Maximum Aperture Required

[pbelarge]

When a manufacturer says that -
"7D requires a lens with a maximum aperture of at least f/5.6 or wider (as reported by the lens) for AF to function."


Does this mean the it has to be set to the aperture for the function to work, or the lens just has to be capable of the requirement and could actually be set at say f/11 and still function?

 

[Derrel]

It means that wide-open, the lens should have a maximum aperture of f/5.6, or wider (such as f/4, or f/3.5 or f/2.8,etc) in order for the lens to supply sufficient light to enable the AF system to function properly.

 

[pbelarge]

It means that wide-open, the lens should have a maximum aperture of f/5.6, or wider (such as f/4, or f/3.5 or f/2.8,etc) in order for the lens to supply sufficient light to enable the AF system to function properly.

Derrel

So, if the lens is say f/2.8, and the camera needs f/5.6 or larger, and I set the lens to f/11 then auto focus does not work and I need to manually focus the shot?

 

[Derrel]

No, it doesn't work that way. The maximum aperture would be f/2.8, and so the lens would be amply fast. The shooting aperture makes no difference, so you could shoot at f/11 or f/16 or even f/22. The only aperture that is matters is the maximum aperture, regardless of what the shooting aperture is.

 

[pbelarge]

Derrel

Thank you very much, now I understand.

Just think I am one point closer to understanding this racket...only 132,394,000 more points and I can start taking some photos. :mrgreen:

 

[cfusionpm]

The reason behind that is that the aperture blades only close (stop down) when the shutter opens. During metering, focusing and such, it's at it's most open aperture.

 

[Pure]

If you notice, when a lens is not attached to a camera, the aperture is closed all the way down to its smallest size, maybe f22 or 36. However when attached to your camera, a small lever on the body pushes a lever on the lens that opens the aperture to its maximum value, maybe 2.8 or 5.6.

Now when you click the shutter that same lever pushes down on the lens' lever and allows the lens' aperture to close just enough, whatever you set on your camera. After, the lever goes up and opens the aperture to its maximum again.

The aperture is forced to open all the way in order to allow for maximum light to enter the camera body, thus allowing it to meter, AF, and allow you to see your subject even when it's dark.

As for the whole f5.6 thing: It's really just a suggestion per say. As long as enough light can travel through the lens and to the AF module, you can AF. You can AF with a lens that has a maximum aperture of f6.3 at 300mm for example, as long as there is enough light. Same principle with fast glass. Even with a 50mm 1.4, if it is too dark, then even that lens cannot AF. And the same principle applies to the ridiculous f.95 lens.

Hope this helped a bit.

 

[JustAnEngineer]

Canon's EF mount is all-electronic, so there aren't any levers, and the apertures of my EF lenses appear to be wide open when they are disconnected from the camera. The part about the lens only irising down when the photo is taken is correct. It is normally wide open. This provides a brighter image for focusing (and in your viewfinder).

The f/5.6 or faster maximum aperture requirement for auto-focus on the EOS 7D is real. The camera and lens combination won't try to auto-focus if the maximum aperture is f/8 or slower. Tamron and Sigma have their f/6.3 lenses set up to auto-focus with your camera, but Canon doesn't make any lenses with that particular aperture, so they state f/5.6 as the minimum. Note that an f/4 lens with the 1.4x teleconverter is an f/5.6 combination. An f/4 lens with the 2x teleconverter is f/8.

Some folks will use a bit of tape to trick the contacts in an extender so that the camera will try to auto-focus with a slower combination, but don't expect the auto-focus to work as well as it does with faster lenses.

With an f/2.8 or faster maximum aperture lens, the precision of the cross-type auto-focus points is enhanced.

 

[pbelarge]

The last 4 posts are the reason I joined the forum. Thank you guys you really helped me to understand what is behind the reasoning of the aperture issue I was struggling with.

Justanengineer, I tried thanking you, but it seems I can only thank 3 people. I will thank you in another thread.

 

[KmH]

The actual AF electronics module is usually in the bottom of the camera. In that case the mirror in the mirror box is only 50% reflective and there is a smaller, secondary mirror behind it that reflects the other 50% of the light down to the AF module.

That is why AF has to be done with the lens as wide open as it can go.

Note that cross-type AF points are usually most effective at apertures of f/2.8 and larger.

Autofocus - Wikipedia, the free encyclopedia