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Wayne

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I guess this does not concern Leica lenses exclusively, but it is a topic I have wondered about for some time.

 

In lens reviews, I notice that review will commonly include the statement: "this lens is a bit soft wide open, but sharpens considerably when stopped down." This is certainly understandable when fast, i.e. 1.2, 1.4.1.8, lenses are being evaluated, but I have also seen the comment included for lenses that are not so fast, i.e. 2.8 through 3.5.

 

Is increased softness at wide open aperture just a quality innately associated with all lens designs? I ask this question because I have also seen the comment: "If you are not going to shoot in low light, bokeh considerations aside, there is no real need for taking on the extra expense of fast glass." Given comparable quality of assembly, materials, and design, is it likely that a 1.4 lens, stopped down to 2.8, will be sharper at 2.8 than a lens that is wide open at 2.8? IOW, in taking on the expense of high quality fast glass, is it likely the quality advantages will continue into use of slower apertures with that lens?

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I guess this does not concern Leica lenses exclusively, but it is a topic I have wondered about for some time.

 

In lens reviews, I notice that review will commonly include the statement: "this lens is a bit soft wide open, but sharpens considerably when stopped down." This is certainly understandable when fast, i.e. 1.2, 1.4.1.8, lenses are being evaluated, but I have also seen the comment included for lenses that are not so fast, i.e. 2.8 through 3.5.

 

Is increased softness at wide open aperture just a quality innately associated with all lens designs? I ask this question because I have also seen the comment: "If you are not going to shoot in low light, bokeh considerations aside, there is no real need for taking on the extra expense of fast glass." Given comparable quality of assembly, materials, and design, is it likely that a 1.4 lens, stopped down to 2.8, will be sharper at 2.8 than a lens that is wide open at 2.8? IOW, in taking on the expense of high quality fast glass, is it likely the quality advantages will continue into use of slower apertures with that lens?

Most lenses shot wide open will be softer than when stopped down 2-3 stops. It mainly has to do with spherical aberration: https://en.wikipedia.org/wiki/Spherical_aberration

 

When comparing two lenses, one with a maximum aperture of 1.4, and one with a maximum aperture of 2.8, both shot at 2.8, it is most likely that the lens with the maximum aperture of 1.4 will be sharper. Because that lens was designed to be shot at 1.4 (albeit with some compromises) stopping it down two stops from wide open increases sharpness pretty significantly. If you were to stop both lenses say, down to 5.6, it is likely that both lenses will resolve a very similar amount of detail.

 

So to answer the last part of your question, the image quality advantages of the faster lens start to diminish as you stop down a lens more and more. Of course you run into a problem with diffraction, but that's another issue.

 

With the faster lens, you do get to the highest resolving power of that lens at a wider aperture. A f/1.4 lens stopped down to f/2 will likely be very very sharp. Where as if you took that f/2.8 lens, and stopped it down, you'll be at f/4. So when I'm shooting any lens...I always stop it down, just a bit: http://www.lensrentals.com/blog/2011/11/stop-it-down-just-a-bit

Edited by Tony Mac
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Generally higher quality glass with advanced design shows less aberations than other glass. Having said that, most aberations diminish as the lenses are stopped down somewhat from wide open. So, in general, a high quality 1.4 stopped down to 2.8 will show better overall IQ than a 2.8 lens of similar design wide open. There are exceptions, of course, depending on how the lens design was executed to produce optimum performance at a given aperture, usually in terms of resolution - central and across the plane, contrast & microcontrast, coma, spherical aberation, color, and flare. Puts, for instance, gives lots of historical design info on optical characteristics in his Compendium...much more than I can absorb and integrate. An example many people can relate to is to notice that with many lenses, shooting wide open a high contrast object...such as a tree brance, roof, or wire against the sky often has tinges of chromatic aberation, which usually disappears when the lens is closed down 1-2 stops. So, yes, there may well be good reason to use a more expensive larger aperture lens, even if it is stopped down a bit, to gain a slight edge in"sharpness" - especially if shooting in color, as often chromatic aberation can be diminished significantly in B&W thru filteration on film or post processing conversion on digital.

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Totally depends on the lens, though every fast 50 lens will be sharper at f/2 than f/1.4

 

Puts says the 50 Lux APSH is sharper at 1.4 than the pre-asph (no slouch) is at 5.6!

 

The 35 cron asph is blistering at f/2, while the ZM 35/2 needs at least 2.8 to get going. By f/4 it can stay with anything and beat almost everything.

 

I think it's a new/old thing. The latest fast lenses, like the FLE 35 and the ZM 35/1.4 are astounding wide open. Still better at f/2, but they will beat many other lenses like the 50 ASPH does. And that lens does not loose it at f/8 or f/11. Still benchmark incredible.

 

On the other end of the spectrum is the CV 35/1.4 which is just not great at all WO, though you can use it. It gets better every stop till f/8.

 

As these guys allude, there are some lenses which really do start to loose it stopping down, but that is not very common at all. You may see this with super speed lenses, like the nocti.

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Is increased softness at wide open aperture just a quality innately associated with all lens designs? I ask this question because I have also seen the comment: "If you are not going to shoot in low light, bokeh considerations aside, there is no real need for taking on the extra expense of fast glass." Given comparable quality of assembly, materials, and design, is it likely that a 1.4 lens, stopped down to 2.8, will be sharper at 2.8 than a lens that is wide open at 2.8? IOW, in taking on the expense of high quality fast glass, is it likely the quality advantages will continue into use of slower apertures with that lens?

Firstly you have to decide whether you are comparing like with like. The latest fast lenses are better than earlier fast lenses - lens design continues to improve and evolve. So comparing a lens like the 50/1.4 Aspheric with older slower designs of as fast or slower aperture isn't a fair comparison and doesn't rally tell you much more than you'd expect - new designs are simply a lot better than old ones (in terms of tester's considerations anyway). As lenses are stopped down they do tend to 'improve' in that aberrations reduce somewhat, or have traditionally.

 

All that said

 

That said, IMO fast lenses should be optimised for use wide open because the prime reason for owning such a fast lens is to use it wide open (others may disagree but they are wrong ;) ). And many modern designs are - an example of a fast lens optimised of use wide-open is the 35/1/4 Aspheric pre-LE which suffers from a marginal amount of focus shift as it is stopped down. The redesign dealt with this but clearly there were/are tradeoffs for optimising performance wide open. That said, I have the pre-FLE and a pre-aspheric lens too - the pre-FLE is better throughout its aperture range than the pre-aspheric although stopped down to say f/8 and the difference is minima,l but still there - showing that lens designs have progressed even though it still appears to be impossible to produce a 35/1.5 as small as the pre-aspheric.

 

As for low light shooting, well its far from the only reason for using a fast aperture and I'm certainly in the camp that thinks that if you want a fast lens its because you want to use it wide-open rather than for esoteric performance reasoning.

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