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Stuart Keasley

Member Since 23 Aug 2008
Offline Last Active Jul 03 2014 06:20 AM
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#347501 A Circular Experiement

Posted by Stuart Keasley on 09 May 2014 - 11:15 AM

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


#347405 Video Recording Formats AVCHD vs AVCHD Progressive

Posted by Stuart Keasley on 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.




#347002 Shooting video & stills on the same dive with a single cam, does it work?

Posted by Stuart Keasley on 29 April 2014 - 12:20 PM

Fish eye would look ugly in video, however wide angle and macro lens choices would be very similar, so no real issue there...

 

However the head set would be a struggle. Video, you need to be thinking about the getting the detail, ins and outs, cut aways, GVs, what you've done and what you need to do build the sequence and tell the story. Trying to mix stills into the middle, if you're as feather brained as me, you'll forget where you are with the video, and won't have sufficient time to concentrate on the stills to get good results there either.

 

The lighten requirements are also very different, as are the settings on the camera... so the more you swap between the two, the more time you'd waste resetting and settling in.

 

So, I'd say keep it simple. If you're doing a video dive, focus on that, stills then think of stills... and even more than that, dedicate each dive to video wide or video macro, stills wide or stills macro. 




#346677 Orcalight Seawolf-22000 lumens in the Maldives

Posted by Stuart Keasley on 23 April 2014 - 11:18 PM

Hi Fergus
 
I agree the Orca SeaWolf light is a fantastic LED light. Great even light and colour rendering. Clever way to transport and use of high capacity rechargeable batteries. One thing I don't like is the opening and closing of the canister. Two hex bolts and only way of opening is use a blunt flathead screw driver next to bolts. I am going to suggest Orca provide some sort of snap latches or even a  thumbscrew. Great light and output is 22K lumen as specified.

OrcaLight have already redesigned the method of opening and securing the lid, have a read of the following for details:

http://wetpixel.com/...showtopic=52724


#346299 Subal S7Q Housing for the Odyssey 7Q

Posted by Stuart Keasley on 15 April 2014 - 10:52 PM

Looks like my prayers have been answered

http://www.subal.com...nitors/S7Q.aspx

My interest is to pair up with the FS700, however Convergent Design have also said the 7Q will support the Sony A7S, and presumably therefore the GH4.


#346097 BMPCC underwater filming setting

Posted by Stuart Keasley on 12 April 2014 - 11:51 PM

That device is to work on a flat surface. If you go diving on rocks you won't have that. And if you go on sand you can have 4-5-7 20 legs but stability comes out of balance not planting the legs in the sand as if it was an anchor. Even sand contains living organisms and should make sure your impact on the environment is as little as possible. That's my perspective at least. Results matter but after am sure am not demanding more than needed out of the seabed

From the Edelkrone web site:

"Works great on uneven surfaces, adapts to any surface"
https://www.edelkron...9/pocket-series

A quadpod has no issue with an uneven surface. It has adjustable legs.

I'm with you, in that I'd rather work with a tripod. But it's a preference thing, Edmond has gone a different route, he's happy with it and is getting good results.


#346084 BMPCC underwater filming setting

Posted by Stuart Keasley on 12 April 2014 - 03:06 PM

Quadpods are more stable and stronger but can take longer to set up.

Tripods are still stable, and are a lot quicker to work with.

You've got some stunning results Edmond, which us really all that matters ;)


#345812 Physically small intervalometer

Posted by Stuart Keasley on 07 April 2014 - 09:53 PM

I'd go with Magic Lantern. Yes, there are risks, however has anyone ever heard anything ever going wrong (when using just timelapse)

And yes, I've used it on a 5d mkii


#345774 Malta

Posted by Stuart Keasley on 07 April 2014 - 04:52 AM

Thank you Stuart ,yes i have been told the same thing (enigmatic) about mauri but due to the problems with organisation (%50 my fault) i couldnt do it. I wish i can dive to the places you recoment one day
Cheers
Lutfu

Just for a tease, filmed quite a while ago now, but here's the Southwold

 




#345578 Vacuum leak detector question

Posted by Stuart Keasley on 03 April 2014 - 07:59 AM

Obviously this needs some extensive testing.  I suggest we send a dedicated and unbiased researcher (that would be me) to the Caribbean for a grueling two-week regimen of test dives.  I will thoroughly document the testing process and advise you of the results upon my return home.  In regard to funding this research, please send a PM and I will send you my paypal address. 
 
Now about this Scotch Mist; does adding lemon peel to a nice whisky really create an enjoyable beverage?  I have my doubts but could probably experiment with that as well during the above two-week research project.  No additional funding would be required for this.  Plus, I could watch for condensation on my glass as I go from an air-conditioned room to the patio and supplement the fogging data.

Can't help with funding, but I'm very happy to supply a pressure cooker (possibly a head to put in it to check for results ;)), along with a small tub of water, some ice cubes and a copy of Nuffield A-level Physics test book (1987 edition), all obviously essential elements for your tests :D


#345562 Vacuum leak detector question

Posted by Stuart Keasley on 03 April 2014 - 12:49 AM

Aluminum conduces heat much more than glass or acrylic so in general terms if any condensation occurs it will be on the walls of the housing and not on the port or the LCD.

 

Obviously the many people who experienced condensation on their glass port in an aluminium housing were just seeing scotch mist.

 

 

 

 

 

 

Edited: Admin




#345531 Vacuum leak detector question

Posted by Stuart Keasley on 02 April 2014 - 10:59 AM

Sorry about that Draq!   I will try to clearly answer the 2 questions posted in this thread using what little expertise I have in the area as a hyperbaric physician.  I will not get drawn into a pettifoggery once again.
 
Your question if the negative pressure generated by the Nauticam system will make it less likely to flood -the answer is yes, because the o-ring is sucked into the mating surfaces of the housing, preventing blow-by. 
 
watboy, your question if an additional benefit of a vacuum system  would be that it would eliminate condensation issues , the answer is no.  It will greatly decrease them though.  If you close your housing at 30°C and 75% relative humidity, condensation will form when the housing wall reaches 25°C.  Under the same conditions, If you pump the housing down negative 10 inches Hg,  condensation will form when the housing wall reaches 18°C.  Starting conditions will certainly modify the results, but not the manner in which you can calculate the benefit. The explanation is below.
 
Condensation forms at the dew point which is when relative humidity (RH)= 100%
 
RH= partial pressure of water vapor (PW) / vapor pressure of water (VP) at a given temp.
 
Vapor pressure of water at a given temp. (VP) is independent of pressure
 
When you decrease the pressure by 10 inches Hg, you are decreasing the pressure inside the housing by 30%
 
Following Dalton's law, decreasing the housing pressure by 30% will decrease the partial pressure of water (PW) by 30% once the temperature equilibrates.  Therefore, relative humidity will decrease by 30%
 
So starting at 30°C and 1atm and a RH of 75%, pumping the housing down to .7atm will yield a RH of 67.5% once the housing equilibrates with the 30°C ambient temp.
 
Using the dew point curve we know that at 30°C and 75% RH  the dew point = 25°C
Using  the dew point curve we know that at 30°C and  67.5% RH the dew point = 18°C
 
So after vacuum there will be a 7°C degree difference in how cold you have to get the housing wall before condensate will form.
 
I will be happy to answer any questions not originating from Interceptor.

Thanks, good to have a nice summary to cut through all the posture and waffle.


#345517 Vacuum leak detector question

Posted by Stuart Keasley on 02 April 2014 - 04:14 AM

Congratulations, you've proved that water requires heat before it will evaporate.

You could get a Nobel prize for that.

As for your other comment, you misunderstand me, it's your tenacity fir being wrong that has beaten me.


#345475 Vacuum leak detector question

Posted by Stuart Keasley on 01 April 2014 - 07:34 AM

Actually no, the camera heating up inside the housing will DECREASE condensation in the camera.  The hot camera heating up the inside of the camera will decrease the realtive humidity (meaning that any condensate that has formed on the walls of the housing will find it easier to evaporate).  You have used the term "jdew point numerous times, this term just means 100% relative humidity.

There is a few where things will react differently

 

- Interior of the camera heats up and generates a temperature differential between the inside of the lens element and the air inside the housing. In this instance, you'll see condensation appearing on the inside element of the lens instead of the inside of the dome port

- If there has been water ingress into the housing during assembly, e.g if some particles of rain dropped in, as the inside of the housing heats up, the pools of water evaporate and then increase the relative humidity inside the housing, dew point therefore increases in response.

 

However in general:

 

- When you assemble the housing the inside and outside temperatures are the same.

- When you enter the water, the outside temperature will be (generally) colder

- The dome port will slowly reduce temperature to match the water, the interior surface of the dome port therefore becomes cold.

 

If the interior temperature of the dome port is at the dew point of the relative humidity of the air inside, condensation will appear. If you're lucky, the heat generated by the camera may be sufficient enough to heat up the air inside the housing, and the interior surface of the dome port, making the condensation disappear (in much the same way as you would blow hot air on a windscreen to demist it)

 

However, whatever is at play, it's all about the dew point.

 

That's the way I understand it, anyway ;)




#345450 Vacuum leak detector question

Posted by Stuart Keasley on 01 April 2014 - 01:58 AM

 

Your educational principles are a bit dated if you think putting someone in a corner to think matters is the way forward.

 

(Metaphorically) going into a quiet corner, without distraction and taking time to think things through is a very effective way of gaining understanding. I suspect you didn't finally get to this conclusion whilst sat on your sofa watching a psychedelic thrash metal performance on the TV ;)

 

 

I have non problems admitting when am wrong. 

 

 

Odd, that you still haven't done so then.

 

 

Like you I said that the dew point that is a temperature would go increase with pressure. 

 

No, you initially said quiet the opposite.

 

 

High pressure means drier air in fact when you breath compressed air you feel lack of moisture as the dew point is pushed further down

This is incorrect, the opposite of what you've now said in terms of dew point, and also incorrect in it's very basis. Also, compressed air is dry for no other reason that it has been filtered during the compression process. 

 

You then changed your tune

 

As in a higher pressure environment you have a higher dew point it feels drier at the same temperature and lower pressure it feels wetter.

 

 

 

Compressing air increases the dew point temperature and has a drying effect

 

 

However you've continued to say that compressing air makes it drier. This is not true, compressing air actually increases the relative humidity, it will feel wetter.

 

 

And the point is exactly that one once you set up your recompression chamber it is dry in side because you are pressurizing things

 

 

Again, incorrect. Recompression chambers don't get drier when you pressurise them. In fact, during the pressurisation process a fog develops inside the chamber (I'm no expert, but would imagine that this is a combination of both increased pressure and increased temperature)

 

On a side note, there's a cave in Tonga Vavau Group called Mariner's Cave. Access is from the seaward side of a cliff face via a 15 metre swim through at a depth of 5 metres. There's a big chamber inside, with breathable air.

 

If you visit the cave around midday, the sun shines in from the north and lights up the inside with an aquamarine blue. That in itself is quite spectacular. But if there's any sort of swell, the surface level of the water inside the cave slowly rises and falls. With each rise, an aquamarine fog forms, each fall, the fog dissipates again. It's quite eerily beautiful, almost as though the cave is breathing.

 

All caused by the volume reducing and therefore pressure increasing on the air inside the cave.

 

Unfortunately I didn't have a camera when I was there, and none of the videos I can find do it justice :(

 

The matters geot confused when your speak about relative humidity as actually what changes is not the % of moisture but the total number of particles of dry air and vapour. Which means now air can hold more moisture before it condenses. I could not work this out myself as the weather station measures the % and therefore reported no changes. The dew point is the absolute humidity not the relative and therefore this now makes sense to me. So I agree that of you pump air out the number of particles reduces and so does the vapour which means that the temperature to which those particles would condense is lower than if I had not created vacuum. Now the fact that if I set the housing outdoor I get 84% humidity and if I do it indoor I get 55% does seem far more important in terms of avoiding condensation but it would seem that if you create a low vacuum you may go in water a tad colder (don't know how to possibly quantify this) before your housing fogs. Now back to work

 

But no matter, big hooray, we got there in the end (shame you didn't read post 76 properly, it said exactly this is nice simple language) :)

 

 

don't know how to possibly quantify this

 

A lot of those links I put up will do the calculation for you. For instance, you've stated an outdoor humidity of 84%, let's put you somewhere nice and warm(ish) with a temperature of 24 C

 

Using the link you put up, and filling in the values (http://www.oxywise.c...ou-calculate-it)

 

The dew point at atmospheric (1 bara) is 19.47 degrees.

If you now suck out 5 Hg (or .16 bar) and so reduce the internal pressure to 0.84 bara the dewpoint now becomes 16.74.

 

So the interior temperature of the housing would need to drop almost 3 degrees lower for condensation to occur once you'd used your vacuum pump. I'd say that's pretty significant.