black and white conversion can be achieved by a number of techniques including:
setting saturation to zero
channel mixer which allows for a range of contrast options by applying a “color filter” effect to the conversion (eg. is you choose red filter as with a red filter on a film camera lens using a B&W film, red objects are made lighter and blue objects such as sky is made darker)
LUTS
many prefer to use layers to provide finer control and set the blending mode of the desaturated “filter” layer to “Color”
healing and clone tools
CLONE mode will just COPY a part of an image to another part of the image
HEALING mode will attempt to apply the textures and colors of a part of an image to another part of the image
this allows you to “paint in” an effect to wherever you wish on the image
this allows the traditional film print process of dodging“ or “burning” to create contrast and emphasis in your image, which is particularly important with B&W imagery.
depending upon your image editor, you will have various effects that can be applied in this manner and you can usually select a “brush” for a paint in style and a feather and intensity (opacity or transparency) as well as flow rate
a mask allows you to select parts of an image where you will later apply an effect to (if it is a white or light grey mask) or not apply a global effect (if it is a black mask)
the skill is in being able to select the regions accurately
some software have AI tools to take some of this often time consuming selection process away
gradient tools
this allows an effect to be applied to the image in a graded manner from zero to maximum effect
you can change the position of the gradient, the rotation and narrow the gradient to create a more dramatic change in the effect (you may want the width to be zero if applying it to a straight horizon)
this can be used to:
bring out over-exposed clouds
add a diffuse sun haze or fog effect
add a less obvious vignette effect by applying it to only select areas of the image
split toning tools
these allow you to add a color tone to the highlights &/or shadow areas
each area can have a different color tone and saturation
where these merge in the tonal scale can be adjusted
vignetting
this may be applied to the original image or to the post-cropped image