October 23, 2014, 08:11:46 AM

Author Topic: Sharpest f stop for a lens?  (Read 8196 times)

alexanderferdinand

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Re: Sharpest f stop for a lens?
« Reply #15 on: September 15, 2013, 03:15:05 AM »
As a rule of thumb: 2 stops closed.
And a old saying says, at f8 a lens has to be good.
REAL GOOD lenses like the new vII zooms of the Canon or the big whites show less difference open vs 2 stops down.

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Re: Sharpest f stop for a lens?
« Reply #15 on: September 15, 2013, 03:15:05 AM »

Chris Jankowski

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Re: Sharpest f stop for a lens?
« Reply #16 on: September 15, 2013, 03:53:45 AM »
A few things worth adding to previous comments:

1.
As you close the aperture, at some point the diffraction effects will start decreasing sharpness.  This depends on the size of the sensor and on an APS-C happens typically from f16.
Here is more information including a calculator:

http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm

2.
Aperture that yields the maximum sharpness often depends on the design of a particular lens.  For example many cheap 70-300 zooms from the film era e.g 20 or 30 years old designs did not become really sharp until f11 especially at the long end of the focal range.

By comparison recently designed lenses, especially the more expensive ones are designed to be tack sharp at maximum aperture at least in the centre and then take only one stop to become sharp across the whole image. Sigma 35/F1.4 HSM and to an extent Canon EF 35/F2.0 IS USM are good examples. 
This was achieved in recent designs due to better design methods, better coatings, use of aspherical lenses and new special types of glass.  This design trend is driven by demand - people pixel peeping.  In the film times the maximum aperture value was often more important, as film had low light sensitivity expressed by ISO value.

3.
It is quite instructive to play with the clickable diagrams with sliders published in the reviews of slrgear.com.  Click on the blur index (sharpness) diagram, wait for it to pop up and play with the aperture and for zooms also with the focal length sliders.

http://slrgear.com/reviews/showcat.php/cat/10

This one is really interesting for EF 50 F1.4 USM:

http://slrgear.com/reviews/showproduct.php/product/140/cat/10

Look at the results for this lens measured on full frame Canon 5D.
At f1.4 only small area in the centre is sharp.  The corners are a mess. The lens becomes very good at F4 and perfect corner to corner at F5.6.

4.
Finally it is worth noting that all lenses have significant manufacturing variations in sharpness. 20% is common.  Here is a very nice blog on this subject by Roger Cicala.

http://www.lensrentals.com/blog/2013/09/there-is-no-perfect-lens

But even 20% variation is not really noticeable in real life, unless by obsessive pixel peepers (:-)).

I hope this helps.

Pi

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Re: Sharpest f stop for a lens?
« Reply #17 on: September 15, 2013, 10:50:59 AM »
Why do I have to say only the sweet spot matters, if I disagree with you that it does matter?

Because the difference, say between f/3.5 and f/5.0, on a decent prime is very hard to see; and has only been measured by PZ, etc., in the center or far away form it, with a perfect focus on a flat target. Sharpness across the frame varies in a ay dependent on the lens (sometimes the max is off center), etc.

takesome1

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Re: Sharpest f stop for a lens?
« Reply #18 on: September 15, 2013, 11:14:49 AM »
Why do I have to say only the sweet spot matters, if I disagree with you that it does matter?

Because the difference, say between f/3.5 and f/5.0, on a decent prime is very hard to see; and has only been measured by PZ, etc., in the center or far away form it, with a perfect focus on a flat target. Sharpness across the frame varies in a ay dependent on the lens (sometimes the max is off center), etc.

That doesn't answer that question.

RGF

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Re: Sharpest f stop for a lens?
« Reply #19 on: September 15, 2013, 02:34:17 PM »

BTW, this is of almost no practical use anyway unless you are shooting a chart.

Sure it is. We spend thousands of dollars on expensive gear to gain a little IQ that for the most part we only see if we pixel peak. Then if we ignore the sweet spot on our lens we ignore something that improves the IQ of our pixel peaking for free.

No, it is not. DOF, noise level, subject movements, border performance, etc., are far more important factors for choosing an f-stop. I would never shoot with the 135L or the 85L at f/2.8 instead of f/4 or vice versa just because some test site tells me that one of the f-stops is 5% sharper than the other. It will be sharper in a tiny plane of focus anyway and it would not necessarily give the best overall look, unless you are shooting a chart.

[\quote]

Agree.  Small difference in optical quality and less diffraction is often less important than other factors such as DOF, higher shutter speed to get less motion, (or slower shutter speed to get motion blur such as rushing water), ...

Suggest you experiment a decide.  Small differences are not that important.

takesome1

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Re: Sharpest f stop for a lens?
« Reply #20 on: September 15, 2013, 02:58:51 PM »

BTW, this is of almost no practical use anyway unless you are shooting a chart.

Sure it is. We spend thousands of dollars on expensive gear to gain a little IQ that for the most part we only see if we pixel peak. Then if we ignore the sweet spot on our lens we ignore something that improves the IQ of our pixel peaking for free.

No, it is not. DOF, noise level, subject movements, border performance, etc., are far more important factors for choosing an f-stop. I would never shoot with the 135L or the 85L at f/2.8 instead of f/4 or vice versa just because some test site tells me that one of the f-stops is 5% sharper than the other. It will be sharper in a tiny plane of focus anyway and it would not necessarily give the best overall look, unless you are shooting a chart.

[\quote]

Agree.  Small difference in optical quality and less diffraction is often less important than other factors such as DOF, higher shutter speed to get less motion, (or slower shutter speed to get motion blur such as rushing water), ...

Suggest you experiment a decide.  Small differences are not that important.

So you agree with PI, just ignore the sweet spot on your lens. It has no practical use.
It appears some people see a use for it, otherwise the OP would have never posted this thread.

By the way, no where in my post did I say that other factors were not more important. These are words PI is using to build an argument that doesn't exist.

ME

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Re: Sharpest f stop for a lens?
« Reply #21 on: September 15, 2013, 05:13:59 PM »
That sounds correct as a very general rule. Not all lenses are created equal. Some have more center sharpness and not as much edge sharpness, which many portrait photographers like. It can depend on the look you are trying to achieve, what dof to use; do you want all of the subjects head/face to be sharp, or both eyes to be sharp, etc. Also, the focal length of the lens, how far away from the subject, how you frame the head, etc can make a big difference. And often one would not want to see every flaw in the person's face, as it might not be a flattering portrait, hence do you really want the portrait to be that sharp? I just experiment with  new lenses and observe photographs taken by other photographers(if the have the exif info) to see how a particular lens will render the subject, whether it be a head shot, landscape, etc. But it sounds ok as a very general rule.
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Re: Sharpest f stop for a lens?
« Reply #21 on: September 15, 2013, 05:13:59 PM »

jdramirez

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Re: Sharpest f stop for a lens?
« Reply #22 on: September 15, 2013, 09:17:20 PM »
I deleted my stupid post... but here is what I was talking about earlier.

http://www.dpreview.com/lensreviews/canon_70-200_2p8_is_usm_ii_c16/5

It is with the 70-200 and you can see sharpness v. vignetting at different focal lengths and apertures.  It is fun to play with, but the real world results are really what you want to consider.
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Pi

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Re: Sharpest f stop for a lens?
« Reply #23 on: September 16, 2013, 11:41:35 AM »
Why do I have to say only the sweet spot matters, if I disagree with you that it does matter?

Because the difference, say between f/3.5 and f/5.0, on a decent prime is very hard to see; and has only been measured by PZ, etc., in the center or far away form it, with a perfect focus on a flat target. Sharpness across the frame varies in a ay dependent on the lens (sometimes the max is off center), etc.

That doesn't answer that question.

It does. The 5% difference between two f-stops that you can find on, say, PZ, is (1) misleading and (2) irrelevant.

Larry

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Re: Sharpest f stop for a lens?
« Reply #24 on: September 16, 2013, 12:27:41 PM »
It all matters. If you ignore one, then two things they compound on your IQ. One may have a greater impact than the other but they all matter.

Sweet spot matters, two examples both I know by experience and using tests;

My 35mm the spot starts at about 1.8 to 2.2, any wider and the quality falls quickly. The narrow DOF of the 35mm is what makes it a really great lens, go less than 1.8 and the IQ falls quickly. Knowing this I can take full advantage of my lens.

We are talking about different things. Of course, you have to know your lens. But the sweet spot for the purpose of this thread is somewhere between f/2.8 and f/4. And you are talking about narrow DOF and f/1.8-2.0. If somebody told you tomorrow that the sharpness across the frame is maximized at f/5.6, would you shoot at f/5.6 instead of at f/1.8 when you want that shallow DOF?  Or would you always shoot at f/5.6 for maximal sharpness even though your object is most likely not flat and not in the center only?

Why do I have to say only the sweet spot matters, if I disagree with you that it does matter?
I would take all in to account and not ignore one. I am saying they all matter.
If you want to ignore all of these things we could just say "F/8 and be there"

+1!

Jamesy

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Re: Sharpest f stop for a lens?
« Reply #25 on: September 16, 2013, 12:50:00 PM »
Prove it for yourself and run controlled tests, tripod mounted, with your different lenses.

The Pro version of Reikan FoCal has an Aperture Sharpness test that plots sharpness over the aperture range for your lenses.  Pretty handy, especially since it tells you about your own copy, not a different copy of the lens which someone else tested.
I ran this test on my lenses and I always thought F8 was the apex of sharpness - not so.
Here some of my results which surprised me:
24-105 (F4.5)
70-200/F4/IS (5.6)
50/1.4 (2.8 )
85/1.8 (4.5)
135L (F5)

As Neuro stated, these results are only for my copies but I always was under the impression that shooting closer to F8 was the sharpest. Now I try to be closer to the above stated f-stops when possible - except with the 135L - that puppy screams to be wide open!

curby

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Re: Sharpest f stop for a lens?
« Reply #26 on: October 03, 2013, 02:37:09 AM »
The OP asked where lenses tended to be sharpest, and if there's a rule of thumb that generally works.

The whole "how much does sharpness matter" debate is irrelevant, because it's still nice to know about patterns in lens performance even if it's not of practical importance for every shot.  There are geeks, enthusiasts, and engineers-at-heart in this hobby that are interested in the science too!

As others have suggested, I often look at reviews to get an idea of where my lens's weak and strong areas are.  That's never the first thing in my mind as I compose a shot, but sometimes it's useful additional information when you have time to set up a shot and carefully consider lens sweet spots along with other factors. 

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Re: Sharpest f stop for a lens?
« Reply #27 on: October 03, 2013, 04:33:51 AM »
So, I'm watching Rod Goodman's video on B&H about headshots.

He said the reason he shoots at 5.6 is that a lens is sharpest two stops from wide open, he shoots with the 70-200 2.8

Is this a myth or true? I have never heard of such a thing before

That's what I love about web based bro-science....so much BS passed off as fact! While it's true that most lenses perform at their best 2 stops down, most photographers hand hold their lenses....which makes stopping down irrelevent, the max resolution anyone can achieve hand held is around 60 lines per mm. Which is lower than most modern L zooms can achive shooting lens charts on a triopd under lab conditions.
Landscape photographer love their tripods becuase they love the max stability and ultimate sharpenss which they get from using this technique. Diffraction arguments are also mute if shooting hand held.
Some lenses are better designs than others, and are sharper wide open than other lenses when stopped down. But that's just the lens, what about the sensor's resolution capabilities? What of the photographer's technique? 

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Re: Sharpest f stop for a lens?
« Reply #27 on: October 03, 2013, 04:33:51 AM »

AcutancePhotography

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Re: Sharpest f stop for a lens?
« Reply #28 on: October 03, 2013, 12:41:17 PM »
- half of your smallest aperture. for example if f22 is your smallest, then f11 is your sharpest


I think you wrote that wrong.  I believe that the correct way is to take the median value of the max and min aperture range and open/close up by that value.  You are not dividing the minimum aperture by 2, but the difference between max and min aperture by 2. 

For example if you have a 5 stop difference between wide open and stopped down (f2.8-f22), then the median value is 3.

Either stop down 3 from max (f8) or open up 3 from the minumum (f8) for sharpness. 

Some say that lenses are sharper 2 stops down.  Others say that lenses are sharper at the median aperture.  They are probably equally right and wrong.  :)

I tend to just stop down by 2 and go from there.  Easier math that way.  LoL
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Re: Sharpest f stop for a lens?
« Reply #29 on: October 03, 2013, 01:03:56 PM »
Prove it for yourself and run controlled tests, tripod mounted, with your different lenses.

The Pro version of Reikan FoCal has an Aperture Sharpness test that plots sharpness over the aperture range for your lenses.  Pretty handy, especially since it tells you about your own copy, not a different copy of the lens which someone else tested.
I ran this test on my lenses and I always thought F8 was the apex of sharpness - not so.
Here some of my results which surprised me:
24-105 (F4.5)
70-200/F4/IS (5.6)
50/1.4 (2.8 )
85/1.8 (4.5)
135L (F5)

As Neuro stated, these results are only for my copies but I always was under the impression that shooting closer to F8 was the sharpest. Now I try to be closer to the above stated f-stops when possible - except with the 135L - that puppy screams to be wide open!

You are looking at center sharpness, not overall sharpness.  As Neuro stated, edges and off center areas are often sharper at a different aperture.  There is a reason why f/8 is often quoted, and that's because the sharpness is usually even across the frame, CA, and other abberations are minimized, so its a safe bet that you can get a excellent image.  Another thing to note is that most lenses have the same sharpness levels at f/8 or f/11, so you don't need a $$$$ lens to match a cheap lens at f/8 - f/11 as far as sharpness goes. Other factors will vary, of course.
 
The reason people buy wide aperture lenses is to get shallow depth of field and more light.  Don't bother if you just want sharpness.
 
Otherwise, if center sharpness is all that matters, down 1-2 stops is usually close to the peak. 

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Re: Sharpest f stop for a lens?
« Reply #29 on: October 03, 2013, 01:03:56 PM »