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On a MacBook computer running OSX, is there an app I can use to create a digital photo file with an exposure of EV -2 ?

I'm looking for something that will simulate the bracketing function of a camera but in an app.

I plan to make a set of photo files with the following EV levels using EV 0 as the base photo:

EV -4, EV -2, EV 0, EV +2 and EV +4

Thanks.

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I'm sure there a many options. One of them would be:

http://rawtherapee.com/downloads

If you not afraid of CLI tools you can look in to something like:

http://www.fmwconcepts.com/imagemagick/exposure/index.php (that is based on imagemagic)

  • Thanks for such a quick response! I'm going to look for rawtherapee tutorials on YouTube to see if it will allow me to make the exposure set. Thanks again. – Emad-ud-deen Jul 28 '16 at 16:37
  • rawtherapee is what I was looking for. It allows me to set the exposure level to -2. – Emad-ud-deen Jul 28 '16 at 22:41
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EV -2 or any other Exposure Value value is not a particular brightness or illuminance level, it is a particular combination of sensitivity (normalized at ISO 100), aperture, and shutter time used to expose a photo. As such, you can't really create a photo "with" an EV of -2. You can only create a photo "using" EV -2.

Two photos can be taken at the same Exposure Value and one could be totally dark while the other might be very bright. It all depends on how much light is in the scene during exposure. Exposure Value is an expression of the mechanics used to capture the light, not an expression of the quantity of the light captured.

Let's use the old rain bucket analogy. Imagine you set out several rain buckets of various dimensions but with the same total volume. Those with larger diameters would equate to wider apertures. Those with more depth would equate to a longer shutter times (because it would take them longer to collect the same volume of water).

If one bucket has twice the surface area at the opening (the diameter would be 1.414 or square root of 2 times larger than the smaller bucket) we could call it f/2 @ 1 minute. Since the other bucket is twice the height we could call it f/2.8 @ 2 minutes. If both buckets are left out under identical rates of rainfall they would both have the same Exposure Value because they would both collect the same amount of rainfall with each combination of opening size and time left in the rain. The wide one would take one minute to fill, the narrow one would take two minutes to fill. Thus both are the same Exposure Value.

But let's say when we put out the narrower bucket for two minutes it's barely sprinkling. The bucket will only have a trace amount of water in the bottom which would equate to a very dark photo. When we later put out the wider bucket for one minute it's during a heavy thunderstorm. The bucket will be almost full which would equate to a very bright image. Yet we used the same EV for both!

The rate at which the rain falls is not an Exposure Value, it is an Illuminance Value or Luminance Value.

From the Wikipedia entry for Exposure Value:

Strictly, EV is not a measure of luminance or illuminance; rather, an EV corresponds to a luminance (or illuminance) for which a camera with a given ISO speed would use the indicated EV to obtain the nominally correct exposure. Nonetheless, it is common practice among photographic equipment manufacturers to express luminance in EV for ISO 100 speed, as when specifying metering range (Ray 2000, 318) or autofocus sensitivity.And the practice is long established; Ray (2002), 592) cites Ulffers (1968) as an early example. Properly, the meter calibration constant as well as the ISO speed should be stated, but this seldom is done.

  • EV -2 could be a 'relative' exposure value. If you have one photo at a particular exposure, you could create another at 2 stops darker or brighter than that. – vclaw Jul 29 '16 at 7:34
  • I would contend that is an EC -2 (exposure compensation) or even EB -2 (exposure bracketing), not EV -2. – Michael C Jul 29 '16 at 12:35
  • I suspect that the question is rooted in a conflation of exposure value with light value. kenrockwell.com/tech/ev.htm Because EV=LV at ISO 100 and because discussion of Sunny 16 normalizes ISO and often involves light values it is easy to see how such a conflation might be easy to make. – user50888 Jul 30 '16 at 17:45
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    For EV=LV at ISO 100 to be true it must be qualified with if it results in an 18% gray medium exposure. – Michael C Jul 30 '16 at 18:19
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    Rockwell is also mistaken that EV -5 is darker than EV 0. Smaller EV values are like f-numbers: the smaller they are the brighter they are. If I use EV 10 and EV 5 on the same exact scene the Ev 5 photo will be five stops brighter than the EV 10 exposure! What KR should have said is that smaller LVs (less light) requires lower (brighter) EVs to produce the same exposure. – Michael C Jul 30 '16 at 18:24

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