Need some enlightment or at least an educated guess!
I want to start doing some long exposures (5 minutes +).
In the past I have use a 3.0 ND filter which gives me a well exposed negative at about 5 minutes which a 6.0 ND the exposure times start at over 1 hour.
The exposure assumes ISO 100 on a bright day, shutter on T.
Any ideas or advise for a 5-7 minutes exposure, or what do I need to add to my ND 3.0 to get there.
well, I am not sure how much neutral density you need, but be aware of reciprocity failure, and also be aware that your normal developing time may result in a great deal more contrast than you expect, depending on what film you are using.
Check the data sheet for starting points. You can download data sheets from the film manufacturers' websites. TMX has unusually good reciprocity characteristics, so if you want really long exposures, you might want to shoot a film that has more normal reciprocity failure, like FP4+ or Plus-X. There's usually a graph or a table on the datasheet, so you can start out with your desired exposure length, and work backward to find the meter reading you want to get that amount of exposure after adding in reciprocity failure.
Tmax100 handles long exposures exceptionally well, just needing a little extra exposure, but no change in development. The shadow detail in very low light levels are exceptional.
My only problem is keeping count - I don't own or wear a watch, but walking round in circles counting out loud has always worked well
The ND factors are ND2 - 1 stop, ND4 2 stops. ND8 - 3 stops
Reciprocity for Tmax-100 starts at about a 4 sec. exp.=increase to 5 sec.
8 sec.=increase to 12 sec.
15 sec.=increase to 25 sec.
30 sec.=increase to 55 sec.
60 sec.=increase to 2 min.
2 min.=increase to 5 min.
4 min.=increase to 12 min.
Like others have said - reciprocity is an issue here. I presume that you are going with neutral denisty to get moving objects removed from the static scene you wnat to capture.
I would suggest you read up on the Ultimate Exposure Computer by Fred Parker on the Web. You will learn that EV's are nothing to be afraid of, and there are a couple of great tables worth printing off.
Say you want to shoot a brightly lit nightime street. That maps usually to EV 7. You are using TMAX100. You are 4x5, so I guess we want something like f/11. 4 minutes at f/11 100asa is EV -1.
So between EV 7 and EV -1 is 8 stops - no direct hit for a simple single filter ND filtration. I here would suggest ND 1.8 plus ND 0.6 to get 8 stops of light loss, and then f/11, for a 4 minute base exposure, before reciprocity should get you a good exposure.
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