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Aperture and AF limits

KarlKrueger
Enthusiast

I'm hoping someone can enlighten me on this topic as everything I researched on the web does not provide the answer I'm looking for. When you look at the autofocus (AF) requirements for a camera you generally see that f/8.0 is usually the smallest opening that can accomodate AF (center point AF). Does that refer to only cross type AF? It does not explain why you can still autofocus beyond that.

 

I have been able to get good auto focusing through the view finder going down to apertures as low as f/16. Clearly there is more in play in here that is not mentioned in the plethora of online articles I've looked at. It seems like you can push the system well beyond f/8.0 and get some sufficient ability to do AF. So what is really happening when you autofocus at small apertures?

8 REPLIES 8

ebiggs1
Legend

On all current cameras the center point is cross type.  All I am aware of anyway.  The f8 limit is imposed by Canon.  The camera is capable of AF at much slower apertures.  You can test this by taping over the AF communication pin on the lens.  A commonly used gimmick to trick slow lenses into working when somebody wants to use a t-con.

 

Why does Canon do this?  They feel AF isn't as reliable or as quick at smaller than f8.  They are probably right, too.  It is a performance standard.

EB
EOS 1DX and 1D Mk IV and several lenses!

As kvbarkley points out... auto-focus always happens with the aperture blades fully retracted (wide open) regardless of what aperture you selected for the exposure.

The aperture limit is stated because if you use focal length modifiers (teleconverters) then it will change the focal ratio possible at "wide open" for that lens. E.g. if you have an variable zoom with a focal ratio range of f/4-5.6 and you use a 2x teleconverter it is now an f/8-11 zoom (at wide-open)... and you can't expect auto-focus to work correctly beyond f/8.

When using a teleconverter, multiply the focal length of the lens by the teleconverter's multiplier (e.g. 1.4 or 2.0) but ALSO multiply the focal ratio by the teleconverter's multiplier.
Tim Campbell
5D III, 5D IV, 60Da

Thank you all for your answers. That explains why you can still autofocus under conditions when it seems you shouldn't be able to.

 

TCampbell brought up the next scenario I was interested in. When you have a teleconverter that now reduces the f stop below f/8.0. If I have a lens that zooms up to 600mm at f/6.3 and 1.4x TC will put the effective aperture to f/9.0. So I guess I should not be able to focus at that setting but might be able to at 400mm where the lens is at f/5.6 - assuming the same focusing distance holds at 600mm.

 

TCampbell I'm not following the latter part of your statement here: When using a teleconverter, multiply the focal length of the lens by the teleconverter's multiplier (e.g. 1.4 or 2.0) but ALSO multiply the focal ratio by the teleconverter's multiplier.

Can you give an example and what purpose does that serve?


@KarlKrueger wrote:

 

 

TCampbell I'm not following the latter part of your statement here: When using a teleconverter, multiply the focal length of the lens by the teleconverter's multiplier (e.g. 1.4 or 2.0) but ALSO multiply the focal ratio by the teleconverter's multiplier.

Can you give an example and what purpose does that serve?


Suppose you have a 70-200mm f/2.8 zoom... and you want to use that with a 2x teleconverter.

 

You would multiple that the 70-200 range by the "2x" multiplier of the teleconverter and that gets you 140-400mm.

But the part that not everyone recognizes is that this also changes the focal ratio of the lens... so you ALSO have to multiply the 2.8 focal ratio by the 2x multiplier and that gets you 5.6.  

 

This means in this example the lens is effectively going to behave as an 140-400mm f/5.6 lens.

 

But let's pick another example where it wont work... such as a 70-300mm f/4-5.6 lens with a 2x teleconverter.

 

In this case the 70-300 X 2x becomes 140-600.

But the focal ratio (in this case it's a range) of f/4-5.6 becomes a range of f/8-11

 

Since it's a variable zoom, it's f/8 at the short end of the range (the 140mm end) but as soon as you start to zoom, the focal ratio immediately increases to a value beyond f/8 and the camera can no longer achieve a reliable focus.

 

A "focal ratio" is simply the focal length of the lens (in mm) divided by the clear aperture (in mm).  At 300mm an f/5.6 focal ratio really means that the physical diameter of the clear aperture is about 54mm. (300 ÷ 54 = 5.6)   When you attach the teleconverter the physical size of the lens barrel and it's aperture opening doesn't actually change -- but you did change the effective focal lenght.  So now when 300 x 2 becomes 600... but the lens barrel still has a 54mm aperture opening the math would be 600mm ÷ 54mm = 11.    So by the math... this is why the focal ratio also increases right along with the focal length when you use a teleconverter.

 

Tim Campbell
5D III, 5D IV, 60Da

Thank you Tim. Now I understand the concept of focal ratio.


@KarlKrueger wrote:

Thank you Tim. Now I understand the concept of focal ratio.


If you look at it mathematically, focal ratios follow an inverse square law. Why does a 2X converter have twice the effect of a 1.4X? It's because 1.4 is approximately the square root of 2.

Bob
Philadelphia, Pennsylvania USA

The very popular 150-60mm super zooms available are another example.  The focal range is f5-6.3.  On most cameras you can not AF past f5.6.  Only the top models can AF down to f8.  How does this work?  The lens reports f5 to the camera at any focal length setting. It is somewhat the same as I told you that the AF pin can be taped off and the camera can focus below f8.

However there is usually a price to pay because in photography there is no free lunch. You give to get.

EB
EOS 1DX and 1D Mk IV and several lenses!

kvbarkley
VIP

What you are missing is that the camera opens the lens aperture up to its widest before focusing. So even if you set your F/1.8 lens to F/16 for the exposure, the camera will still focus at f/1.8, and then step down to f/16 for the picture. This is not new. Even all manual film cameras used to work this way. That is why you need a Depth of Field Preview button

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