Is sharpness function of absolute f/stop or relative f/stop?

New Daddy · Joined: 22 Feb 2011 Posts: 5

I use a number of legacy, MF lenses on my micro four third cameras. As has been repeated reported by other users, all of these lenses are soft wide open.

What I'm wondering now is whether the softness is a function of the absolute f/stop or relative f/stop when these lenses are adapted on a digital camera.

Let me elaborate. I own a few 50mm/1.7 lenses (e.g., Konica Hexanon 50mm/1.7 & Pentax M 50mm/1.7). They are all soft wide open, and gain sharpness stopped down, with varying degree for each lens. Would a Konica 50mm/1.4 or Pentax M 50mm/1.4 exhibit the same degree of softness at 1.7 or would they be sharper than the 1.7s at 1.7? Since the 1.4s are considerably more expensive than 1.7s, I'm only willing to spend premium if they are sharper than the 1.7s at the same aperture.

The answer may be drawn from these lenses' respective MTF, but MTF can't explain how they would perform adapted on a micro four thirds. The softness at wide open is attributed to the fact that the lens is being used on a digital camera with a sensor size smaller than a full frame.

So I'd welcome personal experience from people who have used both 1.7s (or 1.8s) and 1.4s adapted on digital cameras and compared the sharpness.

Thanks.

CarbonR · Posted: Mon Mar 28, 2011 5:30 pm Post subject:

Some lenses have their max. possibilities at f/5.6, others at f/11, it depends mainly on the optical formulae (as far as I've read this) Wink

Orio · Expire: 2012-12-04

Every lens has it's own characteristic and "sweet spot", so one should really check the MTFs to see the optimal aperture.
As a coarse guide, usually two stops after wide open is considered a likely optimal aperture.
Examples: a f/2.8 lens will have sweet spot at f/5.6
a f/1.4 lens will have the sweet spot at f/2.8
This of course is only a generic guide, every actual lens may behave differently, but usually following this guide you don't fall very far from truth.

_

Orio · Expire: 2012-12-04

Addendum: with sweet spot, I mean the top performance of the lens. Which usually happens in the centre of the frame at a given f/stop.
If instead you want better uniformity across the frame, it is usually considered best to stop down one stop further from the sweet spot (which would coarsely mean three stops after wide open). This would slightly worsen the performance in the center, but usually improves performance in the corners, making the rendering of the image more consistent across the frame.
The usage of course will then dictate the choices: typically, a portrait photographer will search for the central sweet spot, while the landscape photographer will trade top performance in the center for uniformity across the frame and quality in the corners.

visualopsins · Expire: 2025-04-11

I have wondered similar -- is my S-M-C Takumar 1.4/50 @ f/4 sharper than my S-M-C Macro-Takumar 4/50 @ f/4? What about f/5.6 and f/8? Testing planned for later this Spring...

I do know my S-M-C Takumar 2.5/135 (version II) is sharper wide open than my S-M-C Takumar 3.5/135 wide open.

Orio · Expire: 2012-12-04

kuuan · Expire: 2014-12-26

comparing Pentax M 1.7/50 to K1.4/50 maybe these charts can help?:
http://www.takinami.com/yoshihiko/photo/lens_test/pentax_normal.html
btw. it seems that these 'normal lenses' tested have highest resolution at f8 regardless of their maximum aperture

another test: http://www.allgeektome.com/pentax/50%27s/resolution.htm

visualopsins · Expire: 2025-04-11

"Picture sharpness also varies with f-number. The optimal f-stop varies with the lens characteristics. For modern standard lenses having 6 or 7 elements, the sharpest image is often obtained around f/5.6�f/8, while for older standard lenses having only 4 elements (Tessar formula) stopping to f/11 will give the sharpest image. The reason the sharpness is best at medium f-numbers is that the sharpness at high f-numbers is constrained by diffraction,[6] whereas at low f-numbers limitations of the lens design known as aberrations will dominate. The larger number of elements in modern lenses allow the designer to compensate for aberrations, allowing the lens to give better pictures at lower f-numbers. Light falloff is also sensitive to f-stop. Many wide-angle lenses will show a significant light falloff (vignetting) at the edges for large apertures. To measure the actual resolution of the lens at the different f-numbers it is necessary to use a standardized measurement chart like the 1951 USAF resolution test chart."

Wikipedia -- F-stop

berraneck

I don�t think there is any relativity between lens sharpness and choice of film or digital - I mean lens is same sharp on both. Am I right? Smile

New Daddy · Joined: 22 Feb 2011 Posts: 5

ryan s · Joined: 26 Sep 2008 Posts: 384 Location: Madison, WI

What's the technical term for this: Lenses for a larger format are less sharp when adapted to a smaller format...at least that's how I understand it. A 645 lens doesn't need to be as sharp as an APS lens, so the 645 lens will be softer on the crop camera, for example.

As for the M 50s against each other, I'll happily take the smaller, cheaper 1.7 version. It's very good wide open, at least on an APS-C camera. Maybe they're not as good on the smaller 4/3 format Confused

That I don't know...

AhamB · Joined: 22 Jun 2008 Posts: 733 Location: Germany