Xpose is a tool for calculating long exposure times in photography. It is particularly usefull when using ND-filters, or when shooting film where you have to take reciprocity failure into account.

Created by Vebjørn Beinnes december 2022, and sporadically updated.

You are welcome to send questions or feedback to xpose.feedback@protonmail.com, and every effort will be made to be of assistance.

The long exposure times calculated by Xpose are intended for guidance only. While every effort has been made to ensure the accuracy of the calculations, perfect results cannot be guaranteed. Please use your own judgement when setting your camera's exposure time, and be aware that there may be other factors that can affect the outcome of your photo. This is particularly true for film photography, where there are a lot of factors affecting the final image, like the developing process, the age of the film, reciprocity failure etc.

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First, you measure the amount of light in your scene using the light meter in your camera or an external light meter.
Then, you set the sliders under *metered settings* to the values the camera or light meter suggested. Next,
set your preferred settings for the actual long exposure with the sliders under *long exposure settings*. Based on these values,
Xpose calculates an appropriate exposure time for your long exposure shot. If you already have a specific
exposure time in mind, you can tweak the other settings until the *calculated exposure time* matches that particular exposure time.

Exposure value, or EV, corresponds to the amount of light in your scene. The higher the number, the brighter the scene.
The EV is calculated as follows, using the aperture value *f*, the shutter speed *ss*, and the *iso* from the metered settings:

EV = log_{2}(f^{ 2}) + log_{2}(1/ss) - log_{2}(iso/100)

Based on the *EV* value from the metered settings, and the aperture value *f2* and *iso2* of your preferred
long exposure settings, Xpose calculates an appropriate exposure time *ET* using the following formula:

ET = (25 * f2^{ 2}) / (2^^{ EV-2} * iso2)

If you are using an ND-filter for the long exposure, the exposure time is doubled for each stop of the ND-filter
(given that the ND-filter was not used for metering the scene). Using the stop-value *n* of the ND-filter, and the already
calculated *ET*, we find the new exposure time *ET _{nd}* compensated for the ND-filter using the following formula:

ET_{nd} = ET * 2^{ n}

If you are shooting on film, exposure times longer than a few seconds typically need to be further extended. Unlike digital camera sensors, film has a variable sensitivity to light. The longer it is exposed to light, the less sensitive it becomes. Each kind of film reacts differently to long exposures. Some are heavily affected and need two or three times the amount of light for exposures longer than a couple of seconds, while other film stocks are barely affected even when exposing for several minutes.

Xpose uses either formulas for reciprocity failure provided by the film manufacturers, or mathematical formulas created to approximate reciprocity failure data from available tables or charts. Please note that the film manufacturers typically only provide information about reciprocity failure compensation for exposure times up to a couple of minutes. Calculated times longer than that are estimated from the available data, and are decreasingly accurate the longer they are. However, this is helped by the fact that it gets decreasingly important to be precise with the exposure time the longer it gets (when shooting film).

Also, note that when shooting color film, the separate color layers of the film might be affected differently by reciprocity failure, which could result in color shifts in the final image. You can use different color filters in front of the lens to counteract this effect.

Below are the formulas Xpose uses to calculate the extended exposure times for each film stock.
The same variables are used in all of the formulas: *T* is the calculated exposure time you would use with a digital camera, while
*T _{c}* is the extended exposure time compensated for reciprocity failure.

Ilford SFX

T_{c} = T^{ 1.43} for T > 1 second

Ilford Pan F+

T_{c} = T^{ 1.33} for T > 1 second

Ilford D100

T_{c} = T^{ 1.26} for T > 1 second

Ilford D400

T_{c} = T^{ 1.41} for T > 1 second

Ilford D3200

T_{c} = T^{ 1.33} for T > 1 second

Ilford FP4+

T_{c} = T^{ 1.26} for T > 1 second

Ilford HP5+

T_{c} = T^{ 1.31} for T > 1 second

Ilford XP2

T_{c} = T^{ 1.31} for T > 1 second

Ilford K100

T_{c} = T^{ 1.26} for T > 1 second

Ilford K400

T_{c} = T^{ 1.30} for T > 1 second

Kodak Portra 160

T_{c} = 0.87 * T^{ 1.358} for T > 2 seconds

Kodak Portra 400

T_{c} = 0.87 * T^{ 1.358} for T > 2 seconds

Kodak Ektar 100

T_{c} = 1.1 * T^{ 1.22} for T > 1 second

Kodak Ektachrome E100

T_{c} = (T + 1)^{ 1/0.97} - 1 for T > 1 second

Kodak T-MAX 100

T_{c} = 1.15 * T^{ 1.12} for T > 1 second

Kodak T-MAX 400

T_{c} = 0.067 * (T + 15.3)^{ 1.825} - 9.7 for 1s < T < 50 s

T_{c} = T^{ 1.24} for T ≥ 50 seconds

Kodak TRI-X

T_{c} = 1.8 * T^{ 1.42} for T > 1 second

Fujifilm Velvia 50

T_{c} = 0.8 * T^{ 1.26} for T > 3 seconds