Why does a wide open lens have less detail than one that's less closed?

[GoingUp]

I was lusting over the 50mm F1.2L from Canon so I read a review and they had a picture of a brick wall at F4 and F1.2.

The brick wall at F4 had a lot more detail than the one at F1.2.

Why is that? Is it because the focusing plane is so narrow?

 

[bobdole369]

At 1.2 the DOF is like 3 inches. Maybe the review missed focus? It's usually the other way round, kinda. Most lenses (not all) have excellent IQ 1-2 stops down from wide open - then it gets worse until like f/11 or so when IQ takes a big dive. The tradeoff is between DOF and focus quality, you get more in focus, but that focus is less effective at smaller apertures. I don't know the physics behind it.

 

[ElFenix]

the edges of a plane perpendicular to the lens axis are a further distance from the lens than the center. at least, that's my theory and i'm sticking to it.

i bet you could test that by focusing at the center, taking a picture, focusing slightly further away, taking a picture, and watch rings of focus on the wall. if the rings of focus don't happen, then i'm wrong.

 

[soydios]

Originally posted by: ElFenix
the edges of a plane perpendicular to the lens axis are a further distance from the lens than the center. at least, that's my theory and i'm sticking to it.

i bet you could test that by focusing at the center, taking a picture, focusing slightly further away, taking a picture, and watch rings of focus on the wall. if the rings of focus don't happen, then i'm wrong.

that probably depends on the lens design. I'm no optics major, but I do remember from physics class that a standard lens has a flat PLANE of focus.

but to answer the OP's question: as you use a larger aperture, you are using more of the outer parts of the glass lens, which is not perfect.

 

[fuzzybabybunny]

Originally posted by: soydios

Originally posted by: ElFenix
the edges of a plane perpendicular to the lens axis are a further distance from the lens than the center. at least, that's my theory and i'm sticking to it.

i bet you could test that by focusing at the center, taking a picture, focusing slightly further away, taking a picture, and watch rings of focus on the wall. if the rings of focus don't happen, then i'm wrong.

that probably depends on the lens design. I'm no optics major, but I do remember from physics class that a standard lens has a flat PLANE of focus.

but to answer the OP's question: as you use a larger aperture, you are using more of the outer parts of the glass lens, which is not perfect.

Pretty much. Old school lenses had spheres of focus. Then they developed lenses with flat planes of focus which are now standard (in the old days these flat plane focus lenses were really expensive when compared to the then standard lenses with spheres of focus).

And yup, opening the aperture uses more of the outer parts of the glass. Cheaper lenses don't have as high a quality at the edges of the glass, so their wide open performance is generally subpar, while very high quality lenses have very good optical performance from center of glass to the edges and can be almost just as sharp wide open as when stopped down.

 

[ElFenix]

Originally posted by: fuzzybabybunny
And yup, opening the aperture uses more of the outer parts of the glass. Cheaper lenses don't have as high a quality at the edges of the glass, so their wide open performance is generally subpar, while very high quality lenses have very good optical performance from center of glass to the edges and can be almost just as sharp wide open as when stopped down.

that was going to be my second answer (well that and light going through the edges must be bent more than light going close through the center, should it?)