Other things to consider before purchasing....

[Barry Johnston]

There are other things to consider before purchasing a lens.

This info is pertinent to my MkII N, and expect it would be similar to other 1D's and other EOS cameras.

The EOS-1D MkII N's AF points are all horizontal-line sensitive. However, bright lenses with a large maximum aperture enable certain AF points to work as cross-type sensors for higher AF precision.

1. With lenses whose maximum aperture is f/2.8 or larger, the AF points highlighted in the diagram (below), will work as cross-type sensors sensitive to both vertical and horizontal lines. The remaining 38 AF points will only be horizontal line sensitive. The cross-type sensor's vertical-line sensitivity is about three times higher than its horizontal-line sensitivity.

2. Other L-series lenses whose maximum aperture is f/4 or larger, the centre AF point will be a cross-type sensor with or without an Extender attached. The remaining 44 AF points will only be horizontal-line sensitive (except with EF70-200mm f/2.8L USM + Extender EF1.4x or EF1.4x II)

(Malurus lamberti - Variegated Fairywren)

There is a list of lenses listed in the instruction manual.

I think this information is worth considering, depending on the type of photography the lens is intended for.

Any other comments or thoughts on this?

Cheers,
Barry.

 

[Asher Kelman]

Hi Barry,

This is beautiful well layed out opening post! One thing you might want to clarify is the perhaps ambiguous word :larger" when applied to f stop. Maybe one can be clearer by using aperture wider than or narrow than.

So with the Canon XTi, for example one might say "using a lens with an aperture of 2.8 or wider, the central focus point works as a cross hair, and is sensitive to both vertical and horizontal detail, whereas for example a lens with a mximum aperture size of only f=4.0 would be not wide enough to have this accurate sensor ever work."

If one doesn't specify that we are talking about large apertures rather than large f stops, I fear we may gert confused.

This, subject, BTW, is so important and is the unfortunate crippling of perfectly wonderful cameras like the Rebel XTi which otherwise would be a great lightweight pro backup!

It sssentially means that using the perfectly superb 70-200 f 4 L IS with the XTi, one would give up the most sophisticated centra focus point!

So birding with an XTi one is compromized even with the best long lenses unless they are f 2.8 or better!

Asher

 

[Barry Johnston]

Hi Asher,
(Happy New Year BTW !!)

Thanks for the note.

The text was taken directly from my Canon Instruction manual, for the MkII N. I am still ploughing my way through it, but it struck me as being extremely relevant when deciding on a lens, which is what I have been doing lately.
As I do mainly sport where the subjects are moving eratically, I really have no alternative but to purchase f/2.8 (or 'Wider') lenses, for greater focusing accuracy. I expect the same would be crucial for birds in flight or other eratically moving subjects (although, most of the larger lenses are all f/4).

I expect a lens with an aperture of f/2.5 would allow the cross-type sensor to work, as it is wider than f/2.8. I am not sure whether this would work for non-Canon lenses though.

By "Larger", I do assume they mean "Wider", but it's a good point. Thanks.

Barry.

 

[Asher Kelman]

Good stuff to point out. Canon has added different degrees of technology when they could just as well have given us build to order choices as in buying a MacBook Pro.

It's a pretty haughty approach to marketing. There is simply no reason why the sensors of the XTi or the 5D shouldn't be state of art. At present, Canon cameras are so good and there are so many models that the seller is the master of the deal.

However, with Canon lenses being easily saleable to recover one's investment, we are entering a new era. Nikon is already a major consideration for anyone who might normally go for 1 camera for sports and everything else. The D3 seems a great competition. It's not that more expensive than the 5D and a lot more value than the 1DMark III with it's extended low light capability.

So for now, we have to accept these limitations perhaps, because we are loyal people. However, this may not last as Nikon get's up to speed.

Asher

 

[Don Ferguson Jr.]

Hi Barry,

This is beautiful well layed out opening post! One thing you might want to clarify is the perhaps ambiguous word :larger" when applied to f stop. Maybe one can be clearer by using aperture wider than or narrow than.

So with the Canon XTi, for example one might say "using a lens with an aperture of 2.8 or wider, the central focus point works as a cross hair, and is sensitive to both vertical and horizontal detail, whereas for example a lens with a mximum aperture size of only f=4.0 would be not wide enough to have this accurate sensor ever work."

If one doesn't specify that we are talking about large apertures rather than large f stops, I fear we may gert confused.

This, subject, BTW, is so important and is the unfortunate crippling of perfectly wonderful cameras like the Rebel XTi which otherwise would be a great lightweight pro backup!

It sssentially means that using the perfectly superb 70-200 f 4 L IS with the XTi, one would give up the most sophisticated centra focus point!

So birding with an XTi one is compromized even with the best long lenses unless they are f 2.8 or better!

Asher

Then again you have Danille using a XT with 400 5.6 lens to get these bird photos .
Does not look like she was compromised
Also I thought the XTi has the same focusing system as the 40 D .
Don

http://www.pbase.com/zylen/root

 

[Asher Kelman]

Well Don,

I guess the differences that Canon engineers tell us about the XTi outer focus points v. central cross hair are like the science that said hummingbirds couldn't fly!

Still, at least for the 5D, I know it is croppled compared to the 1D series.

Asher

 

[Barry Johnston]

Daniella....

Hello Don,

Yes, I agree, Daniella does extremely well with what she has. Her photography is outstanding to say the least, and I can only ever aspire to reach such heights.

Regards,
Barry

 

[Don Ferguson Jr.]

Hello Don,

Yes, I agree, Daniella does extremely well with what she has. Her photography is outstanding to say the least, and I can only ever aspire to reach such heights.

Regards,
Barry

Yes sir ,makes you realize what someone with a lot of ability can do with with the little XT,and she does not even use the XTi that has improved focusing over the XT with her 400 5.6 .
Makes me happy to see what she can do since I just got a new 400 5.6 for my XT .
I would like to have a newer camera but I cannot afford and justify both right now
Regards
Don

 

[Alan T. Price]

aperture vs f-stop

f-stops are just numbers but apertures are diameters.

aperture is the effective or equivalent lens opening corresponding to whatever focal length and f-stop are being used. If it was a simple one-element lens then the effective aperture would be the actual aperture, but it caries when the light path is compressed in a complex lens.

Aperture = focal length / f-stop

That is why apertures should be written as say f/2.8 instead f2.8 or f-2.8 or f_2.8. Doing so reinforces the fact that the aperture is the focal length divided by the f-stop, and that makes it easier to understand that f/4 is a smaller aperture than f/2.8.

In practice, aperture is generally determined by the largest (i.e. the front) lens element diameter. The aperture is no bigger than the front element. e.g. A 600mm f/4 lens will need a front lens element of about 150mm to achieve f/4, since 600/4 = 150. A 400mm f/5.6 needs a front element diameter of about 72mm but a 400mm f/2.8 has to be double that diameter.

- Alan

 

[Alan T. Price]

good point, but...

Barry, you've raised a good point but the limitation is not as severe as it might first seem unless what you are photographing only has vertical contrast edges (like a flag pole). In that case you can rotate the camera a bit, focus, cancel future AF to prevent re-searching, and then restore the camera to its correct orientation. If you did this with the centre AF sensor then you would still be focusing on precisely the same part of the subject. Another option is to employ the full-time manual focus feature of the USM lenses.

In general, the AF system will cope with leaning lines almost as well as horizontal lines even though it might take a little longer to lock on. In your example the top sensor would operate correctly with the line at the top of the bird because it has a horizontal contrast component.

If you enable partial AF point expansion to say 7 points (not possible on the non-1-series bodies except the 5D, and then only with the central sensor and only in AI Servo mode) then you have 7 sensors looking at roughly the same place that are all trying to find a horizintal contrast to latch onto. The selectivity is not as fine as with a single sensor on the same camera but there is a greater chance of AF success. Full auto AF selection is likely to be too unselective unless there is no background or foreground to interfere with subject acquisition.

The reality check is that many people have been using f/4 lenses with and without tele-extenders for years with great success and so the limitation in general has to be more theoretical than real. That's in general but there will be exceptions.

- Alan