[Nayu]

What I am seeing in the video is something called focus breathing. Magnification with external focus primes increases as the lens is focused far-to-near. FOV also decreases resulting in a change of the of relative position of points within the frame. There is also a small change of point-of-view due to extension. I also noticed some camera movement.
Steve

However, looking at the OP's video, I can't determine if we're talking about focus breathing as he focuses closer, or the little sideways jiggle as the focusing is moved.

Is it just the tripod shifting slightly as the lens is touched? Or is the camera super solidly mounted, and the lens does indeed have some slop in its focusing mount, and needs re-greasing to snug it back up again?

Any of the above might be contributing, though my vote is camera motion during the focus pull.
Steve

I was thinking that maybe a direct comparison with the OP's DA 50/1.8 might help sort things out as well.
Steve

Thanks for the replies.

I don't mind the focus breathing as you guys have mentioned -- it's the up/down/left/right movement of the frame(?) that bothers me. There should not be any camera movement, as I have my K-5 on a decent tripod and have confirmed that all knobs are tight. As Steve advised, I have uploaded two videos for a comparison between my A 50/2 and DA 50/1.8, recorded within minutes of each other with the same camera and tripod positioning. I tried to adjust focus quickly to make the effect more obvious in both videos.

First, the A 50/2:

You Tube

You Tube

Then, the DA 50/1.8:

You Tube

You Tube

As seen in those two videos, the frame movement with the A 50/2 is a lot worse than the DA 50/1.8 (if there is any).

[Digitalis]

it's the up/down/left/right movement of the frame(?) that bothers me.

If everything is clamped down tight, there could be some play in the helicoid of the lens. The movement seems to be occurring in a specific point of the focus range.

[stevebrot]

Thanks for the replies.

I don't mind the focus breathing as you guys have mentioned -- it's the up/down/left/right movement of the frame(?) that bothers me. There should not be any camera movement, as I have my K-5 on a decent tripod and have confirmed that all knobs are tight. As Steve advised, I have uploaded two videos for a comparison between my A 50/2 and DA 50/1.8, recorded within minutes of each other with the same camera and tripod positioning. I tried to adjust focus quickly to make the effect more obvious in both videos.

First, the A 50/2:
IMGP9934 - YouTube

Then, the DA 50/1.8:
IMGP9935 - YouTube

As seen in those two videos, the frame movement with the A 50/2 is a lot worse than the DA 50/1.8 (if there is any).

As @Digitalis noted, there is definitely a "catch" with the A 50/2 that throws the optics lens off-axis.

Steve

[Kerrowdown]

Pentax-A 50mm f/2: picture in viewfinder shifting when focus is adjusted

If it were mine... I'd be blaming me and that last dram, as being one too many.