[Alexvc]

Hey! I bought a Pentax K-S2 a year ago and never had any trouble with it. Unfortunately, since a week or so, everytime I take an image with my 50mm f1.8 lens via the viewfinder the results are out of focus. In the viewfinder, it shows the red dot and there is a beep and everything seems sharp, but when I view the actual shot it is always slightly out of focus.

However all the shots taken with live view are sharp.


At first, I thought this was a problem with the lens-body communication pins, but after taking some shots with my 18-55mm the problem remained. So there is something wrong with my camera, but I don't know why (I never dropped it), what is causing the problem and how to solve this issue. I will post some photos tomorrow to show you how bad this is, especially since I am an amateur wildlife photographer and I need a fast and accurate AF, so I don't shoot in live view very often.
Please tell me if you know this problem, know what causes this issue or know how to fix it 😀😗

[CarlosU]

Maybe dirt/dust in the AF sensor or need for AF fine adjustment?

[clickclick]

Just curious, but if you put it manual focus with the viewfinder, ignoring what the camera thinks, and just focus for what looks sharp to you, is it?

[microlight]

Did you initially set up the fine focus adjustment for the kit lens when you got it a year ago? Have you changed any of the fine focus adjustments recently? It sounds like classic back or front focusing, especially if live view is sharp.

[bm75]

Did you accidentally adjusted the diopters of the VF? Just curious: are the focus errors Always the same way (I mean Always backfocus or frontfocus or erratic)?

[Na Horuk]

a) make sure viewfinder diopter is not off
b) make sure AF adjustment is not wrong. Some cameras only have global lens adjustment, some have per-lens adjustments. Start with it off. There are various threads on this forum about AF fine adjustment and how to do it
c) have you fiddled with the focusing screen? If you installed a new focusing screen without shims it might cause some focusing errors
d) which lens are you using? Some lenses have focus breathing, where the focus will shift depending on aperture. This means you can focus correctly at wide open aperture, but the focus will shift for the photo because another aperture is chosen for that photo. Reading the OP, I guess this is probably not the problem here
e) Make sure you are not using filters. Some filters can affect AF, particularly linear polarizers

Also, maybe you want to clean the camera mount and check for damage, just to make sure that is not the problem. Another thing you can do is update the camera firmware to the latest one (lots of threads and even youtube video about how to do this), as it can sometimes solve little hiccups. You can also do one of the Reset options to fully reset all camera settings to default.

The idea about dust on AF sensor is interesting. You can use rocket blower to try to fix that, or you can take camera to a shop for cleaning and tell them about the problem. They should know what tests to run to diagnose the issue

[leekil]

Did you drop the camera?

[clackers]

Try this, Alex ...

Fixing Front and Back Focus - Introduction - In-Depth Articles

[Digitalis]

a) make sure viewfinder diopter is not off

To check this: focus the lens at infinity and look through the viewfinder, if the image on the focus screen is blurry your dioptre setting is wrong - adjust it until the image is sharp.

Make sure you are not using filters. Some filters can affect AF, particularly linear polarizers

The Linear polarizer myth has been debunked since no current DSLR uses beamsplitters anywhere in the optical path of the AF system.

[Alexvc]

Thank you all for your advice, I really appreciate your help

Thanks to you I know that I have a back focussing issue with my ks2. My Phase Autofocus doesn't work properly so I have to do a fine adjustment. I will tell you if it works or not

[Na Horuk]

Thanks to you I know that I have a back focussing issue with my ks2. My Phase Autofocus doesn't work properly so I have to do a fine adjustment. I will tell you if it works or not

Yes, this is a common issue with many digital cameras, that's why they have the adjust function. Keep in mind that its not just a camera issue, but its a lens-camera relationship. Some people had two copies of the same lens on the exact same camera, and one was misfocusing, while the other one wasn't. The problem is that sometimes the lens and the camera are at opposite tolerances.
You might find that many lenses will not have front or back focusing on your camera, but some might. Good luck!

[Alexvc]

Thank you! In my case the back focussing problem was with my 3 lenses (50mm1.8, 18-55mm and 50-300mm) and I had to do a fine adjustment of +8 for each lens, but now the autofocus works absolutely fine with all my lenses so all of you guys really helped my! Thanks a lot for your answers!