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Stuart Keasley

Member Since 23 Aug 2008
Offline Last Active Aug 08 2014 10:34 PM
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Posts I've Made

In Topic: Subal S7Q Housing for the Odyssey 7Q

20 May 2014 - 05:12 AM

Hi Norman,

 

Sorry for the delay in responding, only just back in the country! I've only used the 7Q FS700 Amphibico combo in a studio based environment, I've not managed to get my hands on the 7Q as yet, so I'm having to run that top side linked with HD-SDI cable.

 

I did a review in December, details here:

 

http://wetpixel.com/...=51985&hl=fs700

 

There's also a link to short clip on Vimeo, showing the grading potential of the raw files. 

 

http://www.vimeo.com/82280380

 

My overall opinion, very impressed, can't wait to get a housing for the 7Q so as I can free myself from the top side tether!


In Topic: A Circular Experiement

09 May 2014 - 11:15 AM

If you get a lens is too small for the sensor, you'll get a complete circle...

In Topic: Video Recording Formats AVCHD vs AVCHD Progressive

07 May 2014 - 08:25 AM

Your subjects don't have a great deal of moving detail, so the bit rate isn't going to be pushed. Try throwing a fairly complex image in front of the camera, e.g back lit water droplets hitting a matt black surface, where you're capturing loads of movement, and the cameras trying to cope with a full range of light and exposure, you may see 50p struggle a bit when you look at each individual frame.

 

If you take a 50p clip and render it out as 25p, your video file will have half the amount of frames. This will be done either through interpolation of adjacent frames (which can result in a fairly messy output) or by dropping frames. It depends on how you've set your NLE up.

 

If you render a clip out, the bit rate will be defined by the rendering process, the codec used and the parameters chosen. Throw your AVCHD clip through premier and then output as DNxHD, you'll end up with a 180 Mbps data stream. However you're not going to increase quality.... you can't add data in through the rendering process. As for your intermediary codec, no surprise there at all, each looking at each frame, the 50p footage will have twice as many frames as the 25p.

 

AVCHD is and 8 bit format and highly compressed. It does a great job, which is why it has become so popular, however it does have it's limitations. 8 bit means the image will struggle in low light and highlight, as priority is given to the mid tones where the majority of the important information is assumed to be. Re the compression, as with JPEG vs raw, if you get it right straight out of the box, and don't need to make adjustments, then you'll end up with a pleasing image, however as soon as you start to try to push the image with any form of grade/colour adjustment, you'll very quickly find yourself hitting problems. But in the general case for an untouched clip, it's unlikley that you're going to notice the difference between 25 and 50p.

 

In terms of bit rate, put it into perspective. BBC HD is broadcast at around 10 Mbps, and is viewed on a screen much larger than your PC monitor. You'd be hard pressed to notice, picture quality is still good... but you can be assured that the initial delivery was way higher than that in order to give them the latitude to pull the image around and get the look they wanted (minimum delivery for us for broadcast is DNxHD at 180 Mbps or ProRes at 185).

 

If you are shooting 50p, and then retaining the same frame rate in your end delivery, then there will be a minimal increase in quality when compared to 25p. However if you bring the 50p down to 25p, then you will be faced with either dropping frames and losing half'ish of the data, or interpolating frames and quite possibly getting a smeary image as a result.

I would suggest that you approach each shot individually, and choose the appropriate settings. If it warrants slowing down in post, shoot 50P. If it doesn't, shoot 25P.


In Topic: Video Recording Formats AVCHD vs AVCHD Progressive

07 May 2014 - 03:39 AM

 

Still this would make 25p better but I can't see any difference whatsoever in the examples that I shoot

 

50p has higher compression, 25p will therefore be better. Whether the naked eye can tell the difference is an entirely different thing ;)


In Topic: Video Recording Formats AVCHD vs AVCHD Progressive

07 May 2014 - 02:36 AM

Stuart, I agree completely with you.

 

My reasoning behind my comment, is that to keep the shutter angle at 180, for a 50/60fps vs 25/30fps you would have to double the shutter speed (so losing a stop) - so to compensate you would either have to open the aperture or increase the ISO to keep the exposure correct. Hence my comment about running a lower fps and so keeping the ISO low(er).

 

Thats what I believe anyway!?

Yep, spot on