Focusing with Teleconverters

[Rick50]

I have a Canon 5D3 camera and a canon 300mm F/4 L prime lens I use a fair bit. I also have Kenko 1.4x and 2.0x Teleconverters.

When I focus without a teleconverter (well, with too) I use Live view and zoom in 10X and manually focus (for twilight stuff) and get sharp images. But for some reason I cannot focus as well with the Teleconverters. The lens focus ring just does not work as well. In daylight I can use auto focus and it works well but it's the dark sky stuff.

Anyone have any good reference for accurate focusing?

 

[runnah]

Do you have exposure simulation turned on?

Normally the iris on the lens will open up as wide as it can for focusing purposes. When you hit the shutter the iris will contract to the chosen fstop. When you add a 2x converter it stops the lens down to f/8. This doesn't allow for much light to get in to allow for proper focusing.

 

[Rick50]

Thanks for the reply Runnah. Yes, I do have it on (not intentionally). Since I can focus in daylight by using auto focus I probably should test these Teleconverters for image quality loss. Something to work on this afternoon.

 

[Overread]

A few thoughts;

1) No lens is sharpest wide open (smallest f number - widest aperture) and whilst some are very sharp wide open, close them down even one stop and they will improve in sharpness.
This is important because any TC is going to degrade optical performance - or rather its going to magnify it so that any weakness is increased.

2) 1.4TCs are normally pretty light on image degradation; which makes them very popular as on many high end lenses you still get very good usable photos wide open.

3) 2*TCs are more severe on their effects on image quality and often you will want to close down by one stop when using them. For this reason and for point 4 below 2*TCs are often only used on lenses with a maximum aperture of f2.8 or wider.

4) A 1.4TC takes away 1 stop of aperture - a 2*TC takes away 2 stops. This affects your exposure and also your focusing. IT's for this reason that most Canon cameras won't auto-focus with a 2*TC attached to a lens with an aperture smaller than f2.8 - because the AF stops engaging after the max aperture is smaller than f5.6.
Live-view AF though keeps going to around f8 (its a different system). Meanwhile some newer bodies are starting to focus up to f8 and the 5DMIII might be in this block (I'm not sure - give it a try and see).

Note the AF disables because the cameras AF performance will drop significantly. It's for this reason that you typically won't see the 2*TC used on lenses with apertures smaller than f2.8.


For the lens you have the 1.4TC should work without any problems; AF performance will degrade in dimmer lighting and a little earlier in the day (because its now an f5.6 lens not an f4). The 2*TC will be less impressive in performance, f8 is starting to require very good light to focus in; and the image quality will drop very significantly - plus because you're already at f8 closing down one more stop is really starting to get into apertures where you're restricting your light for the shutter speed.

 

[Rick50]

Boy you sure know your stuff. I just did my test at F/4 (because I will use F/4 on the Moon tomorrow). My results are exactly as you spelled out. The only addition I can add is that I cannot focus as well as Canon does (consistently). The manual focus without the tele's is definitely harder for me with these shaky old hands and bad eye's. So this confirms my earlier decision which is NOT to use a teleconverter if it really counts. And Tuesday it counts.
And when it is OK to screw up I'll stick with the 1.4 tele. It seemed to do better as you said.

Thanks for the info....

 

[Overread]

Practice and you should be good on that lens with a 1.4TC - although yes if at night you might just be pushing the camera and lens that little bit too far focusing wise.

Focusing on modern cameras manually is also hard; they are made with autofocus in mind; although zooming in on the LCD is a very powerful tool for focusing when you've got the opportunity to use it.

 

[Derrel]

Same basic story on the Nikon 300mm f/4 AF-S lens. EASY to get good autofocus using the camera's AF system, with the 1.4x converter added the AF system becomes verrrrry nervous, and jittery, and hand-focusing it becomes very sketchy at longer distances.

One thing overread mentioned is that hand-and-eye focusing of many AF lenses is tricky, because the lenses were designed to be focused by a micro-motor being driven by a computer processor! The actual focusing ring rotation on many AF lenses is very 1)sloppy 2)loosey-goosey and 3)hair-trigger--especially at longer distances! Out near Infinity and coming in closer to say 20 meters, there is often just a few degrees' worth of focus ring travel between...well, the moon, and the front porch! Add the lens issues to the through-the-viewfinder focusing screen nature on modern d-slr bodies, and well, manually focusing many AF lenses is tricky,tricky business under many scenarios. "It's not just you, pal--it's the gear!"

At closer ranges, like say inside of 20 feet, the focusing ring travel on lenses like the 300/4 is much more gradual, and allows easier, more-precise focusing when focusing by hand-and-eye, or by live view.

 

[runnah]

Yeah I tried a 2x with a 100-400 and it was a complete failure.

 

[Rick50]

I also found that if you can find something at the distance your shooting (at night) that is bright use it and auto focus for best results. The 5D3 has a bunch of good focusing modes (from single point to all 61 points) and I'll be using those for the moon. It's pretty easy to verify focus by zooming in.

All good information here. Thanks.