A quick and technical question about autofocus.

Started 3 months ago | Questions
The Ambassador Junior Member • Posts: 37
A quick and technical question about autofocus.

Hello everybody.
I have a "technical" doubt I'd like to share with you.
As an R owner, I don't understand if I have to "trust" lens reviews anymore, when talking about autofocus. To my knowledge, with mirrorless systems, the autofocus mechanism is embedded in the camera and front/back issues shouldn't exist anymore.
Does it still make sense talking about low light autofocus or accuracy?
Will the camera (alone) do 100% of the autofocusing work?

ANSWER:
This question has not been answered yet.
Tazz93
Tazz93 Senior Member • Posts: 2,525
Re: A quick and technical question about autofocus.
1

To answer that in a non-technical way... yes that is relatively accurate. The back and front focus, as you know it, are gone if you no longer use a DSLR. Front/back focusing came from the need to "Sync" two different parts of the system perfectly, the AF sensor and the image sensor.

To keep from going into the weeds on it, if the AF sensor thought it was in perfect focus, but the image to the image sensor was off by just the smallest of small margins, you'd get back/front focus. Now, everything is done from the image sensor so there's no middle man to make a mistake. What the sensor "sees" is what you get. This means there is no room for the conventional front/back focusing of the past. Problem solved.

HOWEVER, it is still possible for systems to fail and not work correctly, so I pull up short in saying, you will not see this problem anymore. Just saying the iteration that we all know and hate, is gone.

As far as low light, I'm not sure I follow you. Front/back focus should be the same in low light as it is in good light, though it may miss more in low light due to the "bad info" going in.

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OP The Ambassador Junior Member • Posts: 37
Re: A quick and technical question about autofocus.

Thank you for the accurate reply.
Yes, front and back focus is now clear.
My other question was related to low light focusing. For what I heard in the reviews, there are some better and worse lens, as regarding this aspect.
I don't understand if this applies to mirrorless too. Thank youu

Tazz93
Tazz93 Senior Member • Posts: 2,525
Re: A quick and technical question about autofocus.

The Ambassador wrote:

Thank you for the accurate reply.
Yes, front and back focus is now clear.
My other question was related to low light focusing. For what I heard in the reviews, there are some better and worse lens, as regarding this aspect.
I don't understand if this applies to mirrorless too. Thank youu

Ah... yes, some lenses are better in low light than others. All things being equal, the faster lenses (i.e. lower f stops like f/1.8, f/1.4, etc) will be able to focus in lower light levels better than the slower ones. AF systems need light, so the faster the lens the better they will focus, in general. This works similar to our eyes. If you walk in to a dark room and squint (imitating slower lenses with smaller openings), you'll notice its much harder to focus your own eyesight. The camera system isn't much different.

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OP The Ambassador Junior Member • Posts: 37
Re: A quick and technical question about autofocus.

Ok, thank you!
So, even in mirrorless systems it makes sense to talk about low light focus performance (because of the lens). And not about the front/back focus.

RDKirk Forum Pro • Posts: 15,878
Re: A quick and technical question about autofocus.
2

The Ambassador wrote:

Ok, thank you!
So, even in mirrorless systems it makes sense to talk about low light focus performance (because of the lens). And not about the front/back focus.

That's true. However, mirrorless provides the benefit of a usable bright screen even in very low light. I think that paradoxically causes some complaints about mirrorless low-light focus simply because the brighter screen encourages us to attempt focusing in lower light than before.

But it also provides alternate means of manual focusing, such as focus peaking, which I've found work quickly and marvelously in far lower light than a DSLR could have managed at all.

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Tazz93
Tazz93 Senior Member • Posts: 2,525
Re: A quick and technical question about autofocus.
1

The Ambassador wrote:

Ok, thank you!
So, even in mirrorless systems it makes sense to talk about low light focus performance (because of the lens). And not about the front/back focus.

I don't want to say the lens is more important because not all focusing systems are equal. Some will be better in low light than others, but mirrorless systems will have a benefit of cutting out the middle man (the mirror) when compared to DSLRs. Since you have to reflect light and pass it around from lens to sensor more is lost in the DSLR than is lost in the mirrorless. Not much, but the mirrorless camera has a small advantage in that area.

So in summary, mirrorless cameras have an advantage on DSLRs in low light, but not massive. A better DSLR can still beat a mirrorless camera, but the mirrorless camera has an advantage built in. Also, faster aperture lenses help to focus in low light.

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JayLT4 Forum Member • Posts: 92
Re: A quick and technical question about autofocus.

I think you still need to look at the focusing of a lens, regardless of the camera body. Look at the initial release of the RF 70-200 which needed a firmware fix released to fix a potential front-focusing issue
It's certainly a less-likely problem now, and something more pertaining to firmware rather then requiring something like micro-adjustment on the body, but the mirrorless world isn't perfect either...but it is better in this regard

[quote]Firmware Version 1.0.6 for the Canon RF 70-200mm f/2.8L incorporates a correction for "a phenomenon when performing AF shooting to capture a subject at close-range with the focus distance approximately set to the Tele-end (200mm), the image may become slightly front-focused[/quote]

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PicPocket Veteran Member • Posts: 4,111
Re: A quick and technical question about autofocus.

The Ambassador wrote:

Ok, thank you!
So, even in mirrorless systems it makes sense to talk about low light focus performance (because of the lens). And not about the front/back focus.

In addition, lenses have their own accuracy and speed aspects too. Even though the sensor can measure how much adjustment is needed to bring the image into focus, it still needs to relay it to lens and expect the lens mechanisms to move the Optics in place. How fast and accurately a given lens can do this, how much tolerance does it have to consider the job done perfectly and how well can it relay back the information to the camera all plays a part it your overall AF experience. This is why lenses can still hunt, miss focus occasionally or need firmware updates to make things work (specially 3rd party lenses)

So front/back focusing due to separate AF sensor is just one improvement in a coherent system. Mirrorless eliminates one point of failure, but you are still dependent on body, lens and how well they can coordinate with each other

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