# What mathematical operation does the raw exposure slider in photoshop do? And how does it differ from adjust exposure when the file has been opened?

If I open a raw file in Photoshop without modifying the raw exposure and compare it to the same file with a raw exposure modification of -0.2; when I inspect the linear values it seems like a different mathematical operation has been applied to each pixel. What is it actually doing? I expected a linear scaling. If I then use the adjustments->exposure to +0.2 I don't get back to the same values why?

What are each of these exposure adjustments actually doing?

• Not sure, but I belive is a multiplication by a factor of 2 ^ x. Where x is the value you give. So, for example, +0.2 EV takes the value of each pixel, and multiplies it by 2 ^ 0.2.
– vsis
Mar 24 '20 at 18:18
• This seems to be the case for the adjustments->exposure version. I see r=146,g=5,b=14 go to r=167,g=5,b=14. But if I do the same thing with the raw exposure slider the numbers are r=168,g=6,b=15! Mar 24 '20 at 18:45
• When using the raw exposure slider what are the minimum and maximum values displayed? 0-255 or 1-256? How about when using adjustments → exposure? In your example, each "raw exposure" value is +1 compared to the "adjustments → exposure" value. Mar 25 '20 at 21:11
• Thanks Michael. It does look like that from these numbers but I was only doing a 0.2 adjustment here. If you increase that to 0.7 or -0.7 they are no longer off by 1. Mar 27 '20 at 11:45

The question is a bit confusing. Photoshop proper doesn't open raw files from consumer digital cameras, what you probably meant is Photoshop plug-in, Adobe CameraRaw (ACR).

The difference is important because the exposure slider in Photoshop (Image - Adjustments - Exposure) operates in a linear fashion (image data numbers are multiplied by 2^SliderValue and clipped to mode maximum - 255, 32768, 1.0) while the slider in ACR works in a linear fashion only up to CameraRaw Process Version 2010 (PV2010, aka PV2). With later PVs the action is non-linear, it is through application of a curve that involves a shoulder.

Even with PV2 a curve is still applied unless you opt out by setting "Linear" in the "Curve" drop-down and bringing Black point, Contrast, and Brightness sliders all to zero. With that, PV2 is forced into a linear mode. The default value of Exposure slider is zero, but, depending on the camera model, some baseline exposure compensation is still applied behind the scene. To quote the part of Adobe specification explaining the concept of the silent baseline exposure compensation, "Camera models vary in the trade-off they make between highlight headroom and shadow noise. Some leave a significant amount of highlight headroom during a normal exposure. This allows significant negative exposure compensation to be applied during raw conversion, but also means normal exposures will contain more shadow noise. Other models leave less headroom during normal exposures. This allows for less negative exposure compensation, but results in lower shadow noise for normal exposures.

Because of these differences, a raw converter needs to vary the zero point of its exposure compensation control from model to model. BaselineExposure specifies by how much (in EV units) to move the zero point. Positive values result in brighter default results, while negative values result in darker default results."

The nature of "raw exposure modification" is different between doing it in the camera and in a raw converter. In a raw converter it's not a true exposure modification (exposure has ended when the shutter was closed, and can't be modified afterwords). What you have in a raw converter is lightness modification. Since modification of lightness is the function of ISO speed, the better name for the slider is "ISO correction".

It seems the formula is something like: `newValue = oldValue * (2 ^ exposureCompensation);`