teleconvertor question?

[wez130]

What does it actually mean when they say you lose 2 stops with a TC?

I have a 2x tc and can shoot at F/4 on my 70-200 F/4L, so does it actually mean that F/4 is actually F/8 or does it mean that my shots are useless below actual F/8 or what?

 

[IanC_UK]

the lens actually becomes a 70-200 F8 with a 2x TC fitted, because the optics in the TC prevent that amount of light from reaching the sensor, you can still shoot at F4 but the pics will be underexposed.


My understanding of it anyway

 

[EdinburghGary]

the lens actually becomes a 70-200 F8 with a 2x TC fitted, because the optics in the TC prevent that amount of light from reaching the sensor, you can still shoot at F4 but the pics will be underexposed.


My understanding of it anyway

So if you stick your exposure comp +2 , it will work as before albeit, longer shutter speed?

 

[IanC_UK]

Possibly, have never tried it though

 

[stylgeo]

the lens actually becomes a 70-200 F8 with a 2x TC fitted, because the optics in the TC prevent that amount of light from reaching the sensor, you can still shoot at F4 but the pics will be underexposed.

That's actually not correct, with a 2X TC the 70-200 f4 becomes a 140-400 f8. You can't use the f/4 because there isn't any, the largest aperture with that set up is f/8. The reason that might show you the f/4 as an option is that you probably don't have the canon TC and the camera doesn't recognise it!

 

[wez130]

the lens actually becomes a 70-200 F8 with a 2x TC fitted, because the optics in the TC prevent that amount of light from reaching the sensor, you can still shoot at F4 but the pics will be underexposed.


My understanding of it anyway

140-400 F/8

But that's what i assumed anyway

So if you stick your exposure comp +2 , it will work as before albeit, longer shutter speed?

Never thought of that one, may have to try it.

 

[wez130]

That's actually not correct, with a 2X TC the 70-200 f4 becomes a 140-400 f8. You can't use the f/4 because there isn't any, the largest aperture with that set up is f/8. The reason that might show you the f/4 as an option is that you probably don't have the canon TC and the camera doesn't recognise it!

i have shot at F/4 with TC on, full functioning AF etc with what looks like OK results (not checked yet), it will still shoot at F/4 but it will infact be F/8

It is also a kenko TC

 

[IanC_UK]

That's actually not correct, with a 2X TC the 70-200 f4 becomes a 140-400 f8. You can't use the f/4 because there isn't any, the largest aperture with that set up is f/8. The reason that might show you the f/4 as an option is that you probably don't have the canon TC and the camera doesn't recognise it!

Its still a 70-200 Lens though LOL

 

[grumpybadger]

No need. Since the camera measures light through the lens, it is already seeing the loss of light. So when it asks the lens to be f/4, it is actually f/8.

 

[stylgeo]

i have shot at F/4 with TC on, full functioning AF etc with what looks like OK results (not checked yet), it will still shoot at F/4 but it will infact be F/8

It is also a kenko TC

What I am saying is that if you were using a Canon TC, it would recognise the combination tc+lens and it would show the correct info, thus not allowing AF since the aperture becomes greater than f/5.6 (unless you are using a 1D) and showing the correct EXIF info, ie 400mm f/8

 

[markta]

Yep the lens would have a maximum aperture of f/8, wouldn't autofocus unless you have a 1 series camera and you'd lose 2 stops of light,

There have been rumours that if you cover some of the pins on the lens/convertor, it'll fool the camera into autofocusing. Never tried this myself.

Mark

 

[SDK^]

Something else to remember.
If you have 2x TC that doesn't report the aperture to the camera then you need to add 2 stops to any aperture setting on the camera.

e.g. - A 70-200L F4 with 2x TC

Camera shows F4 but the actual aperture is F8.

1) You change the aperture setting on the camera to F5.6 but the actual aperture is F11.

2) You set the aperture to F8 but the actual aperture is F16.

etc..

 

[wez130]

What I am saying is that if you were using a Canon TC, it would recognise the combination tc+lens and it would show the correct info, thus not allowing AF since the aperture becomes greater than f/5.6 (unless you are using a 1D) and showing the correct EXIF info, ie 400mm f/8

Yep the lens would have a maximum aperture of f/8, wouldn't autofocus unless you have a 1 series camera and you'd lose 2 stops of light,

There have been rumours that if you cover some of the pins on the lens/convertor, it'll fool the camera into autofocusing. Never tried this myself.

Mark

But it does auto focus without having to do anything with pins etc, it works perfectly fine, albeit a bit slowere focussing than without the TC

Something else to remember.
If you have 2x TC that doesn't report the aperture to the camera then you need to add 2 stops to any aperture setting on the camera.

e.g. - A 70-200L F4 with 2x TC

Camera shows F4 but the actual aperture is F8.

1) You change the aperture setting on the camera to F5.6 but the actual aperture is F11.

2) You set the aperture to F8 but the actual aperture is F16.

etc..

See, this is how i thought it worked.

 

[stylgeo]

Ok, what's happening is that your Kenko TC isn't recognised by your camera. The pins are there to just carry the information from the lens to the camera. So, the camera sees a 70-200mm f/4 lens when in reality the TC would multiply both your focal length and your aperture by twofold. That's why you still have the f/4-f/8 as options and you maintain AF. If you were using a Canon TC, this has full communication with the camera, so your camera would recognise it giving precise readings of the lens+TC combo, giving you correct EXIF and correct aperture readings. You would also lose AF because all cameras except from the 1 series lose AF with apertures smaller than f/5.6. But since your metering is done in the camera and not in the lens, by choosing f/4 the camera would meter the scene as it sees it. But it's a lot darker than f/4 because it's actually f/8 so it compensates for that, even though it thinks that it's f/4.

The aperture is the size of the diaphragm in relation to your focal length. So for your 70-200mm f/4, the size of the diaphragm at 200mm is 200/4 = 50mm. The aperture blades are in the lens, not in the teleconverter, so the actual size of the diaphragm rermains the same, right? As I said before, the aperture is just the relation between your focal length and the size of the diaphragm. So if the teleconverter increases your focal length to 400mm, then your aperture would be (here's some simple maths)

400/x = 50
400/50 = x
x = 8

So your aperture is f/8. It's that simple!