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07-04-2015, 02:33 PM   #1
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List of each lense's sharpest f stop?

Was wondering if anyone had ever compiled a list of each lense's sharpest F stop, often referred to as the sweet spot? I realize the value is subjective, but in reading various lens reviews the f stop given usually is common to the different reviewers.
Searched forum with no luck.
If not and anyone is interested in such a list, I might give it a stab,

07-04-2015, 02:39 PM   #2
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I don't know about for each lens. And I think that would be impacted by if it were used on ff or apsc due to the diffranction limit.

But the conventional wisdom is that most lenses are sharpest when stopped down 2-3 stops from wide open. However, more modern premium lenses tend to break that rule a little early. The DA* 300/4 peaks after stopped down just one stop rather than 2-3.
07-04-2015, 02:48 PM   #3
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You have to look at a professional lens review of the specific lens you are interested in. Check websites like.. um.. Photozone, Lenstip, DXO.. note, though, that you cannot compare the results between different cameras. But you can see which aperture has highest resolution.

On some Pentax cameras you can choose the P mode that has MTF line, and the camera will automatically try to choose the sharpest apertures. But this only works for modern Pentax lenses, I think.
Generally speaking, f8 is safe from diffraction, so it is a favourite because it gives good center and border sharpness as well as a wide DoF. Most lenses tend to be sharpest around 1 or 2 stops from the max aperture But it all depends on the specific lens, some behave differently from others. So check specific reviews.
07-04-2015, 02:56 PM   #4
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Almost all the Pentax consumer primes and zooms, lenses are listed sharpest at ƒ 5.6, but some of the high end fast primes are sharpest at ƒ4. The 31 ltd is sharpest at ƒ2.8 The 77 is sharpest at 4 as is the DA 70 and the DA 35 ƒ2.4. Those are the exceptions to ƒ5.6 being sharpest as I remember then.

Both APS-c and Full Frame are diffraction limited by ƒ8, but combination sharpness and DoF, ƒ8 is definitely the landscape guys sweet spot.


Last edited by normhead; 07-04-2015 at 03:27 PM.
07-04-2015, 07:47 PM   #5
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"Typically, the effects of diffraction softening do not even begin to become apparent until f/11 on FF (f/7.1 on APS-C and f/5.6 on mFT -- 4/3)" Equivalence

because that's where the dof is roughly the same on both formats... both have similar diffraction losses when the dof is the same, so there isn't any advantage either way, it's just different aperture readings on the lenses, you have to shoot accordingly.
07-04-2015, 11:47 PM   #6
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QuoteOriginally posted by normhead Quote
Almost all the Pentax consumer primes and zooms, lenses are listed sharpest at ƒ 5.6, but some of the high end fast primes are sharpest at ƒ4. The 31 ltd is sharpest at ƒ2.8 The 77 is sharpest at 4 as is the DA 70 and the DA 35 ƒ2.4. Those are the exceptions to ƒ5.6 being sharpest as I remember then.

Both APS-c and Full Frame are diffraction limited by ƒ8, but combination sharpness and DoF, ƒ8 is definitely the landscape guys sweet spot.
I can agree to this. Most of my lenses seem to be sharpest at just about 5.6 with very few exceptions (like the 18-55 needing to be like f8).
07-05-2015, 12:00 AM   #7
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QuoteOriginally posted by mdave13 Quote
Was wondering if anyone had ever compiled a list of each lense's sharpest F stop,...If not and anyone is interested in such a list, I might give it a stab,
I suggest just working your way down the reviewed K-mount lenses at photozone, and noting the f stop that delivered their sharpest [centre] point in a list.

If you wanted to be fancy, you could average the readings for the lens centre and the borders + extreme edges, and present just the f stop which yielded the best average sharpness for the lens.

Or go to DxOMark and sort their reviewed K-mount lenses by their sharpness score, then study at which aperture they reached their sharpness peak, according to DxO
07-05-2015, 11:46 AM   #8
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I've created a chart for myself for each lens that summarizes what I think give sharpest results. E.g., the DA 21 f3.2 and DA 50 f1.8 best at f5.6-8. The DA 35 f2.4 at f4-5.6.
The zooms are trickier. Some need to be closed down as I move to the tele end and some opened up. E.g., the Sigma 10-20 f4.5-5.6 is best at f8 at 10mm and f5.6 @20mm.
The DA 18-135 is best at f5.6-8 at 18mm and f8-11 at 135mm.
I also suspect that each lens has its own quirks so that what works with my lens won't necessarily be the same for someone else's.

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