why do some lenses have a built in ND spot filter?

[mike_va 0]

Thanks in advance. Wouldn't that make it less sensitive?

[Overthinker 0]

Mike Va,

 

Some lenses, particularly short focal length ones, may reach their diffraction limit before they could stop down enough to provide brightness control. If you think of a lens like a 4mm and say it needed to stop down to an f/32 aperture to provide an image that was not overexposed. The actual lens diameter used would only be an eighth of a millimeter. At such a small lens diameter the smallest features that could be reproduced on the CCD would be many times larger than the pixels on the same CCD. The image would not look sharp. With advent of 700 TVL cameras some of the more expensive lens manufacturers use a neutral density filter that moves into place when the camera would have needed to close the lens iris more than diffraction limited resolution would allow. This is done to maintain sharpness. The neutral density filter is not used when the light levels are low. Much like a the ICR of many modern cameras. Low cost lenses use a small neutral density "star" pattern in the center of the lens. This type does reduce the light throughput but the amount is small since the "star pattern is only a small percentage of the total lens area.

[mike_va 0]

Mike Va,

 

Some lenses, particularly short focal length ones, may reach their diffraction limit before they could stop down enough to provide brightness control.

That sounds like it makes sense, but I saw a 8-80mm lens for example that had one. I can't understand why they would want to give up the sensitivity at night. It was IR corrected so meant to be used day night.

[Overthinker 0]

Hi Mike.

 

The 4mm was just an example. Very short focal length lenses at high f/ratios suffer from multiple diffraction effects. Actually the F/ratio of the lens is the deciding factor. Of course faster lenses are both harder to produce and design with limited lens aberrations. A typical 1/3 Sony Super HAD II CCD has pixels about 4.6 microns in size. The diameter of the diffraction limited spot size of a perfect optical system is given by 2.44 X wavelength of light used X the lens F/stop. So lets say you have your 8-80 set at 8mm or 80mm it doesn't matter, if the iris stops down more than about f/3.5 the diffraction spot size of the lens will be bigger than the pixels on your CCD sensor. That is if your lens was optically perfect and you were viewing a daylight scene. ( 2.44 x .55 X 3.5 = 4.697microns. Say you used the lens at night with typical IR illumination on... then your perfect lens would start to lose sharpness due to diffraction at about f/2.2. Again this is for a perfect lens. The lens aberrations of most lenses hide this phenomenon to some extent. Now think of my previous f/32 example. Using the diffraction formula the diffraction limited spot size would be 43 microns. Almost 10 times your pixel diameter!

 

So even with an IR corrected lens the resolution of your camera can never equal day light performance since you are using a longer wavelength of light. The ND star helps this problem by keeping the F/ratio from getting too high. Like when someone is close to your camera and IR source and the iris needs to close to maintain the correct exposure. Since the star is maybe 10% of the lens area you lose a little less than that since the star is not completely opaque. I know the 10% seems like a lot however there are many other light losses in the camera system. For a single example you lose 4% at each surface of the glass window protecting your camera. And that does nothing to improve the performance of your camera system.

 

I tend to personally like IR corrected aspheric lenses. Since they really do provide sharper images at wide f/ratios. Also since they are hyperapochromatic they work better in visible light too. Fujinon's IR corrected lenses are easy to spot. The Fujinon name is in red lettering.

[mike_va 0]

Hi Mike.

 

The 4mm was just an example. Very short focal length lenses at high f/ratios suffer from multiple diffraction effects. Actually the F/ratio of the lens is the deciding factor. Of course faster lenses are both harder to produce and design with limited lens aberrations. A typical 1/3 Sony Super HAD II CCD has pixels about 4.6 microns in size. The diameter of the diffraction limited spot size of a perfect optical system is given by 2.44 X wavelength of light used X the lens F/stop. So lets say you have your 8-80 set at 8mm or 80mm it doesn't matter, if the iris stops down more than about f/3.5 the diffraction spot size of the lens will be bigger than the pixels on your CCD sensor. That is if your lens was optically perfect and you were viewing a daylight scene. ( 2.44 x .55 X 3.5 = 4.697microns. Say you used the lens at night with typical IR illumination on... then your perfect lens would start to lose sharpness due to diffraction at about f/2.2. Again this is for a perfect lens. The lens aberrations of most lenses hide this phenomenon to some extent. Now think of my previous f/32 example. Using the diffraction formula the diffraction limited spot size would be 43 microns. Almost 10 times your pixel diameter!

 

So even with an IR corrected lens the resolution of your camera can never equal day light performance since you are using a longer wavelength of light. The ND star helps this problem by keeping the F/ratio from getting too high. Like when someone is close to your camera and IR source and the iris needs to close to maintain the correct exposure. Since the star is maybe 10% of the lens area you lose a little less than that since the star is not completely opaque. I know the 10% seems like a lot however there are many other light losses in the camera system. For a single example you lose 4% at each surface of the glass window protecting your camera. And that does nothing to improve the performance of your camera system.

 

I tend to personally like IR corrected aspheric lenses. Since they really do provide sharper images at wide f/ratios. Also since they are hyperapochromatic they work better in visible light too. Fujinon's IR corrected lenses are easy to spot. The Fujinon name is in red lettering.

Thank you