Understanding Teleconvertors

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Paul
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Hi,

I am thinking about buying a Canon TC.

I know these come in 1.4x and 2x. I know that the 2x seems to affect the IQ more than the 1.4x

What do I loose as far as apeture with the TC's? Is it one stop with the 1.4x and 2 stops with the 2x

I read somewhere that if I use the 2x I will looks autofocus as well - is that correct?

I would mainly be linking it with a 70-200 2.8 (NonIS) or a 75-300

Thanks
Paul
 
Assuming Canon are the same as Nikon and Sigma....1.4 = 1 stop loss and 2x = 2 stops loss.
 
On a Canon consumer grade camera you loose AF if the wide open aperture is less than f/5.6

So on an f/4 lens you can add a x1.4 TC or a x2.0 on a f/2.8 lens

Some you can fool by taping pins.

AF speed is noticably reduced with a TC, I think Canon state by 50% on their benchmark lens (300mm f2.8 L IS) with a x1.4 and more with a 2.0.

Personally I think with 15mp + cameras, simply cropping is a better way.
 
andrewc said:
On a Canon consumer grade camera you loose AF if the wide open aperture is less than f/5.6

So on an f/4 lens you can add a x1.4 TC or a x2.0 on a f/2.8 lens

Some you can fool by taping pins.

AF speed is noticably reduced with a TC, I think Canon state by 50% on their benchmark lens (300mm f2.8 L IS) with a x1.4 and more with a 2.0.

Personally I think with 15mp + cameras, simply cropping is a better way.
Click to expand...

^^^ :thumbs:

Interesting point about cropping instead of using a TC. Quite a few people are now coming to this conclusion with super-high pixel cameras. Better results, and no AF problems.
 
ive got a 2x tele convertor for my 90mm tamron 1:2 macro and I have had better results just cropping an image from 10mp sensor.

the tele convertor just seems to soften the image too much for my liking.
 
HoppyUK said:
^^^ :thumbs:

Interesting point about cropping instead of using a TC. Quite a few people are now coming to this conclusion with super-high pixel cameras. Better results, and no AF problems.
Click to expand...

I've even come across a Wedding Pro who refuses to turn his camera to 'portrait' mode for a shot, simply preferring to crop it later :lol:

Sounds daft I know, but the argument is sound enough that even cropping to just 6mp from 18mp that's easily enough for an A3 print, and there are very few shots even in A3 albums that are both portrait mode and printed bigger than that

I made the same argument in a thread a couple of years ago about cropping being the 'new' telephoto for church shots, but no-one really got it then :(

But think about it now - a 50mm f1.2 at 1000 ISO and 1/50th sec cropped to even 33% of the image is likely to be a sharper less noisey image than using an uncropped image from a 70-200mm f2.8 at either a much slower shutter speed or much higher ISO :)

DD
 
Chaz Photos said:
one point about a TC is you have the exter zoom but you can all so crop this one as well
Click to expand...

That's true enough, but with a TC in the system it has its own optical impact which is never that good, and sometimes very far from it.

Also, TCs only work well with certain lenses. Basically the longer the better seems to be a good rule. With cropping, you can do it with any lens at all.

Not that this is a perfect subsitute for fitting the right lens in the first place, but sometimes that lens doesn't exist, as in Dave's example above, or you don't have it with you, or it's just impractical or impossibly expensive. Eg those super-teles that cost £5k plus and are absolutely massive.

Just as with a TC, to get a decent cropped image you need to have a very good lens to start with. Very good. Personally, I'm thinking about something like a Canon 300L 2.8 (admittedly £3k) which is incredibly sharp and relatively wieldy, on an 18mp 7D. That is already effectively 480mm 2.8 on a 1.6x crop body, and if it will stand 2x enlargement (1/4 the image area) on top that's a 960mm equivalent f/2.8 lens on a 4.5mp camera. Depth of field would be equivalent to f/5.6, which is another bonus as far as I'm concerned.

For birders, who always need longer and longer lenses, that is an absolutely incredible spec. Relatively affordable and manageable, unlike an 800mm f/5.6 prime for example which is £10k. Some of the guys on here are doing this with the 7D, coupling it to a 500L 4, another very sharp lens. They could be getting an effective 1600mm that actually works :eek: :D Don't forget to turn on the IS chaps ;)
 
Since the OP has the 70-200 2.8, I'm frankly surprised the 75-300 sees any use whatsoever. If the extra 1.4x would really be of use, cropping aside, I'd defo get the tele and ditch the 75-300.
 
I have a 2xTC but only use it in conjunction with my 600mm ED doublet scope. The results seem to me to be very good, but it's manual focus only.
 
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