ND filter in front of lights

[david_freed]

 

 

<p >If I have a 1-stop ND gel, does it change the intensity of light hitting my subject if:<br>

a) I place the gel directly in front of the light source, or<br /> b) I instead place the gel right next the subject</p>

<p >My guess is it makes no different in the intensity if (a) the light is filtered first then travels to the subject, or (b) the light travels first to the subject and is then filtered.</p>

<p >Best, David.</p>

 

 

[Matt Laur]

<p>If you use the filter closer to the subject (instead of on the snout of the light), you have some more variables to deal with. Is the entire output of the light striking the back of the ND filter? If not, what's the spill doing ... reflecting around the room a bit? A large sheet of ND material might be rigged to catch all of the output you care about, but it would be a lot less expensive (in terms of material used) to just gel the front of the light - where you might only need a hundred square inches of filter, instead of several square feet.</p>

[david_freed]

<p>I'm using this for an alternative photo process that requires a UV light source and vacuum frame (or contact frame) to hold my transparency and plate. My UV light is too intense so I'm using the ND filter gel to give me longer exposure times. With my set-up, spill-over or ambient light is not an issue.<br>

Based on that, I'm still assuming I can place the gel either close to light or to frame.<br>

Best, David</p>

[Alan Marcus]

<p >Your desire is to dim down the UV and you are interposing ND filter. It makes no difference if the ND is placed in close proximity to the lamp or if the ND is more remote. The key fact is, the light must make one transit through the ND.</p>

<p > </p>

<p >However, UV has a very short wave length. The UV is likely attuned to reduce visible light thus its effects on UV are up for gabs. Likely it will attenuate but maybe the UV value will not be what you expect. </p>

<p > </p>

<p >Likely your UV is florescent. Unlike tungsten lamps these are difficult to dim. However you can buy special dimmers designed for florescent. Also you can impose lots of different semi-transparent material between the lamp and subject. At the hardware store get a sheet of milk plastic, the type used on ceiling florescent fixtures. Or a piece of vellum drawing paper from an art store. Or you can paint the existing florescent lamp with dots of black paint. Just get a flat black and dot away. You can add more paint or remove with a solvent. You can also use black electrical tape cut in circles and dot the glass florescent. </p>

<p > </p>

<p >If its tungsten, don’t paint as these bulbs run hot and paint and close filters and tape can start a fire. </p>