Focus-recompose method and "soft corners"

Started Mar 15, 2014 | Discussions
Ross Warner Senior Member • Posts: 1,323
Focus-recompose method and "soft corners"

Reading endless discussions about "soft corners" and other focus problems in lens tests, it occurs to me to ask if some of these people might be using the focus-recompose method, which many seem to use by default for everyday work. This can cause focus problems, especially in the corners.

For instance, if you're taking a wide angle picture of an off-center lighthouse, DON'T use the center point to focus on the lighthouse and then recompose! Manually move the focus point to where it should be in the scene, then focus!

There are many articles about this problem (try Google), here is a good one:

http://www.mhohner.de/essays/recompose.php

-Ross

rosswarner.com

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Chanthis Regular Member • Posts: 182
That's what the touch screen is for!

Or I guess you can move the focus point. Takes longer.

Ross Warner wrote:

Reading endless discussions about "soft corners" and other focus problems in lens tests, it occurs to me to ask if some of these people might be using the focus-recompose method, which many seem to use by default for everyday work. This can cause focus problems, especially in the corners.

For instance, if you're taking a wide angle picture of an off-center lighthouse, DON'T use the center point to focus on the lighthouse and then recompose! Manually move the focus point to where it should be in the scene, then focus!

There are many articles about this problem (try Google), here is a good one:

http://www.mhohner.de/essays/recompose.php

-Ross

rosswarner.com

sassenach74
sassenach74 Regular Member • Posts: 375
Re: That's what the touch screen is for!
1

Chanthis wrote:

Or I guess you can move the focus point. Takes longer.

Ross Warner wrote:

Reading endless discussions about "soft corners" and other focus problems in lens tests, it occurs to me to ask if some of these people might be using the focus-recompose method, which many seem to use by default for everyday work. This can cause focus problems, especially in the corners.

For instance, if you're taking a wide angle picture of an off-center lighthouse, DON'T use the center point to focus on the lighthouse and then recompose! Manually move the focus point to where it should be in the scene, then focus!

There are many articles about this problem (try Google), here is a good one:

http://www.mhohner.de/essays/recompose.php

-Ross

rosswarner.com

Just tried touching the screen on my A7R, didn't work 

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Neil.

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blue_skies
blue_skies Forum Pro • Posts: 11,532
Re: Focus-recompose method and "soft corners"

Ross Warner wrote:

Reading endless discussions about "soft corners" and other focus problems in lens tests, it occurs to me to ask if some of these people might be using the focus-recompose method, which many seem to use by default for everyday work. This can cause focus problems, especially in the corners.

For instance, if you're taking a wide angle picture of an off-center lighthouse, DON'T use the center point to focus on the lighthouse and then recompose! Manually move the focus point to where it should be in the scene, then focus!

There are many articles about this problem (try Google), here is a good one:

http://www.mhohner.de/essays/recompose.php

-Ross

rosswarner.com

I agree with the concept that "soft corners" can be a result of focus & frame, but the author seems to want to make a general case out of it. I think that it mainly applies to shallow DOF or lenses with a significant field curvature.

The problem with focusing on your subject, being in or off center, is that you move the focal plane towards your subject. The camera will most like catch 1/3rd of DOF in front of your subject and 2/3rd of DOF behind your subject. This means that foreground will go soft when it leaves the DOF area.

If you could move this AF point forward to catch 2/3rd, or even 3/4th of DOF in front of your subject, and 1/4th behind, you will get cleaner foreground and sharper corners.

Of course, the simple way to do so is to STOP DOWN (Doesn't the bracketing app take multi-focus shots in series?)

Or, shoot multi-focus, and blend the images in post (yes, you probably will need a tripod).

Having a touch screen to move the AF point is great, if there is something in the frame to focus on in front of your subject. Focusing on your subject, even off center, will not solve the issue, imho. But then, the focus & frame method would work just as well, assuming sufficient DOF.

But again, pictures that have your subject in focus, are always preferred.

Sharp corners only matter if there is nothing else to see in the image

Get to know your lenses, framing, and subject matter - you will do fine with any method that you master.

Could foreground focus have helped here? Or would the background have gone soft? Which is better?

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Cheers,
Henry

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OP Ross Warner Senior Member • Posts: 1,323
Re: Focus-recompose method and "soft corners"

I'm referring mostly to 24-70 f4 lens tests on A7 and A7R, so no touch screen!

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David Kieltyka
David Kieltyka Veteran Member • Posts: 5,178
Yes

The 24–70mm has a fair amount of field curvature. Corner performance with the lens is actually quite good if you place your subject at a corner and then choose an appropriate focus point. Many people using the lens, though, are landscapers photographing relatively flat-field (2D) subject matter...thus the criticisms.

-Dave-

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NumberOne Veteran Member • Posts: 4,048
Re: That's what the touch screen is for!

Chanthis wrote:

Or I guess you can move the focus point. Takes longer.

Ross Warner wrote:

Reading endless discussions about "soft corners" and other focus problems in lens tests, it occurs to me to ask if some of these people might be using the focus-recompose method, which many seem to use by default for everyday work. This can cause focus problems, especially in the corners.

For instance, if you're taking a wide angle picture of an off-center lighthouse, DON'T use the center point to focus on the lighthouse and then recompose! Manually move the focus point to where it should be in the scene, then focus!

There are many articles about this problem (try Google), here is a good one:

http://www.mhohner.de/essays/recompose.php

-Ross

rosswarner.com

Or customize a button to 'Lock-On AF' - It even works for (slow) moving subjects...

...Anyway, after some hundred shots, I don't see the "soft corner problem" my 24-70mm is supposed to have - Maybe a good lens or perhaps the appropriate technique?!

Just saying...

Best regards,
Pedro

PS - Btw, this is one of the best 24-70 zooms I ever owned and it really performs extraordinary well with the A7R; the same goes to the optical stabilization which is giving me excellent results hand held at 1/6s...

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Chad Hardy
Chad Hardy Senior Member • Posts: 1,192
Re: Focus-recompose method and "soft corners"

Ross Warner wrote:

I'm referring mostly to 24-70 f4 lens tests on A7 and A7R, so no touch screen!

Hi Ross,

If you change the angle enough that DOF is altered then yes. In the end the corners really don't matter a whole lot, and if I hand a lens that was soft in all corners I know this is probably just a limitation of the lens.

However if you have 1 soft corner, it would appear to me something is wrong.

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CraigArnold Contributing Member • Posts: 697
Re: Focus-recompose method and "soft corners"

Ross Warner wrote:

Reading endless discussions about "soft corners" and other focus problems in lens tests, it occurs to me to ask if some of these people might be using the focus-recompose method, which many seem to use by default for everyday work. This can cause focus problems, especially in the corners.

For instance, if you're taking a wide angle picture of an off-center lighthouse, DON'T use the center point to focus on the lighthouse and then recompose! Manually move the focus point to where it should be in the scene, then focus!

There are many articles about this problem (try Google), here is a good one:

http://www.mhohner.de/essays/recompose.php

-Ross

rosswarner.com

And this phenomenon relates specifically to corner softness how?

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OP Ross Warner Senior Member • Posts: 1,323
Re: Focus-recompose method and "soft corners"

CraigArnold wrote:

Ross Warner wrote:

Reading endless discussions about "soft corners" and other focus problems in lens tests, it occurs to me to ask if some of these people might be using the focus-recompose method, which many seem to use by default for everyday work. This can cause focus problems, especially in the corners.

For instance, if you're taking a wide angle picture of an off-center lighthouse, DON'T use the center point to focus on the lighthouse and then recompose! Manually move the focus point to where it should be in the scene, then focus!

There are many articles about this problem (try Google), here is a good one:

http://www.mhohner.de/essays/recompose.php

-Ross

rosswarner.com

And this phenomenon relates specifically to corner softness how?

If you use focus-recompose technique, don't complain about soft corners. The corners are likely to show the worst effect because they don't lie within the focus plane.

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ncsakany
ncsakany Senior Member • Posts: 2,129
Re: That's what the touch screen is for!

sassenach74 wrote:

Just tried touching the screen on my A7R, didn't work

You need to press harder.

sean lancaster
sean lancaster Veteran Member • Posts: 7,309
Re: Focus-recompose method and "soft corners"

Ross Warner wrote:

CraigArnold wrote:

Ross Warner wrote:

Reading endless discussions about "soft corners" and other focus problems in lens tests, it occurs to me to ask if some of these people might be using the focus-recompose method, which many seem to use by default for everyday work. This can cause focus problems, especially in the corners.

For instance, if you're taking a wide angle picture of an off-center lighthouse, DON'T use the center point to focus on the lighthouse and then recompose! Manually move the focus point to where it should be in the scene, then focus!

There are many articles about this problem (try Google), here is a good one:

http://www.mhohner.de/essays/recompose.php

-Ross

rosswarner.com

And this phenomenon relates specifically to corner softness how?

If you use focus-recompose technique, don't complain about soft corners. The corners are likely to show the worst effect because they don't lie within the focus plane.

Focus and recompose is nice for shooting people when wanting to get eyelashes in focus, for example. I do this all the time and the focus peaking on the A7/R is excellent to ensure that the eyelashes stay in focus. Corners never come into play for this type of shooting. If I am shooting f/1.4 - f/2 then I am not worried about corners for my shooting. If I am shooting landscape then I am not going to do focus and recompose because the landscape is not moving. Ideally a tripod is used. But even then, I stop down for landscape so much that I could spin around 10 times after focusing and still get perfect focus on the scene.

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tomhongkong Veteran Member • Posts: 3,602
Re: Focus-recompose method and "soft corners"

Whilst this is a theoretical problem which does exist, it is unlikely to affect many situations.

The example in MHohner's site used a 50mm/f1.4 lens, and assumed that the movement of the focal point was from the centre to the extreme edge of the FOV.  Even then he calculates the focal shift at 30mm, which was within the DOF of the lens at that distance of 39mm for a subject 5m away.  If the subject is at 2m the focus shift is 12cm, almost twice the front DOF of 6.5cm.  I agree with these calculations

The problem is worst with large aperture lenses, longer FLs and short subject distances, and worse with FF.

The largest NEX lens is the 50/1.8.  This has a maximum FOV (diagonally) of 32degrees, so the very most you could move the camera would be 16 degrees.

Using the formula MHohner quotes (which is right) the focus shift at the extreme corner, for an object 2m away is 8cm. (this is less than with a FF lens which has a wider FOV)  This is almost the same as the front DOF of that lens, and is probably visible, but this is the worst case for our cameras.

Using the same lens at f2.8, moving the camera a more normal displacement, say to put the subject on the rule of thirds lines (displacement of 6 degrees), and a subject distance of 2m, gives a focus displacement of 1.1cm, in a front DOF of 12cm which is negligible.

Using your kit lens at around f4 it is hardly ever going to be a problem.

So yes, it is a theoretical problem, but if your corners are blurry you are going to have to look elsewhere for the reason.

(Apologies if my maths is wrong anywhere)

tom

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wwwOKCOMEONcom Contributing Member • Posts: 539
Re: Focus-recompose method and "soft corners"

Ross,
I think the problem is not serious for
wide-angle lens due to greater dof

Godfrey Forum Pro • Posts: 29,403
Re: Focus-recompose method and "soft corners"

Ross Warner wrote:

Reading endless discussions about "soft corners" and other focus problems in lens tests, it occurs to me to ask if some of these people might be using the focus-recompose method, which many seem to use by default for everyday work. This can cause focus problems, especially in the corners. ...

My lens tests are done with the camera on a tripod, rigorously squared and leveled to a target, and I usually shoot two sequences: one focused on a center point, the other on a point about half-way from center to the long edge of the frame, using the maximum magnification by moving the 'hot spot' in the viewfinder so as not to change the orientation of the camera and target.

  • When the two series look equally sharp from aperture setting to aperture setting, I know there is minimal curvature of field and minimal focus shift.
  • When the corners are soft, the corners are soft.
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NomadMark
NomadMark Contributing Member • Posts: 603
Re: Focus-recompose method and "soft corners"

Ross Warner wrote:

I'm referring mostly to 24-70 f4 lens tests on A7 and A7R, so no touch screen!

Or maybe it's just the lens. Nothing to do with field curvature. Perhaps at the wide end, the corners are weak, and that's just a characteristic of the lens.

Check the corners here. Sorry the rest of the frame is not full of rich detail, but the lower corners and boarder is, and there is a disparity at 24mm, even from the boarders to the corners. Everything is at infinity. And no matter where or how you focus, corner, boarder, center and recompose, the results are the same. Weak corners at the wide end.

24mm f4

24mm f8 - corners do not really improve (sorry the framing is not the same as above)

At 35mm, corners look better.

At 50mm (51mm here) corners look great

At 70mm things are starting to soften, but still....they look ok.

I like the lens. It feels great, well build, sharp in most cases...just not at (and around) 24mm, in the corners.

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CraigArnold Contributing Member • Posts: 697
Re: Focus-recompose method and "soft corners"

Ross Warner wrote:

CraigArnold wrote:

Ross Warner wrote:

Reading endless discussions about "soft corners" and other focus problems in lens tests, it occurs to me to ask if some of these people might be using the focus-recompose method, which many seem to use by default for everyday work. This can cause focus problems, especially in the corners.

For instance, if you're taking a wide angle picture of an off-center lighthouse, DON'T use the center point to focus on the lighthouse and then recompose! Manually move the focus point to where it should be in the scene, then focus!

There are many articles about this problem (try Google), here is a good one:

http://www.mhohner.de/essays/recompose.php

-Ross

rosswarner.com

And this phenomenon relates specifically to corner softness how?

If you use focus-recompose technique, don't complain about soft corners. The corners are likely to show the worst effect because they don't lie within the focus plane.

I fear you misunderstand the geometry. It is a plane of focus. Centre and corners will be equally affected. If people are seeing sharp centre and soft corners focus-recomposed has nothing to do with it.

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