Teleconverters & Aperture

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Craft

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Not sure if this is a "basics" question or not, but I'm wondering why aperture changes when a converter is used?

e.g. a lens that has a max aperture of f5.6 with a 1.4 teleconverter is then only capable of f8, why is this ?

thanks!
 
The f number is calculated by the focal length divided by the aperture diameter. When you add a tc you increase the focal length so the f number changes. So the actual aperture diameter doesn't change, but the f number does.
 
the handy maths for working out if you are still going to be able to autofocus.......

A 2x teleconverter can be used on lenses with aperture at least f2.8 (becoming f.5.6)
A 1.4X teleconverter (and usually a 1.7X) can be used on a lens with an aperture of at least f4, (becoming f5.6 or f6.8)
lenses with maximum aperture of f5.6 might not autofocus with teleconverters much more than 1.4X

That said - a lot depends on the light.
 
Craft said:
Not sure if this is a "basics" question or not, but I'm wondering why aperture changes when a converter is used?

e.g. a lens that has a max aperture of f5.6 with a 1.4 teleconverter is then only capable of f8, why is this ?

thanks!
Click to expand...

Basically you are taking a small part of the image and enlarging it over a larger area. As it is enlarged, so the brightness decreases - inverse square law.

For the same reason, check the size of the front element from super-telephotos - they are massive, like the size of your hand, but still only f/4 or so.

DuncanDisorderly said:
Not all lens/camera/teleconverter combinations do the maths correctly.
According to my 5DII, the 24mm f3.5 TS-E stays at f3.5 even with a 2xTC in place.
Exposure needs a bit of experimentation :)
Click to expand...

That's because that lens is fully manual and non-reporting to the camera. It still becomes 48mm f/7 with a 2x extender. Not sure why you'd be using one on that lens though :thinking:
 
Craft said:
Not sure if this is a "basics" question or not, but I'm wondering why aperture changes when a converter is used?

e.g. a lens that has a max aperture of f5.6 with a 1.4 teleconverter is then only capable of f8, why is this ?

thanks!
Click to expand...

The f number is a ratio of the aperture diameter to the focal length of any lens.

Lets say we have a 100mm f4 lens, the diameter of the aperture blades will be (100mm / 4) = 25mm.

Now we add a 2x tele converter to make the lens 200mm, the aperture is still 25mm dia, that cannot change however the focal length has, so we now calculate (200mm / 25mm) = 8, so the focal ratio is now f8

It is as simple to multiply the bare lens maximum aperture by the tele converter's factor, in your example f5.6 x 1.4 = 7.84 (Rounded to f8)
 
HoppyUK said:
That's because that lens is fully manual and non-reporting to the camera. It still becomes 48mm f/7 with a 2x extender. Not sure why you'd be using one on that lens though :thinking:
Click to expand...
I already had the 2xTC and it means I don't have to buy the 50mm TS-e :)
It works VERY well.
The rules of thumb for tilt seem to be the same as when used without the 2xTC.
 
DuncanDisorderly said:
I already had the 2xTC and it means I don't have to buy the 50mm TS-e :)
It works VERY well.
The rules of thumb for tilt seem to be the same as when used without the 2xTC.
Click to expand...

It should work exactly as a 48mm f/7 T&S lens, but it's not a recommended combination. I find it hard to believe that edge sharpness at max shift would be be anything special at all.
 
I must admit I've only used this combo with tilt.
It's plenty sharp enough at the edges with 'normal' levels of tilt.

By 'normal' I mean using my tripod without extending the legs and placing the horizon near the top of the frame oriented in portrait. It doesn't get the tripod legs in shot using a full frame sensor, but it's close. This set-up needs only about 1/4 of the available tilt movement.
 
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