Headline - DSLRs have imprecise focusing due to their mirrors

Started Apr 5, 2014 | Discussions thread
OP (unknown member) Contributing Member • Posts: 969
The tradeoff of more AF precision than is warrented

Bill Robb wrote:

Camley's post is right to the point of this thread. Some folks on this thread claim that a DSLR can't focus precisely with the implication that most DSLR pictures are out of focus.

Well, some people are claiming that some people are claiming that. What I have read is that some people are claiming that the potential for AF error exists in DSLRs because of the way the AF systems work in them, and other people (yourself for example) are claiming that this is a blanket statement that DSLRs cannot focus accurately.

Bill, Please read my original post. I haven't claimed that DSLRs can't focus accurately. I was pointing out statement made by someone else, a statement that I disagreed with. My mistake for not being clear on the original thread.

What is out of focus anyways? If you are off slightly from a theoretical focus point but the elements you care about in the photo are in focus the way you want, is your picture out of focus?

It depends on if your "theoretical focus point" happens to align with what you want in focus or not, doesn't it? For myself, the desirable focus point tends to be the one that I put the AF point on. In my world this would be the theoretical focus point, while in your world it could be anywhere but?

If the theoretical focus point is off by a small fraction and I have setup to have some depth of field it simply doesn't make any difference. The issue is more about how good I am about determining what depth of field is needed versus being off the center point of the focus by just a hair. A photographer's technique is much more in play than a focus to some really precise increment. A typical low cost DSLR consumer snapshot will probably not depend on absolute precision in focus point. The tradeoffs in cost and usability favor a DSLR PDAF versus mirrorless CDAF. Take any desgin and over optimize one parameter and the results probably won't be optimal.

While AF fine tune is available in many higher performance cameras the difference between a fine tune adjustment and not would not be noticeable in typical photographs, especially on lower end cameras with kit lenses.

And for those of us who are using higher end SLRs with high speed lenses at wider apertures, have our AF tuned in and still get slightly out of focus pictures?

I haven't noticed any issues with out of focus pictures with my DSLRs, except when it's my error. The lack of sharpness with a poor lens is much more of an issue. I have seen some small focus errors only when using AF tuning. If I did more marcro work it might be more of an issue. I will be paying more attention now and see if I can identify the effect of small focus errors.

PDAF in a DSLR camera focuses with an appropriate balance of speed and precision. In most cases it just isn't necessary to sacrifice speed of your PDAF and the convenience of an OVF to be able to say you were fractionally more in focus with CDAF. To push one parameter to the extreme at the sacrifice of other characteristics isn't always the best design. See my other post on this point.

On this you are correct, but you have also moved off topic for this thread.

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f8 and be there

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