[kinkindoll]

Hey all,

I am using Pentax K1 with DFA 24-70 F2.8 which is my favourite combo so far. I have a question about the AF focus point. I did the AF adjustment for the back focus issue. I did the calibration based on the center spot which now the AF using the center point is perfect, but i found other AF points are not accurate, the outer AF focus points which far from the center are getting worser and inaccurate. Is this normal and can be fixed by the technician if i bring my camera and lens together to the service centre ?

Thanks

[MarkJerling]

I don't understand how the outer AF points can be "getting worse". Can you post some photos to explain what you mean?

Field curvature could play a role. Have you tested your camera with various lenses? The issue may only relate to this one lens - or not.
Some lenses (zoom lenses especially) have corner softness at certain apertures. Could this be a lens issue rather than a camera issue?

[kinkindoll]

The AF adjustment is based on the center point, the other AF points ehich far off from the center will have the back focus issue. Back focus is the reason of my lens thats why my lens required to do the AF adjustment. Thanks

[MarkJerling]

The AF adjustment is based on the center point, the other AF points ehich far off from the center will have the back focus issue. Back focus is the reason of my lens thats why my lens required to do the AF adjustment. Thanks

As per my previous query: Are you finding this problem only with this lens, or with all your lenses on this camera?

[kinkindoll]

As per my previous query: Are you finding this problem only with this lens, or with all your lenses on this camera?

With all other lenses

[MarkJerling]

Would you like to post some test photos?

[calsan]

I think 99% of the time AF adjustment falls under the 'don't fix it because it's probably not broken' category.
It's unlikely that modern factories are shipping out lenses with focus adjustment so wrong that it may be better adjusted by a home user using graph paper and a ruler.

Also - photographing a flat plane - if the middle point is 'x' meters away, the edge will be 'x plus a bit more' meters away from the lens (the bit more will depend on the focal length), so this is the point of stopping down to an aperture so that it covers the area required to be in focus. Like with a 15mm lens, a subject 1m away in the middle of the frame will be about 1.5m away at the edge of frame and so you need f/11! Which is why fast and wide is not so necessary for architectural photos.

[pschlute]

It's unlikely that modern factories are shipping out lenses with focus adjustment so wrong that it may be better adjusted by a home user using graph paper and a ruler.

Got to disagree here.

More megapixels mean we can spot focus inaccuracies more easily. The advances in sensor resolution have outgrown camera/lens manufacturing tolerances. This is why all DSLR manufacturers offer the ability to Fine Adjust.

But it is the camera/lens combination that may need adjusting, not a camera or a lens on its own. I use modern DFA lenses together with 20 year old FA lenses on my k1. Almost all of them require some adjustment, including some that require the maximum 10 points.

If you don't notice focus inaccuracy, then that works fine for you. But to assume that 99% of the time all is going to be ok for all users is a mistake in my view.

[Photobill]

I have not looked at the specs of the K1 so I don't know how many cross type focus sensors it has. I have found the cross sensors are quicker and more accurate. The center sensor is a cross sensor. A little test you can do is to mount your camera on a sturdy tripod and level it. Point it directly at your refrigerator. Mount an item on the fridge with magnets that you can focus on. (Focus chart) focus with the center sensor first and take a photo of the center of the chart. This shot should be as sharp as as it gets. Now pick another sensor and slide the focus chart so the new sensor is trying to focus at the exact same spot as the previous shot. Once focus is achieved slide the chart back to its starting position and take the photo. (I use back focus and remote trigger) this will allow you to see if there is any variance in the sensors all the photos should ultimately be framed exactly the same and the focus should appear the same from sensor to sensor if all sensors are working correctly. The hardest part of the test is making sure your level & parallel. With this technique it takes out the variances of sharpness of the lens from center to edge. But remember no lens nails focus every time so you need to repeat the test shots a number of times.
I hope this makes since to you its easy for me to do but much harder to explain.
Good luck.

---------- Post added 07-29-18 at 10:29 PM ----------

Got to disagree here.

More megapixels mean we can spot focus inaccuracies more easily. The advances in sensor resolution have outgrown camera/lens manufacturing tolerances. This is why all DSLR manufacturers offer the ability to Fine Adjust.

But it is the camera/lens combination that may need adjusting, not a camera or a lens on its own. I use modern DFA lenses together with 20 year old FA lenses on my k1. Almost all of them require some adjustment, including some that require the maximum 10 points.

If you don't notice focus inaccuracy, then that works fine for you. But to assume that 99% of the time all is going to be ok for all users is a mistake in my view.

I agree
I'd also like to add if your using slower/kit lenses you may not notice front or back focus issues because they cannot achieve the supper thin DOF. If your using faster lenses at low f-stop with that razor thin DOF you will notice a micro front or back focus issue.

[kinkindoll]

The AF system for k1are cross type cross f2.8 except the AFpoint on each side (rectangle one), all the square one is cross type F2.8. The lens i am using is DFA 24-70 F2.8 supposed all the AF point should be accurate but it still require to do the adjustment, the center spot now is fine, others AF are deviated from the center. They are out of focus with back focus problem.

I will try to take some photos for your reference.

[ewig]

may i see your kamera setting ??
please test also with normal kit like 18-55 , i know they are for APS-C but now it seems your default setting is change

[calsan]

Lens Rentals | Blog
"This lens is soft" and other myths

Most people "test" their lenses indoors - with some artificial lights on...or light coming through a window... so don't be surprised when all your lenses "need adjusting" but when you go outside, suddenly your perfect adjustments aren't so perfect... because the wavelengths of the light are different.

Anyway, just saying the factory knows more about the design parameters of the lens and where is the 'best compromise' for that lens - unless your house looks like this, I'd leave it alone:

[krazykat]

Lens Rentals | Blog
"This lens is soft" and other myths

Most people "test" their lenses indoors - with some artificial lights on...or light coming through a window... so don't be surprised when all your lenses "need adjusting" but when you go outside, suddenly your perfect adjustments aren't so perfect... because the wavelengths of the light are different.

Anyway, just saying the factory knows more about the design parameters of the lens and where is the 'best compromise' for that lens - unless your house looks like this, I'd leave it alone:

The problem is a camera and lens are two discrete components that are not tested in combination in a factory. They both have manufacturing tolerances that can, and sometimes do, combine in a way that means that assigning a focus adjustment gives more reliable focus results.


I have encountered this for most, but not all, of my body and lens combinations.

[rawr]

New article seems timely:

How to optimize focusing accuracy with large-aperture lenses / Beautiful Photo-life | RICOH IMAGING