[aaacb]

I decided to fine tune af for my da 35/2.4. Then realized it was actually optimal for distant subjects and it only showed backfocus for closer subjects e.g. 2m. Did the same with the da70. (all with the one camera I have)

I still don't know what to set, because I can't predict all situations in which I'll use a lens. But it is more likely that I'll be able to stop down for distant subjects, so then I guess the often cited 50xfocal lengths makes sense. Maybe another solution would be to move to zooms and call af microadjusting too difficult to bother with

The point of this post is to share my observation that, if you're microadjusting lenses, the testing distance seems very important.

[JohnX]

Convention is that the target fills the frame.

[6BQ5]

I always adjust at the minimum distance since DoF is shallowest at that point. Minimum distance usually turns out to be 10x focal length, I think ... but I'm sure there are lenses, like macro, where that 10x guideline doesn't hold. Distant subjects have the advantage of a deeper DoF. So, even if the adjustment isn't ideal at the longer distance, the increased DoF should help out.

It would be great to enter multiple adjustment points, like 3, for every lens: near, middle, far. That could get complicated with zooms but maybe add another dimension of wide, mid, and zoom. Yeah, complicated.

[clackers]

Convention is that the target fills the frame.

What's supposedly the logic behind this, John?

[Reed]

Depending on the target used, it is recomended to set it for 25 to 50 times the foccal lenght. For DA35/2.4 it should be at lest 85cm from matrix plane mark on your camera

[aaacb]

Depending on the target used, it is recomended to set it for 25 to 50 times the foccal lenght. For DA35/2.4 it should be at lest 85cm from matrix plane mark on your camera

That's what prompted me to start this thread: if I fine tune focus at that distance or so, infinity focus becomes noticeably softer at max aperture. I saw on lensrentals' blog that 35mm in a focal length particularly prone to compromise in af fine tuning:

"When it comes to 35mm and 50mm primes, you’ll often find that making an adjustment at shorter focus distances may cause longer distances to be off. " https://www.lensrentals.com/blog/2016/01/how-to-use-af-microadjustment-on-your-camera/

[victormeldrew]

Well, the tutorials I've seen suggest repeating the process several times at different distances. Either way, how big the target is in the frame will depend on the physical size of the target so shouldn't be important as long as focus can lock on....

But whatever you do will always be a comprimise, no lens will be perfectly linear and so if you have a more commonly used distance and that is no longer accurate then I would adjust a bit. However, DoF is shallower at closer targets and so any adjustment error would show more here than at greater distances.

[Unregistered User 8]

Here is a link to the Pentax Forums Focus adjustment tutorial. Hope it helps.


Fixing Front and Back Focus - Introduction - In-Depth Articles

[kh1234567890]

It would be great to enter multiple adjustment points, like 3, for every lens: near, middle, far. That could get complicated with zooms but maybe add another dimension of wide, mid, and zoom. Yeah, complicated.

That is essentially what AF lenses do. Sliding contacts inside the lens communicate to the body the focus distance zone as well as the focal length set if it is a zoom. This information is then used together with data read out from the lens rom by the AF system. Unless you buy Sigma and their docking modules you normally have no access to the lens rom data and tweaking the microfocus adjustment only shifts any error from one end of the scale to the other as all it is meant to do is to compensate for any variation of the lens mount thickness. The focus distance zone info gets written into the EXIF Makernotes section - check this and you'll be surprised how wide (and often unreliable) the zones are.

In practice, I find that mis-focus at infinity annoys me the most and I set up the AF of my lenses for that.

[Bruce Clark]

Not sure if is just me but I am totally perplexed by this issue of Front/back focus. I am not suggesting that the phenomenon does not exist but I wonder sometimes how important it is. I base my scepticism on several totally unscientific observations I have made from reading several recent posts on the subject.

The majority of complaints arise from photos that are often:

• taken in poor light. (High ISO and Low Shutter Speeds)
• hand held. (Photographers move up, down, front, back and sideways)
• taken of subjects that move. (People and Pets)
• taken at the widest aperture of the lens in question. (f1.4 - f2.8 seems to cause the most problems. Shallowest possible DoF. Not the sharpest aperture for most lenses)
• taken at or near the minimum focusing distance of the lens in question. (Shallowest possible DoF)
• that have a multitude of perfectly legitimate points of possible focus. (Wide angles in a darkened auditorium with multiple actors on stage. How is the camera supposed to decide which eye or whose nose to focus on?)

Other factors that may or may not affect results.

• Cameras do take a certain time to activate the auto-focus mechanism.
• Is the setting to prevent shutter release without focus lock activated?
• Is the camera set to AF.S or AF.C?
• Is the photographer using the Back Button system?
• Is SR turned on and given time to do its thing?
• Is zone or single point focusing applied?

Sometimes I think many of us are too ready to condemn our cameras without properly considering technique. As several have already pointed out, any adjustments should be conducted in a highly controlled manner after very careful testing and evaluation and not on the basis of a few apparently out of focus images.


I am not perfect, nor are my cameras or lenses but there are a few techniques to achieve better focus.

• Use a shutter speed appropriate to the subject and the focal length of the lens.
• Use an aperture appropriate to the subject and required depth of field.
• Let the camera worry about ISO and deal with any noise in PP. TaV mode is the way to go. On flowers I prefer TaV mode at f11 and 1/250th of a second. ISO whatever.
• Avoid wide open unless you have a need for narrow depth of field.
• Avoid auto focus in macro work or in poor lighting.
• Use back button focusing with AF.C
• Use burst mode. Often the second or third shot is sharper.
• Use tripod, remote control or timer and MUP if appropriate.

I have downloaded and printed test patterns to enable me to check for front/back focus. It is just I have yet to feel the need to check and adjust. Yes I do get many out of focus images. Most of them if not all, I put down to poor technique.

[aaacb]

Most of them if not all, I put down to poor technique.

True, many times it's some other reason. I started looking into adjusting front/back focus when I realized af portraits with that lens were consistently showing the subject's ears more in focus than the face.
And after playing around with these adjustments, I agree that the changes are fairly subtle and that the photo needs to be in pretty good shape otherwise for microadjustments to actually make the greatest difference.