Right, so the information below is regarding exposure times for a 5pence camera given to myself by a fellow pupil. I started by working out the focal length of the pinhole, by using the formula: FL = SP x SP x Type of Light. Then I moved on to the F-stop, of which requires the formula of: FL/DofP=FS, then once I gained the F-stop number in this case it was 150. I went through the table below ( F-stop Table) and rounded it up to F180, at the time I was taking a photograph in side so the timings would be the highest to let the most amount in through the pinhole. Then after a conversation with a Photography techinichan I learnt of a certain thing called a false exposure, It can be added If you choose so it just requires a little more calculation. False exposures are due to the film being used at a different speed to which it was manfactured for. Again another formula is required:
Exposure x extraTime= False Exposure, so using the false exposure table and a bit of estimation, I came up with 2 possible exposure times for 1 image, 4-8mins & 42min. Then I wanted to take another Image this time outside, so I used the formula's again and calculated 2 new exposure times for an Image in Hazy Sunshine, 1-2 mins & 3 mins.
Exposure times as a starting point.
Focal length/Diameter of the Pinhole = the F-stop
Metric ( mm ) formula for green light (550 nm)
Focal length = size of pinhole x size of pinhole x 750
So say the Pinhole is 0.4
120mm = 0.4mm x 0.4mm x 750
or an ideal length of 120mm is found.
Constants for other types of light and colors:
Daylight (560nm) - 750
Blue (450nm) - 934
Green (550nm) - 763
Red (650nm) - 647
Infrared (750nm) - 561
My 5pence Pinhole camera has 3 Pinholes of the size of 0.2
0.2 x 0.2 x Daylight - 750 = focal length of 30
Focal length of 30/Diameter of pinhole 0.2 = f-stop 150
F-stop Table
(f150)
f128 f180 f256
sun-15s sun- 30s sun- 60s
haze-30s haze- 1-2min haze- 3-5min
cloudsun-1min cloudsun- 2-4min cloudsun- 6-10min
cloudy&inside- 2-4min cloudy&inside- 4-8min cloudy&inside- 15-30min
In this case f150 is in-between f128 & f180
The weather at the moment is Cloudy, 150 is closer to 180.
New f-stop = f180 at cloudy&inside at 4-8mins
False exposure
However, due to the Film being used at a different speed to which it was manufactured for. The film needs more time exposed to the light. This is called a false exposure.
Exposure : 4-8mins at f180
The formula for this is:
Exposure x either 2 or 6 = False Exposure
False exposure table
1-2mins = exposure x 2
5-6mins = exposure x 6
(there are more, don't worry :D )
4-8 isn't a Number, such a shame.. so for this we'll go half way and take a guess at 7mins.
7 x 6 = 42 mins
Thats 2 tests of speed and time.
4-8mins & 42 mins
Recalculation moment
Now I wanted to take a second Image out side to test the exposures, and contrasts, at the point the light outside is Hazy Sunshine.
f180 at 1-2mins
1.5mins x 2 = 3mins
Again 2 tests:
1-2mins & 3mins
Here is an example of the calculated exposure times using a pinhole camera.
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