[pschlute]

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

When you are checking an image to perform a AF/FA setting, you should be viewing it at 100%. DOF relates to viewing an image at "normal viewing distance".... a 100% view is far closer than that. Can you post a 100% crop to show the problem ?

[TerryL]

Pentax themselves recommend 30-40 times the FL.

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

That’s what I’ll do then. Thank you.

---------- Post added 09-07-20 at 06:18 AM ----------

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.

20 is what I currently use.

[Mooncatt]

If I may jump in for a second, can someone point me to some info on why AF fine tuning is needed from the technical standpoint? My last DSLR didn't have the ability, so this is pretty new to me. I'm just wondering how a camera can think a lens is in focus when the focus point is obviously blurry. I tried searching online and found a lot of how to fine tune, but not anything on why.

[ehrwien]

If I may jump in for a second, can someone point me to some info on why AF fine tuning is needed from the technical standpoint? My last DSLR didn't have the ability, so this is pretty new to me. I'm just wondering how a camera can think a lens is in focus when the focus point is obviously blurry. I tried searching online and found a lot of how to fine tune, but not anything on why.

When you're shooting in Liveview mode, focus will be on point because the focussing points that determine if correct focus is achieved are directly on the very same sensor that will capture the photo. When you're not using Liveview and focussing while looking through the optical viewfinder, you see what the lens sees because the light that enters through the front of the lens will be redirected by a mirror in the camera body to the pentaprism viewfinder in the top section of the camera right into your eye. The mirror in the camera body that redirects the light from the middle of the camera body to the top so you can see what the lens sees, that very mirror is now directly in front of the sensor, so the sensor cannot see what you see, it is just flipping out of the way just in time for the shot after you pressed the shutter button. So the first mirror is not 100% reflective, but will let some light pass through to a secondary mirror behind it, which will send the light down to the bottom of the body where the AF sensor unit is placed. (Look up how a (D)SLR is built; cross-section I found on Wikipedia. It's even a Pentax) In theory the way the light takes from the front of the lens to the sensor, and the way the light takes from the front of the lens down to the AF sensors should be the same (? or at least with some math work out to the same results), but because you now have a whole complex set of moving pieces in between the AF sensors and the lens, minimal differences in tolerances can have a huge effect on the focussing accuracy. Sometimes the same lens needs the same corrections on different bodies, while other lenses might need different corrections on the same bodies.
The good thing is that as long as those little differences in distance are not too long, you can use the fine tuning to correct for it.
This will be most obvious when you're working with small depth of field: with fast aperture lenses, long lenses and near close focussing distance. When focussing faraway things with wide angle lenses at small apertures, you're less likely to notice if the focussing is off a little because of the bigger dof.

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Last edited by ehrwien; 09-07-2020 at 07:31 AM.

[TerryL]

Pentax themselves recommend 30-40 times the FL.

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

Peter,
I want to ask you one more thing. Should I be using live view, or the traditional viewfinder which I normally use?

[c.a.m]

If I may jump in for a second, can someone point me to some info on why AF fine tuning is needed from the technical standpoint?

You may be interested in this article, which describes the basic issues in the section Phase Detection Autofocus: Achieving Better Manual Focus with the Green Hexagon - PentaxForums.com

- Craig

[Mooncatt]

When you're shooting in Liveview mode, focus will be on point because the focussing points that determine if correct focus is achieved are directly on the very same sensor that will capture the photo. When you're not using Liveview and focussing while looking through the optical viewfinder, you see what the lens sees because the light that enters through the front of the lens will be redirected by a mirror in the camera body to the pentaprism viewfinder in the top section of the camera right into your eye. The mirror in the camera body that redirects the light from the middle of the camera body to the top so you can see what the lens sees, that very mirror is now directly in front of the sensor, so the sensor cannot see what you see, it is just flipping out of the way just in time for the shot after you pressed the shutter button. So the first mirror is not 100% reflective, but will let some light pass through to a secondary mirror behind it, which will send the light down to the bottom of the body where the AF sensor unit is placed. (Look up how a (D)SLR is built; cross-section I found on Wikipedia. It's even a Pentax) In theory the way the light takes from the front of the lens to the sensor, and the way the light takes from the front of the lens down to the AF sensors should be the same (? or at least with some math work out to the same results), but because you now have a whole complex set of moving pieces in between the AF sensors and the lens, minimal differences in tolerances can have a huge effect on the focussing accuracy. Sometimes the same lens needs the same corrections on different bodies, while other lenses might need different corrections on the same bodies.
The good thing is that as long as those little differences in distance are not too long, you can use the fine tuning to correct for it.
This will be most obvious when you're working with small depth of field: with fast aperture lenses, long lenses and near close focussing distance. When focussing faraway things with wide angle lenses at small apertures, you're less likely to notice if the focussing is off a little because of the bigger dof.

Or you can just explain it to me directly... Lol. Thanks for the clear and fairly concise explanation.

[stevebrot]

If I may jump in for a second, can someone point me to some info on why AF fine tuning is needed from the technical standpoint?

A technical explanation would take a thousand words or more. The short answer is twofold:

• No global fine tuning would be needed if our cameras experienced no wear in use, meaning small changes to image sensor and main mirror positions over time.
• No per lens fine tuning would be needed if all lenses were perfectly aligned through their full focus and zoom ranges and remained so through their useful lifetime. A lapse in alignment translates to an ambiguous image at the PDAF sensor and possible shift in apparent point of best focus.*

Fine adjustment is a nice thing to have, but not a sure cure for every lens. If a satisfactory adjustment is not possible, a service visit might be in order.


Steve

* No...not a fully accurate description, but good enough without delving into how the PDAF sensor optics and detector array work.

[stevebrot]

Peter,
I want to ask you one more thing. Should I be using live view, or the traditional viewfinder which I normally use?

Not Peter, but Steve knows too. AF fine adjustment is done on the PDAF system, the one used with the optical viewfinder. AF in live view uses the CDAF system and should not require fine tuning.*


Steve

* Cannot be tuned, in actuality.

[CarlosU]

When you are checking an image to perform a AF/FA setting, you should be viewing it at 100%. DOF relates to viewing an image at "normal viewing distance".... a 100% view is far closer than that. Can you post a 100% crop to show the problem ?

These are 100% crops from full size files I just took at 300mm and f4, the first at 15X FL distance (4.5 meters) and the second at 30X FL distance (9 meters). When I use the second image (9 meters distance) for the AF adjustment the differences between contiguous adjustment steps on the camera are almost imperceptible. That is why I prefer to use a shorter distance that gives a swallower DOF and consequently a more precise determination of the focus point.

[pschlute]

If I may jump in for a second, can someone point me to some info on why AF fine tuning is needed from the technical standpoint? My last DSLR didn't have the ability, so this is pretty new to me. I'm just wondering how a camera can think a lens is in focus when the focus point is obviously blurry. I tried searching online and found a lot of how to fine tune, but not anything on why.

Manufacturing tolerances for both cameras and lenses. They are pretty close most of the time, but if you crop a lot or print large you may find those microscopic tolerances have an effect. We never noticed this with film or with early DSLR cameras. With the advent of big MP cameras we do. So all manufacturers supply adjustment menus with their top cameras.

---------- Post added 09-07-20 at 08:48 PM ----------

These are 100% crops from full size files

You will need to link to an external site like Flickr.

This site downsizes your image. They both look the same magnification

[CarlosU]

You will need to link to an external site like Flickr.

This site downsizes your image. They both look the same magnification

That is right, but at least you can see the difference between using the recommended 30X FL distance and a shorter one.

[pschlute]

That is right, but at least you can see the difference between using the recommended 30X FL distance and a shorter one.

Nope, I will be able to make a proper assessment of the AF/FA setting when viewing the image at 100%

Granted , your closer distance image shows that the 6/5/4/3 numbers are more fuzzy, but helps not at all with the -1/0/+1 numbers unless one can view the actual image.

[stevebrot]

These are 100% crops from full size files I just took at 300mm and f4, the first at 15X FL distance (4.5 meters) and the second at 30X FL distance (9 meters). When I use the second image (9 meters distance) for the AF adjustment the differences between contiguous adjustment steps on the camera are almost imperceptible. That is why I prefer to use a shorter distance that gives a swallower DOF and consequently a more precise determination of the focus point.

They both have identical vertical pixel counts. One or both have been resized*, either that or the forum software has done some magic; it does that sometimes.


Steve

* Your caption indicates that the 4.5m image has been resized.

[stevebrot]

That is right, but at least you can see the difference between using the recommended 30X FL distance and a shorter one.

Nope...that I can't. There is no basis for comparison. A 100% crop for the same FOV from 4.5m should have a diagonal twice that of the 9m version.

Addendum: A proper down-sample of the 4.5m version should have resulted in an increase in sharpness and apparent DOF similar to the 9m version. Instead, we are seeing a fuzzier image with less DOF. Something is amiss.


Steve