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Graggs

Member Since 23 Jul 2011
Offline Last Active Jan 17 2014 01:30 AM
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Posts I've Made

In Topic: Anyone tried: Mangrove K-VS-12L12 Lights (18240 Lumens)

06 January 2014 - 11:06 AM

I can't comment on the individual products above or their suitability, but for the benefit of people reading this, here is a quick explanation of CRI / Colour Temperature and other interesting LED light output stuff.

 

The CRI shouldn't be confused with the colour temperature (expressed in °K .... degrees Kelvin)

CRI stands for Colour Rendering Index and is an expression to determine how faithful colours 'look' under a given light source. 

°K .... degrees Kelvin is an expression of colour temperature, the lower the figure, the nearer to the warm end of the spectrum (fire / candle) the higher the figure, the nearer to the Blue end of the spectrum. Daylight being about 5,000°K ~ 6,500°K

 

As an illustration, a Tungsten Halogen light source of around 3,000°K can have a CRI >95 and by comparison a fluorescent light source with a colour temperature of 5,000°K might have a CRI of <75.

 

Part of this is due to the spectrum of light that the light source is emitting. The 'Spectrum' being all the various colours (think rainbow or prism),

In the image below, you can see the Halogen lamp (middle) emits a continuous range of colours whereas the fluorescent (bottom) has whole sections missing (or too dim to be any use). 

301px-Simple_spectroscope.jpg

 

LED's have much the same problem as the fluorescent above, they not only have major deficiencies in certain colours (technically wavelength) BUT also have 'spikes' in some colours - the right hand image below shows our Tungsten light source.

LED_Comp_Sprectrum.jpg

 

LED's are notoriously bad in the CRI department. But they are getting better. You also have to treat the manufacturers CRI readings with caution, as they can be fudged.

 

 

A result of this can be seen below, the image on the left is lit with Tungsten and the right with White Phosphor LED source. Look at the skin tone of the right hand image, the lack of correctly coloured light (or light deficient in certain wavelengths) has rendered the skin wrong. In contrast the blue spike in the LED light source on the right has made the dress look too blue (in fact it's changed the colour of the dress altogether).

LED_Model_Comp.jpg

 

And this is before you shine the light through water with particles in it or take into account the degradation of the LED over time.

 

Another consideration... but way outside the scope of this thread is the sensors ability to record this information, i.e how sensitive it is to various colours or wavelengths.

 

It's all relative of course and most dive sites are NOT offering perfect filming opportunities. But I felt the above distinction might help?

 

A couple of the above images come from this old thread, which is worth a read if you want to expand on the above. It is old but still relevant to some of the points above.

http://www.cinematog...showtopic=52551


In Topic: Testing lenses underwater.

05 January 2014 - 10:13 AM

Did this thread ever get anywhere.. it seems to have died a death?

 

I am having some dome port dilemmas which a Sea & Sea MDX 5II and a 16~35mm USM L lens and I was hoping to find a conclusion here.

 

There are lots of good ideas here, Is anyone interested in trying to resurrect this? 

 

I can knock up a test chart and share it, if we can agree on some content... I propose a combination of the mesh idea of Pete Atkinson and smaller charts positioned at key places.

 

 

 


In Topic: FS: Subal 5DMKII housing kit, with dome port and camera

05 December 2013 - 01:41 AM

I was very close to buying this, but found a setup that suited me closer to home (in England).

 

Josha is an excellent communicator and I wish him luck in selling the above housing, I have no doubt it will suit someone very well indeed.

 

Personally, I would go 'the extra mile' and have it serviced. For anyone that doesn't know, a service tech would look at all the seals - the big 'o' ring at the back is the easy one to check, but all the fiddly little seals around the control knobs are just as likely to let you down.

 

Good luck Josha

 

g


In Topic: Sea and Sea YS-110 strobes, Cables, DX Macro port and focus ring

20 November 2013 - 07:33 AM

Hi

Do you still have the Macro Port bits?

I have the newer lens, but I wonder if any of the parts are compatible?

 

This is the list of bits I need (as I understand it)

 

Canon EF100mmF2.8L Macro IS USM   
DX Macro Port Base  ..... Product No.30105
DX Macro Port 50Ⅱ ..... Product No.30110
Focus Gear ..................... Product No.31155

 

Are there any part numbers on the pieces you have?

 

Cheers

Graggs


In Topic: FS: Subal 5DMKII housing kit, with dome port and camera

02 November 2013 - 01:56 AM

Thanks Josha for your quick answer.

It's good to know it hasn't seen much action, but I fear the age / lack of service and second generation port will make this an expensive proposition for someone?

 

All the best

Graggs