Strange focusing

Hi all.

I was doing a side-by-side comparison between the Tamron A2 135mm F2.5 and Pentax Tak bayonet 135mm F2.5, because I need to trim my collection and want to make a decision.

I noticed this strange behaviour while focusing both lenses...

I try to focus on the far distance building with the little tower (upper right corner in these heavy crops).

Tamron Adaptal-2 135mm F2.5, ISO100, F4, 1/320



SMC Tak bayonet 135mm F2.5, ISO100, F4, 1/400



Both show the rooftop (much closer) in focus while the point of focus (circa infinity) is not in focus, neither is the space between (or the space between roof and camera btw).

I checked, I didn't mix shots. I focused using the automatic focus detection, but to my eyes it should be in focus or close. The focus scale does not lie though, the focus was hitting the edge on both. Going back does not improve focus.

Funny thing is, the roof top is in better focus than when I tried to focus the roof (not a bit, noticeable...).

What the heck is going on?


On a related note, my opinion, based on these copies and the few shots taken is that.
The SMC tak bayonette is far less of a dog than many say. But is it noticeable softer than the SMC-M F3.5 at all apertures (at least F3.5-8 ).
The Tak-bayonette is sharper than the Tamron at F2.5, but this seems to change at higher apertures. However the Tak-bayonette showed PF which I did not see for the Tamron under the same conditions. This is probably a result of the tak-bayonete being a budget lens with poor coatings?

are you using split prism focusing screen?
To be honest, without that I couldn't MF anything but the 400/5.6 I used to own.
Even my Sigma 105/2.8 gave a lot of trouble!
With split prism screen it's much easier. I still miss some shots, but mostly they are fine...

Just the stock screen.
As I said, the focus scale was far from the 10m (give or take) that the roof is...

You may be experiencing this:

https://www.pentaxuser.com/forum/topic/k-7-shims-18721

I replaced my .40mm shim in my K-7 with a .20mm shim and the manual focus is spot on, now.

But Lance, if this was the case, and we assume that is provoking the visible differences in focusing, would they be very noticeable at all focusing distances?

I know I have front focusing issues, allways have had.
I used a chinese split prism for a bit, but that also had problems and focus was not spot on.

I have had shots were I focus is off or nothing is in focus, but nothing like this.

I've also had very different problems with FF. I remember for example the Tammy 24-135mm was almost spot on, while the kit lens was way off (45 angle thingy...)

If you're using auto focus detection, check which focus point you have selected in-camera I suspect it's not centre

It was in the centre focus point.

I'll try a battery of tests to see if I can obtain additional data that may clear up this mystery.

As far as I am aware, the FF/BF issue only affects AF lenses when using AF-S/AF-C focussing, not manual.

Other than that I can't offer any explanation for your problem. Do you have other lenses you can try at infinity?

With older manual lenses the focus point defaults to centre spot, you cannot choose anything else.

K.

There is a shim that adjusts the focus screen accuracy, covered here in this thread:
https://www.pentaxuser.com/forum/topic/k-7-shims-18721

It is one thing to be able to get AF accuracy via the AF adjustment feature, but another to have the actual focus screen reliably and accurately focus when manually focusing (and for that matter have accurate AF confirmation via the focus screen) and hence the shim adjustments between the focus screen and the pentaprism.

I removed the .40mm factory shim and put in a .20mm shim and this corrected my slight manual focus error.

I've had problems with FF with manual lenses. Maybe its the automatic focus detection thats wrong. But I remember using a split prism chinese screen with a 50mm f1.7 and that also was not accurate (but maybe it was the screen's fault).

The split-prism screen would have no effect on the focus indicator (hex/beep).

Sounds like you need to change your shims as Lance suggests.

K.

Well the sensor (photo) is the deciding factor here because you can't move that.

1) use AF and take a photo - you can then see if you need to adjust the AF or not
2) use manual focus (split-prism screen) and take a photo - if the DOF is off you know you need to use shims to adjust that.

ps. it can easily be that both are wrong, also remember that AF sensor is affected by the colour of light indoors it behaves slightly differntly then outdoors

Is your dioptre adjusted properly?