Headline - DSLRs have imprecise focusing due to their mirrors

Started 3 months ago | Discussions thread
Greg A A
Contributing MemberPosts: 607
Curious how it always boils down to the demise of the DLSR - mirrorless insecurity
In reply to Ontario Gone, 3 months ago

Ontario Gone 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? Let's clarify the incorrect statement that "it (the mirror) only gives you imprecise focusing". Also let's correct the misconception that contrast detection is always more accurate than phase detection as this isn't always true.

If either the secondary mirror is slightly ajar for whatever reason, the light path from subject to AF sensor will be slightly further/shorter than the light path from subject to image sensor. This is called parallax and it happens a lot more than anybody notices, perhaps all the time to a degree.

I think with sufficient DOF you won't notice it, but when the play is beyond a certain point, you notice BF/FF. This doesn't happen with mirrorless because there is no change between focusing and imaging, parallax cannot exist. This is one of the ways mirrorless are simpler and easier to manufacture. They have less moving parts, less factors that can go wrong. Below is a quote and a link, perhaps this can set things straight.

There is an overwhelming amount of negative feedback on autofocus issues on such fine tools as the Canon 5DIII, Nikon D800, Pentax K5 and other digital SLR cameras and it seems like most photographers do not seem to understand that the underlying problem is not necessarily with a specific model or type of a camera, but rather with the specific way these cameras acquire focus. If you search on the Internet, you will find thousands of autofocus reports on all kinds of DSLRs dating back 10+ years. Hence, the front focus and back focus issues we see in modern cameras are not anything new – they have been there ever since the first DSLR with a phase detect sensor was created.

For phase detection autofocus to work correctly, the distance between the lens mount and the camera sensor, as well as the distance between the lens mount and the Phase Detect sensor must be identical. If there is even a slight deviation, autofocus will be incorrect. On top of this, if the angle of the secondary mirror is not precisely what it should be, it will also result in autofocus issues. LINK

Multiple mirrors, precise alignment, optical path vs AF path. It seems pretty clear to me this system is weighed down by sever limitations and as soon as on sensor AF tracking speeds up, the DSLR will cease to exist. Afterall, it is this troublesome AF system that makes a DSLR a DSLR. Without those mirrors, it would be mirrorless. This isn't a comment to flame DSLR enthusiasts, it's a call for logic here. This system is archaic and problematic. If not for superior predictive tracking it would have been gone years ago. Cameras like the EM1 and A6000 show us it won't be long.

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"Run to the light, Carol Anne. Run as fast as you can

Mirrors and phase detection AF are proven technologies. How can it be claimed that DSLR phase detect systems don't work? My SLRs and DSLRs focus just fine as do several million others. For someone that is really concerned about AF accuracy newer cameras have AF adjust that can refine this for each lens.

EVFs are not the preferred viewfinder for the majority of camera buyers today. There is a vocal minority on this forum that claims otherwise. Rants on this forum about DSLRs and their inefective mirrors won't change what people buy.

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f8 and be there

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