[Darius4522]

I've just purchased a new "old" lens.... an SMC Pentax A 70-210mm 1:4 lens, but seem to have trouble focusing. Why is this? Does the fact that I wear glasses (not when looking through the view finder) and therefore have to adjust the little thingy above the viewfinder for my sight mean anything? Or, if the little red square in the viewfinder and the accompanying "blip" mean the lens is focussed anyway?

While it is a manual lens with the constant f4 aperture, does playing around with the aperture ring (f4-32) makes any difference?

Sorry to sound so dense and stupid, but I have never before used one of the old lenses on my K10D. Sometime the shots are brilliantly sharp. Other times, while they appear sharp in the viewfinder, they remain fuzzy....

It could because I am trying to 'shoot' chicks in a bird nest (which is on a thinnish branch about 15' high) and the wind constantly moves the nest maybe 6+ inches sometimes, because if I shoot a stationery object the pics are always superbly sharp.

I'm using the AV setting, which I understand is the correct one to use with the old manual lenses.

[Darius4522]

Got an answer in another forum...

In this particular case of trying to shoot chicks in the nest with movement, I suggest you try focus trapping them. Setup so you have a fairly fast shutter speed. Set the switch on the side of the camera to AF-S. Point at the chicks while pressing and holding the shutter button fully closed, as you then focus the lens, it will fire the shutter only when it becomes in focus.....

and it seems to work OK.

[Mike Cash]

You have an "A" lens, so use any mode you want. Just make sure you have the aperture ring set to "A". You were probably misled by advice regarding older M42 (screwmount) lenses to shoot in AV mode.

You should adjust the viewfinder diopter (the little slide thingy) for your eyes. Adjust either with or without glasses, depending on how you shoot. You don't want to adjust it so the image is clear. You want to adjust it so the lines and other markings on the focus screen are clear. Point the camera at a blank wall or the sky or something and it will be easier to do.

[john_mantz]

I also wear glasses and bought the item listed below and it really helps, found it at B&H Photo for about $35.
From Pentax.com...
"Viewfinder Loupe 0-ME53 - With the Viewfinder Loupe attached, the finest details will grab your attention. The 1.2x magnification facilitates the assessment of fine details more accurately. This is ideal for critical focusing (e.g. macro photography), and when using manual focus lenses. "