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300mm F4 IS TC? (1 Viewer)

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I use my 300mm F4 IS with a SIgma TC (1.4) and it works fine with great resolution. I was wondering if anyone has any feedback for selecting a 2X to go with this lens? Do the Canon TC's allow AF i understand on certain lenses like the 100-400 that is lost with the 2X. Any feedback greatly appreciated.
 
Which camera?

Regardless of TC manufacturer, you will lose AF if you use a 2x on the lens with anything other than a 1d (pro) body.
 
Keith Reeder said:
Which camera?

Regardless of TC manufacturer, you will lose AF if you use a 2x on the lens with anything other than a 1d (pro) body.
Click to expand...

Hmm intersting I wonder why Canon did that? I have a 40D and a 20D as a backup BTW. The results with the Sigma 1.4 are really fine.
 
It's not really Canon, it's the physics of phase detection AF.

An f/4 lens with a 2x gives you f/8 (as does a 1.4x on an f/5.6 lens like the 100-400mm): for various rather complex reasons to do with the physical design and light paths of the AF system in non 1d bodies, the AF system simply can't function properly at f/8 and Canon has chosen to make its non-pro bodies not even to try and AF at f/8 because (in theory anyway) the AF system simply will not function properly in those circumstances.

Pro bodies have a lot more space in them, allowing for a bigger mirror box and a consequently much more "forgiving" (at f/8) AF system design, with light paths that will still allow enough light to reach the phase detection sensors in their AF modules to work well.

(Note that it's not just about how much light is reaching the AF sensors, but I don't want to get into discussing the effect of diffraction, circles of confusion and whatnot on phase detection AF).
 
Last edited:
Keith Reeder said:
It's not really Canon, it's the physics of phase detection AF.

An f/4 lens with a 2x gives you f/8 (as does a 1.4x on an f/5.6 lens like the 100-400mm): for various rather complex reasons to do with the physical design and light paths of the AF system in non 1d bodies, the AF system simply can't function properly at f/8 and Canon has chosen to make its non-pro bodies not even to try and AF at f/8 because (in theory anyway) the AF system simply will not function properly in those circumstances.

Pro bodies have a lot more space in them, allowing for a bigger mirror box and a consequently much more "forgiving" (at f/8) AF system design, with light paths that will still allow enough light to reach the phase detection sensors in their AF modules to work well.

(Note that it's not just about how much light is reaching the AF sensors, but I don't want to get into discussing the effect of diffraction, circles of confusion and whatnot on phase detection AF).
Click to expand...

Keith
If the 20D works by taping the pins (my old one worked fine) and a 1D works.......I would have "thought" it feasible for Canon to design the 40D to work.
I must stress I speak "out of ignorance" but have often wondered why they don't overcome this.
 
From experience:

At f8 the 40D, 50D, 60D and 7D do not work with taped pins (or a DGX TC) using the centre point

On the other hand the 500D and 550D work fine.
 
David Smith said:
I would have "thought" it feasible for Canon to design the 40D to work.
Click to expand...
It's because they increased the sensitivity of the 40D (and subsequent bodies) centre AF point, David.

You'll recall that the 40D's centre point is "high precision" when used with f/2.8 lenses: this means (again because of how phase detection works) it places more demands on the "signal" coming though to the AF system, because the 40D AF module contains more optical parts, which means the light coming through has - in effect - to "work harder".

As the linked article above says, the high precision focal point relies especially on wide apertures (note, even if not being used at f/2.8 - presumably because of diffraction and issues around circles of confusion) and a TC makes for a much-narrowed aperture...
 
Keith Reeder said:
It's because they increased the sensitivity of the 40D (and subsequent bodies) centre AF point, David.

You'll recall that the 40D's centre point is "high precision" when used with f/2.8 lenses: this means (again because of how phase detection works) it places more demands on the "signal" coming though to the AF system, because the 40D AF module contains more optical parts, which means the light coming through has - in effect - to "work harder".

As the linked article above says, the high precision focal point relies especially on wide apertures (note, even if not being used at f/2.8 - presumably because of diffraction and issues around circles of confusion) and a TC makes for a much-narrowed aperture...
Click to expand...

Very technical........I'm impressed Keith. I will read and (try) to digest that info......thanks again.
I'm sure my 1st camera (Brownie 127) didn't have phase detection

:-O
 
I have recently used my 300mm F4 IS with a Canon 2x Mk2 converter and was pleasantly surprised by the IQ, it wasn't the pile of mush I was expecting, but actually produced reasonably useable images. However the AF was ponderous at best! I was shooting in very good light against an uncluttered background and still the AF liked to explore it's full range before locking on, that was on a 1D4! On an xxD body I wouldn't bother, just use the 1.4 and crop/or better get closer!
 
The thing is that Canon design these AF sytem so that they work well up to f5.6. they could easily change the electronics so that the Cameras would not stop AF being attempted at f8 (e.g. no need to tape pins etc) but AF would be very poor at times. This would give Canon AF a very poor reputation and would probably lead to a melt down in sales over a time so they only give you a AF that they are confident will deliver good results at all times. To the novice It is worth remembering that f5.6 over f8 does not just give you a bit more light but it gives twice as much!!

This is a similar scenario as to why Canon extenders only fit certain Canon lenses - if Canon do not think that a particular lens is suitable for use with an extender they make sure that their extenders will not even fit (this is not just restricted to the speed of the lens either).

BTW AF can be achieved without taping pins at f8 on Cameras like the 7D by using live mode AF where the image sensor is used to focus, trouble with live view AF is that it can be very slow compared with normal phase detect AF. With live view AF there is no cut off point as to when AF is attempted and it will even attempt to AF f11 or smaller.

With my 300/2.8 I have often used stacked Canon 1.4x and 2x tc to give 840mm f8 and the 7D does attempt to AF because the camera only sees the 2x tc so thinks it is f5.6 but AF is very variable - it is interesting to note with the new MkIII Canon extenders Canon has stopped the ability to stack.
 
johnf3f said:
I have recently used my 300mm F4 IS with a Canon 2x Mk2 converter and was pleasantly surprised by the IQ, it wasn't the pile of mush I was expecting, but actually produced reasonably useable images. However the AF was ponderous at best! I was shooting in very good light against an uncluttered background and still the AF liked to explore it's full range before locking on, that was on a 1D4! On an xxD body I wouldn't bother, just use the 1.4 and crop/or better get closer!
Click to expand...

That was my experience with using a 2x MK2 on a 20D using center focus point. Never thought of trying the others at the time. In good light and with it already nearly in focus it would never lock on. It would shoot miles out as soon as the shutter button was half pressed. I could only use it in manual focus mode. Needless to say the 2x MK2 was returned. The 1.4x MK2 works fine.
 
jalethbridge said:
A good reason to stick with the mk2 converters if you ask me, as well as saving gadquillions of dosh.
Click to expand...
That depends on the performance, from what I am reading the 2x MkIII is a step-up from the MkII even when used on existing lenses (and in particular the 300/2.8). I hope to try one soon and would certainly think about upgrading if I think it is warranted, even though I would lose the ability to stack (and £200 +).
 
I use it with a 30D and it has no AF of course, but creates quite good images in reasonable light and with good technique (hand held mostly). It will even stack well with a 1.4x when I am desperate in VERY good light and makes useable, and good looking images (also mostly hand held). Not been disappointed at all. :) For me manual focus is not (yet) an issue at all. :)
 
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