[c.a.m]

Hi all,

Would like to let you know that I have just posted a paper at our Members' Articles section: Achieving Better Manual Focus with the Green Hexagon - PentaxForums.com

I've been using this technique for a while, and have enjoyed getting spot-on focus sharpness with my manual focus lenses. I finally found some time to write it up.

Please let me know whether this approach has been described elsewhere already. I searched extensively, and found lots of discussion on manual focus and the problems with the 'Green Hexagon' but nothing related to this technique per se.

- Craig


From the paper:

Summary

This article describes an approach to achieve consistently-sharp focus with manual focus lenses while relying on the Green Hexagon focus indication seen in the optical viewfinder of Pentax cameras. The technique involves two main elements:

• Calibrating a lens with the Auto Focus Fine Adjustment function, referenced to the focus point where the Green Hexagon just turns ON.
• In the field, rotating the focus ring from one end of the focus range until the Green Hexagon lights.

By exploiting a camera’s built-in focus detection system, this technique overcomes the uncertainty that is typically associated with the wide Green Hexagon zone. The paper describes and illustrates a quantitative calibration procedure and discusses simpler qualitative calibration approaches.

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Last edited by c.a.m; 01-27-2020 at 06:36 AM. Reason: grammar

[aslyfox]

Always nice when members share knowledge

thanks

[luftfluss]

That is amazing...

[c.a.m]

Always nice when members share knowledge

That is amazing...

Thanks for your feedback!

I hope that some members are able to use some of the information.

Now, if you'll excuse me, I have more lenses to calibrate...jk.

- Craig

[RGlasel]

I have more lenses to calibrate...jk.

I am overwhelmed by the amount of work you put into this, so first, many thanks! I am trying to get my head around the different charts you produced, am I right in thinking that for the M50 f1.4, you could get as good of focus as it gets by focusing from infinity and stopping as soon as the green hexagon comes on, and conversely, with the 28 f3.5, by focusing from MFD and stopping as soon as the green hexagon comes on, even without AFFA? If a lens is calibrated with AFFA, then the best focus is roughly in the middle of focusing through the green hexagon, regardless of if you are focusing from infinity or MFD? Yet another question, I have my viewfinder set so that the selected focus point lights up in red to confirm it is in focus, do you know if that focus confirmation works the same as the green hexagon? I get the impression that both methods of focus confirmation work the same at the same position of the lens, but I don't have anything to back it up.

[c.a.m]

many thanks!

Thank you! I'm happy to contribute to this great community.

Thanks also for the questions. I hope I can clarify things.

for the M50 f1.4, you could get as good of focus as it gets by focusing from infinity and stopping as soon as the green hexagon comes on, and conversely, with the 28 f3.5, by focusing from MFD and stopping as soon as the green hexagon comes on

Yes, this part is what I have found. The key is to stop your focus rotation just when the hexagon comes ON because that's the spot that corresponds to the AFFA setting.

even without AFFA?

This is the tricky part, I think. No AFFA means that no adjustment value has been entered in the camera, and the AFFA setting stays at its default value of '0' (or at whatever value was last entered into the camera in AFFA 'Apply All').

In this case, if you were to focus from infinity and stop just when the Hex lights up, the camera is saying "You can stop focusing now, I'm good, we're sharp" but the focus would be less than optimal because an AFFA value of zero doesn't correct the errors in the camera's focus detection brain. As seen in Figure 4 (for the 50/1.4), at an AFFA setting of zero (the default), the red curve is at about 7.0, a bit below the acceptable threshold of 8.5. If you were to focus from MFD and again stop when the hex turns on, the blue curve shows a mis-focused case at around 3.5, or really front-focused.

But, in the case of my 28/3.5 (Figure 9), without any AFFA, focusing from MFD to hex ON actually is not bad -- the focus quality is above the threshold and focus is good. You could even focus into the green hex zone a bit and still be well-focused. Focusing from infinity, though, ends up mis-focused at a quality of 5-6. The problem with not doing any AFFA calibration is that none of this is known -- we'd be guessing at the right focus position on the lens. Depending on the focal length, aperture, PDAF errors, etc, we might be ok and hit the focus spot-on. But, this is generally not the case in my experience.

If a lens is calibrated with AFFA, then the best focus is roughly in the middle of focusing through the green hexagon, regardless of if you are focusing from infinity or MFD?

It depends on the extent of the PDAF error and the depth of field. As we know, some AF lens/camera combinations don't require any AFFA, while others need AFFA near or at the end of the AFFA range.

The AFFA calibration process means that you're finding out how much error there is in the PDAF chain, and telling the camera what the correction factor should be. To ensure that we get a definite indication of the best AFFA correction, we use the point where the Hex turns ON as the reference point.

Figure 5 shows several cases of focusing with the calibrated lens. An AFFA value of +4 was entered in the camera's setting, and the best focus was achieved at the Hex ON point at Shot 7. Shots 9-11 show the effect of focusing 'through' the Hex to stop somewhere in the middle -- the images are mis-focused. Shot 7 is the best-focused image; that's where the hex illuminated and focusing was stopped.

the selected focus point lights up in red to confirm it is in focus, do you know if that focus confirmation works the same as the green hexagon?

My K-3 II is set up the same way. I believe that the small red square indicates which focus point is in use but does not have the same function as the Green Hexagon focus indicator. When focusing, the red square lights up immediately even if focus has not been attained. Usually, the two indicators seem to light up almost at the same time if the lens focuses quickly.


- Craig

[stevebrot]

Yes, this part is what I have found. The key is to stop your focus rotation just when the hexagon comes ON because that's the spot that corresponds to the AFFA setting.

The "edge" of the green hexagon the serves as a convenient offset from the calibrated plane of focus, right? Question...what happens at field distances where the hexagon "dwell" is narrower? (Please forgive if I missed that part. There is a ton of high concentrated stuff.)


Steve

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Last edited by stevebrot; 01-27-2020 at 09:14 PM.

[Aaron28]

very nice great work! I depend a lot on the hexagon with manual glass (with AF its focus priority so no shot until focus is obtained) and in general gets focus correct but on occasion not so much....in particular with longer focal lengths (could be the cheap m42 adapters I have...there is some 'play').....have never done a fine adjustment with any lens......may take the time to give it a whirl on some of my more tricky glass.....thanks for taking the time to lay this out for everyone....much appreciated

[c.a.m]

It is a convenient offset from the calibrated plane of focus, right?

If I understand you correctly, yes I think your elegant interpretation is right. We're using the hexagon's illumination point as a convenient indicator, and using an AFFA setting to tell the camera to shift or offset the green band a bit.

(Please forgive if I missed that part. There is a ton of high concentrated stuff.)

No worries. Happy to get any feedback or questions.

(I appreciate your comments over the years in response to many posts/issues that PF members have raised about the focus confirmation, etc)

- Craig

[c.a.m]

very nice great work!

Thanks kindly.

in general get focus correct but on occasion not so much....in particular with longer focal lengths

I suspect that some lenses simply don't need need any calibration and they play nice with the camera's PDAF system. Also, I think that depth of field definitely has a role to play in determining focus sharpness. A very shallow DOF probably sits 'inside' of the green focus indication band, so sometimes focus will be missed depending on where you stop inside the band.


- Craig

[c.a.m]

Question...what happens at field distances where the hexagon "dwell" is narrower?

Oops, I neglected to respond here.

Do you mean where the dwell is narrower in terms of the physical rotational distance on the lens scale?

I've applied this approach on my calibrated MF lenses in the field, at various distances, and haven't noticed any issues or special cases. I'll have to think whether the DOF plays a role in the sensitivity of the green hex band.


- Craig

[Aaron28]

Thanks kindly.

you bet

A very shallow DOF probably sits 'inside' of the green focus indication band, so sometimes focus will be missed depending on where your stop inside the band.

can believe that to be the case as in those instances i'm wide open and dealing with very narrow dof....trying to get a half to 3/4 of a foot at 20-25 right would be a good example I guess as that's typical of 200/f4 for me

[stevebrot]

Do you mean where the dwell is narrower in terms of the physical rotational distance on the lens scale?

Yes...this tends to narrow as distance increases in the physical sense on the lens scale, if nothing else. Beyond that, I will have to do some thinking...


Steve

[yucatanPentax]

So much to think about in your article. I only had time to scan through it just now and caught several important learning points for me. I'm going to read and re-read it in depth soon to see what I can do to get "tack sharp" photos more often, as I feel like I too often miss the mark.

Thank you!

[texandrews]

Fabulous addition to our knowledge base. Kudos.