Gamma Correction in Adobe Photoshop

Adobe Photoshop 2.5.1 and 3.0.5

Adobe Photoshop for the PC and Adboe Photoshop for the Macintosh do two different things with Gamma and we aren't sure why. We think the reason is that Photoshop was first developed on the Macintosh and Adobe implemented the gamma functionality incorrectly. Then when they ported to Windows they fixed the functionality but decided to leave the Macintosh version the way it was.

Windows Photoshop

In the Windows version, the Gamma setting is found under the File->Preferences->Monitor Setup option. The gamma value that is set here, really represents the gamma correction that has already been applied to the file which you are currently viewing. For example, if you were to scan an image and set the gamma correction of the scanner to 1.8, then the file created would be gamma corrected to 1.8. In order for Photoshop to display correctly a file which has been gamma corrected to 1.8 already, Photoshop must complete the correction. Photoshop assumes that full correction requires a gamma of roughly 2.5. Therefore setting the Gamma in the Monitor Setup to 1.8 tells Photoshop to further correct by 2.5 / 1.8, or about 1.4 in this case. The assumption of 2.5, we think, can be overridden by the Monitor Calibration option also found on the File->Preferences->Monitor Setup dialog box. Some PC video cards apply gamma correction in adjustable amounts, sometimes for each channel (R,G and B) separately. Users with such a video card may need to adjust the Monitor Calibration to compensate.

The Gamma setting, then, does not say anything about the monitor, but rather about the file you are looking at. Unfortunately, the name of the menu option may lead you to believe otherwise.

Changes in the Gamma setting, then, will have a visual effect (the values in the frame buffer are affected) for images stored in RGB and other device dependent modes. Higher gamma settings will show darker images (because Photoshop is told that they need less correction) and lower gamma settings will show lighter images (because this tells Photoshop that they require more gamma correction.) Images stored in Lab mode will not be affected by changes in Gamma. This is logical since the Lab color space is not device dependent and does not depend on gamma.

Macintosh Photoshop

As stated above, we believe there is an error, or at least an excercise of bad judgement, in the gamma implementation on the Macintosh version. Simply making the gamma have a different effect on different platforms is bad judgement, but of the two ways in which the gamma seting behaves, the behavior on the Windows versions is far more logical.

The gamma setting is found under the File->Preferences->Monitor Setup menu. Adobe tells us that the gamma value should be set to the value stored in the Gamma Correction control panel from Knoll Software which is included with Photoshop 3.0.x. This control panel affects everything displayed on the Mac, not just the current image in Photoshop. The idea is to calibrate your system with this control panel, and then enter the result into Photoshop's Gamma setting. This is a very good idea, but the implementation is not so good. (We also have some reservations about whether or not you can really use this control panel to calibrate your system!)

After much arm-twisting, Adobe admitted ...

On the Mac, RGB images are assumed to have the same gamma as the monitor -- i.e., we can just pump the data straight to the monitor. You can use the Gamma control panel (or some other calibration tool) to adjust the behavior of the video board in converting the numbers into voltages in order to achieve a particular gamma response. Monitor Setup should contain the value that reflects how the monitor is behaving.
Well, there is a gamma Look Up Table (LUT) and a DAC which reside in Macintosh hardware, so we'll assume that Adobe didn't really mean "pump the data straight to the monitor" but at least we now know why the gamma setting comes under the Monitor Setup on the Windows version - Adobe just copied the Mac menu structure to the PC version even though the operations are not analogous.

A little background. Gamma correction happens in three stages.

So what does the Mac version do with Gamma? In RGB mode, it does nothing! Mac Photoshop in RGB Mode essentially assumes that the image file is gamma corrected for the Macintosh. Adobe says the RGB data is sent "straight through to the monitor" which we will assume means straight through to the LUT and DAC and then the monitor, since there is no way to avoid this. This means that RGB image data will be gamma corrected with a value of 1.4 (or 2.5/the value in the gamma control panel). Thus an image with a gamma of 1.0 (which SGI's performer library likes) will appear to be too dark when viewed with Photoshop in RGB Mode. In RGB Mode, there is no way to change this behavior. The gamma setting in RGB Mode has absolutely no effect because it refers to Monitor Gamma, or more correctly "System Gamma" where "System" includes the monitor, and all hardware that image data passes through to get there.

In Lab mode, the data must first be converted to RGB in order to be displayed. (Until someone makes an Lab monitor, this must be true!) So Mac Photoshop uses this gamma in the conversion from Lab to RGB and thus changing the gamma setting in Photoshop will change the appearance of an Lab image! This is a rather different approach than that taken in Windows Photoshop. In Windows Photoshop, the data is thought to be "real", or representative of the real-world color information. This "real" data is modified to fit your display system (monitor and other hardware) In Mac Photoshop, the data is not considered "real" or representative of anything in particular. It is the way your system displays this data that is treated as "real" and the data will be modified to match. This is a serious problem if you ever hope to show your data on a different system with a very different System Gamma! The other system will have to be recalibrated for your image unless it can know your File Gamma and its own System Gamma, and correct accordingly.

The Lab color space is device independent and can be used to store data that is meant to represent the real color of some real-world object. The RGB color space (and CMYK) is device dependent and is always a function of the system on which it is being viewed. Therefore to have the gamma setting in Photoshop alter the Lab data, means that the data can not represent anything "real" no matter how good your film, scanner or other image generation equipment is!

The gamma setting then, affects the conversion to RGB from Lab and CMYK in Mac Photoshop. Photoshop will apply a gamma correction equal to this value before sending the image data to the Mac hardware (which includes its own gamma Look Up Table set to 2.5/1.8 = 1.4 and can be overwritten by the gamma control panel.)