. Do you ever do any green screen work?

Ken Campbell April 10th, 2009 07:36 AM

Thanks Leslie!

Ron, glad to help. As for greenscreen, I do some stuff, mostly talking heads and interviews, against the greenscreen. I try not to when I can do without it, as it is a real PITA.

Sandy Tedford April 24th, 2009 07:28 PM

shooting on grey
 
Hello,

any advice for shooting on grey. For some reason the camera seems to be struggling with holding focus on the subject.

cant work out why.... any ideas?

thanks

Sandy

Ken Campbell April 25th, 2009 12:48 AM

Hi Sandy, the only time I tried shooting against a grey background I used the white background without any light hitting it directly. As you light the subject the background gets less light and you get grey. FWIW, that is how I also get pure black, by shooting against the black paper background and lighting only the subject while controlling the spill onto the background. Here is my latest video against a black background YouTube - Tony Cordaro playing "Hot Frets" with a Masotti X100M Classic

As for focus, I never use autofocus because I always get a shift when I don't want one. When working with solid backgrounds, the camera will almost never focus on the background because the cameras tend to look for strong contrasting lines for focus. Moreover, with solid backgrounds I tend to work in the very wide angle range with infinity focus so that my subject(s) are always in focus. If you don't have the option to go manual I think avoiding complex constrasts where you don't need them may help?

Sandy Tedford April 27th, 2009 03:35 PM

thanks for the reply,

we are getting there slowly and i think it is getting better. We may have a problem with the camera rather than the background.

ill test a bit more and get back to you.

appreciating your help!

thanks

Sandy


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