Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, Sarah Thompson 
<sarah@...> wrote:
>
> c1asia wrote:
> > can you explain IR high-pass and low-pass filters to me? i'm not
> > familiar with IR.  thanks.
> >
> >   
> OK, it's easiest to start with a bit of basic physics. Light is 
part of 
> the electromagnetic spectrum, along with radio waves, microwaves, X-
rays 
> and gamma rays. Like all the other forms of electromagnetic 
radiation, 
> light (visible or otherwise) has a characteristic frequency. Radio 
> receivers tune to a particular frequency by filtering out other 
> frequencies that aren't relevant -- similarly, the human eye 
perceives 
> colour with three sets of colour receptors, one tuned for red, 
another 
> tuned for green and another for blue. In terms of frequency, red is 
the 
> lowest that is visible, then green, with blue being the highest. 
> Infrared light isn't visible because the frequency is too low for 
it to 
> be detected by any of the three sets of receptors -- the name 
basically 
> literally means 'lower frequency than red'. In photography, we 
generally 
> deal with what is known as short wavelength IR, also known as near-
IR. 
> This is close to visible light in frequency, and should not be 
confused 
> with 'seeing heat' -- the thermal imager footage often seen on TV 
is 
> actually much lower frequency (longer wavelength) than the kind of 
light 
> normally dealt with in infrared photography.
> 
> Conventional film is only significantly sensitive to visible light, 
so 
> no special technology is needed in cameras or lenses in order to 
capture 
> visible light images. However, silicon imagers, including both the 
CCD- 
> and CMOS-based sensors used in digital cameras, are actually 
sensitive 
> well into the infrared as well as to visible light. This is a 
> consequence of the physics involved, it's not something that is 
> generally deliberately engineered in. Normally, this infrared 
> sensitivity isn't something you want, because it can tend to cause 
weird 
> colour shifts, particularly in strong daylight (where there is a 
*lot* 
> of infrared from the sun). As a consequence, nearly all digital 
cameras 
> contain an infrared high pass filter -- literally, this is a piece 
of 
> specially treated glass that typically looks like it has a slight 
> cyan-blue cast to the naked eye, which blocks infrared very 
strongly, 
> whilst passing through visible light more or less unaltered. As I 
> mentioned in my previous post, the Better Light doesn't have a high 
pass 
> filter on its sensor, so you have to use an actual filter attached 
to 
> the lens. This means that you can take this off and replace it with 
an 
> infrared low pass filter that blocks visible light and passes 
infrared 
> -- this is key to getting good sharpness in infrared, because 
lenses 
> have a slight focus shift toward the IR, so it is difficult to get 
both 
> IR and visible light in focus at the same time.
> 
> Hope this helps,
> Sarah
>


thanks.  i hadn't heard the terms "high pass" and "low pass" but now 
i understand.  i've always heard it as "IR cut filter" and "IR pass 
filter" where the former cuts IR and the latter cuts visible light.  
i read somewhere (can't remember where) that with the newer DSLRs 
with live view, if you convert them to dedicated IR cameras, you can 
focus more accurately with the live view on IR than with the lens.  
the reasoning is that the focus is based right off the sensor rather 
than through mirrors and eyepiece.

i'm still waiting for iraw mono -- supposedly a superior raw 
converter for modified digital cameras using dedicated 830nm low pass 
filters.  see http://khromagery.com.au/IRaw.html