Canon 300 f/4 and 1.4x II

[jeffersonposter]

I have a 300 f/4 IS and was shooting with the 1.4x II on a 5D at Road Atlanta the other weekend. When doing some post processing in PS I noticed the max. aperture was f/5.7 not f/5.6. the shot was f/10, ISO 125. The 5D auto focus, as you know is f/5.6 max. I did not see a problem with auto focus, but I do not care much for the 1.4x II on this lens, camera combination. Without the 1.4x II the max. apeture is f/4 where it should be. Have not tried this combo on the 30D yet, but interested if any one elese has seen this, or does this lens need to go back to Canon for some fine tuning? Very sharpwithout the 1.4x. Have not had this glass but a couple of weeks.


JeffersonPoster

[Daniel Browning]

When doing some post processing in PS I noticed the max. aperture was f/5.7 not f/5.6.

This is one of those funny things about photography. Here are the true f-numbers:

1.41421
1.9999~
2.82842
3.9999~
5.65685
7.9999~
11.31370

Of course, it would be a bit of a pain to write down all those numbers every time, so we round them like so:

f/1.4
f/2
f/2.8
f/4
f/5.6
f/8
f/11

The only problem is f/5.6. It was not rounded correctly. 5.65 should be rounded to 5.7, not 5.6. But for whatever reason, 5.6 is the number we use. Every once and a while, though, someone (especially computers) will do the rounding correctly, and you'll see f/5.7. I suspect that is what's happening in your case: normally the camera knows to pretend that 5.65 is rounded to 5.6, but with the TC it forgets to do it the wrong way, so you see f/5.7 (the correct number).

[HiFiGuy1]

Daniel,


Thanks for the lesson, once again. These appear to be related to the square root of two. Is that true, and if so why?


Sq. rt. of:


2 = 1.414


4 = 2


8 = 2.828


16 = 4


etc.

[Ehcalum]

I believe you will only be able to use the center af poin on the 30D.

[lculpin]

So a little of topic, but Daniel, how the hell do you know that??? :P

[Daniel Browning]

You're welcome.

These appear to be related to the square root of two. Is that true, and if so why?

Yes.


In order to double the intensity of light falling on the film, the surface area of the iris[1] of the lens must double. To double the area, the diameter must increase by a factor of sqrt(2). They figured that since they're going to use sqrt(2) factors, they might as well use the sqrt(2) sequence for "whole stop" markers as a convention:

sqrt(2)^1 = f/1.4
sqrt(2)^2 = f/2
sqrt(2)^3 = f/2.8
sqrt(2)^4 = f/4

[1] I'm using the term "iris" instead of "aperture" to avoid confusion, because most readers think aperture and f-number are the same thing.

So a little of topic, but Daniel, how the hell do you know that??? :P

[] Same way I learned everything else: reading books and talking to people.

[lculpin]

That's actually really logical when you put it that way


Now what were we talking about again? :P

[wickerprints]

What do you mean by "1.9999~"?


My degree is in mathematics and I do not recognize your use of the ~ symbol.


If you meant to write 1.9999... as in the 9 repeats indefinitely, then you may as well write 2, because the two are equal.

[jeffersonposter]

That makes sense. I had noticed that with a 200 f/2.8 (2.82) and the 1.4x PS shows f/4 (3.999~) did the right thing. 300 f/4 (3.999~) and the 1.4x f/5.56 (5.56585), did the right thing. I do use center focus point most of the time on both the 30D and the 5D. Does the camera care that much (5.7)?


Thanks for the information,


JeffersonPoster

[Daniel Browning]

If you meant to write 1.9999... as in the 9 repeats indefinitely, then you may as well write 2, because the two are equal.

Pfft. I don't know where you got your degree, but on this planet, 1+1=1.999 and sqrt(2)^2=1.999.


Wait a second... let me check... oh, I see we're on planet Earth. I guess you're right. 1+1=2 and sqrt(2)^2=2. Thanks for the correction. []

Does the camera care that much (5.7)?

Manufacturing tolerances can be a third stop or more, anyway, due to things such as aperture activation variance. (I.e. you set f/8, but the camera accidentally goes to f/7.5 or f/8.5 because the mechanics aren't precise enough.)