Applying Filter Factor to Time.

[Bruce Osgood]

Suppose you want to shoot a scene at f-16 with a filter added, regardless of how long the shutter must be open.
Your meter indicates an f-16 exposure would be 3 seconds.
Your filter has a factor of 2 so you factor the filter and learn you need 1 more f-stop of exposure.... but you want to keep the f-16.
The answer is 6 seconds but how do you plot an equation of 1 (factor) X 3 (seconds) = 6 (seconds)?

Thanks,

 

[spijker]

2^1(stop) x 3 = 6 ("^" = to the power)
or
2 (factor) x 3 = 6

"1" is not the factor but the number of stops. 2 is the factor.

factor = 2^number_of_stops
2^0 = 1, so zero stops is a factor of 1 (= no change)

Hope this helps

 

[David A. Goldfarb]

And if you adjust the time to compensate for the filter, don't forget reciprocity after accounting for the filter factor.

 

[Bruce Osgood]

Spijker
Head of the class. Thank you very much. It helps a great deal

And David thanks for the reminder.

I'm trying to put together a sheet for my Tablet where things are calculated accurately rather than scratching my head trying to remember what I forgot.

 

[David A. Goldfarb]

Post it when you've got it set up and working, and I bet some others could use it too.

 

[nsurit]

Although I haven't use it, this seems like it might be helpful. https://itunes.apple.com/us/app/reciprocity-timer/id459691262?mt=8

I wonder if there are other iPhone apps to solve this problem. My guess is there are others out there in APUG land who use an iPhone. I'm not app crazy, however I do have a light meter app on my phone which I do use. Bill Barber

 

[David A. Goldfarb]

Ike Eisenlord had a reciprocity calculator called "Film Timer" which also included various recommendations for development adjustment based on reciprocity compensation to adjust contrast, but I don't see it listed currently.

 

[ic-racer]

Simple numbers to remember that most darkroom workers know:
add one stop = original time x 2
add 1/2 stop = original time x 1.4
add 1/3 stop = original time x 1.26

To subtract a stop then just divide by the above numbers.

For example, the original time is 2 milliseconds, to add 1/2 stop, multiply by 1.4 to get 2.8 milliseconds = 1/357 of a sec.

 

[DREW WILEY]

Not all pan films have the same kind of "pan" sensitivity, so filter factors aren't set in stone. If it's something I need a calculator or tablet for,
it's getting too complicated for spontaneity in the field. At long exposures, the game changes in any number of ways. Save your smart phone
for what it does best - skipping on a pond just like any other nice flat piece of detritus. The most I've ever needed was to simply write the filter
factor (or in my case, EV factor) on the filter box. ... don't even look at that anymore. Easy enough to remember.

 

[Doremus Scudder]

Not all pan films have the same kind of "pan" sensitivity, so filter factors aren't set in stone. If it's something I need a calculator or tablet for, it's getting too complicated for spontaneity in the field. At long exposures, the game changes in any number of ways. Save your smart phone for what it does best - skipping on a pond just like any other nice flat piece of detritus. The most I've ever needed was to simply write the filter
factor (or in my case, EV factor) on the filter box. ... don't even look at that anymore. Easy enough to remember.

Amen!

Plus the fact that light sources and the colors in your subject are rarely the same from shot to shot... Filter factors are approximate anyway; just read the manufacturers' tech sheets.

I'm going to advocate taking meter readings through the filter. Yes, this adds the mismatch of meter spectral sensitivity to the mix, but has the advantage of allowing one to really spot mergers that may not be readily apparent.

Problems with spectral response really only show up with extremely strong filters. E.g., with a red #25 filter, I'll add more exposure than the meter says, because I know it's reading up to a stop low when just seeing red light, and, for some films, I'll reduce development: this latter because the film itself yields more contrast with just red light than white. Some films are fine, some need a bit more... Testing for this is relatively straightforward and, once done, allow you to forget factors completely and just use your spot meter. That's what I do.

FWIW, anyone shooting with a TTL meter and a small/medium format camera usually does this too. Since there aren't a lot of caveats about the practice out there, I imagine even without testing and compensation factors, it works just fine most of the time.

However, I do use my smart phone for a number of things - telephoning, dictionary, Google, maps, etc., but never in the field when photographing except to call my wife to come pick me up I use flat stones for skipping across the pond... not so many sharp corners.

Best,

Doremus

 

[Andrew O'Neill]

Not all pan films have the same kind of "pan" sensitivity, so filter factors aren't set in stone.

So true. I have my own factors for TMY-2 and HP5+. Some development alteration may also be required, depending on the filter used.

 

[Xmas]

If it is more than deep yellow Id bracket in two stop intervals, then one shot will be less than 2 stops out.

Simpler than keeping film spectral responses in mind and remembering to swap film box top memos.

 

[BradS]

to apply a filter factor to time, simply multiply. So, if you meter reading says f/16 @ 1/125 seconds and yiu want to stay at f/16 then multiply time by filter factor.... (1/125) * 2 = 2/125 ~ 1/60

 

[Alan Klein]

And if you adjust the time to compensate for the filter, don't forget reciprocity after accounting for the filter factor.

But once you Add time to correct for reciprocity by adding time instead of aperture, you have longer time so the reciprocity isn't right again. I could never get a straight answer to this.

 

[Xmas]

But once you Add time to correct for reciprocity by adding time instead of aperture, you have longer time so the reciprocity isn't right again. I could never get a straight answer to this.

If you increase time into the next reciprocity zone then expect underexposure.

Using aperture to adjust for failure is simplest if the reciprocity is changing quickly. Otherwise iterate. calculations.

If your meter has not got a good calculator for compensation you may need to make one up paper stuck to plastic.
You need a flash light to work 'dark' anyways.

Film with fast varying is difficult to work with as any metering error is magnified. A curse with ULF as bracketing is another dark slide or two...

 

[DREW WILEY]

Filters not only affect reciprocity in terms of the sheer density of the filter, but can behave differently at long exposures than in more typical applications. In other words, the filter factor itself can change with respect to film at long exposure. And different kinds of film respond differently in this respect. Most people aren't aware of this fact. So if you're using strong contrast filters for long exposure work, test and test
some more. The rules change.