Extenders, f-stops and autofocus points

jonathan_ed

jonathan_ed

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Jonathan
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Morning everyone

I am currently using a 300m f/4 IS, often with a 2x extender mkiii (on a 1d mk iv). Now obviously this reduces the f4 to a f8, and i only get get the central AF point to use. My question is, if i used a a lens with a constant aperture of 2.8, such as the 300 f 2.8 or 120-300 f 2.8 would i get more AF points to choose instead to help composition?
 
Hi james

I know it'd act as 5.6, I'm asking would I get more auto focus points, as currently only have the centre point and doesn't help trying get a good composition
 
In honesty I can't say I've noticed. I've got it set to the full 10fps on the custom settings. Never actually counted what it's doing, if I'm doing props aviation and shooting at low shutter speeds of 1/125 - 1/200 I don't know of it's churn out that many per second anyway, if I'm shooting jets at 1/2000 it seems to fly through no problem
 
You are not focusing at f8 on another lens.

When you focus with a lens, the lens is wide open to allow the most light to enter the lens. If your camera setting is f8 or f11 etc, the aperture is only stopped down the moment you press the shutter.

Whilst have an extender attached doesn't physically change to position of the aperture blades on the lens, less light is entering the lens, therefore not providing enough light for your outer focus points to work.
 
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jonathan_ed said:
Hi james

I know it'd act as 5.6, I'm asking would I get more auto focus points, as currently only have the centre point and doesn't help trying get a good composition
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All the AF points are available at 5.6 but horizontal line sensitive only.

andrewc said:
Quick question for a fellow 1D4 300mm IS L owner. Does your camera not provide full FPS with this combo? Two I've tried don't provide the full frame rate and less so with IS enabled.
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Never had this issue on my MkIV but my 1DX won't run at full chat with the 300 f4 on it.
 
ecoleman said:
You are not focusing at f8 on another lens.

When you focus with a lens, the lens is wide open to allow the most light to enter the lens. If your camera setting is f8 or f11 etc, the aperture is only stopped down the moment you press the shutter.

Whilst have an extender attached doesn't physically change to position of the aperture blades on the lens, less light is entering the lens, therefore not providing enough light for your outer focus points to work.
Click to expand...



Ahhh... Thanks for clearing that up. :)
 
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