[TerryL]

Hello,
I’m about to fine tune the focus on my FA Limiteds and my DFA 50 and really want to get to it right. I’ve heard all different distances, such as 20XFL, and 50XFL. Now what should I use? Thank you very much.

[zapp]

Different corrections apply for fairly close distances up to 1m and longer distances. Pentax does well accounting for these changes automatically as well as for different adjustment for changing aperture. If you use Sigma with a dock, you are offered three distance settings with individual corrections. For me not even one worked. So more is not always better.
For the ltds and the dfa 1.5 - 2 m will work fine. Make sure that your target is large enough to judge perfect focus correctly. If in doubt use the distance that you will use most often with open aperture.
Afterwards check you focus sensor. On oblique objects it s not easy to know where the system assumes best focus. The result may look off, but calibration could be perfect.
Play and have fun - it is definitely worth it.

[DeadJohn]

I don't think you need to worry too much about the exact distance. 7 feet / 2 meters seems to be a good default but I suggest adjusting it based on how you use the lens. If you frequently shoot up close use less distance while fine tuning.

[maw]

Hi TerryL,

This summer I double-checked some of my lenses with the SpyderLensCal, they say exactly what is shown below, I hope it will be useful to you.

Optimal distance = distance used frequently / focal length x 30~50, e.g. 35 mm x 30~50 = 105~175 cm.

Ciao Mario

[stevebrot]

Many sources recommend 20x the focal length. For most lenses, this should be greater than the minimum focus distance. If you are using a particular device, the instructions generally are helpful.* The important part is to be in range where the FOV is adequate to use your device and where the step increments of the focus motor will not have an exaggerated effect. If too far, increased DOF will degrade the precision of the results, particularly with wide angles. If too near, parallax may be an issue when reading the scale. (Hint: devices with a slant target are fundamentally flawed due to providing an ambiguous image to the camera. The intent is to make it as easy as possible for the system to attain a consistent focus point, not make it harder.)

I personally use a flat target with no slant scale and adjust based on whether manual focus in magnified live view is required to correct.


Steve

* SpyderLENSCAL recommends 25-55x focal length, LensAlign recommends 25-50x and also provides an online tool that provides a distance range based on a number of parameters. For 50mm on FF and f/1.4, the range is 125-250cm.

[SpecialK]

Hello,
I’m about to fine tune the focus on my FA Limiteds and my DFA 50 and really want to get to it right. I’ve heard all different distances, such as 20XFL, and 50XFL. Now what should I use? Thank you very much.

I used the grass-filled crack in the sidewalk, about 4 feet away, when adjusting with a Sigma 30. I think I did 2 shots, twice. Bingo.

[pschlute]

Hello,
I’m about to fine tune the focus on my FA Limiteds and my DFA 50 and really want to get to it right. I’ve heard all different distances, such as 20XFL, and 50XFL. Now what should I use? Thank you very much.

Pentax themselves recommend 30-40 times the FL.

I see no reason to doubt them. This is through experience

[stevebrot]

Pentax themselves recommend 30-40 times the FL.

I did not know this. Can you provide a reference? Perhaps they had a few other tips?

Addendum: Belay that request! I found it! (It took several tries, but Google is still my friend.)

How to optimize focusing accuracy with large-aperture lenses / explore | RICOH IMAGING

Their procedure is similar to what I do.


Steve

[ehrwien]

Can you provide a reference?

Might be from here: How to optimize focusing accuracy with large-aperture lenses / explore | RICOH IMAGING

[CarlosU]

Depth of field (DOF) should be the variable to consider. You should do AF fine adjustment at maximum aperture so you have the smallest DOF possible. Then, you can calculate the DOF for your lens (lots of DOF calculators online) at maximum aperture and different distances. Then choose a distance that gives a DOF around 3 centimeters. More than that and it becomes difficult to pinpoint the focus plane.

For example, if you are adjusting AF for the 77 f1.8 you get a DOF of 3cm at a distance of 165cm, which is by the way 21 times the focal length of the lens.

For slower lenses it can be harder to determine the exact focus plane because of the larger DOF, and it becomes impractical to reduce the distance to achieve a swallower DOF.

[stevebrot]

For example, if you are adjusting AF for the 77 f1.8 you get a DOF of 3cm at a distance of 165cm, which is by the way 21 times the focal length of the lens.

So for 24x36mm FF we go with 130cm? What is the magic with 3cm DOF?


Steve

[alfa75ts]

Depth of field (DOF) should be the variable to consider. You should do AF fine adjustment at maximum aperture so you have the smallest DOF possible. Then, you can calculate the DOF for your lens (lots of DOF calculators online) at maximum aperture and different distances. Then choose a distance that gives a DOF around 3 centimeters. More than that and it becomes difficult to pinpoint the focus plane.

That's the way I do it. Perhaps it's not exactly accurate at greater distances but the object you're focusing on will still be within the DoF and therefore in focus.

What is the magic with 3cm DOF?

Just that it normally fits on the scale and is easy to see the sharp bit between the fuzzy bits. If you don't like 3cm try 1/2" to 2" it will still work.

[CarlosU]

So for 24x36mm FF we go with 130cm? What is the magic with 3cm DOF?


Steve

At least for me, with a DOF between 3 and 4 cms I still can estimate with some accuracy where the focus plane is, but beyond that I begin to guess the exact point of focus, which is crucial when doing the AF fine adjustment. The bigger the DOF the harder it is to determine where the exact focus plane is. That is using a plain flat focus target or a dedicated device like Datacolor Spyder Lenscal, which I have.

When I tried using the recommended 30 to 40 times FL distance I always ended with a DOF so big that most of the scale in the Datacolor device looked in focus.

I'll give you an example. I have a Sigma 100-300 f4 zoom. When adjusting AF at 300mm the recommended subject distance is 30 times the focal length, which is 9000mm or 9 meters. But with an APS-C sensor, a 300mm at f4 and a subject distance of 9 meters gives you aprox. 14cm of DOF. To me, that is almost useless to determine the exact focus plane. The same setting but at 4.5 meters distance gives a DOF of 3.4 cms, enough to see where the focus point is, and this is at half the recommended shooting distance.

[Ian Stuart Forsyth]

There are many things that I would consider for how you are going to calibrate your fine tune AF adjustments.
I usually tune it for my most common working distance I am going to use the lens at, there is also how much you are going to stop down the lens.

One of the problems is that lenses can have different amount of focus shift depending on how you are going to use them. If you are using a zoom this can introduce even more shifting variables that if you want critical focus then you may decide to use a AF correction that is a best fit for all of the conditions you are going to use the lens and camera in. For me this was the best solution and for times that i needed to shoot at MFD I recorded the AF correction and stored that setting on the lens hood so that i can quickly change it in the field.

[stevebrot]

I'll give you an example. I have a Sigma 100-300 f4 zoom. When adjusting AF at 300mm the recommended subject distance is 30 times the focal length, which is 9000mm or 9 meters. But with an APS-C sensor, a 300mm at f4 and a subject distance of 9 meters gives you aprox. 14cm of DOF. To me, that is almost useless to determine the exact focus plane.

I personally don't use a slant target and use magnified live view as a check, but your explanation is not the reason why.

Having said that, remember that DOF numbers are fairly arbitrary. There are rules for viewing. The expectation is for an 8x10" print viewed at 10" by a person having average visual acuity.* For those conditions objects within the calculated DOF range will appear to have acceptable focus. The emphasis is on the word acceptable. When reviewing on the camera rear LCD, the lower magnification there exaggerates DOF making focus evaluation difficult. Magnification defeats a calculated DOF. The solution is to review using a higher magnification. 100% will provide a 1:1 pixel-to-pixel mapping on the rear LCD, which should prove adequate.** There is no advantage to going higher.


Steve

* Sorry for using the traditional English units here. The more correct version involves a stated circle of confusion and 25cm viewing distance.

** This is basically the same as doing manual focus at 100% using magnified live view.