Canon 1Dx-No AF above f/8

[Alis]

Is anyone concerned about this issue of Canon 1Dx not having AF above f/8? I know why it does not but I am not sure if it is a big issue. Appreciate any help as I have pre-ordered it and am waiting to get it (sometimes) soon

Thanks!

[rpcrowe]

I think that you might have mis-interpreted the "issue of Canon 1Dx not having AF above f/8". What this is referring to is that the 1Dx and other full frame cameras cannot auto focus using a lens of LESS than a MAXIMUM f/8 aperture. It does not mean that using a lens stopped down to f/8 or smaller will not auto focus. Example: The 70-200mm f/4L IS lens stopped down to f/16 will autofocus because, the maximum aperture is still f/4.

The ability to auto focus at f/8 is better than any of the Canon 1.6x crop cameras such as the 60D, 7D and Rebel series which cannot retain auto focus when the maximum aperture of a lens is less than f/5.6.

What this means in real life is that the 1.6x cameras cannot retain auto focus using a 1.4x TC on a lens such as the 400mm f/5.6L because the TC will reduce the maximum aperture to f/8. However the 1D series of cameras CAN retain auto focus using the 400mm f/5.6L lens with the 1.4x TC. The case in which the 1dx could not retain autofocus would be if you mounted a 2x TC on the 400mm 5.6L lens which would make it a maximum aperture of f/11...

This is really a moot question because I don't believe I would wver want to use a 2x TC on the 400mm f/5.6L lens. However, the 1Dx COULD retain autofocus using the 70-200mm f/4L IS lens with a 2x TC because the lens would have a maximum aperture of f/8.

BTW: although this would be possible, I don't think that I would want to do this...

[Colin Southern]

Hi Ali,

+1 to Richards post.

I might add that I've read conflicting reports of the 1Dx AF specs; some imply that it's less sensitive than previous models, whereas others are saying its more sensitive.

Contrast -v- Phase modes are probably adding to the confusion.

Bottom line is that if you're not using 2x TC on F4 lenses then it's not going to be an issue anyway.

Personally, I'd be surprised if the AF on the 1Dx was less sensitive than previous models.

[Alis]

Many thanks, Richard and Colin. Very helpful.

I guess I did not get what this cute lady was explaining:

http://youtu.be/2WuUmbAcK_c?t=23m59s

I thought you might find it helpful as well. She explains why but looks like she is not sure about the reason!

P.S: What really made me worried last night was that I read a post by some bird photographers that were saying that they are very disappointed at Canon for ditching them and that they are switching to Nikon as a result. Did not understand that part either

[Black Pearl]

It would seem mad for Canon to step back from the performance of the 1D MkIV and while there are conflicting reports online the base spec doesn't mention a f8 capable centre point.
Oddly I've been talking about this at work recently as a customer has just bought a Mark4 specifically because of this. He might get a 1x if it proves to be f8 capable but for bird Photography he wanted the 1.3 crop and the ability to use a converter on his lenses.

[Colin Southern]

Hear ya go Ali,

'Cause I luv ya buddy!

https://docs.google.com/open?id=0B2g...W1rWnZaemRqNkU

[Colin Southern]

It would seem mad for Canon to step back from the performance of the 1D MkIV and while there are conflicting reports online the base spec doesn't mention a f8 capable centre point.
Oddly I've been talking about this at work recently as a customer has just bought a Mark4 specifically because of this. He might get a 1x if it proves to be f8 capable but for bird Photography he wanted the 1.3 crop and the ability to use a converter on his lenses.

Just been having more of a read around and it does indeed look like AF @ F8 is no more. Having just said that, there are a raft of new AF modes that I think will more than compensate.

[herbert]

Using current Canon specifications the 1Dx does not focus at F8 maximum aperture in phase detect mode.

Phase detect works by splitting parts of the incoming image into two. A comparison between the left and right is then used to calculate where to move the lens focus to align the two. The difference between the two can be used to calculate how far to move the focus and in which direction. This is why phase detect focussing is very fast. It knows where to move the lens to achieve focus. By comparison contrast detect just moves the lens forward/backward until the highest contrast is achieved. This results in hunting. The following page has some great info and a nice diagram:

http://graphics.stanford.edu/courses...tofocusPD.html

The reason for no phase detect at F8 is that when the image is split into two the left and right image must land on the autofocus sensor strips. If they did not build the sensor with the strips in the necessary place with the right sensitivity then no software is ever going to fix this. It will require a redesign of the sensor.

Why they left it out is up to speculation. Perhaps they compromised to cram in all the auto-focus points. 61 points is a lot. There may not have been space for the F8 sensitive lines. A more optimistic view is that they are building a different camera specifically for use with long lenses with F8 points. I am an optimist today and so believe that a multi national with 180000 employees and millions in market research budget did not 'forget' the F8. They would have calculated how much money they can make with different cameras in their line up for the next 3 years and picked the specifications they thought the market wanted.

Canon seem to be addressing this issue by saying it is not an issue. In the following rose-tinted interview with a wildlife photographer the F8 issue is addressed:

http://cpn.canon-europe.com/content/...he_eos_1d_x.do

One area I thought I might have a problem with is the loss of focusing at f/8. The previous [camera], the EOS-1D Mark IV can focus from the centre point when using an EF500mm f/4L IS II USM (or EF600mm f/4L IS II USM) and an EF2x III extender or EF800mm f/5.6L IS USM and an EF1.4x III Extender, but the EOS-1D X cannot. A lot of people have been talking about this as a problem. The reality is that, in use, I have not found it a problem at all. I have found that if I need that focal length, then I should either be moving closer to my subject, or I can use manual focus accurately. The viewfinder is so clear that it is easy to see when something is sharp in the frame, and the new magnify button, which I have set to 100%, makes it very quick to check that I got the focus right.

So there you have it. If you need to focus at F8 then move closer or focus manually. Thanks Canon.

Nikon also are addressing this issue. They announced their D4 camera a few weeks after Canon and made a big song-and-dance about the fact it has 9 autofocus points that are F8 sensitive. This is a switch from previously since their prior cameras do try to autofocus at F8 anyway. It is just they do not do it very well. I believe Nikon cameras have no coding to tell the camera not to bother so it always tries to focus. Canon on the other hand add code to tell the camera to not bother. This can be tricked by taping over the contacts on the lens so that the camera does not know the maximum aperture. In this case they work like Nikon, I.e. not very well.

@Colin,

I am not sure what you mean about changed focus sensitivity. I believe previous 1D cameras had 1 F2.8 sensor and could focus at -1EV. The 1Dx has 5 F2.8 sensors in a vertical strip in the middle, 41 F4 points and can focus at -2EV. All other points are F5.6 sensitive. Here the F-numbers refer to the aperture needed to get the maximum performance from the point. I do not know if the F2.8 and F4 points also work at F5.6. It would depend on how long the phase detector strips are on the sensor. Note that wide aperture lenses blur out of focus points more. Thus the detector strips have to be further apart to work at wide apertures.

For reference Nikon do not have any F2.8 sensitive points, or even F4 points. All are F5.6 points. But it is still easy to read stuff on the web that states that Nikon focussing is better. However since there is no resource on the web that tests focus accuracy in a lab controlled consistent manner we are subjected to anecdotal 'proofs'. The D4 also focuses at -2EV.

I am happy for anyone to tell me I have my Nikon facts wrong and I'll change my post. I mainly follow Canon technology.

Alex

[Colin Southern]

Thanks Alex,

I'm not strong on this stuff to be honest. So if the 1Ds3 AF sensor is good for -1EV, and the 1Dx AF sensor is good for -2EV, which is more sensitive? (ie how do I interpret -1EV or -2EV in relation to AF sensitivity?).

[herbert]

Sensitive is a bit vague.

The -2EV means it can focus in lower light. This level of light would be a good full moon.

The fact that it has 5 F2.8 points means it is has more of the sensitive focus points. This is because F2.8 points have a narrower depth of field (the usual aperture-to-DoF link applies to autofocus sensors too). So if the sensor accuracy is within 1/3 of the depth-of-field, which is Canon's specification, then the F2.8 sensor will be more accurate.

One other thing that makes auto-focus sensors more sensitive is the number of strips used for the left/right parts of the sensor. Usually each strip is a single row of pixels. Focus is achieved by aligning the peak intensity for the left and right side. However if you use two strips on each side that are offset at half a pixel you get more precision on the location of the peak intensity. So you can align the two sides with twice the accuracy. The 1Dx has dual line sensors for all points. This is quite an upgrade since most other Canon sensors have only a few. Here's a good article on Canon autofocus:

http://www.the-digital-picture.com/P...Explained.aspx

On xxD bodies and the 7D, three vertical points have the dual line vertical sensors (center top, center, and center bottom), and data from those three inputs drives the extreme defocus detection. On the 1D X, all of the vertical lines (i.e., all 61 AF points) have the dual line zig zag arrangement, and thus the whole AF sensor acts as one big extreme defocus detector.

The article includes a picture of the 1Dx sensor if you are interested.

Focus tracking is apparently better too. This is because the light meter is now a 100,000 pixel RGB sensor which provides feedback to the focussing unit. This aids focus tracking. For example it can track a red object. Previously light metering was done using greyscale pixels and so this cannot help tracking specific coloured objects.

Overall I expect the 1Dx to have amazing focussing. Just not at F8. It is a major upgrade from all previous Canon cameras (except maybe the Canon Eye Control autofocus).

I've done a lot of reading on this issue because I have had focus problems with some of my images that 'got away'. I want to avoid it happening and so tried to find out how the focus system works.

For anyone worried about this discussion I think it is fair to say that autofocus usually works great and you shouldn't have to worry about it. The situations where you push up against the limits are:

- Focussing on small objects (relative to the frame)
- Focussing in low light
- Focussing on low contrast objects
- Focussing on fast moving objects

Given that I shoot birds in flight in the overcast UK I experience all of the above nearly every time I am out.

Alex

[Alis]

Hear ya go Ali,

'Cause I luv ya buddy!

https://docs.google.com/open?id=0B2g...W1rWnZaemRqNkU

Thanks, "mate" This is great! I will come back if I have questions after reading it

[Alis]

Something funny just happened: I have a search alert in Google for "Canon 1Dx" and I just got an alert on my own post here

Never happened before!

[Colin Southern]

Thanks, "mate" This is great! I will come back if I have questions after reading it

No worries. And if you have any questions, I'm sure Alex will be happy to answer them

[Colin Southern]

The -2EV means it can focus in lower light.

That's the bit that confuses me; on one hand we're saying it can focus in lower light, but on the other hand we're saying that it can "only focus with a lens with a max aperture of F5.6 instead of F8?

In reality I should add that for me, it'll be a non-issue as the only time it would bite be in the bum would be if I was using my EF 70-200/2.8 with a 2.0x TC AND a 1.4x TC.