Quick question about reciprocity

[thegman]

Hi all,
Let's say I'm shooting Astia for long exposures, and my exposure time is 2 minutes. The data sheet says to add 1/3rd of a stop to correct for reciprocity failure.

So if I wanted to correct with exposure time rather than aperture, would that be 2 minutes and 20 seconds? As the last stop getting to 2 minutes, would have been an increment of 1 minute? And 20 seconds is 1/3rd of a minute?

Thanks

Garry

 

[Philly]

And 20 seconds is 1/3rd of a minute?

Thanks

Garry

That means that for a two minute exposure you should add 40 seconds.

 

[charjohncarter]

I would check the reciprocity chart for Astia, because I believe that you have to add more time when the exposures start to get longer than 10 minutes. At least that is the way it is with some other films.

 

[Roger Hicks]

That means that for a two minute exposure you should add 40 seconds.

That's my reading too.

Cheers,

R.

 

[thegman]

Great, that's easy to work out, it's basically 1/3rd or 2/3rds or whatever of the time you've already calculated. Astia was just an example as I happened to be reading about it. I'll probably end up using Velvia, and my exposures will be < 1 min I expect.

Thanks to all.

Garry

 

[degruyl]

Just to be clear, I am reading : http://www.fujifilm.com/products/professional_films/pdf/astia_100f_datasheet.pdf on Page 2, the note about "Long and Multiple Exposures" says that you should also use a filter 5b to color correct for the reciprocity failure. In addition to the 1/3rd stop aperture correction. (Or time, as was mentioned).

 

[thegman]

Yes, I may have to pick up one of those filters, shame all my lenses are different size filter threads and one (the 15mm CV) does not have one at all!

 

[not_in_good_order]

If you are open to using a film other than Astia, Provia 100F requires no corrections until you go beyond 128 seconds.

 

[thegman]

Yes, Velvia 100 too I believe, I've got some Velvia 50 in the fridge to use, which I think needs correction after 2 seconds, so that'll give me some practice. Next time I'll get Velvia 100, I can't tell the difference between that and Velvia 50.

 

[Dwig]

... In addition to the 1/3rd stop aperture correction. (Or time, as was mentioned).

No, its 1/3 stop aperture correction. If you attempt to correct by extending the time the calculation gets much more complex.

In the example quoted earlier in the thread, a 1/3 stop increase of a 2 minute (120 second) exposure would be an additional 40 seconds or 160 seconds total. This addition exposure time would require even more correction for reciprocity failure requiring a further increase (~10-15sec?). That additional increase would require an even further increase. In theory this repeats indefinitely, but in practice one additional iteration is enough.

 

[degruyl]

No, its 1/3 stop aperture correction. If you attempt to correct by extending the time the calculation gets much more complex.

In the example quoted earlier in the thread, a 1/3 stop increase of a 2 minute (120 second) exposure would be an additional 40 seconds or 160 seconds total. This addition exposure time would require even more correction for reciprocity failure requiring a further increase (~10-15sec?). That additional increase would require an even further increase. In theory this repeats indefinitely, but in practice one additional iteration is enough.

He's right, of course. (unless you are looking at the kodak datasheet for tri-x, for example, which states "use +3 aperture or 1200 seconds" and -30% developing time, when the exposure is metered at 100 seconds.)

I wanted to write something witty about Zeno's paradox, but apparently I failed.

 

[zenza]

I wouldn't stress too much. When shooting in the minutes at night, a mere 10-20 seconds isn't that big of a deal. I usually shoot for overexposure and the negs/slides come back looking perfect. In you're case I'd try to stay between 2 and 3 minutes and not worry much beyond that.

 

[charjohncarter]

When I use Efke IR820, the reciprocity factor is so cumulative as Dwig pointed out, that the film almost self exposes. After 5-7 seconds you can almost just leave it for whatever. I would not try that with color or a Tmax film, but one like Efke IR 820 which has the worst reciprocity factor I've ever seen you can hardly miss, as long as you go long: (but be careful that there are not too many highlights (IR) as you are now dealing with two reciprocity factors). Shadows and highlights (or in Efke IR 820's case foliage) react to reciprocity differently, I suppose that is why you have to change your developments.

 

[angeloks]

No, its 1/3 stop aperture correction. If you attempt to correct by extending the time the calculation gets much more complex.

In the example quoted earlier in the thread, a 1/3 stop increase of a 2 minute (120 second) exposure would be an additional 40 seconds or 160 seconds total. This addition exposure time would require even more correction for reciprocity failure requiring a further increase (~10-15sec?). That additional increase would require an even further increase. In theory this repeats indefinitely, but in practice one additional iteration is enough.

Let's say you have a 120 seconds exposure. You'll add 120*1/3 to correct the reciprocity failure, but, as you said, you'll have to correct for that additional time : it's a geometrical series.

From Wikipedia :

120 + 120*1/3 + 120*1/3*1/3 + ... = 120/(1-1/3) = 180

If you wanted to correct for 2/3 of a stop, you'd replace the 1/3 by 2/3 and get 360 seconds. If the correction is more than a stop, the serie will diverge...

 

[thegman]

Maybe I'll just correct with aperture instead!

I'll probably try both in reality, I just wanted to understand what precisely 1/3rd of a stop meant, if it was a 1/3rd of the last stop or the next stop if you see what I mean. I understand now. At least I think I do.

 

[Juan Valdenebro]

Because of the few stops of tonal range in slide film, and because bracketing with long exposures can be long and boring, you should test a roll before the real shots... Pick low light places and bracket some slow scenes both using aperture and speed: a few shots for 1 second, a few for 2 and 5 seconds, for 10 and 30, for 1 and 2 minutes... You'll master the thing after one single roll, and during the real shots you won't need to guess or bracket a lot...

Cheers,

Juan

 

[thegman]

My shots will be taken 20 mins from my flat, so it's no big deal if they don't turn out great, I can go back any time I like. I know negative film like Ektar would make it easier, but I like Velvia too much..