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  #21  
Old 23rd February 2021, 01:08 PM
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10 seconds to 11 seconds is approx 1/7th of a stop (10s + 1/7th-stop = 11.04s). 1/6th of a stop would be 11.2s so 11s is 0.2s out, which is some silly fraction of a stop. [OK, I had to calculate it - it's 1/39th of a stop at 11s].

I believe that repeatability is more important than absolute precision (up to a point!) so as long as 10s + 1/6th of a stop always sets 11 seconds (which I am sure it does) the fact that it is 1/7th of a stop over 10s rather than 1/6th I feel, is less important.
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Originally Posted by JOReynolds View Post
Exposure times < 10 sec are displayed as decimal fractions, for example 1.0, 1.1, 1.2, 1.4.... Above 10 sec they are 11, 12, 14, 15, 17, 20, 22.... The actual rounding error is never more than 1.2%, which is negligible in photographic terms.

Another advantage of logarithmic setting is that the whole timing range, from 1.0 to 99 sec, takes just over 2 revolutions of the knob.

I'm 76 and I have to admit that it took me a while to get the hang of log scaling. I understand from SDS that several schools are using Timer 3 in their darkrooms and that they learn to use log time setting from the start.

Just for clarity, guys, because the numbers you’ve given don’t tally in my head.
When making a test-strip at 1/6 intervals manually above 10sec. and the exposures are rounded up/down to the nearest whole, what will the deviation be between each of the neighbouring test-strip exposures?
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Last edited by MartyNL; 23rd February 2021 at 01:16 PM.
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  #22  
Old 23rd February 2021, 01:54 PM
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I’ve had a go and there are some noticeable differences, relatively, between the exposures of the two systems. However, a calculation still needs to be made about the difference between each of the test-strip exposures on the timer 3 side, in terms of f-stops.
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Old 23rd February 2021, 02:53 PM
JOReynolds JOReynolds is offline
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I’ve had a go and there are some noticeable differences, relatively, between the exposures of the two systems. However, a calculation still needs to be made about the difference between each of the test-strip exposures on the timer 3 side, in terms of f-stops.
Which two systems do you mean?
Your spreadsheet is interesting but does not indicate a meaningful error in the timings. Timer 3 is a instrument intended for photographic printing, not a chronograph. In tests with two classes of sharp-eyed sixth-form students I discovered that none of them could distinguish the density of two patches with 5% exposure time differences on Multigrade lV with a grade 5 filter. But a few correctly identified patches exposed 10% (1/6 stop) differently.

Although I could dust off my Macbeth densitometer and repeat the experiment with an old Omron industrial timeswitch scaled in milliseconds, I don't think it would prove anything.
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  #24  
Old 23rd February 2021, 03:15 PM
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Which two systems do you mean?
Your spreadsheet is interesting but does not indicate a meaningful error in the timings. Timer 3 is a instrument intended for photographic printing, not a chronograph. In tests with two classes of sharp-eyed sixth-form students I discovered that none of them could distinguish the density of two patches with 5% exposure time differences on Multigrade lV with a grade 5 filter. But a few correctly identified patches exposed 10% (1/6 stop) differently.

Although I could dust off my Macbeth densitometer and repeat the experiment with an old Omron industrial timeswitch scaled in milliseconds, I don't think it would prove anything.
The left figures are f-stop times as printed as a table in way beyond monochrome book to aid printers without an f-stop timer.
The figures on the right represent the rounding up/down of the f-stop times from the table.

Surely the 1/6 separation between test-strip exposures would be gone?
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Old 23rd February 2021, 04:08 PM
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Just think of "log scaling" as another term for f/stops but applied to time instead of aperture. Add 1 stop = twice the time, add 1/2 a stop = 1.4 times the time etc.

Also, if people want buttons, point them at the FADU F-Stop Timer android app - it has all the (virtual) buttons anyone could want - across three screens . Sometimes, simplicity is welcome.
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Old 23rd February 2021, 05:05 PM
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Sorry Marty, I'm not 100% sure I follow you. I am not familiar with the timer (and I had to stop watching the video because of the very annoying "music").

To take the absolutely worst-case scenario, consider you wanted to set 10.5s for some reason. You would have to set 11s as your starting value: this is an error of 0.5s which is 1/15th of a stop from 10.5s to 11s. I suspect this is not detectable by the human eye in practice.

The f-stop values you have in your chart, the corresponding nearest whole second and the error in stops are as follows:

Chart-Value => RH-Value => Error in stops
10.1 => 10 => -1/70th
11.3 => 11 => -1/26th
12.7 => 13 => +1/30th
14.3 => 14 => -1/33rd
16.0 => 16 => 0
18.0 => 18 => 0
20.2 => 20 => -1/70th

0.3 of a second might sound like a lot, but it isn't much at all when you consider it as a percentage of 11.3, 12.7 or 14.3 seconds.
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Old 23rd February 2021, 05:28 PM
JOReynolds JOReynolds is offline
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Default Accurate exposure

Thanks, Bob. I am afraid that the discussion was getting a bit silly.
The most significant source of errors in printing is the aperture setting of well-used enlarger lenses. The aperture ring usually has detents - click positions - at the main positions (F/4.0, F/5.6, F/8.0 etc). But these wear with use.
I was checking the calibration of an Ilford 500 head on a de Vere enlarger with an old 80mm Rokkor, with a Maplin Lux meter. I found I could not return to a consistent brightness on stop-down after focusing because there was so much 'slop' in the mechanism. Even if I made sure that the aperture ring was always set clockwise, viewed from below, the indicated Lux was very variable. Yet exquisite B&W prints have been made since Victorian times, with all these limitations and without log-scaled digital timers.
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Old 23rd February 2021, 05:59 PM
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It has to do with the difference between the tones on a test strip.
I believe that test strips produced with f-stop times to 1/10 of a second, will be visibly more evenly and equally spaced, than test strips produced to f-stop times rounded up/down to the nearest second.
It also begs the question that if the unit of a tenth of a second was redundant in practice, then why include them in the table in the first place? Why not just round off all of the numbers above 10 seconds, up or down, to the nearest second?

Hopefully that's a bit clearer?
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Old 23rd February 2021, 06:39 PM
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It has to do with the difference between the tones on a test strip.
I believe that test strips produced with f-stop times to 1/10 of a second, will be visibly more evenly and equally spaced, than test strips produced to f-stop times rounded up/down to the nearest second.
It also begs the question that if the unit of a tenth of a second was redundant in practice, then why include them in the table in the first place? Why not just round off all of the numbers above 10 seconds, up or down, to the nearest second?

Hopefully that's a bit clearer?
OK - I see what you are saying, but the figures I have calculated suggest that that is not the case in practice.

Yes, armed with a densitometer you will measure more accurate density increments using intervals rounded to the nearest 0.1s instead of the nearest 1s. But in practice, I do not believe you will see any difference - you cannot see a 1/26th of a stop difference in exposure even under ideal conditions and excluding all the other variables in development, emulsion batch characteristics, mains voltage fluctuations, etc. It's easy enough to do the experiment and see.

This was at the heart of my earlier observation that I will take repeatability over accuracy (within reason) any time. It makes no difference if it is 11.3 seconds or 11.0 seconds as long as I always get the same value for the same configuration. 1/26th of a stop is so far within reason for me that there is no contest.

I didn't write that table, but I can tell you why the FADU F-Stop Timer uses 1/10th second display accuracy when you increment by 1/12th of a stop, or by 1 whole stop for all display times - because making it not do that would just add extra programming steps. Sometimes people do things because it's simpler than not doing them (especially me ).

Last edited by Bob; 23rd February 2021 at 06:44 PM.
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