ALEXA Mini=no IR filter?

[christian mann]

Hi there,

 

looking at the ALEXA Mini which has built in ND filters I wonder why ARRI wouldn't use built-in IRNDs if the sensor was sensitive to infrared pollution....

Does ARRI want to support Tiffen and Schneider by not using built-in IRNDs or is the Mini simply not sensitive to IR pollution?

 

Has anyone tested this maybe?

 

Thanx

 

[Brian Drysdale]

I suspect it's just because time has moved on and given these uses on the Arri web site, having built in filters make sense, as against using external ND filters.

 

"The body design is optimized for use with new-generation brushless gimbals, multicopters and other specialized rigs. It is compact enough in the lens direction to allow the use of standard PL mount lenses even on lightweight and space-constrained rigs, such as gyro-stabilized aerial systems."

Edited April 29, 2016 by Brian Drysdale

[christian mann]

sure it makes sense.

but why not have built-in IRND instead of straight ND ?

[Brian Drysdale]

It seems they use Full Spectrum Neutral Density Filters, which have scratch issues and are only manufactured in small sizes..

 

https://www.arri.com/forum/viewtopic.php?t=105&p=206

[Satsuki Murashige]

The built-in NDs do filter out IR. Shot with a Mini and Amira yesterday, we used the internal NDs up to 2.1 combined with regular Schneider NDs 0.3-0.6. No IR problems.

[Adrian Sierkowski]

I always assumed the IR filtration, in that case, might be on the sensor proper?

[Brian Drysdale]

It's the filter they use, it's explained in the link above.

[christian mann]

But isn't the ARRI guy making contradicting comments?

 

Looking at the graph it shows that the internal FSND 1.2 blocks IR light. So why would I use an additional IRND like he recommends? Wouldn't that result in a greener image?

If the internal FSND 1.2 and 2.1 block Infrared light the use of an additional IRND makes only sense in combination with the internal FSND 0.6 (which doesn't block Infrared I assume).

Then again why would you call the internal 0.6 filter Full Spectrum ND if it doesn't block Infrared light??

 

Any thoughts on that ?

[Satsuki Murashige]

I think what the guy is saying is that the Arri internal FSNDs filter both visible light and IR light in equal proportion. So even an internal ND2.1 when used alone doesn't need any additional IR filtration because the IR is reduced to essentially invisible levels.

 

When using external NDs (either instead of or in addition to) the internal NDs, then he recommends using IRNDs from ND1.2 and above. Below that, IRNDs are not necessary in his opinion.

 

Since you really only need to carry 0.3 and 0.6 of external NDs to have a full set when combining with the internal NDs, you should not really need to ever use IRNDs with these cameras. But some DPs are old school and don't want to use internal NDs because they feel they affect image sharpness, or they simply prefer to filter in front of the lens. So the options remain for both working styles.