[Poit]

I have read reviews on various lenses, such as the FA43, that state centre sharpness at wide apertures (eg f2.8) is great / excellent / tack sharp etc etc BUT corners are soft.

Perhaps a naive or dumb question, but I have been wondering if selecting an off-centre AF point in the camera's AF array (say, for example, the top-right AF point in the K-3's AF matrix) effectively moves the 'centre sharpness' of the lens to that point?

To put it another way, if a lens suffers from 'edge softness' at f2.8, will it suffer from that softness regardless of which AF point in the camera's AF you use, or does moving the AF point also move the centre sharpness to that point?

I've jusy acquired an FA43, and I'd like to know if, when composing shots at wide apertures, I should be accounting for that edge softness or if it will be negated by AF point selection.

Thanks in advance, and forgive my ignorance

[stevebrot]

Perhaps a naive or dumb question, but I have been wondering if selecting an off-centre AF point in the camera's AF array (say, for example, the top-right AF point in the K-3's AF matrix) effectively moves the 'centre sharpness' of the lens to that point?

No. Center sharpness is measured at the lens axis (the center) while edge sharpness is measure further out towards the "edge" of the frame. This is the case regardless of focus technique and is true even when the lens not mounted to the camera.

As for how to use your FA 43...simply shoot with it the same as with any other lens. The lens may be softer at the edges (most are), but there is nothing you can do to compensate short of taking care to shoot at apertures where the lens is more uniformly sharp.


Steve

[Poit]

Thanks Stevebrot.

So if I want a dramatically shallow depth of field with an off-centre subject the IQ will suffer from degradation regardless?

[jatrax]

So if I want a dramatically shallow depth of field with an off-centre subject the IQ will suffer from degradation regardless?

Short answer is yes. However, you are seriously over thinking this. Just because the lens is not as sharp on the edges as it is in the center does not mean the IQ is degraded. It only means that on a particular test the lens center is sharper than the edges. Very few lenses (if any) except those designed for flat field imaging such as a macro lens will be equally sharp at all points.

Also if you are using the lens on an APS-C camera you are only using the center portion anyway.

[robjmitchell]

There are two factors at play.
There is a difference between the sharpness of the centre and the corners.
Then there is field curvature that can cause the corners to be out of focus relative to the centre. Essentially the focus is set at the same distance from the lens, but when viewing a flat surface the corners move outside the focus because they are further away than the centre of the surface. Field curvature can be corrected for optically, hence flat field macro lenses but there can be penalties.
Obviously field curvature can affect sharpness results if the testing method does not focus separately for the centre vs corners.
To answer your question, using different AF points will help ensure you subject is in focus and hence sharper.
As for the 43ltd wide open, it has more than enough sharpness for portraiture and a slightly dreamy quality that isn't seen once stopped down.(instead you get 3D pixie dust)
My favourite lens by the way!

[stevebrot]

Short answer is yes. However, you are seriously over thinking this. Just because the lens is not as sharp on the edges as it is in the center does not mean the IQ is degraded.

I could not have said this better. One of my favorite lenses is my KMZ Helios-44M 58/2. Made in the former Soviet Union, it was derived from the Carl Zeiss Biotar 58/2. Both lenses share the characteristic of having quite adequate center sharpness at all apertures, even wide open. That consistent center sharpness comes at the cost of noticeably soft corners for 35mm FF and fairly soft corners for APS-C. Despite this "fault", my Helios provides consistently cool images. Go figure ...must be the superb bokeh...


Steve

---------- Post added 10-17-14 at 07:47 PM ----------

So if I want a dramatically shallow depth of field with an off-centre subject the IQ will suffer from degradation regardless?

The subject here is off-center and the DOF is shallow. The amount of image degradation is, I suppose, up for discussion.




Yes, the above was taken with the soft-in-the-corners Helios-44M.


Steve

[bschriver11]

Your never going to position a subject in the corners anyway,if your using the rule of thirds your subject would be between the center and corner and should be fine.If you were photographing a large building with a fast aperture that fills the frame then the soft corners would be noticeable,fixed by simply stopping down.I laugh when i read lens reviews and they bash an ultra fast lens that has soft corners,you are isolating a subject from the background so the corners are out of focus anyway.

[vrrattko]

Hi Poit, don't worry about your FA43 ....stopped down it is sharpest lens (corners included) from all lenses I ever tried....sharper in the corners then FA31 in my experience....

[kh1234567890]

I think that people who rant on about the 'lovely dreamy quality' or 'soft corners which you never use' or 'it's the field curvature, you know' are in denial that they'd spent lots of money on a cr*p lens. If a lens cost me the price of a quick meal then I am prepared to accept some shortcomings, although they'll annoy me every time I use it. I can fix colour and contrast in post-processing but not fuzziness. And as for the 'it's OK once you stop it down to f/16' argument - the bottom of a beer bottle would probably be sharp if stopped down enough ...

[Poit]

Thanks for the responses & advice.

I think I probably was reading too much into it. I had a play with the FA43 yesterday and it really is quite a marvelous little lens.

Moral of the story: pixel peeping reviews are no substitute for real world experience!

[KDAFA]

... I had a play with the FA43 yesterday and it really is quite a marvelous little lens.

Moral of the story: pixel peeping reviews are no substitute for real world experience!

You have grasped a crucially vital point.

Enjoy your FA43 - once you learn its behaviour and unique strengths, and understand how to take advantage of it, you will be in a good position to create BEAUTIFUL shots - the sort of magical pictures that you don't soon forget. And in that moment, as you drink in the sheer beauty of the picture, the technical measurements will seem quite irrelevant, actually.... I'm not kidding here.

---------- Post added 10-24-14 at 10:03 PM ----------

I could not have said this better. One of my favorite lenses is my KMZ Helios-44M 58/2. Made in the former Soviet Union, it was derived from the Carl Zeiss Biotar 58/2. Both lenses share the characteristic of having quite adequate center sharpness at all apertures, even wide open. That consistent center sharpness comes at the cost of noticeably soft corners for 35mm FF and fairly soft corners for APS-C. Despite this "fault", my Helios provides consistently cool images. Go figure ...must be the superb bokeh...


Steve

---------- Post added 10-17-14 at 07:47 PM ----------



The subject here is off-center and the DOF is shallow. The amount of image degradation is, I suppose, up for discussion.




Yes, the above was taken with the soft-in-the-corners Helios-44M.


Steve

That is a beautiful shot.

And it illustrates your point well.

---------- Post added 10-24-14 at 10:04 PM ----------

Your never going to position a subject in the corners anyway,if your using the rule of thirds your subject would be between the center and corner and should be fine.If you were photographing a large building with a fast aperture that fills the frame then the soft corners would be noticeable,fixed by simply stopping down.I laugh when i read lens reviews and they bash an ultra fast lens that has soft corners,you are isolating a subject from the background so the corners are out of focus anyway.

Couldn't have explained it better myself.

---------- Post added 10-24-14 at 10:07 PM ----------

Hi Poit, don't worry about your FA43 ....stopped down it is sharpest lens (corners included) from all lenses I ever tried....

Agreed. When I examine my stopped-down photos, I find the FA43's sharpness on par with even the DA35 Macro LTD - that's really saying something.

[lytrytyr]

Perhaps a naive or dumb question, but I have been wondering if selecting an off-centre AF point in the camera's AF array (say, for example, the top-right AF point in the K-3's AF matrix) effectively moves the 'centre sharpness' of the lens to that point?

To put it another way, if a lens suffers from 'edge softness' at f2.8, will it suffer from that softness regardless of which AF point in the camera's AF you use,

There's another issue going on here that no one has brought up yet:
The AF will have a harder time finding focus if it's working with a part of the image field where the lens is softer.

So while selecting an off-center focus point won't move the lens's "centre sharpness",
it may end up giving soft images, regardless of field curvature,
just because the AF can't find the right focus point.

[stevebrot]

The AF will have a harder time finding focus if it's working with a part of the image field where the lens is softer.

This is compounded by the camera not having any focus points in the corners


Steve

(...sorry, could not resist...)

[lytrytyr]

This is compounded by the camera not having any focus points in the corners

But with a lot of lenses out there, you don't have to go that far from the center at full aperture.
So even Pentax has AF points at the soft spots!

[osv]

There's another issue going on here that no one has brought up yet:
The AF will have a harder time finding focus if it's working with a part of the image field where the lens is softer.

So while selecting an off-center focus point won't move the lens's "centre sharpness",
it may end up giving soft images, regardless of field curvature,
just because the AF can't find the right focus point.

field curvature behaves exactly like that in some lenses i've looked at... the edges keep improving as they are stopped down, but they never match the center sharpness, no matter where the focus point is.

some lenses will get razor sharp on the sides, when focused there, but the perfect focus point on the edge is slightly different than the perfect focus point in the center... in a perfect lens, those two things would match up exactly, the entire field would be flat and sharp.

what if you are shooting down a narrow european street, with tall buildings on each side... since you want the entire length of the street to be sharp, is a flat field better than a lens with slightly different perfect focus points.