For travel - 20mm or 25mm Panasonic?

Started Apr 10, 2013 | Discussions thread
NZ Scott
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Re: Curvature Of Field
In reply to David Kieltyka, Apr 12, 2013

David Kieltyka wrote:

NZ Scott wrote:

To be honest, I did have higher hopes for the 17/1.8. I think for the price it should have f1.4 aperture, and it would also be nice if it were just a smidgen sharper. However, it's still a very good lens.

In my use of the two lenses (17mm f/1.8 & 20mm f/1.7) I can't say I've noticed any differences in actual resolving capability. Of course the 20 is a little longer--in the M43 format 3mm makes a noticeable difference at the wide end--so the slightly greater magnification compared to the 17 might give the impression of greater sharpness. But what the 17 does have compared to the 20 is more field curvature. This can make it less effective than the 20 for shots of fairly distant subject matter where you want the corners as sharp as the center. But if you actually focus the 17 on closer subjects using the corner focus points, those corners are plenty sharp...even wide open.

It's gotta be pretty difficult to design a fast rectilinear 17mm lens, at least one mere mortals can afford, that doesn't have some degree of field curvature.

-Dave-

Yes, I noticed this myself when I tested my lens immediately after purchase. I wonder if it is one reason why the 17/1.8 has scored lower than it might on some sharpness tests. If you are shooting a flat object and focus in the centre of the plane then the corners will be slightly out of focus due to field curvature, giving the impression that the lens lacks resolving power in the corners. However, if you actually set the focus point in the corners then the corners tend to be sharp.

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