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 Time lapse and a maths problem 
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Joined: Mon May 09, 2011 5:36 am
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Post Time lapse and a maths problem
A few years ago, I did some tidal time lapse by the coast and I really liked the interval time I chose – the speed of the incoming tide was just right. Unfortunately, I didn't write down what interval time. Today, I'm going to do some more tidal time lapse from the exact same spot and I would really like to know the exact time interval I used last time. Ive dug up the original Raw files from the last tidal shoot and I note that they indicate what time each photo was taken. Though determining the time interval from the individual times of the photos is not straight forward. I admit that I am hopeless at maths so that doesn't help!

If anyone here is decent at maths, Ive listed the times of the first 30 shots in the older time lapse sequence to see if someone can determine what time interval (in seconds) was used.

1. 9.37pm
2. 9.37pm
3. 9.38pm
4. 9.39pm
5. 9.40pm
6. 9.40pm
7. 9.41pm
8. 9.42pm
9. 9.43pm
10. 9.43pm
11. 9.44pm
12. 9.45pm
13. 9.46pm
14. 9.46pm
15. 9.47pm
16. 9.48pm
17. 9.49pm
18. 9.49pm
19. 9.50pm
20. 9.51pm
21. 9.52pm
22. 9.52pm
23. 9.53pm
24. 9.54pm
25. 9.55pm
26. 9.55pm
27. 9.56pm
28. 9.57pm
29. 9.58pm
30. 9.58pm

Btw, I know it seems unusually late in the evening to be shooting the tide (judging from those times) but the time was not set correctly on the camera. Even so, it should still be indicative of what time interval was used.

Edit - also if it helps, there is a total of 219 photo files. First photo was taken at 9.37pm and the last photo was taken at 12.20am.


Sat May 16, 2015 7:35 pm
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Post Re: Time lapse and a maths problem
I'm going to say 1 frame every 45 seconds - it has to be more than 30 as you have two consecutive minutes where only one frame is taken.

Timt

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Sat May 16, 2015 10:16 pm
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Post Re: Time lapse and a maths problem
Easy - it's 42 seconds...

1. 58 - 37 = 21 minutes total.

2, 21 x 60 = 1260 seconds total for 30 frames.

3. 1260 / 30 = 42 seconds per frame


Sat May 16, 2015 11:22 pm
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Post Re: Time lapse and a maths problem
Thanks guys. That certainly puts my maths skills to shame.

A tidal chart on a weather website indicates 2.57pm at 'High' for today's date. Ive used this same website before and I seem to recall that 'High' is when the tide starts to come in, not the peak of high tide like I would expect. I also remember from previous occasions that the tide would start to come in about an hour after the specified time indicated in the website. Additionally, the area that I want to photograph receives water about half an hour after the tide begins to come in. So based on all that, I went out there this afternoon (at about 2.48pm) and to my surprise, it was high tide! Huh? Almost everything was submerged. And then to top it off, some guy was fishing in the exact spot where I wanted to photograph....! Sunny days are so rare at the moment....I hope I get another one soon.


Sun May 17, 2015 12:43 am
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Location: UK
Post Re: Time lapse and a maths problem
Good luck on your next outing. Also take into account British Summer Time when reading your tide chart.

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Sun May 17, 2015 7:21 am
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Post Re: Time lapse and a maths problem
MikeA wrote:
Also take into account British Summer Time when reading your tide chart.


Hopefully, I can make use of that information here in Australia!


Sun May 17, 2015 7:14 pm
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Post Re: Time lapse and a maths problem
tribal-warrior wrote:
MikeA wrote:
Also take into account British Summer Time when reading your tide chart.


Hopefully, I can make use of that information here in Australia!


Then you will need to take UCT into consideration.... LOL.

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Sun May 17, 2015 11:43 pm
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Joined: Thu Mar 10, 2011 11:38 pm
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Location: Exmouth, Western Australia
Post Re: Time lapse and a maths problem
High tide is when the tide is as 'in' as it's going to get and just starting to go out.

Your best bet is to find the nearest fishing/boating shop and get a copy of the local tide tables. The experts in the shop should be able to tell you the variance between the times on the table and the times for other places nearby.

Kit


Mon May 18, 2015 5:41 am
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Post Re: Time lapse and a maths problem
i was with pixelbot it had to be longer than 30 secs and shorter than 60 and magic lantern goes up in five second increments on my canon :-)
thanks to geraldft for the maths lesson otherwise i still be a little off
might have to write that equation down somewhere :-)


Mon May 18, 2015 6:06 am
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Post Re: Time lapse and a maths problem
I don't mean to be contentious, especially since I am new here, but I believe that the interval would be a little more than 42 seconds; while there are 30 times listed, there are only 29 intervals. If you use the edited information that there were 219 shots (and therefore 218 intervals) in 163 minutes you would get:

163 x 60 = 9780 seconds.
9780 / 218 = 44.86 seconds
Basically 45 seconds


Tue May 19, 2015 11:19 am
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Post Re: Time lapse and a maths problem
Nyea Heh... Nerd snigger. You are correct - I overlooked the distinction between intervals and frames. Divided by 29 gives 43.45 seconds. Of course this too is an approximation because we don't have accurate second times - it assumes the first exposure occurred right at the beginning of the first minute and the last exposure occurred at the end of the last minute. For instance if the first exposure was 15 seconds into the first minute and last exposure 15 seconds before the end of the period, then we take off 30 seconds and get 1260 - 30 = 1230 then / 29 = 42.41

Also during the length of shooting, temperature changes may have effected the time intervals, as well as inaccuracies in the camera. Hence while the overall average time interval may be a little higher, this is still the best estimation for this particular section...

Not that any number between 40 and 50 is going to make a big difference to the result... ;)


Thu May 21, 2015 12:43 am
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Post Re: Time lapse and a maths problem
float or int for 2 years projects??? ;-))


Thu May 21, 2015 1:56 am
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Post Re: Time lapse and a maths problem
kruppTown wrote:
float or int for 2 years projects??? ;-))


Too much camera movement with float ;)

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Thu May 21, 2015 3:11 am
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