Canon Community Canon Community
 


Reply
Frequent Contributor
Posts: 41
Registered: ‎10-01-2017

Re: Wanting more zoom...

Sorry for the delayed response. If I wasn't on disability the price tag of that lens wouldn't scare me off...but I would have to wait until I was in one of my bi-polar shopping spree moods...lol...

Having been looking at the 1.4 II extender, wish it didn't take a full stop out as I will be doing a lot of lower light shooting. 

Hmmm....

Esteemed Contributor
Posts: 3,186
Registered: ‎06-11-2013

Re: Wanting more zoom...


inkjunkie wrote:

 

Having been looking at the 1.4 II extender, wish it didn't take a full stop out as I will be doing a lot of lower light shooting. 

Hmmm....


That's really just math.  Also the choice of "1.4" has mathematical significance.

 

The focal ratio of any lens is simply the focal length of the lens (in millimeters) divided by the clear aperture (in millimeters).

 

So if you had a 100mm lens and it's widest possible aperture opening was 25mm then that would be an "f/4" lens because 100 ÷ 25 = 4.

 

If you then add a 1.4x tele-converter, it multiplies the focal length by 1.4x... so the 100mm lens effectively becomes a 140mm lens.  

 

However... the 100mm still only has a 25mm aperture opening... the tele-converter can make the focal lenght longer, but it can't make the lens physically wider.  So now when you divide 140mm (the new effective focal length) by the 25mm aperture opening (because that part didn't change) you get 140 ÷ 25 = 5.6.

 

Notice how the lens dropped from an f/4 to an f/5.6 (which is exactly one full f-stop) by adding a 1.4x extender.  That only happens because the multiplier is 1.4.  

 

It turns out 1.4 (the real value is 1.41421...) is the square root of 2.  Whenever you change the size of a circle by the square root of 2 (either multiply the diameter by the square root of 2 to make it larger ... or divide the diameter by the square root of 2 to make it smaller) you will EXACTLY double or halve the area of that circle.

 

And since this particular circle is the aperture opening which allows the light to flow through the lens, you exactly double or halve the amount of light that can arrive at the sensor (assuming the shutter speed wasn't changed).

 

So the 1.4x multiplier was selected because it happens to be the square root of 2 (well... a rounded off value for the square root of 2) and it changes the exposure by exactly one full f-stop so you don't have to worry about fractional f-stop changes.

 

A 2x change to a circle's diameter will quadruple it's area (if making it bigger) or reduce it to one quarter (if making it smaller).  So a 2x extender changes the camera's effective f-stop by 2 full stops.

 

Basically you muliply BOTH the focal length and the focal ratio by the multiplier of the tele-converter.

 

This is true of all teleconverters.

 

Tim Campbell
5D III, 5D IV, 60Da
VIP
Posts: 8,469
Registered: ‎12-07-2012

Re: Wanting more zoom...

"Having been looking at the 1.4 II extender, ...."

 

The 1.4 tel-con works with the 70-200mm f2.8 but it doesn't work with all of Canon's lenses. So be careful there if you have another tele or zoom in mind.

A lot less stuff for 2018 but still a lot of Canon stuff. Along with, a lot of other stuff.
Esteemed Contributor
Posts: 3,931
Registered: ‎06-25-2014

Re: Wanting more zoom...


inkjunkie wrote:

Sorry for the delayed response. If I wasn't on disability the price tag of that lens wouldn't scare me off...but I would have to wait until I was in one of my bi-polar shopping spree moods...lol...

Having been looking at the 1.4 II extender, wish it didn't take a full stop out as I will be doing a lot of lower light shooting. 

Hmmm....


Yes, the extender turns an f/2.8 lens into an f/4. But you mainly need f/2.8 indoors, and you mainly need the extender outdoors. So it sort of works out.

Bob
Boston, Massachusetts USA
powered by Lithium

LIKE US on Facebook FOLLOW US on Twitter WATCH US on YouTube CONNECT WITH US on Linkedin WATCH US on Vimeo FOLLOW US on Instagram SHOP CANON at the Canon Online Store
© Canon U.S.A., Inc.   |    Terms of Use   |    Privacy Statement