Math with two extenders (1.4 + 2.0)

[pontus_lindqvist2]

Hi all,

 

Just bought myself a second Canon extender today (2x, I already had the 1.4x).

Using my 70-200 2.8 IS with both a 20D and 1D mk III body I get the following

results which confuse me a bit:

 

<br><br>1. Using 70-200 + 1.4x gives me a maximum possible aperture of 4.0

<br>2. Using 70-200 + 2.0x gives me a maximum possible aperture of 5.6

<br>3. Using 70-200 + 1.4x + 2.0x gives me a maximum possible aperture of

<b>5.6</b>

<br><br>

It is the third example above I don't understand. I would expect the maximum

aperture to be 8.0 instead of 5.6 since I loose one step from the 1.4x and two

from the 2.0x. Or is my math all wrong here?

 

.pli

[walang_pangalan]

In the EOS system, the lens senses the existence of the extender, not the body. And because the lens can't "see" past the first extender, when the body asks the lens who it is and what kind of properties it has, the lens can only report the effect of the 1.4x, even if the 2.0 is mounted as well.

[j.kivekas]

2.8 (1.4)-> 4<br>

4 (2) -> 8.0 .... (yeah, I think you are right)<br>

Propbably the body sees only the converter with bigger X. You can always shoot grey surface and compare on the screen in the lab-mode looking at the brightness value.

[j.kivekas]

If Walang has got it right the order in which you stack the converters should change the aperture realized by the EOS system, am I right?

[Matt Laur]

While this is interesting (academically), in practical terms, isn't it a little pointless? I just mean in terms of the degradation of the image, and the likely inability to get useful focusing behavior - it seems that stacking TCs is just way more trouble than it's worth. But then, I'm a nerd, and I'll try anything once, too. But since that's probably - what... $1000 worth of TCs, isn't another, longer lens looking a little more useful, at that point? Just sayin'.

[walang_pangalan]

The lens computer can only "see" the extender directly connected to it, so if you change "realized" to "reported" you have it right. Of course, if you put two converters on, the optics are f/8 regardless of order or what the lens computer thinks.

[pontus_lindqvist2]

Hi all,<br><br>

 

I guess your theory about the inability to see multiple extenders might be right. But something doesn't add up anyhow. When using both extenders it is physically <u>only</u> possible to use this order (anything else won't fit):<br><br>

 

Camera - 1.4x - 2.0x - Lens<br><br>

 

Thus, it seems very strange that the camera thinks it has a 2.0x converter on (i.e. 5.6 instead of 4.0). Might be that the camera asks the lens and the lens returns what is attached to it somehow...

<br><br>

Matt: Well, it might be academic but I still find it kind of interesting. Don't know if I'll be using the setup much but it's interesting to try if it works don't you think?

<br><br>

 

.pli

[robin_sibson1]

Walang has it nearly right. When both Extenders are in use the one nearer to the lens is the 2x, not the 1.4x, so the original f/2.8 lens is reported as f/5.6. The logic is extremely simple: the lens looks for open/closed circuit between the extra pin nearest the standard ones, and either or both of the other two. There's no electronics in the Extender. Aha, you'll say, but that gives three options apart from "nothing there", not just two. Quite right. The third option signal the presence of the Life Size Converter to the 50/2.5 macro lens.

[pjmeade]

Can the two extenders be mounted in tandem?

 

I think I would just save for a longer lens.

[robin_sibson1]

Peter, the Extender 2x II (but not the original version) can be stacked ahead of (that is, towards the lens relative to) any Canon Extender, but no other stacking is possible except by the use of extension tubes as spacers, in most case losing infinity focus. Users of very long telephoto lenses have reported results from stacked Extenders that they regard as acceptable. However, the standards that pixel-peeping is pushing us towards are so high that the Extender 2x on its own now gets a pretty bad press with all but a very few lenses, whereas there are plenty of enthusiastic users of the 1.4x.

[john_hanlon3]

I sometimes use a Canon 200 f/2.8L with both the Canon 2X & 1.4X extenders mounted on a Rebel XT using a mounting ring and tripod with ball head. Whilst there is some loss of sharpness, I still find the results perfectly acceptable for my amateur needs. I use this combination to photograph small birds in my back yard. The max aperture is f/5.6 which makes for a dim viewfinder and the autofocus is unreliable so I focus manually. These little birds move very quickly so getting a picture with the bird actually in the frame is , I think, an achievement in itself!

[mark_chappell]

<I> Users of very long telephoto lenses have reported results from stacked Extenders that

they regard as acceptable.</i><P>

 

I've done this occasionally with a 500/4. It's a pain to focus sharply (an f11 combination that

requires manual focus). I have not done rigorous tests but I believe the results -- if good

focus is achieved -- are better than using just the 2X and then upscaling. Either way, image

quality is at best OK -- often fine for the web but not for any substantial enlargement. The

2X by itself on the 500/4 can yield excellent results if stopped down a bit from wide open.

[John Crowe]

I have had fun mounting two 2x converters and a 2x and a 1.4x but it really is only 'fun'. No real point in doing it. I finally sold all my 2x converters and prefer cropping images from a 1.4x instead. Much better quality.

 

 

While the Canon camera may not be telling you the truth about the aperture the light meter will not not be lieing. Your camera will be adjusting the shutter speed for the extra stop of light it is missing from the additional converter. Back in the old days Canon manual cameras did not know that converters were mounted, and the appropriately adjusted shutter speed simply indicated the reality. So meter a blank wall with the 2x then meter it again with the 1.4x as well and you will see that you are left with a shutter speed 1 stop slower than with the 2x alone.

[marknagel]

I have stacked my Canon 1.4x and Tamron 2X and the results are good with my 300/2.8is. I'm looking at it now, and I can only stack it Camera-2X-1.4X-Lens, again that is the Tamron 2X. With these stacked, the aperture reads f/4 and AF works fine, probably faster than my 100mm Macro, but slower then 300mm_1.4x only, not much though.

 

I thought I tried it with the Canon 2X II and could not stack them, I returned it, so I can't confirm that, but I was almost positive they did not stack.

 

The bottom shots on this link are done with the Canon 2X and Tamron 2X stacked, the 1.4 and Tam 2X is a bit sharper, but I don't have anything handy to post. As you can see, not to bad, these are 100% crops.

 

http://www.nagelhome.com/phototest/Teletest.jpg

 

 

M

[robin_sibson1]

Mark N, your 2x Tamron converter presumably has the glass not extending at the front, and with no glass-free space at the back, so it's unsurprising that it fits in the order you descrbe but not otherwise. The Canon Extender 2x II has glass extending at the front but a glass-free space at the back, and definitely stacks on the Extender 1.4x with the 1.4x closer to the camera. That's what the documentation says, but, better still, I've just stacked them like that.