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iangreenhalgh1
Joined: 18 Mar 2011
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Expire: 2014-01-07
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 Posted: Sun Apr 14, 2013 2:16 pm Post subject: Calculating exposure for long film exposures |
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iangreenhalgh1 wrote:
Hi folks
I want to try shooting some waterfalls on 6x9 and was wondering how to calculate the exposure. Let's say the metering of the scene gives me 1 second at f32. If I add a ND4 filter, that is cutting the light by 4 stops, so, is it correct that this will turn the 1 second without filter into 16 seconds? If I added a second ND4 filter would this make the exposure 256 seconds?
Film has a reciprocity characteristic, do I need to factor this in after calculating the exposure?
I take it I need to find the reciprocity figures for the particular film I'm using? I will be using Ilford FP4, anyone know how to calculate the reciprocity failure for fp4?
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iangreenhalgh1
Joined: 18 Mar 2011
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| Posted: Sun Apr 14, 2013 2:23 pm Post subject: |
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iangreenhalgh1 wrote:
I found this, seems quite simple:
| Quote: |
If you wish to calculate a corrected exposure time based on the ILFORD chart, use the formula
Ec=Em^1.48
Where Ec is the corrected exposure, and Em is the measured exposure, in seconds. Measured exposures of one second or less do not require any compensation.
Note that this formula is based on the chart. The chart was determined from experimentation. Times calculated past the chart should be good estimates, but are not based on ILFORD experiments.
David Carper ILFORD Technical Service |
So, if I'm working this out right, I just multiply my measured exposure time by 1.48.
Let's say the metered time was 1 sec at f32, add on two ND4 filters to cut 8 stops, that gives 256 secs. 256 x 1.48 = 379sec, which is 6.3 minutes.
Hmm, looks like I can just open the shutter and leave it for a while, won't be needing a stopwatch if the exposures are going to be on the order of several minutes.
Am I calculating this right?
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berraneck
Joined: 24 May 2009
Posts: 972
Location: prague, czech republic
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| Posted: Sun Apr 14, 2013 2:29 pm Post subject: |
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berraneck wrote:
hi ian
at first, ND4 filter is 2 stops correction, not 4. you can simply use the NDx number to recalculate exposure directly. that means 2x ND4 filters = 4 stops = 16x longer time
every film has reciprocity failure. I would trust Ilford datasheet:
http://www.ilfordphoto.com/Webfiles/2010712125850702.pdf
if you have 1s exposure -> that is 16s with 2xND4 -> that is ~60s final exposure time including reciprocity failure
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iangreenhalgh1
Joined: 18 Mar 2011
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| Posted: Sun Apr 14, 2013 2:46 pm Post subject: |
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iangreenhalgh1 wrote:
Aha, many thanks, so the ND number is for calculation, rather than denoting the number of stops, that is good to know, would have completely ruined my calculations not knowing that! Two filters is 4x4 so 16, so as you say, my 1 second becomes 16 secs. 16 x 1.48 would give me 24 secs.
So it sounds to me like I should bracket a few exposures to see what I get. Perhaps 20, 30 and 40 seconds?
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berraneck
Joined: 24 May 2009
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| Posted: Sun Apr 14, 2013 3:14 pm Post subject: |
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berraneck wrote:
well I used this chart from datasheet and it gives me different exposure time. I think 1.48x prolongation is too few for correct exposure with standard emulsion.
if you intend to use bracketing I would go with 3exposures: -1EV, measured, +1EV
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sichko
Joined: 20 Jun 2008
Posts: 2475
Location: South West UK
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| Posted: Sun Apr 14, 2013 3:45 pm Post subject: |
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sichko wrote:
Ian, the caret (^) means "raise to the power of" not "multiply". So, in your example 16^1.48 = 60.5, wheras multiplying gives a lower value : 16*1.48 = 23.7.
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rawhead
Joined: 09 Feb 2009
Posts: 1525
Location: Boston, MA
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| Posted: Sun Apr 14, 2013 8:26 pm Post subject: |
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rawhead wrote:
not every reciprocity failure is equal, though. E.g., Fuji Acros (B&W) and Provia 100F (E6) have so little failure that you don't need to take it into consideration at all up to 15 - 30 minutes.
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iangreenhalgh1
Joined: 18 Mar 2011
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| Posted: Sun Apr 14, 2013 10:59 pm Post subject: |
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iangreenhalgh1 wrote:
Aha, I see, it's logarithmic so I need to plot a graph and print it out to refer to in the field. The 1.48 figure came from Ilford Technical Support, it is for fp4.
Does anyone know how to plot a graph in software? Or perhaps a table would be better?
Maybe Ilford already has a graph for fp4?
To bracket +/-1EV, I would halve or double the time, right? For instance, 10, 20, and 40 secs?
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sichko
Joined: 20 Jun 2008
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Location: South West UK
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| Posted: Sun Apr 14, 2013 11:43 pm Post subject: |
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sichko wrote:
| iangreenhalgh1 wrote: |
| Does anyone know how to plot a graph in software? |
Do you have a copy of Microsoft Excel ? If not, what about Open Office (free !) ?
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iangreenhalgh1
Joined: 18 Mar 2011
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| Posted: Mon Apr 15, 2013 1:49 am Post subject: |
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iangreenhalgh1 wrote:
I have Open Office, never done much with spreadsheets though.
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rawhead
Joined: 09 Feb 2009
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| Posted: Mon Apr 15, 2013 4:03 am Post subject: |
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rawhead wrote:
| iangreenhalgh1 wrote: |
Aha, I see, it's logarithmic so I need to plot a graph and print it out to refer to in the field. The 1.48 figure came from Ilford Technical Support, it is for fp4.
Does anyone know how to plot a graph in software? Or perhaps a table would be better? |
http://fooplot.com/
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Sony α7R, Pentax 67II, Kiev-60, Hasselblad 203FE, 903SWC, Graflex Norita 66, Mamiya M645 1000s, Burke & James 8x10, Graflex Pacemaker Speed Graphic (4x5 and 3x4), Century Graphic (2x3), R.B. Graflex Seried D, Rolleiflex SL66E, Rolleiflex 2.8C Xenotar, Mamiya C330f, a few M42, six P6, three OM, four Hasselblad, two Pentax 67, two Mamiya 645, one Noritar, and a sprinkle of EF. Oh, and an Aero Ektar and Leica Noctilux |
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