Focus at Infinity F 5.6 vs 22

[dan_shoe]

When I set my Canon EFS 17-85 zoom lens to its maximum length, and have the

focus ring set to infinity, I get a slightly sharper image at F-22 than at

F-5.6, for a subject several miles away. I used an E0S20D body on a tripod. It

would seem if the lens focus ring was set exactly to infinity, a wide aperture

should be as sharp as a narrow one. Any explanations as to why not? thanks. -Dan

[geoffs1]

Many lenses focus a bit past infinity (to allow for mechanical imprecision and thermal effects), so it may be that you're actually focused past your subject.

 

I'd try using C.Fn-4 to set the focus on the "*" button (so you can shoot w/o changing the focus position). Then use single-shot focus and the center-point to auto-focus on your subject. Then shoot the two shots at f/22 and f/5.6.

 

Cheers,

 

Geoff S.

[rainer_t]

Firstly ... was the setting to infinity really correct?

 

Secondly ... at f/22 the image sharpness is already suffering from

diffraction on a crop 1.6 size sensor.

Better retry with f/11, f/13, f/16.

[pc1]

I'm with Rainer T. start with f/8 - f/16 . things might look a bit better .. PC

[gdanmitchell]

On that lens the wide aperture should be _sharper_ than the narrower one, since defraction begins to degrade sharpness after about f/8 or so. I'd

even be very cautious about f/11 on a crop body, unless you are willing to give up a slight bit of sharpness in the plane of focus in order to gain a

bit of DOF. About the only use for f/22 on a crop sensor body is to test for dust on the sensor... :-)

 

Sounds like you may have a focus issue at infinity. Or not.

 

This could be a camera/lens issue, but it could also be technique related. A good starting point might be to let the camera AF on a distant subject;

then turn AF off and try to manually focus slightly closer and farther away and see if things improve.

 

Also, if you are manually setting the lens to the infinity mark on the barrel, as was pointed out above, you may actually not be selecting the best

adjustment point. The AF system often finds a spot that is slightly different from the marking on the barrel when shooting a subject at "infinity."

 

Dan

[bobatkins]

"It would seem if the lens focus ring was set exactly to infinity, a wide aperture should be as sharp as a narrow one"

 

Why? What makes you think that?

 

Typically sharpness peaks at mid apertures. Wide open and fully stopped down the image will be softer. Try shooting at f8-f11

[dcheung]

my tests with that lens has shown maximum sharpness to be attained at about f11 for 24-85mm. At the 17-24mm range, the sharpest aperture seems to be f8

[dave_holland]

Two competing issues,

 

1. Astygmatism and minor defects of the lens are partially negated by stopping down.

 

2. Increased diffraction at the smallest aperture.

 

As above, your sweet spot should be at f8-11.

 

Sounds to me like your lens defects wide open are more dominant than diffraction effects closed down. I don't think it will be a focus issue, as focus mostly occurs before the shutter stops down anyway, so it should be the same at both apertures.

 

No matter, shoot at f8. For a landscape you don't want to focus at infinity anyway.

 

Dave

[bernardwest]

<i>No matter, shoot at f8. For a landscape you don't want to focus at infinity anyway.</i><br><br>

 

Not so Dave. Checkout <a href=http://jimdoty.com/Tips/Depth_of_Field/More_DOF/dof_merklinger/dof_merklinger.html>Merklinger</a>