Depth of Field Questions

[Cecilman]

Please forgive the question but I have recently purchased the Pentax K-x, coming from Pentax ME Super, and am thrilled that I could use my old lenses. My problem is that I don't really notice much blur in the background when that is the result I'm going for with the Pentax 18 - 55 kit lens that came with the K-x in Av mode. I also use a Sigma 70- 250 zoom lens with the a on the aperature ring and have no problem with setting dof on that lens with the camera. Being fairly new to the world of photography I thought I would ask . I also have an smc Pentax 50 mm full manual, no problem there. I notice that there is not much change in the size of the opening when looking through the lens with it removed from the camera, maybe for the lens its normal. Anyway thanks for listening. Cecil.

[DevinCatron]

i have the same 50mm lense for my k200d. i think your problem is maybe the aperture. what aperture are you using most of the time? on my 50mm i use 2.8 and i get plenty of blur and on my kit lense on 3.5 theres plenty

[i speak in math]

You probably already know that larger apertures create shorter depth of fields. But, if you play around with a dof calculator, you will see that the wider the angle of the lens, the deeper the dof. So, when you shoot at 18mm, you will have a hard time getting the background out of focus unless your focus point is very close to the lens. Zoom in but step back and keep the aperture as wide as it will go. Focus on stuff closer to you with lots of room behind the subject.

[Cecilman]

Thanks for advice will give it a go, and see what happens. Cecil