[hjoseph7]

The other day I posted a picture of a snowy scene that looked rather muddy. I took the picture with my $800 Pentax 31mm f1.8 lens. I wasn't sure why the picture looked so bad, so I figured I must have chosen the wrong picture to download ? I went back and looked at every picture I took that day and almost every picture looked like it was taken with a toy camera and lens ? Could it be that I drank too much coffee that day, or could it be something else ? Today I decided to perform a couple of test and I noticed that the lens was back focusing. It took a +10 to correct the error ! The thing is I had this lens now for over 5 years and don't remember it acting up like this ? Did something unusual happen to this lens, or maybe it's the camera ? A +10 sort of got me nervous.

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Last edited by hjoseph7; 03-04-2021 at 11:44 PM.

[Not a Number]

Did you change to spot metering?

[jatrax]

Check that all the screws on the camera mount plate are tight.

[bwgv001]

clean your camera, there might be dirt on the autofocus mechanism. I did have that problem once.

[stevebrot]

Back-Focusing is it the camera or lens ?

Does this happen with other lenses?


Steve

[UncleVanya]

As a general answer no rr red elated to this specific situation: it’s both. The same lens with different bodies can have drastically different adjustments required. However a group of lenses with the same adjustment required on one body will cluster around a number on all bodies, the number itself will differ based on how close to exact spec the body is.

Example. Lens 1 on camera A needs -3. Lens 2 on camera A needs -3. Lens 3 on camera A needs +2. Lens 4 on camera A needs no adjustment.

On camera B all the lenses need an additional -2. Lens 4 no longer is perfect, lens 3 appears perfect now.

Obviously it isn’t quite this cut and dry. Maybe one lens is really -3.1 and another is -3.5 and in camera the adjustment is rounded to -3 for both. But on another camera with only a small shift, say - -0.3 more. The -3.1 lens might still end up as a -3 but the -3.5 lens might change to -4 in the adjustments needed.

As others have said a dirty af sensor can throw off accuracy also.

[pschlute]

A +10 sort of got me nervous.

Back or front focussing can be a problem of both the camera and the lens. My K1 requires BF adjustment of between +6 to +10 on all my lenses so I am assuming the camera is largely responsible. But the AF-FA menu is there to correct exactly this, and I would not be bothered by a single lens requiring +10.

My 31mm LTD also needs +10 !

[hjoseph7]

Thanks I'll try all of these (tightening front plate,cleaning sensor, other lenses) today, I'll keep you posted...

[pschlute]

Thanks I'll try all of these (tightening front plate,cleaning sensor, other lenses) today, I'll keep you posted...

The AF -sensor is at the bottom of the mirror box. You will need to use the cleaning menu to raise the mirror. Just give everything a good blast with a rocket blower.

[UncleVanya]

Thanks I'll try all of these (tightening front plate,cleaning sensor, other lenses) today, I'll keep you posted...

The part that worries is that it may have changed. Try the fa sensor and compare a few lenses to see if they all behave badly now and used to work ok.

[hjoseph7]

Last summer I bought one of those lens cap holder/keeper things, because I was afraid i would lose my very expensive 31mm f1.8 lens cap. Apparently the strap for the lens cap holder was rubbing up against the focusing rings at the front of the lens causing erratic focusing behavior. I turned off the fine focusing and now the lens is working fine !

[pschlute]

Last summer I bought one of those lens cap holder/keeper things, because I was afraid i would lose my very expensive 31mm f1.8 lens cap. Apparently the strap for the lens cap holder was rubbing up against the focusing rings at the front of the lens causing erratic focusing behavior. I turned off the fine focusing and now the lens is working fine !

All's well that ends well...as the saying goes. Glad you got it sorted.