calculating "darkness factor" of IR filter

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lucien

Senior Member
Hi I ordered a 720nm IR filter and I wanted to calculate how "Dark" the filter is going to be because I want to calculate how long I have to expose for . Sort of like ND filters have a # based system. ND 1 , ND 6 is there a formula for that? Do I have to find out by trial and error? 720nm = Nd 7? Something like that. I spoke to the conversion people and they don't have such a formula.


thanks,
 
Fred Kingston_RIP

Fred Kingston_RIP

Senior Member
Kolari says a 720nm filter is 10 or more stops difference. I'd start with 10 stops and go from there. Your first project should be to determine how many stops your filter is, along with practicing how to focus/meter a scene, adjust your camera's settings, and then add the filter and shoot. Realizing that it's almost impossible to see thru the viewfinder with the filter in place, it take some practice in focusing and then screwing on the filter without moving the focus. Use that time/effort to test your filter's darkness with your camera's different metering modes.

I put the over/under at about 50 shots doing the above before you get tired of that and start looking for a second camera to convert. ;)
 
L

lucien

Senior Member
thanks alot. I have the long exposure app. A few of them actually I just need the ND value/equivalent. White balance I already figured out. I took 4 pictures of green things ranging from leaves to grass and they are now custom presets 1-4, #1 being the greenest. I'll bring a tripod and delay shutter release. As a focus work around. Won't the green dot show up with the filter on? For confirmation
 
Fred Kingston_RIP

Fred Kingston_RIP

Senior Member
I don't have a D7000 series camera so can't answer that specifically but I don't think it does. The biggest reason folks go with the camera conversion is because of the focus issues... Older lenses used to have focus scales on the lens' barrel to adjust for the focus variance in IR's focusing distances. Camera converters, in addition to replacing the sensor's filter, typically adjust the camera for a specific lens... In some cases, you even send in the lens with the camera.

I wouldn't worry too much about the WB settings. It's only useful for pixel peeping in the field on the small jpg image in the camera. As soon as you do any post-processing, the WB gets thrown out as soon as you convert to B&W.

Post-processing is where all the action happens in IR, IMO...
 
hark

hark

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lucien said:
thanks alot. I have the long exposure app. A few of them actually I just need the ND value/equivalent. White balance I already figured out. I took 4 pictures of green things ranging from leaves to grass and they are now custom presets 1-4, #1 being the greenest. I'll bring a tripod and delay shutter release. As a focus work around. Won't the green dot show up with the filter on? For confirmation
Click to expand...

Do you mean the green dot that indicates if something is in focus? I believe with the 10-stop ND filter I have, I had to focus without the filter, switch to manual focus, then screw on the filter before firing the shutter. Otherwise, it simulates being in such a low light condition that the camera isn't able to achieve focus. If it is only a 3-stop filter, then it *should* still be able to focus with the filter in place - but again it will be dependent on how much available light is there.
 
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lucien

Senior Member
OK thanks, I'll prefocus and try not to shake the camera while installing the filter. The long exposure apps are giving 4 seconds if I use a 10 stop nd filter (also known as ND100) and shutter speed of 1/250. That is pretty reasonable to me in terms of wait time. Iso doesn't have to be 100 I'll take up to 400. And I'm assuming aperture is a non factor.
 
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hark

hark

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lucien said:
OK thanks, I'll prefocus and try not to shake the camera while installing the filter
Click to expand...

As long as you don't touch the focusing ring, the filter should be pretty simple to install without changing focus. I don't have a lens in front of me, but I don't think you can change focus just by holding the front element of the lens and turning. If it's a push-pull lens, then you might risk pushing it in further if you aren't careful.
 
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480sparky

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L

lucien

Senior Member
In response to Cindy, with a push/pull I could just mark the barrel with a marker. Something that will show. I edited post # 6 to reflect what the long exposure apps were telling me.


I'm in the process of securing a D80 for $50 on kijjijji
 
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hark

hark

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lucien said:
OK thanks, I'll prefocus and try not to shake the camera while installing the filter. The long exposure apps are giving 4 seconds if I use a 10 stop nd filter (also known as ND100) and shutter speed of 1/250. That is pretty reasonable to me in terms of wait time. Iso doesn't have to be 100 I'll take up to 400. And I'm assuming aperture is a non factor.
Click to expand...

lucien said:
In response to Cindy, with a push/pull I could just mark the barrel with a marker. Something that will show. I edited post # 6 to reflect what the long exposure apps were telling me.

I'm in the process of securing a D80 for $50 on kijjijji
Click to expand...

If you change your aperture, that will directly affect your exposure. Aperture, shutter speed, and ISO all work in tandem to control exposure. If you change any one of them, your exposure will also change.
 
L

lucien

Senior Member
Got it thanks, what I meant by aperture being a non factor. It won't how would I say be affected once all the other factors are set. The exposure apps don't factor aperture. It's ok I don't want a lecture about the exposure triangle. At those long exposures F4-8 will come out the same is what I'm trying to get at
 
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