Why oh why do I keep seeing weird arguments about "depth of field" (just as a concept).
It appears when people talk about fast lenses.
Why buy a fast lens? For a thinner DoF, of course!
No! No, you buy a fast lens so you can get more light, so you can shoot in darker conditions, so you can use faster shutter speed, so you can use lower ISO, because fast aperture on a lens is often indicative of good overall lens performance, because you can always stop down but you cannot open beyond the max. There are many reasons to get fast lenses, but DoF should NOT be among them. First of all, the difference between f2.4 and f1.7 is not big in terms of DoF. It is a big difference in regards to light captured, though.
It appears when people talk about APS-C vs FF.
Why FF? Because you get a thinner DoF, of course!
No! Stop it! Yes, FF can have its perks. Yes, FF tends to indicate a higher quality camera (not anymore, not really, but let's pretend). Valid arguments are about noise performance, AF, wide angle capabilities, etc. But not about DoF. Even worse, here people start talking about DoF equivalence, as if the DoF is the most important thing and everything else, aperture, shutter speed, ISO, is just centered around making the thinnest possible DoF. Please, people, stop this. FF has its pros and cons, as does APS-C. DoF is the least important thing.
Thinner DoF means better photos!
No, it does not. Making a portrait with a thin DoF can be useful, but if it is too thin it becomes distracting. The person's eyebrows and nose are suddenly out of focus, the surroundings turn into mush, suddenly the photo becomes a mess of blurry bits instead of a portrait of a whole person. Portraits are more than just the eyes! It is not romantic, it is not sexy, it is not "deep" to take photos with overly-sharp and saturated eyes, with everything else being blurred out.
Can DoF be used creatively? Sure. Just as split toning and HDR. But please, calm down with the DoF. It is not that important, ever. Nobody will look at your photo and say "this is garbage, but it would be good if you had 2mm less stuff in focus." A lens at f2.8 and f1.4 will not have a big difference in DoF with near focusing. I often shoot wide open, but usually it is to gather more light, to allow faster shutter, or to have a more pleasing OoF bokeh (since aperture blades don't turn it hexagonal, so a slower lens can be useful because it has a wider DoF and round bokeh), not because I am hoping the DoF will be thinner.
Here are examples of my usage of thin DoF:
500px / Photo "Flower solitude" by Stolpulus II 500px / Photo "Magical mood" by Stolpulus II
Notice that these have ISO 1600 and one is still dark. And notice also that the photo would be better if the DoF were wider, if more stuff would be in focus.
Extreme subject isolation is rarely useful, but often it is distracting. But I also find myself sometimes shooting "wide open aperture in bright sunlight to make an artistic photo." Those photos are rarely any good.
And yes, I know DoF is related to aperture, how nearby you are focusing (which is usually related to what focal length you are using), even to sensor size (in some convoluted, mathematical way), but people keep talking about it as if it is the one most important thing. More important than ISO performance, than DR, than AF. It is not. Of all the variables in photography, DoF has to be one of the least important. The exception being maybe landscape and street photography where you usually want the exact opposite - maximum DoF. And tilt-shift is not helping with its "miniatures" gimmick. Shift lenses were usually used to improve the sharpness, to get more stuff in focus, not less. The miniatures effect is just a funny addition, its not the reason to buy shift lenses/adapters.
If you want more blur, just buy a "soft" lens or put vaseline on a filter. I don't know why thin DoF has become such a fetish among so many photographers. Its probably related to expensive, fast primes being so sought-after and a status symbol, rather than just a tool. Hey, I'd like a f1.2 lens for Pentax, I'd use it in the dark. And I wish Pentax had some more faster lenses, but I want light and quality, not DoF.
Ugh, I had to write this rant, I had to vent a little