Yesterday a rather nice Pentaxian on one of the many FB Pentax communities put me onto something I wasn't aware of before... 'Focus Shift'. I mean I was
kinda aware of it, in a way that I stopped relying on using FA (Fine Adjustments) and Focus Charts so much because it became apparent to me one of a few things would happen;
1) Set up the Focus Chart test equipment up on another day and results may differ (possibly due to not having extremely accurate and fixed points for the equipment to be housed in-conjunction with focus chart etc.
2) Over time perhaps the lens 'changed' and those values no longer hold true.
3) Different apertures require different values, why are we only calibrating to wide open apertures? (especially if this is not an aperture many shoot at).
Point No.3 is what this video is all about below, and I never knew it had a specific name 'til now, Focus Shift. Please watch, I think you'll find the video informative;
Ok so I thought I would put my recently acquired FA77 through the Focus Chart testing this morning and see what values I got through the following apertures, f1.8, f2.8, f4, f5.6 and f8. I had actually never done the focus chart with the FA77 yet, but I was happy to see that my 'out in the wild' adjustment of +3 for f1.8 was confirmed correct when actually doing the focus chart test, below are the rest of the outcomes;
As you can see I also tested HD DA 35 Macro as well, and perhaps true to what this fella was saying in the video that Macro lenses are superior to non Macro lenses in this regard, the variance for Focus Shift was far less. Maybe I shouldn't have sold my DFA 100 after all...
Just as this guy talks about in the video, there doesn't seem to be a brand of camera yet that allows for FA values to change accordingly throughout the aperture range. Perhaps however with Pentax we are a little graced with a somewhat workaround. Here's what I did...
You'll notice that with my FA77, +3 was fine for apertures ranging 1.8-2.8, and that +5 would be better for apertures 4-5.6, and lastly f8 requiring +8. You could do this;
Create a User Mode, call it 'FA77 f1.8-2.8' and set the AF Fine Adjustment (Custom Menu, Item 25 for the K-1) to being '3 Apply One' (and in this case for me set to +3). Then... create a second User Mode, call this one 'FA77 f4-5.6' and set the AF Fine Adjustments to being '2 Apply All' (and for me this time the setting would be +5). Once those User Modes are saved and stored you now at least have two user modes to go between that increase accuracy of the shot at varying apertures, you just need to toggle a User Mode if you drift out of the range too much.
It's a shame that you can't use '2 Apply All' and save
that setting value per User Mode, but unfortunately it appears at least with my K-1 that during power downs and power ups it recalls
only the last setting value used. Hence why you would use both '3 Apply One' and '2 Apply All' to get differing values for the
same lens.
If shooting at f8 I would just have to manually increase the FA value for the second User Mode (ie '2 Apply All') or even just use Manual Focus (as things are a little more forgiving at this more stopped down aperture). Write down on the lens cap F8=+8 as a mental note or something.
Things are a little easier for my HD DA 35 Macro, I could indeed just set it to '+2' and have a single User Mode ('3 Apply One') as a 'compromise between wide open (2.8) to f8 (as it doesn't alter that much).
It's not the best workaround, I mean I guess if you really use a lot of different lenses and they all need varying degrees of different FA values then this idea goes out the window. Perhaps tho there is a theme to your lenses, that the more you stop down the more the FA value needs to change, perhaps that knowledge in itself might be handy in the field. Moving simply up from f1.8 to f8, bump the FA value up quickly also, and just don't sweat it too much etc.
I am curious however to see if other users are also discovering this issue with their own lenses, would be quite fascinating to gather large amounts of data on what FA values people are finding they are using on common lenses, perhaps there being a very common adjustment theme for specific lenses!
Cheers!
Bruce