Silly 10D Focussing Question...

[Cadwell]

OK, I don't think I've asked a truly silly question for a while but this has been bugging me and I guess I am due one. Here goes (please be gentle with me).

First some facts as I understand them

1) A 10D (and 300D/dRebel) cannot autofocus with a lens which has a maximum aperture of less than f/5.6.

2) A Sigma 50-500mm (Bigma) has a maximum aperture of f/6.3 at 500mm.

3) A 10D (and 300D/dRebel) can autofocus at 500mm with a Bigma.

So how does that work? At 500mm the camera is dealing with a lens with an effective maximum aperture of f/6.3 which according to the manual it can't autofocus with; yet it does.

[robertwgross]

Are you considering that the Sigma lens has a different maximum aperture of f/4 at the short end? Or are you only considering the f/6.3 long end?

---Bob Gross---

[Cadwell]

Are you considering that the Sigma lens has a different maximum aperture of f/4 at the short end? Or are you only considering the f/6.3 long end?

---Bob Gross---

Oh, I have no problem with the fact that it can autofocus at the short end. It's what happens at the long end that bothers me. After all, the lens doesn't change back to the short end to focus and then flip back to full zoom to shoot so the aperture at which it autofocuses is f/6.3 when you're pointing it at something at 500mm.

[Scottes]

From what I understand, the Sigma lies, and tells the camera that it's at f/5.6.

At least that's what CDS told me, so blame him if I'm wrong. :)

[Panza]

But why have Canon disabled the AF for apertures larger than 5.6 when it obviuosly can work ?
Is it because the AF at large apertures becomes sub-standard to such a degree that Canon thinks it is best to have MF instead ?

[dn7elson]

But why have Canon disabled the AF for apertures larger than 5.6 when it obviuosly can work ?
Is it because the AF at large apertures becomes sub-standard to such a degree that Canon thinks it is best to have MF instead ?

The AF definitely slows as the amount of light decreases (smaller aperture). It would be my guess that Canon stopped the AF there for a couple of reasons 1) limits of reliable and fast AF, and 2) to further differentiate from the higher priced bodies (AKA marketing decision, similar to the firmware disabling in the DRebel vs. the 10D).

[Cadwell]

OK... so are we saying that the Sigma lies to the camera but that it doesn't matter because the 10D can AF at apertures smaller than f/5.6 but that Canon has crippled that functionality in software?

[Scottes]

Yep.

I use my 100-400 L at 400mm quite often with extension tubes. At 400mm it's f/5.6 - even with a 12mm tube I see problems with AF - more so than usual with the 100-400. And it's extremely noticable with a 36mm tube on it.

Many people here have added 1.4 TCs and "taped" them. So at f/8 it *will* AF sometimes, but people have mentioned that it hunts a LOT more.

I'd say that the 10D can *reliably* focus at f/5.6, would probably work OK at f/6.3 but you'd see more problems. So Canon just drew a line in the sand and said "f/5.6" and that's that.

How many people would complain if Canon never set a limit, but had all sorts of problems when people tried to AF at f/8? The complaints would be enormous.

[Cadwell]

Oh I'm not complaining, just trying to understand. The 100-400L is the slowest lens I have so it's not a problem for me. There is obviously going to be a margin where things work/sort of work/won't work.

I have no need of the tape trick. I can get to 600mm quite happily with my Sigma 120-300mm and a 2xTC. I did try it once with a 2x and a 70-200mm f/4 just to see, but the camera failed to AF at all so I gave it up as a bad idea.

[CyberDyneSystems]

It's not that Canon is crippling anything...

Canon has merely installed a cut off point for what they feel is reliable AF..

There ahs to be some point at which the AF will no longer function reliably? Right? So rather than throw caution to the wind and have to deal with thousands of uneducated consumers complaining about "funky" af.. Canon has rated each camera for where it feels the aperture can still provide "flawless" AF performance.

The camera simply will not event try to AF beyond this rating.

In the case of all but the 1 and EOS3,.. this is f/5.6 (this is true of many other manufactures by the way,. not just Canon)

One of the reasons they have a "cut off" is to protect the lens itself. With poor AF function.. hunting and worse can occur,.. a lot.. this erratic activity can literally burn out a focus motor if one ignores the erratic behavior and continues to push things.

Canon prevents this for the consumer by refusing to attempt to AF when the requirements are not met. Again.. as do most manufactures.

Now depending on quality and more importantly.. magnification.. some lenses will AF on a given body beyond it's f/5.6 rating.

Sigma is one manufacturer that has pushed this envelope quite succesfully with the 50-500mm... (and also wth the 170-500mm) essentially.. the 50-500mm is using "The tape trick" on a much better level.. instaed of providing the EOS with NO info.. it provides cheated info..

So.. f.6.3 is NOT f/6.3 all across the board.. (though many would argue it is) because the AF is not just about light and contrast. Some f/6.3 can AF some can't.. Some lenses will AF with the tape trick, some won't.. some 10D's will with the same lens,. some won't etc...

Anything beyond the f/5.6 is "gravy" but it is not guaranteed.

[samdring]

Agree with everything said so far but to help me better understand whether my own thoughts are correct...

On AF lenses the AF process takes place at full aperture regardless of selected aperture.

Even if a non-constant aperture lens is at its long (and reduced aperture) end, AF still takes place at full (short end) aperture but the lens prevents camera from allowing selection of that full aperture. On shutter release, it stops down to max selectable aperture.

So any lens that has at least 5.6 at any part of its range will AF?

Whoops this could be embarrassing :wink:

[Scottes]

Samdring, no. A lens AFs at it's maximum aperture for that particular length. A lens is listed at it's maximum aperture range, such as a zoom with f/4-5.6. So halfway through the zoom range it's max aperture may be around f/4.5. At this length it will AF at f/4.5.

Think of this as the lens having an max aperture opening of a particular fixed size. As lens length increases, the ratio between this fized size and the length of the lens increases. The lens length increases but the opening stays the same, thus the aperture changes, since aperture is a ratio of the opening to the length.

[CyberDyneSystems]

The reason being that the lens CAN'T open to it's max aperture at the short end when it is zoomed to the long end. It's physically not possible.. if it could.. then you'd have a constant aperture lens, as opposed to variable.

But you ARE correct in assuming that the AF does take place with the aperture "wide open" at it's largest Possible opening.. in the case of a Variable zoom like the 100-400mm @ 400mm this means f/5.6.

In fact.. "Wide open" is just how these EOS AF lenses sit at idle all the time. Whether it's on your camera or sitting on a shelf.. the aperture is wide open. It is not untill you either press the shutter release OR the "Depth of feild preview" bution that the lens actually stops down.. no matter how the camera is set.