Pentax Q + PK adaptor + 50mm f2 DOF?

Hello,
I am trying to understand the depth of field of having a 50mm f2 PKA lens on a Pentax Q.

If I understand correctly there is a crop factor of 5.5x so the 50mm would be a 275mm f2 but on a much smaller sensor. Would the depth of field be super short though i.e the bookeh should be amazing no?

Please direct me if someone's taken pictures with such thing. 275mm is too long for portrait but that means a 28mm f2.8 (thinking the old K lenses) would start to be decent no?

instead of editing my post (and in case anyone is interested), I have found this:
http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm

to have the equivalent DOF of 150mm f2.8(on APS sensor) on the pentax Q + adaptor we would need a 58mm f1.1 lens...

so not possible...
so basically I am not sure the interest of having an Q + adaptor.. if you were to use your 300mm on it to shoot the moon, maybe, but otherwise isn't it the same as cropping?

It's still interesting
I've got the 50/1.4 on the adapter - and the DOF is still very shallow with lovely bokeh!

Manual focus is extremely sensitive at f/1.4 though! The focus peaking helps, but you need a tripod really

Matt

The depth of field of a lens is always the same. When the size of the sensor changes, the angle of view changes, but the depth of field remains constant.

Cameras with small sensor have huge depth of field due to the short focal length used to give a "normal" view.

That's an explanation that I finally understand.

well that's an explanation I dont understand..!?

It simply means that a given lens always has the same depth of field, regardless of the sensor size. But, using a small sensor, the field of view is narrower because most of the image seen by the lens is outside the sensor size, giving an enlargement effect compared to the larger sensor.

Actually, if you use a 50 mm lens and a 200 mm lens on an APS-C sensor to take a picture without changing the point of view or the camera angle, with the same opening, and you crop the picture taken with the 50 mm lens to get the same image you have with the 200 mm lens, the depth of field will be exactly the same for both lens.

Also, the depth of field is largely dependent upon the enlargement ratio. The more you enlarge a picture, the shallower the depth of field.

DOF depends on only one thing, the Scale Ratio, sometimes called the Magnification Ratio.

Scale Ratio is now many times your sensor size will fit into the subject size. Which is altered by how close you are to the subject and the size of your subject, your sensor size is a constant (obviously).

Putting a 50mm lens onto a Pentax Q will only alter how close you need to be to the subject to frame the same shot. If your framing the same shot on any camera with any focal length lens the DOF will be exactly the same with all combinations. Once the Scale Ratio is known the DOF can be read off a table such as this one.




Even the effect of aperture is Scale Ratio, this time how large the aperture is compared to the size of your sensor, as the aperture is (usually) smaller than the sensor or film it mitigates the effect of the lens Scale Ratio, you get a wider DOF with smaller apertures.