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sweet spot for a lens ?
Old 09-04-2006, 08:17 AM   #1 (permalink)
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How does one find the sweet spot of a lens ? I have not been able to see any real difference .
Mike
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Re: sweet spot for a lens ?
Old 09-04-2006, 09:09 AM   #2 (permalink)
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There are two methods that I know of; quantitative or qualitative. The former involves all sort of MTF charts and physics, while the latter means just shooting a bunch of images at different apertures and focal lengths (if it's a zoom) until that 'something special' shows up. I couldn't tell you how the first one works and I'll bet you've been working on the second. That's my personal method. And a whole bunch of fun to boot... I look for sharpness (200% blowups in PS), contrast, and color rendition to make my decision. YMMV.

For my Nikkor 180mm f/2.8 prime, that happens to be f/4.0. The 85mm f/1.4 has the biggest sweet spot I've ever seen from f/2.0 to f/8. At least to my eyes

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Re: sweet spot for a lens ?
Old 09-04-2006, 09:17 AM   #3 (permalink)
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[quote=That_Look_Photo]How does one find the sweet spot of a lens ? I have not been able to see any real difference .
Mike

In general and in most cases it's usually two stops from the widest aperture point. For instance, a 2.8 lens would have a sweet spot at 5.6.

In lamen terms and in some cases shooting wide open on lenses you might have a strong center sharp area while the edges start to go soft. You will find this a lot more often in your cheap chimpy bimbo lenses, but even expensive lenses might depart from sharp from center, top and bottom sharpness. Therefore when it comes together at the lens's optimum aperture you will have the "sweet spot" while in control of dof.

Now take that into consideration and then you have focal length to factor in on various zoom lenses. For instance, an 80-200 at 80 mm might have a sweet spot that is different then at 200. Also variable aperture zoom lenses will obviously be different as well.


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The Sweet Spot--the Myth, the Legend, the Truth...
Old 09-04-2006, 10:11 AM   #4 (permalink)
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There are many myths and legends as to what a lens sweet-spot is or isn't, but in reality, unless you check manufacture's specifications or have an accurate bench-test method, the sweet spot will vary from lens to lens and from manufacturer to manufacturer.

First, let's look at the legend and a fact--Leica. Leica invented the 35mm format, not Kodak. Kodak invented film. According to Leica, all Leica lenses have a sweet-spot throughout the entire aperture range. Leica even makes an F/1.0 lens--they have the best glass, they also are appropriately priced. With that said, the entire range of F/stops (apertures) on a Leica are sweet.

Now let's look at the myths--

1. Many say that the sweet-spot on a lens is two to three stops up from the widest aperture of the lens. While this could be, it's not scientifically proven, only myth.

2. Many say the sweet-spot on a lens is F/8 to F/11, again, this is not scientifically proven, only myth.

3. Many say it's the middle F/stop of the lens, again, this is not scientifically proven, only myth.

4. Zooom lenses have a better sweet spot at the middle focal-length, middle aperture, i.e., a 70-200mm zoom has the best sweet spot at 140mm at the middle aperture, again, this is not scientifically proven, only myth.

Now, with all the three myths, let's look at a lens with the maximum aperture of F/2.8 and a minimum aperture of F/22, focal length of 70-200mm (non-Leica). If we apply myth #1, then the sweet-spot will be between F/5.6 to F/8. If we apply myth #2, the sweet-spot will be F/8 to F/11. If we apply myth #3, the sweet-spot is F/8. If we apply myth #4, the sweet spot for the lens is at F/8 at 140mm.

What all three myths have in common is F/8. Perhaps that's why photojournalists always say, "F/8 and be there?" Nope, that's not why, so don't go there, that has more to do with exposure and the Sunny Sixteen rule and the old days of Tri-X pan film--another thread!

The whole concept of the sweet-spot is the characteristics and physics of the lens as all lenses have a sharper center with the edges becoming softer and because of this, their sharpest point is not wide-open, and at smaller apertures the lenses suffer from diffraction.

There are so many variables that determine the sweet-spot, focal length (magnification), zoom vs prime, glass quality, resolving power, contrast of the lens glass, longitudinal and lateral chromatic aberration, refractive index, low-dispersion glass, volume of fluorite in a lens, diffraction and accutance.

Bottom line, know your equipment. You won't go wrong with the sweet-spot at F/8 on any lens--now the real question, do you want the sharpest image you can get with every subject? Of course not. You can use the sweet-spot knowledge to your advantage, on a subject with not so porcelain skin you'd shoot with a wider-aperture to not only avoid the sweet-spot, but to reduce your depth-of-field (DOF) for a more soft image of your subject. On the other hand, shooting products or a model with superb, porcelain skin, shoot at the sweet-spot of your lens.

In the end, the manufacturers can tell you for each lens the specific "optimum aperture" is for their specific lens, also know as the sweet-spot, or the compromise between chromatic abberation and lens diffraction. Trust me, it's in their engineering notes and probably filed with their patents.

Thanks, wishing you the best, and thanks for being a part of the Glamour 1™ community and family, rg sends!
(Image of the Spurs shot with Leica 70-180mm zoom lens at F/2.8)


(Images below shot w/Leica 100mm prime lens, somewhere between F/4-F/8.0)
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Last edited by rolandogomez; 09-04-2006 at 10:24 AM..
 
Re: sweet spot for a lens ?
Old 09-04-2006, 11:24 AM   #5 (permalink)
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Unless you're a professional lens test-chart photographer (and then I pity you), don't worry about it too much. Even the term sweet spot could mean different things to different photographers. Manufacturers always have to balance between resolving power and gradation, as you can't optimize for both at the same time. High resolving power comes with low contrast and vice-versa (high contrast comes with low resolving power). Let the lens designers worry about it.

From a physics standpoint, nearly all modern lenses are diffraction-limited once you get beyond f/8. Unless you need the DOF there's no reason from a sharpness standpoint to stop down beyond that aperture. Likewise, there are very, very few lenses that are sharpest when used wide open. But so what? If shooting wide open allows you to overcome camera shake then I guarantee that the image will look sharper than if you had stopped down to where the lens supposedly performs best.

The only way to really answer your own question is to get the bench-test results (MTF and other charts) for a given lens and read it. After you've done that you should crumple it up, throw it in the trash, and go out and shoot. Working on your composition and reminding yourself to not stab the shutter release with your finger will be a much better use of your time than reading lab reports.

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Myths can be in general
Old 09-04-2006, 12:07 PM   #6 (permalink)
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RG

Appreciate your scientific definitions of the sweet spot but in general your definition of myths are simple "in general" situations to what most have found to be good starting points for "various" lenses on the sweet spot.

It's true about Leica, but myth or not, it's common for good starting points for the popular "in general" lenses to be about two stops. Doesn't mean exactly but there is no need to rack our brains to get so specific with the sweet spot.

My own bench tests from a visual stand point on everything from the past 21 years and every lens used from Zeis to Mamiya to Tamron with Canon and Nikkor in between has hit the sweet spot between 2 and 2.5 stops from the widest aperture almost every single time, regardless of manufacturer's lenses bench tests. This is why you hear it constantly repeated in lamen terms and it's why I prefer to generalize the sweet spot in most cases.

It's always best with the individual and his or her camera to do their own field test regardless of what companies say their lenses will do. Sweet spots will vary per individual tastes, cameras, film or digital, 1.5x factor or full frame sensors, digital noise, mirror alignment, etc, for their view points.

Most importantly a photographer should not be limited to the sweet spot and can very easily limit him/her to constantly thinking they have to be shooting at the "sweet spot".

For instance the original poster has never been able to tell a difference and now he will probably be limiting himself trying to find a sweet spot thinking he will become a better photographer for it. ;-)

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beliefs vary....
Old 09-04-2006, 12:25 PM   #7 (permalink)
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JT,

I was merely pointing out what are "beliefs" out there that are not grounded, only assumed. As noted, the best way to know the actual sweet-spot for sure is to ask or seek info from the manufacturer. I like Chip's mentality, just fu#$%g shoot! When I shoot, I don't worry about the sweet-spot on my lens anymore than on my scanner. I know my equipment and I know what it will deliver with different combination for different scenarios. I do know practically any lens will have the sharpest point somewhere around F/8, so it's safe to say F/8.0 is a happy medium.

In the end, what works best for you applies to everyone here. While you've done your "bench tests" it's based on subjective testing, i.e., your eyes, your lenses, your equipment, your subjects, your working environments, your lighiting, your post production and so forth. No one is saying you're wrong, to the contrary, knowing your equipment and what works best for you is always the ideal solution. Just like diopter adjustments for camera viewfinders are adjusted for the user, bench tests vary too, even at manufacture level with scientific equipment and bench test experts.

Depending on what I'm shooting, where I'm shooting, and whom I'm shooting determines my sweet-spot, and as Chip pointed out, I rarely worry about it, I shoot off instinct, or in my case, 27-years of photographic experience with every brand of equipment out there, from 35mm to larger formats.

In the end, use what works best for you and learn your equipment along the way while developing your style. Practice, practice, practice. I practice my craft everytime I release the shutter--photography never ends and it's always subjective.

I wish you the best, rg sends!


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Re: beliefs vary....
Old 09-04-2006, 12:28 PM   #8 (permalink)
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Quote:
Originally Posted by rolandogomez
I like Chip's mentality, just fu#$%g shoot!
I hear that. ;-)

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Re: sweet spot for a lens ?
Old 09-08-2006, 03:29 PM   #9 (permalink)
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and this advice hits the sweet spot!

well said, sir!

Quote:
Originally Posted by ChipBulgin
Unless you're a professional lens test-chart photographer (and then I pity you), don't...
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