Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, Sarah Thompson 
<sarah@...> wrote:
>
> Just my $.02 worth, as someone who likes to shoot B&W IR landscapes 
with 
> a Better Light scan back and a large format camera.
> 
> I'm quite fond of shooting infrared, partly (mostly?) because it's 
> really easy to do with a a Better Light scan back. The Better Light 
has 
> no built-in IR filter, so you normally need to use an IR high-pass 
> filter in front of or behind the lens for visible light work. I 
like to 
> swap this for a (visible light opaque) IR low pass filter, which 
makes 
> it possible to shoot very high quality IR.
> 
> The first thing I'll say is that I've not really seen any problems 
with 
> any of the glass I own with regard to IR -- whilst it would be 
difficult 
> to get a sharp result shooting IR and colour *at the same time*, 
using 
> an IR low pass filter and refocussing gives pin-sharp results. 
There 
> *is* a focus shift, however, which can be troublesome at times, but 
the 
> focus meter capability of the back makes it possible to tweak the 
focus 
> and get it right even though it's impossible to actually focus 
> conventionally (i.e. no visible light to focus with). I've had the 
> sharpest results with my 150mm Fujinon and my 90mm Super Angulon, 
but 
> then, they are my sharpest lenses anyway (not that my 47mm Super 
Angulon 
> XL or my 210mm Caltar are exactly blurred either, mind you). As 
regards 
> f stops, the usual 'stop down at least 2 stops from wide open, but 
don't 
> go too far or you'll get diffraction losses' advice works for IR 
just 
> fine, and (I suspect) even more so, because the longer wavelengths 
> involved will be affected more by smaller apertures, at least in 
theory. 
> I've heard people say that IR gives dreamy, slightly blurred images 
> inherently -- from experience, I can say for certain that this 
isn't 
> fundamental to IR because I've had some truly pin-sharp results. It 
> probably *is* inherent in some IR shooting techniques, particularly 
IR 
> film, however. There's a bigish print on my office wall of Split 
Rock at 
> Joshua Tree National Monument where pretty much every grain in the 
> house-sized rock is clearly visible.
> 
> My workflow is pretty simple: take the filter off, set the lens 
wide 
> open, focus and set shifts, swings and tilts conventionally (I use 
a 
> reflex finder as a matter of preference), insert the back, stop 
down, 
> put the filter in place, put the back in, refocus, shoot.
> 
> To return to the original point, rather than IR for a moment, depth 
of 
> field can certainly be an issue with large format cameras shooting 
> landscapes. The best sharpness is generally found at around 2 to 4 
stops 
> narrower than wide open for most LF lenses, but this rarely gives 
you 
> depth of field that would have, for example, a meadow of flowers in 
> focus from a couple of metres to the horizon. This is particularly 
> noticeable with a 'standard' lens or short telephoto (150mm or 
210mm 
> with a Better Light, a bit longer than that for 4x5 film due to the 
> slight crop factor), though it's still noticeable with a 90mm. 
> Ultrawides like the crazy 47mm Super Angulon I use sometimes do 
> obviously help, but you don't always want to shoot with something 
that 
> extreme. I've had very good results from using a 90mm Super Angulon 
with 
> a small amount of tilt to rotate the plane of focus  -- this is the 
> *real* secret of pin-sharp large format landscapes. Stopping down 
to 
> f/64 will indeed give far more depth of field, but it does very 
> noticeable soften the image. It's not so noticeable with film, but 
it's 
> *very* noticeable with a modern digital sensor, so tilts and swings 
are 
> where it's at in practical terms unfortunately. (And I mean 
> *unfortunately* -- getting them right is difficult and time 
consuming)
> 
> [s]
>


can you explain IR high-pass and low-pass filters to me?  i'm not 
familiar with IR.  thanks.