Focus adjustment


womble

Link Posted 11/02/2018 - 21:27
I noticed that with my new toy -- the FA31 -- the focus was often not where I wanted it to be. For example, I photographed a portrait bust, and focused on the eyes, but the back of the hair was nice and sharp. So, I spent some time today testing and adjusting the lens for my K-3 and finally decided that +8 gave the best results. That seems, on a scale of -10 to +10 to be quite a bit... Is this a common finding when adjusting the focus for AF lenses? (I spend so much time shooting old manual focus lenses this isn't something I have ever done much.)

Many thanks, Kris.
Kris Lockyear
It is an illusion that photos are made with the camera… they are made with the eye, heart and head. Henri Cartier-Bresson
Lots of film bodies, a couple of digital ones, too many lenses (mainly older glass) and a Horseman LE 5x4.

My website

Fishbones

Link Posted 11/02/2018 - 22:21
I don't think its that unusual, mine had a +3 but my 43mm had +8 on my old K3. They are better behaved on my new body but so long as they are sharp on your K3 after adjustment I wouldn't worry. The option would be to sent both lens and camera off for re-cal. but that maybe a bigger risk. How are you finding it? I am using mine as a full time landscape lens at the moment but hope to get something a little wider at some point. It performs really well for me I just wish it were WR rated with the weather we are having.

Tony
"Twelve significant photographs in any one year is a good crop." — Ansel Adams

womble

Link Posted 11/02/2018 - 22:46
When I can nail the focus, I'm really liking it. More than the 43 which on APS-C is an odd focal length. The 31 is pretty close to a "standard" lens on APS-C. I'll load up my LX soon and try it out as it was supposed to be.

K.
Kris Lockyear
It is an illusion that photos are made with the camera… they are made with the eye, heart and head. Henri Cartier-Bresson
Lots of film bodies, a couple of digital ones, too many lenses (mainly older glass) and a Horseman LE 5x4.

My website

pschlute

Link Posted 11/02/2018 - 22:52
My 31 Ltd requires +10 on the K1
Peter



My Flickr page

Fishbones

Link Posted 11/02/2018 - 23:14
Life was so much harder on my K10d using the hack program and having to remember the values for different lenses as it only saved the one setting.

Maybe someone has a magic method that makes calibrating easy, I always never feel confident I have nailed it but use my lenses mostly at small apertures so never really notice a problem.
"Twelve significant photographs in any one year is a good crop." — Ansel Adams

pschlute

Link Posted 12/02/2018 - 00:35
Fishbones wrote:


Maybe someone has a magic method that makes calibrating easy,

Tripod
focus chart at 45 degree
natural light
widest aperture
single centre point AF point

And the most important one : do 10 tests for each setting, making sure the lens has to focus from infinity or closest focus distance each time.

If you are getting 7 or 8 (or higher),out of 10 correctly focussed then save that AF adjustment setting for that lens. Do not rely on one or two test attempts.
Last Edited by pschlute on 12/02/2018 - 00:35

davidstorm

Link Posted 13/02/2018 - 00:14
Hi Kris

There is no magic method, but at least with a prime lens you can nail the adjustment. I prefer setting on a tripod in good natural light and focusing on a subject with good contrast such as a magazine page with writing on it, set at a 45 degree angle. With zooms it's a nightmare as the tendency to front or back focus can vary with the focal length, so it's just a case of getting the best compromise.

I wouldn't worry at all if your lens needs +8 adjustment, as long as it takes sharp images. My DA 100 WR Macro needs a big adjustment on my K-3, which may be a reason why I was able to buy it fairly cheaply from the previous owner, but it is tack sharp once adjusted correctly.

Regards
David
My Website http://imagesbydavidstorm.foliopic.com

Flickr

Some cameras, some lenses, some bits 'n' bobs

Algernon

Link Posted 13/02/2018 - 10:28
Ricehigh has a good method. Sadly Photobucket have messed up the photos in the article, but the text is there....

http://ricehigh.blogspot.com/2012/01/focus-adjust-k-5-and-pentax-af-lenses.html

Be careful focussing at 45deg the spot focus can move about a bit. Something like this that can be folded vertical is better.

I just fold a triangular target out of A4 paper (looks like a roof) and place it on the A4 test paper then move it out of the way after I've focussed then take a shot.

The main requirement is that the target is parallel to the sensor. 45deg is also a bit too much the camera can slip so a bit flatter 35-40deg is better. I use a small adjustable angle finder (spirit level) to check they are parallel.

This is the setup with the target pushed out of the way. Squiggly pen marks help in the checking. The metal disks are magnets to hold the test paper in place on a metal table.




The Ready Brek characters card is great for manual focus because the coarse screen printing used causes moire in the viewfinder/screen.

The lens was a DA 35mm f/2.4 so big depth of field. It was also f/8 I was checking for focus shift.


--
Half Man... Half Pentax ... Half Cucumber

Pentax K-1 + K-5 and some other stuff

Algi
Last Edited by Algernon on 13/02/2018 - 10:59

Algernon

Link Posted 13/02/2018 - 11:08
To make sure that the centre of the target is in line. Tie a piece of string to the tripod column and pull it through the centre line of the target. Twist the target until it lines up.

--
Half Man... Half Pentax ... Half Cucumber

Pentax K-1 + K-5 and some other stuff

Algi
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