Photoshop Pitfalls

There are a number of seemingly obvious commands and operations in Photoshop which may lead to results that may differ from what the user expects. In some cases results could prove disastrous for the unwary, while other issues are simply minor annoyances. Below is a small list of some of the issues to be aware of.  

Add Noise & CMYK

The addition of noise after retouching or for banding removal in gradations is a common and often necessary practice. When working in CMYK the Add Noise filter will add to all four colour channels, which may not be desirable in all situations. It is generally best to apply the noise only to the CMY channels and not the K channel, using a separate layer and unchecking the K channel in the Advanced Blending section of the Layer Style Blending Options command.

Black Point Compensation, Photoshop 5

BPC is an Adobe specific option independent of the ICC profile, that is generally used with the Relative Colorimetric rendering intent such as in a RGB to CMYK colour conversion (a similar approach is used in Perceptual rendering so BPC is not used for this rendering intent). With most profile conversions to CMYK, using BPC is generally recommended. Some profiles do not work well with BPC and in these rare cases this option should be avoided (this may also be related to Photoshop 5). This will be fairly obvious, as shadow detail will be lost and or ink limits blown.

As a side note, it is recommended that Perceptual rendering be used instead of Relative Colorimetric when performing conversions to CMYK in software that does not offer BPC or a similar option.

Converting CMYK files to Grayscale Mode

The assigned or assumed CMYK source profile will affect the conversion to Grayscale mode, which can lead to elements which were solid K being translated via their Lab value rather than their CMYK value (this is not a concern with legacy Custom CMYK settings). This results in solid black elements being converted to a tint of black instead of being a solid. I favour simply converting to RGB first or using the Apply Image command to stamp the original CMYK modes files K channel in Darken mode over the final grayscale mode conversion (Layer Style Blending Options Blend If sliders can be used to further restrict the blend). 

Desaturate

The desaturate command found under the image adjustments menu is designed to work in RGB mode and not in CMYK (the same applies to desaturating via the hue/saturation command in CMYK). If using the command on a CMYK image the resulting colour numbers will not have enough cyan to produce a neutral. This is a well known issue for prepress operators that are used to editing the press gray balance manually, although it may be a surprise to users that are more familiar with RGB editing in idealized colour editing spaces such as sRGB or Adobe RGB etc.

Edit, Fill - Black

The Fill command under the Edit menu or via shift/delete (Mac) or shift/backspace (PC) has an option to fill with a preset black option. This command works as expected in all modes except CMYK. When used in CMYK mode, the colour build for black will use a rich black with a 400% total ink cover - rather than a sensible ink limited amount suitable for a press. The obvious solution is to avoid this command and foster the use of the foreground colour set to 0cmy100k or to use the default 'D' rich black when a multi-ink black is required. This is a documented feature which appears to offer little benefit to CMYK users.

Exclude Overlapping Path Areas

The Exclude Overlapping Path Areas option found on the far right of the pen tool options bar was not explicitly known in earlier versions of the pen tool prior to v6, although this is the type of path behaviour that most users are familiar with. It is recommended that one use this option as the default pen tool path behaviour when generating clipping paths.

Fade & Linked Layer Masks

The Fade command is available directly after most tool edits, image adjustments and filtering operations and is a very useful technique for altering the strength or blending of the previous edit. It appears that a linked layer mask can sometimes disable the fade command when used with common filters such as USM, although paint tools and image adjustments can be faded with a linked layer mask so this limitation only affects filters. To work around this apparently random bug, undo the filter, unlink the mask from the layer and reapply the filter. The fade command should now be available directly after the filtering step.

Hue/Saturation Command & Normal Blend Mode

Due to the inferior results of increasing saturation with the saturation slider in normal blend mode while in RGB or CMYK, it is recommended that the Hue/Saturation command be used in an adjustment layer set to either Color or Saturation blend mode (or that the fade command is directly used after the edit if one is not using an adjustment layer). The saturation slider in Adobe Camera Raw or Lightroom does not suffer from this same limitation and behaves in a similar fashion to Lab mode saturation boosting.

Info Palette & Proof Colour

The Proof Colour option found under the info palette can be useful for a small proportion of users in specific situations, most users have little need of this option. If one is proofing a device in the same colour mode as the image (say an RGB inkjet profile), it can be confusing to see the files numbers "misrepresented" as other values. Adobe indicate that this option is in use by using italic text in the info display when proof colour is chosen.

Lab Mode, Gray Working Space & Painting in Lab Channels

Photoshop uses the Gray Working Space found in Color Settings to display single channels in full colour mode images (not in the composite channel gamma as some users expect). There appears to be a bug when working in Lab mode where painting in individual channels with the brush tool results in tones that are lighter or darker than the sampled tone (the clone tool is not affected). Simply using Grayscale Gamma 2.2 in the Gray Working Space will correct this issue. Another work around that does not involve changing the Gray Working Space is to convert the Lab document to Multichannel mode, editing the separate channels and then returning to Lab mode.

Transform/Free Transform

For common layer transforms, the Edit/Transform or Edit/Free Transform command is very useful as one can combine multiple transform steps in one operation. Unfortunately, the command appears to have a bug that results in inferior quality interpolation when performing rotations. Despite the theory stating otherwise, superior results can be obtained by using the Image/Rotate Canvas/Arbitrary command and the Image/Image Size command (two separate interpolations, as opposed to combining the rotation and scaling in one operation via Free Transfrom). Many images may not display this issue and it may only become a major concern when the original image contains fine repeating regular patterns or detail, such as found in images of fabric (as moiré can be introduced in digital photography, scanning, image editing, page layout and final pre press output stages of print production).

Use Proof Set-up

The Use Proof Set-up checkbox found in the save/save as dialog will perform an 'on the fly' colour conversion from the current image profile to the current soft proof profile when saving the image. Although this command is useful for some proofing workflows as a shortcut, it is probably best avoided by most users (ideally this would not be visible unless selected in Advanced General Preferences).