Originally posted by GUB Some people have been discussing the difficulty of focusing Pentax 28mms recently.
As one of those people, I can add I have spent even more time trying out various MF lenses on my K3 and KP cameras. The latest being a K 35mm F3.5. It too has the same type of problem I have experienced with the A 28mm. I have come to the conclusion that that the small, compact lenses are harder to focus through the viewfinder because of three reasons.
First is the short focus throw of the lenses. My K 28mm f3.5 has a large body and also a relatively large focus throw. My HD DA 35mm Ltd also has a (very) long focus throw. Neither lens is hard to focus through the viewfinder. Indeed, none of my macro lenses are very hard to focus.
Second is the the nature of the modern viewfinders and their screens. They are designed for use with AF lenses which are also relatively slow. This provides a nice bright image for viewing, but is rubbish for manually focusing. The focus confirmation light helps, but it is easy to overlook, and it only flashes briefly. APSc cameras are even worse because of the small viewfinder image. If you compare looking through an old film camera like an MX or ME Super with their large bright finders and screens designed for manual focussing you will easily see what I mean. The difference is like night and day.
Third is the limitation of our own eyesights.. As we get older our eyes deteriorate. So are we certain that our dioptre adjustments are set correctly? I have had to adjust mine recently. It is easy to forget that one's eyes are getting worse as it is a gradual decline that one may not notice for a long time. The other problem with our eyes is that they can find it hard to focus on something if the contrast between it and its background is not that great. Staring through the viewfinder for longer and concentrating harder does not help for me as my eyes will still struggle to see which part of the scene is in or out of focus. Try for long enough and my vision will go swimmy and nothing will look in focus!. Bright lenses though old bright film viewfinders do not present this type of problem.
Having said all that though, I think there may well be infinity focus issues with some of the lenses themselves. I have just sent my A 35mm f2 to be re-calibrated. I would have done it myself, but I can't get the vanity ring off. The lens is so far out of calibration, it almost has macro capabilities at close range!
These focusing issues I have found to apply to not just 28mm or 35mm lenses, but also to telephoto ones. I have given up trying to use MF lenses above 55mm on my KP/K3 cameras, unless they are macro ones. It is just easier to use either my X-T1 or X-H1. The latter recently acquired for just this task. I am not prepared to give up using more standard lenses on my Pentax cameras though, because a) I generally prefer to use an optical viewfinder and b) I just can't get some of the colours I like out of Fuji's without significant post processing. That is another story though!
Your post is very helpful I feel, regarding these matters, and with a little bit of work, hopefully we can start to enjoy these old lenses more, rather than remain being frustrated by them.