Many will just go grey scale, while others will desaturate. This is really the best method. Starting with this digital image.
Pentax *ist D, SMC Pentax-M 85mm f:2 lens, Bowens Prolite 100, 1m softbox. Bowens Prolite 50 background, and reflector
You can play with the channel mixer. Here are some preset settings which I found on-line:
Image)Adjustments)Channel Mixer and check the monochrome box. Play with the top three sliders. Try to make them add up to 100%.
After tweaking the Channel Mixers, using the Agfapan 100 settings I got this result. Select the monochrome radio button at the bottom of the Channel Mixer dialog box. You may want to adjust manual levels, or brightness and contrast to fine tune. This is like changing paper grades in a darkroom.
Here’s some film emulation suggested settings:
Agfa 200X: 18,41,41
Agfapan 25: 25,39,36
Agfapan 100: 21,40,39
Agfapan 400: 20,41,39
Ilford Delta 100: 21,42,37
Ilford Delta 400: 22,42,36
Ilford Delta 400 Pro: 31,36,33
Ilford FP4: 28,41,31
Ilford HP5: 23,37,40
Ilford Pan F: 33,36,31
Ilford SFX: 36,31,33
Ilford XP2 Super: 21,42,37
Kodak Tmax 100: 24,37,39
Kodak Tmax 400: 27,36,37
Kodak Tri-X: 25,35,40
And these basic ones:
Normal Contrast: 43,33,30
High Contrast: 40,34,60
or alternatively try this …….
Step 1. Take your original colour image as one layer. Below it add a 50% grey fill layer. Change the blend mode of your original colour image to luminosity. You should now have a true B&W tonal image.
Step 2. Above of your colour image add a 20% (light) grey layer. Change the blend mode of this layer to colour burn and adjust the opacity to suit. (somewhere around 50%). If you want to tint your image pick a 20% tone of say blue instead…..
Step 3. Above these layers create a levels adjustment layer and adjust to suit. Drag the end sliders in until where the graph starts to rise. Do the same with a curves adjustment layer if you fancy..
(because all of these are done in layers you can waste endless hours playing with them….but surely you have better things to do?….)
Many modern dSLR have a B&W mode, and you can use your B&W contrast filters, but you are commited to B&W when you do that.