Depth of Field 2

Expose
9-Jan-2005

Introduction

Expose is a program for photographers that computes recommended exposure settings based on film speed, f stop or shutter speed, filter, subject matter and lighting. It can operate in either aperture or shutter priority mode as selected by the A and S buttons on the command bar. In aperture priority mode, you enter an aperture and Expose computes and displays the corresponding shutter speed; in shutter priority mode, you enter a shutter speed and Expose computes the aperture.

Expose incorporates information from a number of standard exposure guides regarding the optimal exposure for a wide range of situations. Although Expose displays recommended exposures with a precision of a tenth of stop, these numbers are only intended as a rough guide to correct exposure. For the best results bracketing your exposures by up to 2 stops in either direction, especially for difficult subjects, is strongly recommended.

While most modern 35mm cameras have built-in exposure meters, many larger format cameras do not. Even if you have a meter, it is still useful to have a reference you can use to confirm its readings. Furthermore, there are many situations that are very difficult to meter such as photographing the full moon, sunsets and night scenes, or high contrast subjects -- for these situations, more consistent results can often be obtained by bracketing based on standard exposure guidelines. Adjusting these standard settings for film speed, filters, and other factors can be tricky, and you may not want to carry a printed exposure guide around with you, and this is what makes Expose such a handy tool.

ISO

Tap the drop down box to select your ISO film speed. Expose remembers this and all other settings you entered the last time you used it.

Aperture

In aperture priority mode, tap the drop down box to enter your aperture f stop; Expose will compute and display the shutter speed (see below). An exposure compensation in stops and tenths of stops may also be displayed next to the f stop to indicate an exposure correction to be applied. You can apply this correction with your camera's exposure compensation dial; positive values indicate that increased exposure is required.

To set aperture priority mode, tap the A button on the command bar.

Shutter

In shutter priority mode, tap the drop down box to enter your shutter speed; Expose will compute the f stop (see above). An exposure compensation in stops and tenths of a stop may also be displayed next to the f stop to indicate an exposure correction to be applied. You can apply this correction with your camera's exposure compensation dial; positive values indicate that increased exposure is required.

To set shutter priority mode, tap the S button on the command bar.

In either aperture or shutter priority mode, Expose computes displays the EV (Exposure Value) number next to the shutter speed. Each EV number represents all the equivalent combinations of f stop and shutter speed for a given film speed -- lower EV numbers correspond to darker scenes. For example, for ISO 100 film, EV 3 stands for f2.8 at 1sec., f4 at 2sec., f5.6 at 4sec., and so on. The EV value displayed is computed based on the scene brightness level, the film speed, and any filter and reciprocity failure corrections..

Filter

Select the name of the filter you are using, if any. The required exposure correction is reflected in the recommended exposure settings. You can customize the list of filters by editing the file Filters.txt (see below).

Film

Select the film you are using -- this information is used to compute exposure corrections for very long exposures based on tabulated reciprocity failure corrections. Selecting the Generic entry at the beginning of the list disables this correction. You can customize the list of films by editing the file Reciprocity.txt (see below).

Level

This setting lets you specify an overall scene illumination level. Levels are displayed as a graduated set of 21 small rectangles ranging from white (level 0) to black (level 20), with white corresponding to the brightest illumination level (full sun) and black representing the darkest (partial moonlight). Simply tap the rectangle for the level you want to select, or you can drag the stylus across the rectangles to scan for the desired level. Each time you change levels, the Scenes control (see below) is updated to display a list of scenes typical of the level you have selected, and the f stop or shutter speed is automatically recomputed and redisplayed.

Scenes

Each time you select a new level, a list of scenes typical of that level of illumination is displayed here. You can customize the list of scenes by editing the file Levels.txt (see below).

Customizing Exposure

You can customize both the list of filters, films, and the list of scenes to suit your individual needs. The filter list is contained in the text file Filters.txt; the film reciprocity data is contained in the text file Reciprocity.txt; and the scene data is contained in the text file Levels.txt. All of these files must must be located in the same folder as the Exposure program, Exposure.exe.

Filters.txt consists of one line per filter up to a maximum of 100 filters. Each line starts with the name of the filter in quotes and is followed by a positive integer representing the number of stops of extra exposure necessary to compensate for using the filter. Enter the filter correction as a decimal number.

Reciprocity.txt consists of one line per filter up to a maximum of 100 films. Each line starts with the name of the film in quotes and is followed by a series of up to 16 exposure times (in seconds) and exposure corrections (in stops). These values should be entered as decimal numbers.

Levels.txt consists of one line per exposure level up to a maximum of 40 levels. Each line contains scene information for one brightness level (each level is one full stop darker than the previous level). The first line always corresponds to the brightest level, namely full sunlight.

Changes in these files are not reflected until the next time you start Expose.

Additional Tips for Photographing Tricky Subjects

Lightning at Night

To compute the recommended aperture using Exposure, select shutter priority mode and set the shutter speed to 8 seconds; then locate the scene brightness level for lightning (about 2/3 of the way to the right) and a good starting f stop will be displayed.

With your camera on a tripod and the shutter set to Bulb, make a time exposure that includes one or more lightning flashes.

Safety Tips

If the thunder comes less than five seconds after a lightning flash, it is striking less than a mile away and you should quickly leave any exposed position.

Never shoot from under a tree, from high ground, or near large metal objects or power lines.

Using a non-metallic tripod, wearing insulated shoes, and other similar measures do not provide any protection from being struck by lightning.

Shooting from inside a car using a window clamp as a camera support is relatively safe. Shooting through an open window from inside a building is also relatively safe.

Fireworks at Night

To compute a recommended aperture using Exposure, select shutter priority mode and set the shutter speed to 8 seconds; then locate the scene brightness level for fireworks (a little more than half of the way to the right) and a good starting f stop will be displayed.

With your camera on a tripod and the shutter set to Bulb, make a time exposure that includes one or more bursts.

Star Trails and Meteor Showers

Stars rotate around a point in the sky near the north star at a rate of roughly 15 degrees per hour.

For long exposures (e.g. 1-2 hours), use slower film (e.g. ISO 200) as high speed film has a tendency to fog from ambient light.

For short exposures (e.g. 5-10 minutes), use faster film (e.g. ISO 1000) as low speed film will not register.

Use a tripod and shoot with your shutter wide open.

A good foreground can make your photograph much more dramatic.

The Moon

The following is courtesy of Michael Myers (http://www.mhmyers.com) - please see his web site for more detailed recommendations:

The brightness of the moon varies with its phase -- the brightest part of a quarter moon is not nearly as bright as the brightest part of a full moon.

The exposure varies considerably with the elevation of the moon in the sky. Whatever the phase, the moon is brightest at its maximum elevation, and is roughly two stops dimmer just after it rises or before it sets, depending on atmospheric conditions.

Except for a full moon, the brightness varies from the bright outer edge to the dark shadowed side and you cannot properly expose all of it.

Stained Glass from the Inside

Add 3 stops to the outdoor exposure.

Glassware in Windows

Add 1 stop to the outdoor exposure.

This program may be redistributed free of charge as long as the entire program and its accompanying help file are copied intact, including this notice.

Jonathan Sachs
12 Ash Street
Cambridge, MA 02138

jsachs@dl-c.com
http://www.dl-c.com