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 Author Topic: Sharpness wide open  (Read 3498 times)
JB Rasor
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 « on: January 18, 2014, 06:25:10 AM » Reply

I'm in situations where, due to dim light, I need to use a large aperture to shoot a distant subject. In other words, f1.4 or f2.8 for a landscape, where the closest object in view is well over 30 feet away, and the horizon may be hundreds of meters.

My question is probably a bit complex, but what happens, mathematically/scientifically or even in layman's terms, when shooting a distant object at a large aperture? Are the edges a bit softer than they'd be at f8? Is there a warping effect of softness around the center of the frame? Or is the closest object focused on sharp and very distant objects slightly sharp? I'm familiar with depth of field and optics on a pretty good level, but I've never really had this thought addressed before.

I heard Michael Reichmann say that a perfect lens is theoretically sharpest at its widest aperture. But I've always believed when shooting distant subjects it's better to stop down. True? False? Enlighten me please.

Thanks all!
JB Rasor
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ErikKaffehr
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 « Reply #1 on: January 18, 2014, 07:02:57 AM » Reply

Hi,

A perfect lens would be sharpest at maximum aperture. Such lenses do exist, a few of them are circling the earth.

A normal lens is not perfect, they are suffering from aberrations. Stopping down a lens normally decreases aberrations, making the lens better. Stopping down increases diffraction, which is cause by light waves bending around the aperture. When you stop down a lens there is an aperture where the sum of aberrations and diffraction is lowest, that is the optimum aperture. It is often around f/5.6 or f/8.

Best regards
Erik

I'm in situations where, due to dim light, I need to use a large aperture to shoot a distant subject. In other words, f1.4 or f2.8 for a landscape, where the closest object in view is well over 30 feet away, and the horizon may be hundreds of meters.

My question is probably a bit complex, but what happens, mathematically/scientifically or even in layman's terms, when shooting a distant object at a large aperture? Are the edges a bit softer than they'd be at f8? Is there a warping effect of softness around the center of the frame? Or is the closest object focused on sharp and very distant objects slightly sharp? I'm familiar with depth of field and optics on a pretty good level, but I've never really had this thought addressed before.

I heard Michael Reichmann say that a perfect lens is theoretically sharpest at its widest aperture. But I've always believed when shooting distant subjects it's better to stop down. True? False? Enlighten me please.

Thanks all!
JB Rasor
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Tony Jay
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 « Reply #2 on: January 19, 2014, 02:57:36 AM » Reply

Nice summary from Eric.

The dot pitch of the sensels on the sensor also play a role.
Sensors with larger sensels allow one to stop down,perhaps a stop or two, more without the effects of diffraction becoming obvious.
Actual diffraction caused by the lens diaphragm is not affected.

Some small  cameras with apparently big resolution are diffraction limited with the lens wide open.
So, in digital photography, the size of the light sensitive captor (sensel) is much smaller than the finest grained film.
Therefore, theoretically better resolution is possible size-for-size, but diffraction then becomes the limiting factor i fth esensels are made too small.
This is one reason why medium-format digital backs still command a premium price - large sensels and lots of them.
Increased dynamic range is also an advantage of the larger sensors.

Another corollary of all of this is that cameras with large sensors and sensels theoretically can use lenses that are of lesser quality than cameras with smaller sensors and correspondingly small sensels.
In practice though it is often the case that current lens designs for medium-format cameras are manufactured to a higher standard than those for 35mm (and smaller) cameras and so the medium-format cameras can potentially outperform their smaller cousins.

I won't mention anything in detail about anti-aliasing filters apart from to say that most medium format-backs do not have one and most 35mm (and smaller) camera sensors do and so that again influences the apparent resolution of the cameras.

This was not mean't to be a head-to-head comparing different camera formats rather a small explanation of the fact that it is pointless to talk about the behaviour of lenses without taking into account what the lens is focusing on - in this case a digital sensor for nearly all of us.

Tony Jay
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Petrus
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 « Reply #3 on: January 19, 2014, 04:22:35 AM » Reply

My question is probably a bit complex, 1) but what happens, mathematically/scientifically or even in layman's terms, when shooting a distant object at a large aperture? 2) Are the edges a bit softer than they'd be at f8? 3) Is there a warping effect of softness around the center of the frame? 4) Or is the closest object focused on sharp and very distant objects slightly sharp?

I heard Michael Reichmann say that a perfect lens is theoretically sharpest at its widest aperture. But I've always believed when shooting distant subjects it's better to stop down. 5) True? 6) False? Enlighten me please.

Thanks all!
JB Rasor

1) same thing as when shooting any object at the largest aperture: depth of field is at minimum and focus errors come apparent as do lens imperfections.

2) Most likely yes, as all commercially available lenses to general public are not sharpest at full open. Different aberrations always manifest themselves first in the corners.

3) Well, what ever you want to call it. Usually there is more softness and color fringing away from the center, but also non-flat plane of focus which makes center sharp but corners not.

4) If the lens is properly focused to infinity, then the horizon (if it is 100m or more distant) should be sharpest. If the lens is out of adjustment or deliberately or by mistake focused closer than infinity then something closer will be in better focus and horizon not. Just like with any focusing distance. Many lenses will focus past infinity (for several reasons), then nothing will be in focus in normal circumstances. So turning the focus ring against the infinity end stop might not be the best way to focus for landscapes shot at maximum aperture.

5) True, for maximum sharpness something like f/5.6-f/8 is best. If more DOF is needed just close down even to f/11 or f/16 without worry. Theoretically sharpest means such a lens could be theoretically built, but in practice none are, if we are talking about normal relatively fast lenses, not something f/8...

6) False, generally. Of course it also depends on the DOF needed and if short exposure times (handheld) are more important the maxim lens sharpness.
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JB Rasor
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 « Reply #4 on: January 19, 2014, 04:45:11 AM » Reply

Thank you very much gentlemen! Excellent summaries. I was looking at the depth of field chart on my lens models, which on the surface seemed to answer my original question, but you guys really broke it down for me. Have a great Sunday!

JB
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Christoph C. Feldhaim
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There is no rule! No - wait ...

 « Reply #5 on: January 19, 2014, 08:47:03 AM » Reply

You don't need to be too anxious about stopping down a lot.
Though it is true, that diffraction starts to kick in at some F-Stop,
it is relatively benign to sharpening in post.

Example:
I made some landscape shots with my Fuji X-E2:
16 Megapixels, Pixel Pitch 4,82µ

According to this Wikipedia Article the Gaussian approximation to the Airy disk is about one-third the Airy disk radius, i.e. 0.42*wavelength*F-Stop.
Since visible light is somewhere between 400 and 700 nm wavelength
at F22 the Gaussian approximation lies somewhere between
0.42*400*22 = 3696 nm ≈ 3.70µ and 0.42*700*22 nm  = 6468 nm ≈ 6.47µ

For my sensors pixel pitch this means I will start with a sharpening radius between
3,70µ/4,82µ≈0,8 pixels and 6.47µ/4,82µ≈1,3 pixels which ends up in a
rule of thumbs of starting with about 1/15th to 1/30th of my F-Stop in pixels for the initial sharpening radius to eliminate diffraction on my specific sensor.
Since focus blur and other effects also come into play the radius might differ, but you get something to start with.

Hope thats not too far off, just wanted to take away the fear from the diffraction monsters lands beyond F8 ....

Cheers
~Chris

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Alan Klein
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 « Reply #6 on: April 21, 2014, 03:47:10 PM » Reply

I'm in situations where, due to dim light, I need to use a large aperture to shoot a distant subject. In other words, f1.4 or f2.8 for a landscape, where the closest object in view is well over 30 feet away, and the horizon may be hundreds of meters.

My question is probably a bit complex, but what happens, mathematically/scientifically or even in layman's terms, when shooting a distant object at a large aperture? Are the edges a bit softer than they'd be at f8? Is there a warping effect of softness around the center of the frame? Or is the closest object focused on sharp and very distant objects slightly sharp? I'm familiar with depth of field and optics on a pretty good level, but I've never really had this thought addressed before.

I heard Michael Reichmann say that a perfect lens is theoretically sharpest at its widest aperture. But I've always believed when shooting distant subjects it's better to stop down. True? False? Enlighten me please.

Thanks all!
JB Rasor

1.  Calculate and select the proper f stop to get the objects you want to be in focus to be in focus.  Don't worry about diffraction or lens sharpness.  If the subjects aren't in focus, having less diffraction or a so-called sharper setting won't matter.  The picture won't be good anyway.  What I do is calculate the f stop I need for the DOF required for what I want to be in focus. Then I stop down one additional stop for safe measure.

2. I always use a tripod so I can decrease shutter speed for the proper exposure.  If that's 1 second, so be it.

Bottom line.  Damn the diffraction and sharper lens setting.  Work on the f stop first for the DOF you need.
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cjogo
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 « Reply #7 on: May 23, 2014, 01:46:53 PM » Reply

I don't think I have ever shot a landscape at less than  F 8-11 -- even with a good Zeiss lens -- but I  shoot without a tripod 95% of the time.
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bill t.
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 « Reply #8 on: June 02, 2014, 08:31:30 PM » Reply

And you have to remember that truly perfect, prime focus is in a very shallow plane (possibly curved) out in front of the camera, even when you are stopped down.  For instance, when I shoot focus stacks at f8 with a 50mm lens, to truly carry excellent focus out to infinity, I need to shoot separate exposure brackets out as far as about 30 meters and then one at infinity as well.  When I pixel-peep an image focused at 30 meters, I see that the 3km distant mountains behind it are not quite in sharp focus, and vice-versa for an image shot focused at infinity.  I bet those orbiting f-zilch lenses need separate focusing points for the tops of tall buildings, versus the first floors.
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Jim Kasson
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 « Reply #9 on: June 02, 2014, 09:23:22 PM » Reply

A perfect lens would be sharpest at maximum aperture. Such lenses do exist, a few of them are circling the earth.

Nicely played, Erik. I missed this when you first posted it.

Jim
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Misirlou
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 « Reply #10 on: June 03, 2014, 03:05:01 PM » Reply

Nicely played, Erik. I missed this when you first posted it.

Jim

Although, a mirror and a lens aren't exactly the same thing.
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Jim Kasson
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 « Reply #11 on: June 03, 2014, 03:33:50 PM » Reply

Although, a mirror and a lens aren't exactly the same thing.

How about the ones trained on the Earth?

And, in the old days when they were thicker on the ground, didn't we talk about "cat" (for catadioptric) lenses, calling the whole optical system a lens, even if it had reflective and refractive components?

Jim
 « Last Edit: June 03, 2014, 03:36:45 PM by Jim Kasson » Logged

BernardLanguillier
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 « Reply #12 on: June 03, 2014, 04:49:33 PM » Reply

You need to look for a lens with a flat field. That will probably be the most important characteristics for your application.

Cheers,
Bernard
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A few images online here!
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