FilterCompute is an iPhone app for photographers who need a simple and fast method to perform difficult filter exposure calculations in the field. There are two types of calculations for which FilterCompute excels: Exposure difference between two parts of a scene, and exposure compensation calculation for a high density filter.
Scenario 1: Select the correct graduated filter
You’re looking at a scene with mountains and clouds. You want to know the exposure difference between the mountains and the clouds to even out the illumination with one of your graduated filters.
Select the camera icon on the FilterCompute screen. Aim the focus area on the clouds, press Exposure 1. Then aim at the mountains, press Exposure 2. The screen gives you the exposures and the number of stops between them.
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You’re done! You know the filter strength, and you can just quit the app since you don’t need to use the calculator.
Scenario 2: Find the exposure time for a 10-stop filter
You’re looking at a 20′ waterfall. You have your tripod, and you want to blur the motion of the water with a dark filter.
Select the camera icon on the FilterCompute screen. Aim the focus area on the waterfall, press Exposure 1. You’re not taking an exposure difference, so just press Save to transfer the exposure to the calculation screen.
Tap the entry field at the top of the screen under the label Filter compensation factor. Enter the filter factor you want to use (say it’s 10), and press return. The Deficit field, which is highlighted in red, now shows 10. This means that you need to adjust one or more exposure parameters to achieve a zero deficit so that the camera will produce the correct exposure with the 10-stop filter.
Now tap any one of the three exposure parameters (ISO, fStop, or exposure time) to adjust the Deficit field. You can either tap the parameter’s +/- buttons, or enter a new value directly in the text field. When you decrease the ISO, the deficit increases, which means you need to increase the exposure time even more (this is why you have a tripod and a 10x filter, after all). Although the default is to adjust fStops (or fStop equivalents) in 1.0 intervals, you can choose 1/3 or 1/4 intervals for finer tuning.
Once you achieve a zero deficit, you can take your picture with the 10x filter mounted, and have an excellent chance of taking a shot that will at least be close to the exposure you had hoped for. The whole computational process probably took less time than the final exposure time for the shot!
Go to the Apple Store for FilterCompute.
FilterCompute Free
A free version of FilterCompute is available for more limited calculations:
- Displays ads
- Can’t take exposure readings with the built-in camera (you need to manually enter the exposure data)
- No fractional fStops