Since I discovered that my Tiffen dark orange filter fits perfectly on my Hasselblad's Zeiss lens I've been experimenting with getting a nice, dark sky effect (without using post-processing). First, you need a deep blue sky to get a dark and dramatic tone. Second, the filter factor seems a bit too extreme for this #21 filter. Most guides have recommended giving 1.5 to 2.5 extra stops of exposure when using this filter. My experiments have shown that one stop is enough to compensate for the filter placed on the end of your lens. For instance, two extra stops on my 8x10 setup seemed overexposed. Over the weekend, I shot a roll of Fuji Neopan Acros 100 film in the Hasselblad. I bracketed my shots and have found that I'm happiest with just one stop of extra exposure when using the filter with Fuji Acros. I developed these in Rodinal 1+50 at 20C for 13.5 minutes. The midtown sky behind the Chrysler building was a bold and clear blue for this shot and this is the exact effect I was looking for out of a dark orange filter.
The sky was less of a dark blue here and there were wispy clouds framing the building. Still an improved look over a bland, white sky I would normally get with no filter and B&W film.
The sky surrounding the Untied Nations building on the East River was an intense blue and I waited a bit for the thin clouds to line up with the building. Another good effect.
This shot is just me goofing around while deciding if I wanted to walk over to the UN building or not. The orange filter provides a nice contrast boost on the buildings here, but very little darkending of a light blue sky.
The bottom line is that I need less extra exposure than the manufacturer suggests and if you want a dramatic sky and properly exposed buildings, it helps to have a nice deep blue sky day. Now I'm just waiting for another fluffy cloud day in the city...