Has anyone used a Pentax A on a K10D?

Hello,
I just bought a 50mm 1:2 Pentax A lens to use on my K10D. When I attach the lens the menu pops up & asks for focal length, is this normal? I select 77mm as there dosn't appear to be a 75mm. I've set the lens to auto & it seems to work fine although I understand it will only work in manual focus mode. Has anyone used one of these before on a K10D? Is 77mm ok to set as focal length? Any advice would be most welcome.
Thank you
Caz

You should select 50mm because that is the focal length of the lens, if you select anything else it will confuse the camera and not give you the correct readings.
Jack.

Oh! I understood that as it was designed for a film SLR I shold apply the x1.5 rule for digital. I bought it for the purpose of using a lower f stop & for portaits where I expected it would behave more like a 75mm. Now I'm more confused.
Caz

Caz,

I have used one but on my K20D. I started a thread Test Shots SMC Pentax-A 1:2 50mm Lens but I was only taking flower shots and then some landscapes. I eventually got some decent results but found it hard work. Not sure if this helps at all?

Jeanette

Yes it was made for a 35mm film camera I have the A50mm 1.7 but it is still a 50mm lens, you only convert it because the sensor is smaller than film which makes it 75mm in digital but the actual lens is a 50mm so that's what you tell the camera, I also have a M28mm and a M40mm but I tell the camera that it is a 28mm and a 40mm, I may not have explained this very well and I'm sure someone will but you should always tell the camera the actual focal length of the lens.
Jack

Your camera will only ask for focal lenght when SR is on.

You are right: it will behave as a 75mm but it is still 50. Lots a writing about this subject already where you can find better technical descriptions of why this is so than I can give you. So it is good for portraits.

Edit: jackitec explained it while I was typing!

Caz

The 1.5 rule is really only indicating to you that the field of view you will get from mounting the 50mm on your digital camera is the same field of view that you would see if you mounted a 75mm lens on a film camera.

The focal length of a lens is a physical measurement pertinent to that lens only, and does not change depending on what camera you try to mount it on.

So it will behave like a 75mm lens would on film, but it remains a 50mm.

I hope that's clear.

Caz,

It would be reasonable to apply the 1.5x crop factor as your angle of view with the smaller sensor does indeed appear to 'increase' your focal length by 1.5.

A 50mm lens is still a 50mm lens though. Pentax have tried to make it easy for us with entering the settings, no need to do any maths, just enter the focal length value as marked on the lens

The 50mm will give you the field of view of a 75mm, but the depth-of-field would be the same as if you were using the 50mm on a film camera, the sensor size will not change that. That's what I think anyway, I could be wrong there....

Caz, I think you got the message now, it's all about maths not my strongest point, anyway its a great lens you have and you should get some nice photos with it so lets see what you can do with it,
good luck
Jack.

Hi all,
Thanks that's very helpful. I have now set the FL to 50mm. I'm still not sure I understand why the DOF would remain as 50mm. Still it does seem to be a nice clear optic. I must admit I'm not sure how this is going to work out but it's worth a shot as they say. After all a 75mm prime lens would cost much more. I clearly have much to learn.
Caz

Caz, DOF is a whole new kettle of fish nothing to do with 50mm and a 75mm lens would be about 112.5mm on digital, still confused, so am I.
Jack.

Thank you all. I've turned the SR off & as Joe S said it no longer asks for the FL. I'll give it a go as soon as I get a chance. Looks like fun....
Caz

Maybe it will help if you think about it as basicly an in camera crop where only the center part of the picture is left. In this case nothing happens to DOF

Hi Caz,

I've just bought the same lens for my K20D..very good it is too.

BTW - generally you're best keeping the SR turned ON

simon

Once you have correctly set the focal length for a manual focus lens (50mm in the case of a 50mm lens), and if it is the only MF lens you have, you need do nothing more than press the shutter button to turn off the 'Select focal length' menu because the camera will 'remember' your last setting and use it ever time until you change it.