Tmax 100 reciprocity

[Chuck_P]

I want to venture into the area of reciprocity exposures but have never really done it before. I use Tmax 100 and although I have the reciprocity table from Kodak for TMX, I have a question about the table. How do you determine the adjustment if the indicated exposure time is between 1 sec and 10 sec, say it's 6 sec, or between 10 sec and 100 sec, say it's 60 sec..................is there a method to calculate that or is it just a WAG, a wild ass guess?

 

[koraks]

You could make a plot; put the exposure time as a logarithmic scale on the x-axis and then plot the compensation on the y-axis. Connect the dots in a smooth line. That way you have a fairly convenient look-up device where you can just locate your exposure time on X and then work out approximately how much compensation you need by finding the intercept with the plot.

I personally wouldn't bother, however, and just give +2/3 for 60 seconds based on the fact that it's somewhere in-between 10 and 100. It's not going to make a real life difference to try and be more accurate than that. Heck, I might even give +1 for good measure or just +1/2 if I feel that missing out on a tiny bit of shadow detail wouldn't hurt.

Keep in mind that reciprocity compensation also increases contrast in the negative. You compensate for the shadow areas, but the highlights (especially in a contrasty scene) won't be needing as much compensation. As a result, you're giving them more exposure than you might realize. Especially on a film like TMAX100 and with the purpose of either scanning or printing on VC paper I wouldn't worry too much about this.

This contrast issue tends to become a problem with alt. process printing, where you'd have to compensate for the reciprocity failures in the shadows, but then the highlights run amok, so you try and reduce development, but that in turn requires a bit more exposure to get sufficient shadow detail...and before you know it, you're in a catch-22 situation where the exposure time essentially becomes infinite and the required development time approaches zero...

Anyway, like I said, don't worry about this for regular printing/scanning processes and scenes that aren't too extreme in terms of both contrast and reciprocity failure. Besides, if you're using TMX, you won't be doing alt. process printing directly from the negatives anyway.

 

[Ian C]

Without more detailed information about the relationship between the metered time and the correction in stops, I’d construct a simple linear model for each time interval in the table. They should give a reasonable estimate of the exposure correction required.

Using (t, Δ) to represent a point containing time t and Δ to represent the difference in stops, we have points (1 second, 1/3 stop) and (10 seconds, 1/2 stop). Using linear regression, we get

Δ(t) = 0.0185185t + 0.3148148 for t within the interval from 1 to 10 seconds

At t = 1 sec

Δ(1 sec) = 0.0185185(1 sec) + 0.31414 = 1/3 stop

and at 10 seconds,

Δ(10 sec) = 0.0185185(1 sec) + 0.31414 = 1/2 stop

So, for a time of t = 6 seconds, we get

Δ(6 sec) = 0.0185185(6 sec) + 0.31414 = 0.4259 stop


For metered times from 10 seconds to 100 seconds,

Δ(t) = 0.0055555t + 0.44444

Δ(10 sec) = 0.0055555(10 sec) + 0.44444 = 1/2 stop and

Δ(100 sec) = 0.0055555(100 sec) + 0.44444 = 1 stop

At 60 seconds, the model predicts

Δ(60 sec) = 0.0055555(60 sec) + 0.44444 = 0.78 stop

As noted in post #2, since this is negative film and the time intervals are relatively small, the exposure correction isn’t overly fussy. For most uses, the model in this post (post #3) is overkill, but it can be done if wanted.

 

[Doremus Scudder]

FWIW, here's my reciprocity table for TMX that I made shortly after the new TMX and TMY were introduced. It's based on Howard Bond's tables and adjusted with my testing. It may be a good starting point for you. Note that I base my exposures on shadow values.

Indicated -------- Adjusted
Exposure --------- Exposure
--- 1 sec ----------- 1 sec
--- 2 sec ----------- 2.5 sec
--- 4 sec ----------- 5 sec
--- 8 sec ----------- 10 sec
--- 15 sec ---------- 21 sec
--- 30 sec ---------- 50 sec
--- 1 min ----------- 2 min
--- 4 min ----------- 12.5 min
--- 8 min ----------- 37 min
--- 15 min ---------- 100 min
--- 30 min ---------- 250 min
--- 60 min ---------- 720 min

The really long adjustments (after 4 minutes) I have from a publication from a group on astrophotography. They seem to work well, but likely could be refined.

Hope this helps,

Doremus

 

[DREW WILEY]

It's also related to what color contrast filters might be in place, since they might differentially affect long exposures. But the difference within the little window between 1 and 10 seconds is almost negligible. And according to my experience, what Doremus just posted looks appropriate, at least up to 30 sec or so. At one time, the original version of TMax was available coated on astronomical glass plates, so there might be some hard data somewhere adaptable to very long exposures (along with actual testing of course, since the emulsion has somewhat changed).

 

[MattKing]

Thread title typo tweaked

 

[Doremus Scudder]

... I might also add that erring on the side of overexposure with long exposures provides a good safety buffer. Keep in mind that overexposing a 30-second exposure by 15 more seconds (and the associated reciprocity failure) barely gets you a 1/3-stop overexposure.

Best,

Doremus

 

[Chuck_P]

Thank you all for those detailed responses.

 

[Lee L]

I posted some charts here that include TMX: https://www.photrio.com/forum/threads/reciprocity-misbehavior.6192/post-83997
These are based on the same data from Howard Bond that Doremus just posted.

 

[Alan Edward Klein]

Slightly off topic., What are the adjustments for Tmax 400?

 

[Doremus Scudder]

My TMY reciprocity failure correction times:

Indicated -------- Adjusted
Exposure --------- Exposure
--- 1 sec ----------- 1 sec
--- 2 sec ----------- 2.5 sec
--- 4 sec ----------- 5 sec
--- 8 sec ----------- 11 sec
--- 15 sec ---------- 22 sec
--- 30 sec ---------- 50 sec
--- 1 min ----------- 1.75 min
--- 2 min ----------- 4 min
--- 4 min ----------- 9 min
--- 8 min ----------- 21 min
--- 15 min ---------- 45 min
--- 30 min ---------- 120 min
--- 60 min ---------- 300 min

Slightly better than TMX at the longer exposures.

Best,

Doremus

 

[Alan Edward Klein]

Thanks.

 

[Rich Ullsmith]

You guys are funny. Any error in metering (and there always is error) is magnified on a logarithmic scale. If the metering is off 1/4 stop, then the calculated exposure will be off by minutes. Yes, WAG is a reasonable approach with pinhole. It helps to have multiple cameras with the same pinhole diameters.

 

[Doremus Scudder]

Well, reciprocity failure needs to be corrected for somehow. Having a table helps get one in the ballpark at least. Even with a metering error, some correction is better than none. And, one can improve their metering technique with experience and err on the side of overexposure when in doubt.

Doremus