Reciprocity and Bellows Extension
Reciprocity
Usually, exposure time and the amount of light have a pretty stable relationship, and your lightmeter can tell you that relationship helping you get the best exposure for your image.
However, this all goes out the window when your exposure time goes above one second. You can no longer go by the time given on your light meter and have to figure out another exposure time. The only trouble is, there's no relationship anymore. There's no correlation or formula between the amount of light and the exposure time and so it's damn near impossible to work out how much time you need, and each film reacts differently.
Luckily, there are apps you can use to help with this. You can input your film type and what you lightmeter is telling you do do, and it will work out your new exposure time. The app is simply called 'reciprocity' and is available for both android and apple.
When you've applied reciprocity, you should develop your film at N-1 as the light tones can be affected by this.
Bellows Extension
Bellows extension is a factor you need to apply when your bellows move beyond a focus on infinity. This is when the standards are far apart, often used when taking images of close ups.
Because the bellows are longer, the light to the film takes that little bit longer too, and you could end up underexposing. To stop this, we must make a calculation to see how we substitute the extra light needed.
First we must measure the distance between the front and back standards in mm, and know how far your focal length is.
The formula is as follows;
bellow extension ² / focal length ² = a factor
for example, you have a bellows extension of 250mm and a focal length of 150mm
250² / 150² =
62500 / 22500=
x2.7 (round up to 3)
Each factor of 1 = a half a stop.
So, with a factor of 3 like above, 1/2 x 3 = 1 1/2 stops to increase the exposure by.
You could change the exposure time rather than the aperture if you wish to keep the same depth of field.
This should be calculated before reciprocity.
Usually, exposure time and the amount of light have a pretty stable relationship, and your lightmeter can tell you that relationship helping you get the best exposure for your image.
However, this all goes out the window when your exposure time goes above one second. You can no longer go by the time given on your light meter and have to figure out another exposure time. The only trouble is, there's no relationship anymore. There's no correlation or formula between the amount of light and the exposure time and so it's damn near impossible to work out how much time you need, and each film reacts differently.
Luckily, there are apps you can use to help with this. You can input your film type and what you lightmeter is telling you do do, and it will work out your new exposure time. The app is simply called 'reciprocity' and is available for both android and apple.
When you've applied reciprocity, you should develop your film at N-1 as the light tones can be affected by this.
Bellows Extension
Bellows extension is a factor you need to apply when your bellows move beyond a focus on infinity. This is when the standards are far apart, often used when taking images of close ups.
Because the bellows are longer, the light to the film takes that little bit longer too, and you could end up underexposing. To stop this, we must make a calculation to see how we substitute the extra light needed.
First we must measure the distance between the front and back standards in mm, and know how far your focal length is.
The formula is as follows;
bellow extension ² / focal length ² = a factor
for example, you have a bellows extension of 250mm and a focal length of 150mm
250² / 150² =
62500 / 22500=
x2.7 (round up to 3)
Each factor of 1 = a half a stop.
So, with a factor of 3 like above, 1/2 x 3 = 1 1/2 stops to increase the exposure by.
You could change the exposure time rather than the aperture if you wish to keep the same depth of field.
This should be calculated before reciprocity.
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