Well I might be a bit slow, and you might all be aware of these apps but I found one (many available actually) for my Android phone today. I expect those of you with iPhone can find one too via whatever mechanism Apple force you down.
The one I found is pretty good, with specific cameras and lenses being selectable.
You select the camera, then the lens, then the aperture and your subject distance in either metres, feet, inches or cm so useful for macro work too.
Hot the calculate button and it spits out the Near and far limits of focus, giving you the depth of field in the gap.
So for my 5Dmk2, 180 macro at say f11 and 20cm, the available DOF is just 1mm. Moving the aperture to f22, it doubles to a massive 2mm. Moving the subject distance to 30cm, it doubles again to 4mm. So as you can see, when working with things like butterflies, it is not surprising it is not always possible to get things sharp from front to back.
Moving to the 7D and 500 at minimum focus distance, and at f11, the available DOF is only 28mm, so if you are taking images of birds you can see why shooting wide open is not always sensible.
Another feature I like is that you have the option of the longer lenses with converters on as well so you can select 420mm (300 + 1.4TC) or 700 for the 500 +1.4.
Whereas moving to say a landscape, with the 5D and 24mm at f11, with say the horizon at 3km, the near focus limit comes down to 1.72m, so no need for infinity and you can select a much closer distance known as the hyperfocal distance.
Now - my big caveat to working anything photographically with numbers on a technical basis is check and check again with your own experiences, and don't hang your hopes on three decimal places. Techno nerd geekery is all fine and dandy - but check it out for yourself. It couldn't be easier with digital.
But as a quick and easily available aid, it does help to remove some guesswork.
It would be good to hear from you if you have any favourite and useful phone apps that you use. I have previously blogged about sunrise/sunset times and angles, as well as weather and tides.
Exciting day tomorrow for me - might take some images so will share if I do.