What is the technical reason for Canon DSLR's not auto-focusing during video?

[jemanner]

I am aware the DSLR focus sensor is not functional when in live view/video mode, but cannot the image sensor somehow serve this function? Mirrorless cameras can focus, albeit slowly, real-time in video mode, presumably via contrast detection via the image sensor. Why not this same function on a DSLR such as the 5D III or 1DX?

[Samsen]

As I understand it, the mirror in DSLRs is the problem for AFing in video mode.

Sony for the first time got with this with a clear design that is a Transparent Mirror that though sites on the way of passing light, but allows part of light to pass through it so sensors can auto focus. Be aware that the Transparent mirror though has given a lot of credit to Sony but is actually invention of Canon from many decades back (Oh gush Don't we all Love Canon for a good Reason, Simply the smartest and best).

Well no a days many other manufacturers also use translucent mirror so autofocusing on video mode is allowed on newer Canon DSLRs even some of the cheap entry level one.

[ScottyP]

Mirrorless might be the thing of the future or might not, but for now, meh.  There are very few lenses for mirrorless cameras on any brand (Sony, Nikon, Canon, etc..) unless you use an adapter and then screw on a regular big lens.  Since one big point of mirrorless is smaller size, the big lens spoils that, and just makes the little camera look stupid. 

Also, the unmoving translucent mirror:

 1.) necessitates an electronic (not real thru-lens) viewfinder, which is a second,  tiny LCD screen in the eyehole, which kills battery life, and which many photographers find to be an abomination anyway, and

2.)  eats up some of the light available to the sensor (like mirrored sunglasses in front of your lens!), thus crippling your low light ISO performance. 

[jemanner]

Thank you for the replies. Not sure I am yet 100% sure why auto focusing cannot be incorporated into a DSLR with the mirror up and utilizing the image sensor and contrast focusing, but it is what it is, at least at this juncture.

As far as there being a lack of available lenses for mirrorless cameras, yes compared to the DSLR world. However, although a loyal (since 1962) Canon SLR/DSLR person, I also own an Olympus OM-D EM-5 for a carry-around camera. Between Olympus, Panasonic, Voigtlander, and others, there are quite a good selection of lenses to choose from for Oly Micro 4/3 bodies.

BTW, still have the first Canon camera purchased, a Canonflex RP. Canon's first SLR. Still as functional as when it was new. Several other film bodies accompany it in storage, such as the A-1, F1, and EF. All revolutionary cameras in their day.

[jfo]

---

@jemanner wrote:

Thank you for the replies. Not sure I am yet 100% sure why auto focusing cannot be incorporated into a DSLR with the mirror up and utilizing the image sensor and contrast focusing, but it is what it is, at least at this juncture.

---

The T4i has this feature.  DSLR's were made for stills, and video features are slowly being incorporated.  Just because features exist doesn't mean it won't take research, engineering, and marketing costs to figure out whether it makes sense to include them. 

[jemanner]

If video auto-focusing is in the T4i and not in higher end products (5DIII, 1DX), then marketing is suspect. Don't know much about them, but doesn't Canon have "C" series cameras specifically for video? So probably they don't wish the 5DIII or 1DX to compete.

[Samsen]

Ok let me clarify a few misconceptions here:

1- There is no such thing as dedicated lens for mirrorless cameras. When Sony Alpha 55 came out I was one of the first to buy it only for that reason (Though I am born Canonist so Canon please forgive my cheating on you). I love that Sony and I am using all kind of lens on that including older Minolta 500mm that is fully compatible with beautiful picture result. Lens has nothing to do with mirror type in camera body.

2- I think on above topic, you are mistaking the dedicated Canon lenses made for DSLRs with Fine Video capabilities that is the STM line of lens. These are specially made lenses as far as motor in the lens is concern. If you have ever tried to capture some HD video with a DSLR that has Auto focusing capability, you know it very well that motor sound of lens is very annoying and is heard quit loudly. Well if you know about silent garage door openers (Having elastic belt drive instead of metallic noisy chain) you know silence can be added when needed. Canon did that with STM lens so now the sound of AFing motor of lens will not be picked up by sensitive, in camera, microphone so you have a much better audio recording. Otherwise you can use any older lens even with these newer cameras like T4i and as long as you are not video recording or use an external sound recording system, you will have no problem in using any lens that fits EFs mount.

3- I must have not mentioned it well in my first post that  when lens is down (Rest location) the portion of light reflected up goes into pentaprisum to be seen on viewfinder (Creating a right side up, then right side right and not left image) and a small part of same light is reflected on the AF sensor so AFing can be achieved. Now think of Live view or Video Recording. Lens is locked up that means light directly falls on the sensor to form a digital image to be processed as needed. There is no light getting to AF sensor so camera can not do the Autofocusing. But with Transparent mirror lens light reaches to both side as lens is not moving up during picture taking so light can travel to sensor via non opaque mirror, and at the same time, being fix, some portion of light going up (Via reflection of mirror) does the rest of magic.

4- The reason that T4i, the most basic entry level Canon's line of DSLR does provide that Autofocusing with video recording, also doing a lot of other nice functions such as touch sensitive LCD screen, Mobile LCD, etc etc, is because this is a much newer technology product as opposed to 5D III or 1Dx. And yes one year in today's sophisticated digital Camera  life (Very similar to computers) is probably equal to 20-25 years of human life. So no wander you feel really odd to use you camera after few years even though everything works. And though its my gut feeling only but I am sure that I am not dreaming when telling you to be prepare to see a replacement of 5D III very soon, sooner than what you think and definitely not in traditional way with 5D line of cameras.

5- As for nostalgic section: Yes I do feel exactly what you say. I am also still have my Canon FTb that was my first camera, received when being a young boy when my father notice I am playing too frequently with his fancy Zeiss. Everything works in that camera and still I can take fantastic extremely long exposure time images of dark sky when shutter is fully opened with no fear of Sensor Noise, or take the camera to extreme cold place and be sure that I will come back home with some pictures in my bag.

Hope this helped.

[jfo]

Samsen has some great points about technology progression and why the T4i can handle auto-focus features.

---

@jemanner wrote:

If video auto-focusing is in the T4i and not in higher end products (5DIII, 1DX), then marketing is suspect. Don't know much about them, but doesn't Canon have "C" series cameras specifically for video? So probably they don't wish the 5DIII or 1DX to compete.

---

Of course they don't want to make the 5DIII or 1DX compete! Not every camera can do everything, nor should it; why would there be a need for different camera models then?  Marketing is doing exactly what they should be in creating/segmenting different products for different uses. 

There are already plenty of still photographers out there who are pretty vocal that they don't want or need any video capability at all in their DSLR.  In addition to other tradeoffs, it increases costs and makes the user interface more complex.  If users need advanced video capabilities, they should get a product focused on video, not a DSLR that happens to take great video.

[jemanner]

"There are already plenty of still photographers out there who are pretty vocal that they don't want or need any video capability at all in their DSLR.  In addition to other tradeoffs, it increases costs and makes the user interface more complex.  If users need advanced video capabilities, they should get a product focused on video, not a DSLR that happens to take great video."

I hear you, still some diehard/purist folks out there. However, the 5DIII and 1DX do have exceptional video capabilities, just not auto-focus. My personal preference is an instrument which will do both extremely well. Why carry two cameras or more when one will do? As far as the interface being more complex, compared to film days the interfaces are already orders of magnitude more complex and include video. All I'm asking for is auto-focus in the Canon high-end DLSR series. My original post was not to debate this issue, but the technical reason why the feature cannot be included.

[jfo]

---

@jemanner wrote:

Why carry two cameras or more when one will do? As far as the interface being more complex, compared to film days the interfaces are already orders of magnitude more complex and include video. All I'm asking for is auto-focus in the Canon high-end DLSR series.

---

Gotcha.  I shoot both stills and video, so I'm all for great video in a DSLR camera too.  

My point is that there might or might not be a purely technical reason, but there's certainly a business/marketing reason supporting the technical.  People don't necessarily want to pay for continuous auto-focus in their DSLR, thus its not feasible.  i.e. maybe it can be done, but you don't want to see the price tag for it.

One camera simply can't do everything. It might seem like "just auto-focus" and that's "All [you're] looking for", but after auto-focus (or instead of it), what's to say that Canon shouldn't put in a swivel-touchscreen LCD, XLR audio connections, and 4K recording, 120fps, etc. and also the 1DX's (and 7D's) continuous still shooting speed into the 5DIII?  I'd love any or all that included for the same price, but then it wouldn't be a 5DIII. 

Sure the user interface is more complex now than in the past, but that's not liberty to keep adding more and more features.  The risk is getting one of these:

Of course, all this changes as time goes on and technology advances, so maybe these features/changes are coming soon.