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Inverso

Get it on Google Play

If you are not using Android, don’t be sad! A web version is now also available at inversoweb.printerattic.com/

Inverso is a free Android app geared towards photographers using large format cameras and medium format cameras using focusing bellows. Although various apps already exist to aid the photographer in making exposure adjustments, I didn’t find one that suited my work flow best.

The app currently features two tools that are integral to the work flow of using a technical view camera / field camera: determine exposure corrections and accurately time the new exposure time.

Exposure correction

In contrast to other apps available on the market, Inverso computes exposure times that are corrected for bellows extension, filter factor and reciprocity failure not only for continuous light, but also for flash illumination and combined illumination.

Timer function

The timer function is designed with photographers in mind and by default features a delayed start so that you don’t have to start the timer and push the shutter release at the same time. In addition to this an optional audible signal can be given at the start and end of the exposure time.This serves as an extra reminder for very long exposure times and potentially increases accuracy for relatively short exposure times, as recent research has shown that response times are lower for audible cues.

User manual

Using Inverso is very straight forward. This manual will quickly get you started and also explain the features under the hood. Let’s get started.

1. Setting the illumination mode

Three illumination scenarios are supported: continuous light, continuous + flash and flash. The first time you open the app, ‘continuous light’ will be set by default. The default can be changed in the preferences as will be shown later. You can toggle through the modes by repeatedly tapping the illumination mode button (Figure 1).

Figure 1: the main screen. Here exposure corrections are calculated.

In ‘continuous light’ mode, Inverso will determine the corrected exposure time based on the measured exposure time, the filter compensation factor, the focal length and the bellows extension, and the reciprocity failure of the selected film stock. In ‘flash’ mode, the illumination is assumed to be dominated by the flash light source. Because the flash duration is typically much shorter than the exposure time, the exposure is controlled by changing the flash power or by changing the aperture. In ‘continuous + flash’ mode, a combination of an adjusted exposure time and a change in flash power/aperture is suggested. The changed exposure time is shown in the default location, while the additional changes are shown in a dialog at the bottom of the screen.

Whenever possible, a change in flash power is preferred. If this is not possible, tap on the ‘OPEN APERTURE’ button in the dialog. A change of the aperture will affect both the ambient illumination and the flash illumination. Therefore, Inverso calculates the new ‘measured’ exposure time that corresponds to the opened aperture to balance the ambient illumination.

2. Setting the measured exposure time

Type the measured exposure time in seconds in the designated text field. Exposure times shorter than 1 seconds can by typed in as the corresponding fraction. To do so, hit the toggle in front of the text field so that it reads ‘1/ ‘. For example, 1/4 seconds can be entered by typing a 4 in the text field, when the fraction toggle is enabled. For reference, see Figure 2.

Figure 2: Shutter speeds shorter than 1 second can be entered as fractions as shown here.

3. Enter the filter compensation

Enter the filter compensation factor expressed in f-stops in the designated text field. Filter manufacturers use different designations to express the filter compensation factor on their products. Common are the filter factor on a linear scale, the optical density (log10) and the filter factor in f-stops.  For reference, you may use the values in Table 1 as a starting point. Always check with the manufacturer for the correct values for your product.

Table 1: Typical filter compensation factors in f-stops.
Filter Compensation (f-stops)
0.3 ND 1
0.6 ND 2
0.9 ND 3
circular polariser 1.67
yellow filter 0.33
green filter 1.33
orange filter 0.67 – 1
red filter 2

4. Set the focal length and the bellows extension

Light falls off with with the square of the distance between the rear nodal point of the lens and the film plane. For most lenses, the rear nodal point is located close to the center of the lens. In practice, it suffices to measure the distance between the lens board and the film plane when the lens is focused to infinity (Figure 3). This gives the physical focal length of the lens. For rear-focus or telephoto lens designs, this distance can differ significantly from the effective focal length that is indicated on the lens barrel. Refocus the lens to bring the subject into focus. Remeasure the distance between the lens board and the film plane. This distance is the bellows extension.

Figure 3: Measure the distance between the center of the lens and the film plane.

5. Choose the film stock

Each film stock responds differently to long exposures. While for exposure times between 1/1000 s and 1 s the density change of the film to exposure dose can be considered linear, for extremely short and longer exposure times this linear relation is no longer valid and we speak of reciprocity failure. These non-linear responses have to be determined through experimentation, but starting points are typically provided by the manufacturers in tabular form. Inverso uses interpolation functions to accurately represent these relations which are accurate up to 1/3rd stop in the worst case, but are typically within 1/10th stop. In practice this should not pose any problems. Warnings will appear when the accuracy of the interpolation can no longer be guaranteed.

The data published by the manufacturers is not always accurate. These may be for older versions of the emulsion (as is the case for Kodak T-MAX 400), or be very generic (as is the case for all Ilford films). Therefore, Inverso relies on data published by distinguished and experiences workers in the field. In the table below the sources of the data are cited, including a notion of the to be expected accuracy.

Table 2: Sources for reciprocity failure relations.
Film stock Source Note
Kodak T-MAX 100 Howard Bond Considered the best data available online for this specific film.
Kodak T-MAX 400 Howard Bond Considered the best data available online for this specific film.
Kodak Tri-X 320 Howard Bond Considered the best data available online for this specific film.
Kodak Portra 160 LF Forums Found to be accurate by forum users.
Kodak Portra 400 LF Forums Found to be accurate by forum users.
Kodak Ektar 100 eyeintheworld Accuracy unknown. According to Courter’s procedure. Data wanted.
Ilford SFX 200 Harman Technical Service / David Abberley Single parameter power law. Preliminary data of unknown accuracy.
Ilford Delta 100 Harman Technical Service / David Abberley Single parameter power law. Preliminary data of unknown accuracy.
Ilford Delta 400 Harman Technical Service / David Abberley Single parameter power law. Preliminary data of unknown accuracy.
Ilford Delta 3200 Harman Technical Service / David Abberley Single parameter power law. Preliminary data of unknown accuracy.
Ilford Pan F+ Harman Technical Service / David Abberley Single parameter power law. Preliminary data of unknown accuracy.
Ilford FP4+ (Gainer) Patrick Gainer Considered a better set than that published by Ilford. Additional testing may be required.
Ilford HP5+ (Ilford) Harman Technical Service / David Abberley Single parameter power law. Preliminary data of unknown accuracy.
Ilford HP5+ (Bond) Howard Bond Considered the best data available online for this specific film.
Ilford (other) Ilford Curve provided by Ilford for all their films. Okay starting point, but known to be faulty. Will be replaced as soon as possible.
Fuji Neopan Acros Fujifilm Acros requires very little correction.
Fuji Velvia 50 F295.com Data gives data points between those made available by Fujifilm.
Fuji Velvia 100 Fujifilm Based on four data points provided by Fujifilm. Corrections are minor up to 4 minutes.
Fuji Provia 100F Christopher Courter Based on test under daylight and moonlight.
Fuji Provia 400X Fujifilm Based on four data points provided by Fujifilm. Corrections are minor up to 2 minutes.
Fuji Superia 200 Fujifilm Based on four data points provided by Fujifilm.
Fuji Superia X-TRA400 Fujifilm Based on four data points provided by Fujifilm.
Fuji Superia X-TRA800 Fujifilm Based on four data points provided by Fujifilm.
Fuji Superia 1600 Fujifilm Based on four data points provided by Fujifilm.
Fuji Pro 160NS Fujifilm Based on two (!) data points provided by Fujifilm. Needs further testing.
Fuji Pro 400H Fujifilm Based on thee (!) data points provided by Fujifilm. Needs further testing.
Foma Fomapan 100 Classic Foma Based on four data points provided by Foma.
Foma Fomapan 200 Creative Foma Based on four data points provided by Foma.
Foma Fomapan 400 Action Foma Based on four data points provided by Foma.
Foma Retropan 320 Soft Foma Based on four data points provided by Foma.
Rollei Retro 100 TONAL Maco-Direct / Rollei Based on six data points provided by Rollei. Unknown reliability.
Rollei Infrared 400 Maco-Direct / Rollei Based on six data points provided by Rollei. Unknown reliability.
Rollei RPX 25 Maco-Direct / Rollei Based on five data points provided by Rollei. Unknown reliability.
Bergger Pancro 400 Bergger Based on three data points provided by Bergger. Unknown reliability.
Kentmere K100 Harman Technical Service / David Abberley Single parameter power law. Preliminary data of unknown accuracy.
Kentmere K400 Harman Technical Service / David Abberley Single parameter power law. Preliminary data of unknown accuracy.

Nota bene: when the exposure rounding is set to ‘Auto’, the exposure times are rounded to the closest available shutter time with either an overexposure or an underexposure bias. For positive material (slides), Inverso biases the exposure towards underexposure, while for negative material, the bias is towards overexposure.

6. Tap ‘Adjust!’ to correct the measured exposure time

This is when Inverso works its algorithms to determine the exposure corrections taking into account the light source, the measured exposure time, the filter compensation, the bellows extension factor and the reciprocity failure of the film. The corrected exposure time is displayed above the button. For exposure times shorter than 1/500 s, the unrounded exposure time is shown and a warning is displayed. The user is warned that high intensity reciprocity failure is not corrected for. For exposure times between 1/500 s and 1 s, the exposure time is rounded depending on the selected rounding preference. For shutter speeds longer than 1 second, the exposure time is rounded to 1 decimal place.

Exposure rounding

Large format lenses typically feature shutter times of 1 s, 1/2 s, 1/4 s, 1/8 s, 1/15 s, 1/30 s, 1/60 s, 1/125 s, 1/250 s and for the smaller shutters either 1/400 s or 1/500 s. To allow exposure times to be set to one of the available settings, Inverso has 5 different behaviors for rounding the corrected exposure time.

Overexpose

In this mode the exposure times are bias towards overexposure. When using negative material, slight overexposure is favored such that shadow detail is retained. Highlights can easily be recovered when the film is scanned or in the wet darkroom by burning in. The exposure error can be at most 1 stop.

Underexpose

For positive material, the behavior is reversed and underexposure is favored to avoid blowing out the highlights. The exposure error can be at most 1 stop.

Closest

In this mode the exposure time is rounded to the closest exposure time available on the lens. The exposure error can in this case be at most 1/2 stop.

Do not round

The name says already says it. The unrounded exposure time is displayed and the choice is left to the photographer to round in either direction or change the aperture or illumination in order to end up precisely at one of the available shutter speeds.

Auto

In automatic mode, the film stock type determines the behavior. For negative material the rounding is bias towards overexposure, while for positive material the rounding is biased towards underexpose.

Aperture or flash power corrections

When using flash illumination or a combination of continuous light and flash, Inverso will suggest to either increase the flash power or open the aperture. These corrections are rounded in steps of 1/3 stop. If a change in aperture also affects the continuous light exposure, the new exposure time is automatically calculated.

Figure 4: an expert summary is given upon tapping on the calculated exposure time.

Exposure correction details

If you tap the calculated time, a small expert summary (Figure 4) will be shown that indicates the individual corrections that are included. The factor is given in linear scale, while the value between brackets is the equivalent in stops. This information may be useful is you wish to not change the exposure time, but only compensate for bellows extension by opening the aperture.

7. Time it!

Figure 5: Timer function of Inverso in portrait mode.

If the exposure time is longer than one second, you will have to switch to either T or B mode on the lens and countdown the exposure time yourself. To aid in this, the ‘Time it!’ button will take you to the built-in timer as shown in Figure 5. When you hit the start button, the timer will start either directly or with a delay. The shown exposure time is unrounded.

Figure 6: Delayed start of the timer.

Pushing both the timer button and the shutter release at the same time can be cumbersome and lead to unaccurate timings. Therefore, a delayed start of 3 seconds is set by default (Figure 6). The delayed start can be switched off completely and the delay time can be changed via the app preferences. Optionally, a warning sound is played at the beginning and at the end of the exposure time. This can be switched on or off by toggling the ‘Silent/Beep’ button on the bottom left of the screen. When the delayed start is toggled on, a countdown will also be shown in the last few seconds of the countdown. If the beeps are switched on, beeps will be given for every full second to indicate that the exposure time is ending soon (Figure 7). This feature was added after beta testing and was found to be useful for very long exposures.

Figure 7: Countdown timer for long exposures.

8. Changing the app preferences

Via the menu that is available on the main screen, you can go into the app preferences. Here you can change the default rounding behavior (overexpose, underexpose, closest, do not round and auto), change the default illumination mode (continuous light, continuous + flash or flash) and the default film. In addition, the timer can be configured to either use a delayed start and set the corresponding delay time in seconds.

Figure 8: Inverso in night mode. At the bottom of the screen the quick help is visible.

The main screen by default shows a light grey bar in the bottom. When you drag this bar up, the quick help is shown. This short summary, gives you the most important information required to arrive at a correct exposure time. If you do not need this quick help, you can choose to hide if by default by switching it off in the app preference.

In long exposure times in the dark, you do not want any stray light of your phone screen to mess with the exposure. Therefore a night mode is available which changes the color scheme to a darker theme as shown in Figure 8.

Questions, suggestions or bugs to report?

If you have questions or suggestions to improve Inverso, please drop me a line! Found a bug? Please let me know. Include a detailed description of what triggered the unexpected behavior, the version of Inverso you are using and your Android version.

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