Is it the weather or the 50L lens?

The weather at that time is quite clear. I can see the tower quite clearly without any fog. But what I don't understand is this is the best result I got by under and over it a few stops ( by changing the aperture). The shoot was taken using a 5D MK II 50mm f1.2 using f7.1 at 1/1000 AWB at noon.
The filter use for this lens is B+W UV filter. I've clean both the lens and camera beforehand. If it's the sensor/lens problem then in any condition I'd get a similar result. And I didnt' shoot at min aperture.

May I know what's the real cause of it?

Looks like the weather. You can always punch up contrast in post processing.

I didn't know anyone used the 50L at anything less than F1.2! (joking of course)

Was it raining? Our visual systems are pretty good at editing that out, but cameras don't. The clue is that the top of the tower is worse.

If the above weather suggestions turn out to be false (though I think it's the weather most likely, too), then:
- Have you shot the same scene with another lens to rule that out?
- Is the lens fogging up due to going from indoor->outdoor or vise versa
- Did you try shooting it with the filter removed?

melcat wrote in post #12451166
Was it raining? Our visual systems are pretty good at editing that out, but cameras don't. The clue is that the top of the tower is worse.

No it's not. But it's not a bright day either.Probably a bit cloudly. Oh gee, I don't even know of eyes are that smart, a lot smarter than the moderm DSLR .

phreeky wrote in post #12451214
If the above weather suggestions turn out to be false (though I think it's the weather most likely, too), then:
- Have you shot the same scene with another lens to rule that out?
- Is the lens fogging up due to going from indoor->outdoor or vise versa
- Did you try shooting it with the filter removed?

Never thought of that. But anyway, That's my widest lens at that time. Thus don't even bother to switch to 135L.

No. It's quite clear if I shoot a nearly/smaller subject i.e nearby building.

Oh, I never did that also. But I recall I've checked the filter and the lens it doesn't show any sign of fogging.

Would a ND filter would help in this case( In my opinion probably no)?

I'd say it's due to the atmospherics.
It doesn't look like a great day - fairly overcast would be my guess (edit - oops...you did mention that) and you were probably shooting back towards where the sun was hidden behind the clouds.
Is this straight out of the camera or have you processed? My guess it isn't processed.
I've had a play with it and a minor tweak to the contrast makes a big improvement.
If you want, I'll post it up for you to look at.

philwillmedia wrote in post #12451271
I'd say it's due to the atmospherics.
It doesn't look like a great day - fairly overcast would be my guess (edit - oops...you did mention that) and you were probably shooting back towards where the sun was hidden behind the clouds.
Is this straight out of the camera or have you processed? My guess it isn't processed.
I've had a play with it and a minor tweak to the contrast makes a big improvement.
If you want, I'll post it up for you to look at.

Hmm, I think you're right.

I can't recall where's the sun, probably I can't see a clear sky.

Yes, It's indeed not undergo any processing except resize and watermark

Oh, that'd be great .

philwillmedia wrote in post #12451271
...If you want, I'll post it up for you to look at.

danielyamseng wrote in post #12451565
...Oh, that'd be great .

As requested by the OP

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Phil that's pretty fast . That's looks much better, but still can't get good color saturation( partly due to the weather). Thanks, pal .

Let me guess, you just reduce the contrast level via photoshop and not using the Curve for adjustment. I think for this image either way the result is similar.

Thus can I concluded that the the time and weather is vital for a good quality image. Postprocessing won't help much if this two factor is not favorable to us.

Anyway, nowaday I'm too lazy to do post processing for landsacpe pictures, I would rather get it right from the camera, but looks like the weather condition is not really favorable to me.

Another thing is the top and bottom of the tower has different brightness. I guess adding a ND filter to even out the lighting doesn't help much either, is it?

no probs...
I just did a slight adjustment to the brightness/contrast and just gave the curves a minor tweak.
Because the building is grey and it was an overcast day, the lightng was flat and therefore what little colour you've got isn't going to look that great. Dull colours on dull day equals not good for colour saturation.
It may be worth giving a black and white conversion a try.
No matter how "right" you get it in camera, nearly every pic you take, whether it's jpeg or raw, will need some sort of processing.
With the uneven lighting top to bottom, I'm not sure if a ND filter would help.

It looks like a combination of backlighting of the building against a bright sky and potentially some interference from the UV filter. Even high quality filters can cause some lack of contrast when angled against the light "just right".