Headline - DSLRs have imprecise focusing due to their mirrors

Started 4 months ago | Discussions thread
Greg A A
Contributing MemberPosts: 711
Its inconvenient when facts get in the way of a hypothesis
In reply to stevo23, 4 months ago

stevo23 wrote:

Greg A A wrote:

stevo23 wrote:

Greg A A wrote:

Leonard Migliore wrote:

Greg A A wrote:

peevee1 wrote:

Live view on DSLR is not "just like mirrorless camera", it is awfully slow. And mirror does not give you better image quality, it only gives you imprecise focusing.

Interesting premise; DSLRs have imprecise focusing because of their mirror.This is one of the wildest claims I have yet heard. Can you back that up with some facts?

Well, the phase detect autofocus assembly is at the bottom of the mirror box. It senses focus indirectly and the path length can be affected by mechanical inaccuracy. That's why the first thing you do when you have focus problems with a DSLR is to focus in live view; that's the only way to focus directly on the sensor. And it's why micro focus adjust is such a valuable feature.

So I don't see it as a wild claim at all.

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Leonard Migliore

Sometimes measured results show that a theory doesn't always take all the actual factors into account. Take a look at these measurements of focus accuracy measured wiith current cameras. This shows that phase detect can be more accurate and more consistent than contrast detect. This was the case with the D800, 5D MkII, 7D, and a split decision on the 6D.

"The contrast detect results on the 5Dmk2 and 7D show a big focus error with a reasonable amount of variability between the shots too. This just translates to out of focus shots – plain and simple" - from: http://www.reikan.co.uk/focalweb/index.php/2012/12/af-consistency-comparison-nikon-canon-phase-detect-contrast-detect/

Does anyone have actual measured AF performance data that differs from what was done here?

Let's stop a minute - DSLR "can" have imprecise focus due to the nature of phase detect autofocus and the mirror alignment/calibration aspect of it. But it need not be so.

Let's not start a flame war with "yes it does" vs. "no it doesn't". The answer is "yes it can" but also "it doesn't have to."

It's really that simple.

I agree with you there. However, how often do mirrors become misaligned?

Good question. And how often do they come from the factory poorly calibrated? I doubt they change much over time, but I'm sure that the micro adjust feature is there to deal with the difference in lenses as well as poorly factory calibrated units.

Let's clarify the incorrect statement that "it (the mirror) only gives you imprecise focusing".

I thought I did.

Also let's correct the misconception that contrast detection is always more accurate than phase detection as this isn't always true.

That's making an assumption as well. I do know that micro adjust has no impact on my D600's contrast detect autofocus, but it does impact the phase detect autofocus.

I also know that lenses play a role as well since each lens that you micro-adjust in your DSLR is stored and always has it's own value - rarely are two exactly alike. And linearity of focus has an impact in a lens's focus range. This may be one source above statement.

You are making hypothetical claims. There's no assumption in my statement. Measured results show that contrast detection isn't always more accurate than phase detection. I didn't say that phase detection was always more accurate than contrast detection.  Clearly the mirror doesn't only give you imprecise focusing as this was proven with several cameras.

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f8 and be there

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