[paranoia23]

i bought a pack of filters on ebay ... there should be a IR filter in the package but i dont know if it really is a IR filter ?!
i own a hoya rm90 which is pretty black... bit this noname thing looks like a red filter ?!
i can even see the blue led from my pc through this filter!

[VoiceOfReason]

Does it say the wavelength of light that passes through it in nanometers is?

[paranoia23]

there is nothing on the filter itself

[VoiceOfReason]

There are some that are red + IR. I have one that is 590nm and it looks like a red filter. You can see visible light through it, but it also does IR as well. Getting exposure right on a DSLR with it is a real PITA, and in fact I simply use my 720nm IR filter rather than it on my K5 when doing IR pictures because I get way too much exposure on the light that falls between 590nm and whatever the IR cut filter is on the sensor. I'll be using that one when I get my K-01 in and get it full spectrum converted.

In the example of the IR 590 filter it will allow all light at 590nm and above to pass while blocking most if not all below that. The blue LED most likely looks red when you look at it because it's still putting out some light at other spectrums and the filter is still passing a little light in the blue spectrum.

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Last edited by VoiceOfReason; 01-14-2013 at 09:52 AM.

[HSV]

It could be, what we call in optics, a "longpass". It let's through wavelengths that are, let's say, 650nm and upwards (until 2000nm or something ridiculous like that). Obviously, a small percentage of the other wavelengths will pass though as well (which might include your blue LED, which is about 475nm).

Our eye, and the CMOS sensor (which is silicon based) are still very sensitive to light at 650nm, that's why you see red when looking through.

[paranoia23]

i read that most people use 720nm filters ... i only have a 900nm hoya and this reddish thing

"longpass" well yes i understand that a filter has a curve and doesnt filter for exaple 719nm but passes 721nm.
anyway the IR is defined upon from 780nm ... photografic from 700nm (atleast in german standards).. so filters with a cutoff at around 600nm aint a IR.

i would like to have a 720 or 780 filter but for example the hoya R72 is about 50 euros.

[Ben_Edict]

i read that most people use 720nm filters ... i only have a 900nm hoya and this reddish thing

"longpass" well yes i understand that a filter has a curve and doesnt filter for exaple 719nm but passes 721nm.
anyway the IR is defined upon from 780nm ... photografic from 700nm (atleast in german standards).. so filters with a cutoff at around 600nm aint a IR.

i would like to have a 720 or 780 filter but for example the hoya R72 is about 50 euros.

NO a filter starting at 610nm would be a RG610 (by Schott denomination) and cannot be called IR. Obviously it would let through IR too (unless expensively coated to reflect IR (in which case it would be a dichroitic filter) - but I can hardly image that to be the case. So, it is a simple rip-off. The same as the "multi-coated" UV filters I once bought cheaply. They are hardly single coated... But who would take the seller responsible for such a cheap item?

Anyway a real IR is near black and should let pass only a tiny amount of visible light.

Ben

[Ben_Edict]

There are some that are red + IR. I have one that is 590nm and it looks like a red filter. You can see visible light through it, but it also does IR as well. ...
In the example of the IR 590 filter it will allow all light at 590nm and above to pass while blocking most if not all below that. The blue LED most likely looks red when you look at it because it's still putting out some light at other spectrums and the filter is still passing a little light in the blue spectrum.

That is exactly the behaviour you would expect from any coloured glass filter. Having a nominal filter edge of 590 nm is lets everythin longer, including reds and infrareds pass. So, nothing special about it. As these filters are made from dyed glass and are not coated to have a hard edge they will let through a diminishing amount of wavengthes below the nominal edge value, but the intensity fall-off will lead to severe shifts in the perceived colour.

That is basically the same behaviour as a capacitor when you feed different electrical waves through it (think: music signals fed into a loudspeaker). It will let through the shorter wavelengthes and filter out the longer ones with a 6db/octave fall-off. So the capacitor is a good analogy to a glass filter's behaviour.

Ben

[paranoia23]

as i said it aint a IR ... but could i use this red filter for anything? better long distance view through haze?!
because if i dont i will break it for a new distance ring

[VoiceOfReason]

If you have a camera converted to IR then using it could make for some interesting pictures. Google IR plus color photography and you will see what I mean. If yours isn't and you were planning on simply doing long exposures then it's pretty much not going to be useful.

[paranoia23]

i just recognized that this mysterious red filter tends to change its color when i hold it in front of my monitor and rotate it ... from red to blue... like a circular pol. is changing from blue to yellow if u hold it backwards and turn it on a screen.. ???

what a mystery

[excanonfd]

Back in the days of shooting with film, Red 25 was commonly used for IR photography. If you look up the Wratten number at Wiki Wratten number - Wikipedia, the free encyclopedia, Red 25 and Red 70 are considered as IR filters. You had to focus the lens, move the focus point to the IR mark on the lens, compensate for the Red 25 exposure and shoot. Made for some pretty interesting black and white images.