Tmax 100 Reciprocity - beyond 2 hours

[brett_kosmider]

I am shooting extended exposures using Tmax 100 and I've found Marcus Kazmierczak's

exposure calculator to be quite useful (http://www.mkaz.com/photo/tools/expcalc.html)

but it won't calculate reciprocity out past one hour. Does anyone have info on Tmax 100

reciprocity for, say, 2, 3 or more hours?

 

Thanks,

Brett Kosmider

[pics]

For such long exposures the only sure thing is lots of experimentation and good notes.

[Dave Luttmann]

Brett,

 

What are you photographing? You may find that doing some tests with gas hypersensitization could maybe cut you reciprocity failure in half or more.

[brett_kosmider]

I'm exploring motion within nature and part of that is celestial motion. I frame a portion

of the sky within a landscape and depending on the phase of the moon I'll get a dramatic

landscape (that still looks like night, not a full exposure that looks like daylight at night)

and star trails. But the problem has been gauging the exposure to accomplish the desired

effect (ie long enough star trails). I know that, the sky is equal to 180 degrees and the

earth moves at 15 degrees per hour, so a good arc may be 45 or 60 degrees = 3 or 4

hours.

 

I'll have to experiment some more, just though maybe there were some tables out there to

assist me.

 

Thanks

BK

[patrick_gainer]

Yes, but the stars are moving. They are not standing in one spot on the film to make use reciprocity calculations based on stationary objects. Look up Michael Covington or Michael K. Gainer. However, if you want a formula for more standard reciprocity corrections, it is:

 

Tc = Tm + (0.07 * Tm^1.62)

 

In words, "Corrected exposure time equals measured time plus measured time raised to the 1.62 power plus measured time." Measured time would be the time you measure with a light meter. I guess you could work backwards from an actual time and calculate the measured time if the "law" of reciprocity were truly obeyed.

[patrick_gainer]

I didn't read your question closely enough. Depending on where you are with respect to city lights, you will reach a point of diminishing returns. I don't think any calculation of reciprocal effects will help unless you can calculate the exposure that would cause the landscape to drown out the skyscape if there were no linear reciprocity. My guess is that exposure corrected for reciprocity would be the point of diminishing return of the record you want.

 

In the equation I provided, the factor 0.07 is specific to 100TMX. The power 1.62 is common to all films for which I have seen data.

 

The longer the focal length for a given actual aperture, the better it will be for your purpose. An f/4 lens with an opening of 2 inches will capture more starlight than an f/2 lens with a 1 inch aperture. This is perhaps the best way to get longer star trails while not overexposing the landscape. The trouble then is that you may not get the field of view you want. Then you may have to go to LF film for the ultimate. A 12" lens at f/6.8 has about 3 times the starlight gathering capability as a 2" lens at f/2.

[patrick_gainer]

I should have added "but will take about 1.7 times as long to register the landscape. Thus, for the same landscape exposure, your startrails will be 1.7 times longer and you will make about another magnitude of stars visible by using a 12" f/6.8 lens instead of a 2" f/2 lens.

[brett_kosmider]

Patrick,

 

Thanks for the insight! I will have to mull that over for a while and see what I can come up

with.

 

Thanks