I suspect that the high filter factor you see with the filter in front of the lens has to do with the spectral sensitivity of the metering sensor. The R87 filter likely blocks in excess of 99 percent of the visible light thus the high filter factor. The factory sensor filter blocks the IR light, to which, it seems, the unfiltered sensor is sensitive. I'm guessing that it will take some experimentation to determine the correction factor from the 'normal' meter reading. The proportion of IR light to visible light is quite variable and is strongly affected by time of day. Cheers, James Sent from David's iPad On May 31, 2012, at 9:43 PM, Alberto Tirado <fotodiseno2003@xxxxxxxxx> wrote: >> Send your XT to LifePixel (www.lifepixel.com). They'll swap out your >> senser filter with an R87, or other specialty IR filter they sell. >> You'll then be able to shoot 1/400 @f8 AND look through your viewfinder. >> > > > This has me thinking ! > > If an r87 filter has a compensation of, say, 8 or 16 steps, how come you can convert your camera, in which they place an r87 filter on the sensor, and then go about happy-shooting? > My "reasoning" is that an r87 has a certain density, no matter if it is in front of the lens or behind it. > > > So how can you shoot at higher speeds with a camera conversion, but not with a filter? > > **************** > > Alberto Tirado >
