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Flats and UHC filter....

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I am now using a light pad for flats, placing it on the aperture and using AV mode to expose it a third of the way across.

Last week I used it for the first time, shooting at iso 800 it exposed at 4000th sec. Not much leeway here as it is the fastest speed on my DSLR. (exposed nicely a third of the way across though).

Same process last night, iso 800, exposed a third of the way across, the only difference being I had a UHC filter attached. The shutter speed was a 400th sec.

Thats a ten fold reduction in shutter speed. I may not be calculating correctly but I think that means using a UHC filter needs a 10 fold increase in shutter time to get the same light to the sensor? 

Does this sound right? I actually experimented with shorter subs last night (5 mins, with filter) and it came out allright, but I was shocked at the different in exposure time the filter makes. Hmmmmm...


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Fortunately it doesn't work like this. I have found that a Baader UHC-s will  need +1.5 Ev to a none filtered exposure and +1 Ev for IDAS D1 or P2. The DSLR sensor calculates the exposure on the basis of full spectrum visible light but in AP you want specific bands that the filter will let through.


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