Probably obvious optics question

[jl-lb.ms]

This is going to sound like (probably because it is) a dumb question, but here goes. Is there any substantial optical difference between lenses when they're stopped down; e.g. a 50 3.5 versus a 50 1.2 say, and both stopped down to f16? Does all that extra glass have any affect when stopped down?

thanks,

John

 

[keithwms]

Not a dumb question at all.

The most quantitative answer I can give is: consult the MTF charts.

Usually, MTF charts (e.g. at photodo) will list the numbers wide open and stopped down to f/8. What you will see is that the performance wide open can be very different between two different lenses, but by f/8 the performance of those different lenses is already starting to converge. Compare the noctilux at f/8 to the summi at f/8. Not much difference really, at least not until the very edge of the frame.

When stopping down as far as f/16 in small format, I doubt that there will be many meaningful differences left, in terms of lp/mm detail. At f/16, diffraction softening is already in play. Peak performance is going to be more around f/8, typically (though with some RF lenses you can get fantastic performance very close to wide open).

Now, setting MTF aside... what about real world performance? Here there can be a big difference between a fast lens and a slow lens, even if both of them are equally stopped down. There is something you said which can be very important: "all that extra glass." Indeed the faster lens, with wider max aperture, will generally be more prone to certain issues like flare and ghosting. So you should always use a hood when possible, of course. But even so, some fast lenses really don't deal well with front lighting. A hood helps but it's not a cure-all.

 

[Ronald M]

Slower lenses are almost always better at 5.6 /8 / 11.

If I compare modern fast and slow, there is not much difference.

 

[bmattock]

In very general terms, I agree with Ronald M and Keith.

Lenses are a compromise, like all of photography. Faster lenses are more prone to various kinds of aberration, unless one spends a tremendous amount of money. Stopped down to their 'sweet spot', they may (or may not) behave the same as a slower lens.

One also has to consider differences in optical formula. A 'tessar' lens design will exhibit different characteristics than a 'double-gauss' lens design. Stopped down, they may resemble each other, but that's a guess.

 

[ferider]

Our friend Erwin says:

"It is very difficult to design a 1.4 lens with a performance equal to the best 2/50mm lenses. Oblique rays in the sagital plane play havock with all good intentions of the designer. Do not forget that a 1.4 design admits twice the amount of light energy and aberrations grow disproportionally. Many of the specific 1.4 aberrations errors do not improve when stopping down, making a 1.4 more of a compromise than a 2/50, which is a much more evolved type of lens design."

And he is right, IMO, also for modern lenses.

Best,

Roland.