Lens Question for Inspeqtor

[BuckSkin]

This is quoted from this thread:
"PSE Cropping Question"

photoshopelementsandmore.com/post/74088/thread

Inspeqtor:  "I used my Sigma 150-600MM lens with my Sigma 2.0 TC"

In good light, will your Sigma 150-600MM lens with the Sigma 2.0 TeleConverter consistently autofocus at 600mm, or do you have to manual focus ?

I have two 2.0 TeleConverters, a Tamron and a Kepcor; both have the circuit contacts; and, lens information transfers reliably through either, and also when both are doubled up.

However, whether singly or paired, autofocus lurches and hunts around and fails miserably.

This is when using my 50-500mm Sigma, but holds true regardless of which of our lens I may be using.

I thought, possibly, that the same brand lens/TC combination might fare better with autofocus.

I am also curious whether a Canon camera/Canon TC/ Canon lens combination will autofocus.

[alexr]

I am going to explain this really badly, so my apologies, but the issue is something like this:
It is a mix of factors - the teleconverter, the lens and the camera all play their part. I don't believe brands will make any difference.
Using a teleconverter changes the maximum aperture of your lens. A 1.4x convertor changes it by 1 stop, a 2x teleconvertor by 2 stops. So if the maximum aperture for the original lens (and assume by definition you are always at the high end of the quoted range) is f/5.6, this become 2 stops worse which is f/11. (using the stop scale of f/2.8, f/4, f/5,6, f/8, f/11, f/16 etc), To do auto focus, cameras work at the maximum aperture, and every camera will have a maximum value that it works at. My Canon 7D will stop auto-focusing if the lens doesn't go below f/5.6, although more recent Canon DSLRs will go to f/8. So for me when I put my 2x teleconverter on my f/4-f/5.6 zoom lens I know it won't autofocus. I would need a lens that at maximum zoom was f/2.8 or better.
Hope that helps. I'm sure someone will correct me if I am wrong.

[BuckSkin]

Nov 1, 2020 13:03:40 GMT alexr said:

I am going to explain this really badly, so my apologies, but the issue is something like this:

Using a teleconverter changes the maximum aperture of your lens. (using the stop scale of f/2.8, f/4, f/5,6, f/8, f/11, f/16 etc)To do auto focus, cameras work at the maximum aperture

Just going by the numbers on your scale, by your use of the phrase "maximum aperture", would "maximum" be f/2.8 or f/16 ? (I hope my question makes sense)

My Canon 7D will stop auto-focusing if the lens doesn't go below f/5.6

Forgive me, but I am struggling to understand this sentence.

(I reworded it a bit) "My Canon 7D will stop auto-focusing when the lens/TC combination doesn't go below f/ ___ (fill in the blank) " --- If it goes "below", would that be a larger or smaller f/number ?

Hope that helps.

It sure does; Thanks !

[Chris]

Buckskin, cameras usually focus at the maximum aperture which means the widest pupil diameter or the smallest F number*. eg F2.8 for some lenses. This is to allow the maximum amount of light into the camera so the photographer can see the subject clearly, and so that the auto focus will work properly. Then as you press the shutter it will stop down to the required aperture during the exposure. So putting 1 or 2 teleconverters on the camera reduces the effective F number and much less light reaches the auto focus sensor and the focus will not work properly. If your lens has a maximum aperture of F 2.8 then adding a 2x teleconverter could make it effectively F 5.6 (about 2 stops darker, that means about 25% of the original light will reach the sensor). Adding 2 teleconverters will make an F 2.8 lens effectively about F 11.

Kind regards
Chris


* F Number = Lens Focal Length/Pupil Diameter
en.wikipedia.org/wiki/F-number#:~:text=In%20optics%2C%20the%20f%2Dnumber,is%20very%20important%20in%20photography.

[BuckSkin]

Thanks.
I grasp that smaller numbers mean bigger opening and larger numbers mean smaller opening.

I guess where I have trouble sometimes is that I was always led to understand that the various sizes of the aperture opening were the F\ numbers and yet there are other definitions as well, hence my confusion at times.

Is this "Pupil Diameter" you mention the same as the aperture opening ?


As for the tele-extenders having electrical continuity, I have deduced that that is for communicating the various lens variables more so than to power the focus motor.

[pontiac1940]

BuckSkin
Not sure of this will be of assistance. 
I wrote it over 15 years ago and it's been update a couple of times. Includes some basic aperture info. 
Clive

[BuckSkin]

Nov 4, 2020 0:56:47 GMT pontiac1940 said:

I wrote it over 15 years ago

My gosh, you were just a child.....

Thanks; I will read every word,

[Chris]

Nov 3, 2020 23:16:58 GMT BuckSkin said:

Thanks.
I grasp that smaller numbers mean bigger opening and larger numbers mean smaller opening.

I guess where I have trouble sometimes is that I was always led to understand that the various sizes of the aperture opening were the F\ numbers and yet there are other definitions as well, hence my confusion at times.

Is this "Pupil Diameter" you mention the same as the aperture opening ?


As for the tele-extenders having electrical continuity, I have deduced that that is for communicating the various lens variables more so than to power the focus motor.

Yes, smaller F numbers mean larger aperture openings. Large F numbers mean smaller apertures.

The pupil diameter of the lens is the dame as the diameter of the open camera diaphragm. The word "aperture" basically means an opening through which light passes, and its directly related to the diameter of the open diaphragm. So if a photographer speaks of a large aperture he means a large opening or a small F number. Knowing the F number is more useful to photographers, because it is a function of the relative amount of light falling on the film or sensor, and is used to calculate the correct exposure*.

So in photographic terms, if someone asks about the "aperture" they usually refer to the F number. However, as you mention, it can get confusing. I hope this helps.

Kind regards
Chris

* F Number = Lens Focal Length/Pupil Diameter
en.wikipedia.org/wiki/F-number#:~:text=In%20optics%2C%20the%20f%2Dnumber,is%20very%20important%20in%20photography.

[pontiac1940]

Nov 4, 2020 6:25:16 GMT BuckSkin said:

you were just a child.....

HA HA! Yes! 
That would have been my second childhood!