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next PZ lens test report: Tokina AF 16-28mm f/2.8 AT-X Pro FX
#11
[quote name='Brightcolours' timestamp='1297950995' post='6191']

If you have focus shift, you simply have a miss focussed image. The camera focusses wide open, and when the aperture closes for exposure, the focus shifts. And you get out of focus results.



Field curvature, you get in focus centers and out of focus edges. Usually only noticeable when the corners are supposed to be clearly in the same focal plane as the center, but OOF.

[/quote]



Thanks... What about the DOF dependencies? With the closed the aperture (lets say @ f/10) the focus shift arises during exposure but at the same time the DOF is increased. So depending on the shifted distance, it can happen that we cannot recognize the focus shift on the final image at all, right?



Regarding the field curvature: Is it constant on the edges regardless of the aperture value?



Serkan
#12
Hi Klaus



and thanx a lot for the review. Certainly an interesting lens for FF. I was thinking about the QA issue as well and came up with this question: Does anybody know, what happens to the faulty lenses that are returned to Tokina, are they taken out of circulation or are they just shipped to the next unsuspecting customer. If the latter is the case, that would mean, over time it gets more and more difficult to actually be supplied with a decent sample at all, because the supply chain is watered down with returned lemons. Hope I am not seeing ghosts here...
#13
[quote name='PuxaVida' timestamp='1297947635' post='6190']

And just out of curiosity... how can we distinguish residual spherical aberration (focus shift at closed aperture) from field curvature?

[/quote]

At full aperture, field curvature should be at its highest and focus shift would be zero.

At a small aperture, field curvature is reduced and focus shift would be at its highest.





[quote name='PuxaVida' timestamp='1297947635' post='6190']

Is the field curvature effected by the closing of the aperture?

[/quote]

Ignoring the increase of DOF, field curvature should reduce when stopping down (e.g. imagine a lens with a smaller aperture and glass) because light needs to refract less.





[quote name='PuxaVida' timestamp='1297954061' post='6192']

Thanks... What about the DOF dependencies? With the closed the aperture (lets say @ f/10) the focus shift arises during exposure but at the same time the DOF is increased. So depending on the shifted distance, it can happen that we cannot recognize the focus shift on the final image at all, right?

[/quote]

In a word, yes... in practice I see very little of this. For example the 24L II has a strong field curvature but I rarely shoot flat subjects (at 1.4) or focus-recompose with this lens. When I'm stopping down, it's almost always for landscape shots... so f/11 at 24mm gives a hyperfocal distance after a few feet and field curvature is not an issue.





[quote name='PuxaVida' timestamp='1297954061' post='6192']

Regarding the field curvature: Is it constant on the edges regardless of the aperture value?

[/quote]

Like I said earlier, I can't see how this can be the case, even if we don't account for the DOF increase. Field curvature is a result of refraction, as the aperture gets smaller, refraction is less... therefore field curvature should be less.



GTW
#14
My first pleasant surprise was the price. The street price in my area is about$950, much less than the $1,400 I am seeing guessed at by uninformed people!

This makes the Tokina lens about half the price of the Canon, and it is a better lens on most counts!

I bought the lens recently because of dissatisfaction with my Canon 16-35 f2.8 LII!

After using the lens I can say it has performed as well as I hoped!

The greatest improvements I have found are in resolution, distortion, and color fringing at the edges and in the corners!

Clarity of the image is improved, especially at the edges and in the corners.

The Canon I had tended to smear fine detail in the corners, even stopped down to the sharpest aperture!

Distortion is extremely well controlled.

Another big improvement was the reduction in color fringing at contrast points and in the corners and edges.

Yes, it is a heavy lens. But it outperforms any other wide angle zoom I have used.

There are no threads, so I can't use a filter on the lens unless held in front, but I am working on an answer for this.

All things considered, a very satisfying lens to use!
#15
Nice lens but they should add a built-in ND filter or a slot to put any filters.



For example you need ND4 or ND8 for taking "silky" waterfalls or car lightpaths at night
  


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