Wanting more zoom...

[inkjunkie]

I have an EF 70-200 f/2.8L lens that will be on a 5D Mark IV body. I realize the I can use this lens with either of the Extenders, the version II if I remember correctly. Have read that this will choke back the Aperture some. One of the things I will be doing with my gear is taking pictures at the local race track, many times at night. The track does have lights but last year when I was using one of my old lenses with an aperture of f/5.6 the pictures were far from stellar. Hoping to find a 300mm, or longer, in the f/2.8 area. Does such a lens even exist? Thanks to our ISP pretty much shutting down at 4p.m. not having much luck searching.
Thanks in advance for any help

[TCampbell]

---

@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.

[ebiggs1]

"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.

[RobertTheFat]

---

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.

[TCampbell]

I own the original EF 300mm f/2.8L IS USM (which wasn't cheap... but that was around $4k) but the current version II is now more like $6k.  The 400mm f/2.8 is basically $10k.

After the 70-200mm f/2.8 zoom... the only two f/2.8 lenses are those primes (300 & 400) and everything else is a zoom with f/4 or a variable f/4.5-5.6... so you loose a stop or two of light (and also they cost more.)

You can push the ISO pretty high on the 5D IV (which is what i would do) and you can still clean up the noise with decent software.