the newest leica book has lens test data on every leica lens and while i haven't had a chance to ponder the charts all that much, the feeling i get is that lens performance is more a function of construction than maximum lens opening. Most of the Leica lenses seem to perform best when stopped down a bit, even the smaller aparature ones.
is a fast lens, stopped down, just as good as a slow lens at the same aperture?
It depends on the lenses and the situation.
For example…
At f/16, I see a significant difference in image quality between my fast normal lens on my small format camera and my slow normal lens on my medium format camera. The medium format image is so much better.
At f/16, I see very little difference in image quality between my high-priced 50mm f/1.4 lens and my low-priced 50mm f/1.8 lens.
At f/16, the close-up images produced by my high-quality 55mm f/3.5 macro lens are so much better than those produced by my high-quality 50mm f/1.4 lens.
At f/5.6, the close-up images produced by my 105mm f/2.8 macro lens have more image detail than those produced by my 105mm f/2.5 lens. However, the portrait images produced by my 105mm f/2.5 lens are better than the portrait images produced by my 105mm f/2.8 macro lens.
Impossible to generalize, Ralph. There is no law in optics that says a faster lens will perform better or worse at f/8 than a slower lens. It depends on the lens.
. . . At f/16, I see a significant difference in image quality between my fast normal lens on my small format camera and my slow normal lens on my medium format camera. The medium format image is so much better.
At f/16, I see very little difference in image quality between my high-priced 50mm f/1.4 lens and my low-priced 50mm f/1.8 lens. . . .
At f/16 any decent 50mm lens on a 35mm camera should be noticably diffraction limited. A good MF lens should be conspicuously better at f/16.
The old rule of thumb: "The best definition is obtained 2-stops down from wide open" still holds for modern lenses.
Larger apertures are subject to aberrations (coma, spherical etc.) that can be eliminated by stopping down a couple of stops. The exceptions are lateral chromatic and distortion.
Stopping down beyond this degrades the image by diffraction. This point is called the diffraction limit. Further stopping down causes increased degradation because of diffraction.
The smaller the physical size of the aperture the greater the diffraction caused degradation. The wave length of light is constant so focal length, format etc come into play.
So stop down 2-stops for the best definition. Stop down more if you need the depth-of-field but you will lose some sharpness at the focused subject plane.
The reason(s) for buying a faster lens vs the slower lens, even though it costs more, in response to someone else's comment earlier in the thread:
The faster lens is easier to focus in low light
The faster lens can let you use a faster shutter speed in low light, enabling you to capture an image you couldn't with the slower lens
The faster lens may have a more aesthetically pleasing appearance at or near wide-open than the slower lens does
The faster lens will let you blur out backgrounds and foregrounds more than the slower lens does (see item above)
and then there's always, perhaps the most important reason of all-
the faster lens proves you're a better photographer because you can afford more expensive equipment, and gives you an excuse to toss around terms like bokeh, circle of confusion, and diffraction limited... (cough cough)
Adv Reply
Originally Posted by RalphLambrecht
