B&W film reciprocity calculations

[spoolman]

Hello All:I'm finding the exposure calculations for reciprocity failure for most films to be not very accurate.Is there a formula or calculation that can be used to more accurately pinpoint the adjusted exposure time for most black and white roll and sheet films.

Thanks,

Doug

[Ray Heath]

g'day spool

why do you think the calculations are not very accurate?

what result do you get?

what result do you want?

Ray

[gainer]

Originally Posted by spoolman

Hello All:I'm finding the exposure calculations for reciprocity failure for most films to be not very accurate.Is there a formula or calculation that can be used to more accurately pinpoint the adjusted exposure time for most black and white roll and sheet films.

Thanks,

Doug

They're like women: all the same but a heck of a lot different, thanks be.

The reciprocity behavior of all the films for which I have seen data fall on parallel straight lines on log-log graph paper. That is the way they are both all the same, yet all different. Look in www.unblinkingeye.com for an article called "LIRF is Lurking at your F-Stop" by some guy named Gainer. LIRF stands for "low intensity reciprocity failure."

The fact that all their lines are parallel means that if you know accurately one point on the line you can draw the chart for all the others. The slope of the line is 1.62 on log-log graph paper.

[Ian Grant]

Your best bet is download the film manufacturers data sheets. They list the reciprocity adjustments needed. There isn't a common calculation that holds good for all films,Tmax films behave quite differently to conventional films.

A lot can also depend on the accuracy of you light meter in low light levels.

Ian

[Lee L]

Here's a 111 post thread on many things related to reciprocity, some of it regarding the accuracy of manufacturer's data and formulae, including Gainer's method: http://www.apug.org/forums/forum37/1...sbehavior.html

Lee

[gainer]

The problem is usually in trying to write an equation that describes the entire exposure instead of the part that differs from what the meter reads. If the equation only describes the additional exposure needed to correct for reciprocity "failure" then even the TMax and Ilford T-grain films are parallel on log-log graph paper.

[Peter Williams]

You can always just shoot Fuji Acros and stop worrying about reciprocity forever.

[rob champagne]

if you need to refer to reciprocity charts then it means that you are working in low light or night time. When you are doing that, the way you think about your image changes. For example, a deep deep shadow placed on zone II or III will probably be way brighter in the result than it was in reality. So, if you want to retain the low light or night time appearance of the scene in your final result, it is better to expose for the highlights and let the shadows fall where they fall. Also only adjust for reciprocity based on your highlight values.

So meter something you want on a zone VII or VIII and work out the exposure to place it on that zone and then if that exposure is into reciprocity, adjust for that time.

I have found that this works really well using HP5+ published reciprocity figures.

If you meter a deep shadow and try and place it on zone II or III you will invariably be making it too light and by doing so you are giving it too much exposure and the exposure required to do that is a long way into reciprocity and when you correct for that, you push the hightlights off the end of the curve. So meter and expose for highlights when doing low light/night time photography and use manufacturers recommended figures for reciprocity correction.

To recap: Your judgement about zones or placement of values in low light conditions is screwed by your subconcious perception of how things should be on film based on daylight conditions. Metering and exposing for highlights remedies this(mostly).

[Nicholas Lindan]

Originally Posted by spoolman

Hello All:I'm finding the exposure calculations for reciprocity failure for most films to be not very accurate

Reciprocity failure is proportional to light intensity. There is more failure in the shadows than in the highlights and so the contrast will increase. You need to meter and reciprocity correct the shadows. Developing for the highlights then has to compensate for both the scene luminance range and the lessened reciprocity failure in the highlights.

A film that is well characterized for reciprocity failure will give a list of exposure time increases and corresponding reductions in development time. This will be based on the standard 7 stop range and different subjects will require different treatment.

The three factors for success are experimentation, experience and luck. Pretty much like the rest of life.

[keithwms]

I discovered some time ago that the astronomers used to trade very reliable and handy reciprocity tables.

Anyway look here:

http://mkaz.com/photo/tools/reciprocity.html

(which has several links at the bottom. N.b. the Fuji 64T in this chart is not the current 64T which has somewhat better recip. characteristics)

At some point I just put a bunch of these on my phone. I decided it was easier than working out a way to run a calculation on the thing.

Incidentally, I use reciprocity charts for night stuff but more often for macro. Take 64T to 5:1 indoors and you quickly see why (even though it has quite remarkable reciprocity).