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Vincent !!8LCSE0Zp1mL
Normally I wouldn't post this, since its more of a gear thread, However, since this lens was pretty seriously debated on here I figured I would.

I sold my Nikon 50mm f1.4 Af-d
Bought the Sigma 50mm f1.4 HSM
Cost difference is about $220 CAD if you buy them both new.

AF speed is actually very close on my D200 (D300 is at nikon because my battery grip is fucking up) The Sigma is obviously faster but not by much. (D50 and D70 is more noticeable, suprisingly it also is accurate AF on my IR camera)
Biggest difference is in the accuracy, the sigma is dead on all the time, where the Nikon version seems to not be able to make up its mind, sometimes it focuses behind a bit, sometimes in front, af-c is dangerous with this lens.

The Nikon 50 f1.4 has been a weird lens for me, it takes great pictures when the AF is accurate, however it seems to be hit or miss for getting that critical accuracy at f1.4.
EXIF data available. Clickhereto show/hide.
Camera-Specific Properties:Equipment MakeNIKON CORPORATIONCamera ModelNIKON D50Camera SoftwareVer.1.00Maximum Lens Aperturef/1.4Sensing MethodOne-Chip Color AreaColor Filter Array Pattern670Focal Length (35mm Equiv)75 mmImage-Specific Properties:Horizontal Resolution240 dpiVertical Resolution240 dpiImage Created2008:10:21 15:19:15Exposure Time1/2000 secF-Numberf/1.4Exposure ProgramAperture PriorityISO Speed Rating200Lens Aperturef/1.4Exposure Bias-1/3 EVMetering ModePatternLight SourceUnknownFlashNo FlashFocal Length50.00 mmRenderingNormalExposure ModeAutoWhite BalanceAutoScene Capture TypeStandardGain ControlNoneContrastNormalSaturationNormalSharpnessNormalSubject Distance RangeUnknown
>> Anonymous
inb4shitorm
>> Vincent !!8LCSE0Zp1mL
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BTW none of these pictures have had any sharpening or CA correction on them, I simply corrected the WB and evened out the histogram.

The Sigma 50mm f1.4, looks a lot like a bigger 30mm f1.4, Except for the petal hood the 50mm is just bigger.
It is a huge lens though, and the filter size is 77mm, Plus the glass looks awesome, it takes up the whole front of the lens!

As for image quality, the Sigma is a definite improvement over the Nikon, far far sharper on my crop bodies, and its much more predictable in its focusing.
Contrast seems good though I was shooting on my D50 so it tends to process the images in camera a bit.

Its close focus is not very close (See this pic) 1:7.9 or something brutal.

Camera-Specific Properties:Equipment MakeNIKON CORPORATIONCamera ModelNIKON D50Camera SoftwareVer.1.00Maximum Lens Aperturef/1.4Sensing MethodOne-Chip Color AreaColor Filter Array Pattern670Focal Length (35mm Equiv)75 mmImage-Specific Properties:Horizontal Resolution240 dpiVertical Resolution240 dpiImage Created2008:10:20 17:26:09Exposure Time1/500 secF-Numberf/2.0Exposure ProgramAperture PriorityISO Speed Rating200Lens Aperturef/2.0Exposure Bias-1/3 EVMetering ModePatternLight SourceUnknownFlashNo FlashFocal Length50.00 mmRenderingNormalExposure ModeAutoWhite BalanceAutoScene Capture TypeStandardGain ControlNoneContrastNormalSaturationNormalSharpnessNormalSubject Distance RangeUnknown
>> Anonymous
moar
>> Vincent !!8LCSE0Zp1mL
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Stopping it down slightly just gets it even sharper.

Camera-Specific Properties:Equipment MakeNIKON CORPORATIONCamera ModelNIKON D50Camera SoftwareVer.1.00Maximum Lens Aperturef/1.4Sensing MethodOne-Chip Color AreaColor Filter Array Pattern670Focal Length (35mm Equiv)75 mmImage-Specific Properties:Horizontal Resolution240 dpiVertical Resolution240 dpiImage Created2008:10:20 17:26:57Exposure Time1/2000 secF-Numberf/2.2Exposure ProgramAperture PriorityISO Speed Rating200Lens Aperturef/2.2Exposure Bias0 EVMetering ModePatternLight SourceUnknownFlashNo FlashFocal Length50.00 mmRenderingNormalExposure ModeAutoWhite BalanceAutoScene Capture TypeStandardGain ControlNoneContrastSoftSaturationNormalSharpnessNormalSubject Distance RangeUnknown
>> Vincent !!8LCSE0Zp1mL
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The plant was about 3-4metres away from me.
Bokeh is nice, really smooth looking.

Camera-Specific Properties:Equipment MakeNIKON CORPORATIONCamera ModelNIKON D50Camera SoftwareVer.1.00Maximum Lens Aperturef/1.4Sensing MethodOne-Chip Color AreaColor Filter Array Pattern670Focal Length (35mm Equiv)75 mmImage-Specific Properties:Horizontal Resolution240 dpiVertical Resolution240 dpiImage Created2008:10:20 17:27:42Exposure Time1/3200 secF-Numberf/1.4Exposure ProgramAperture PriorityISO Speed Rating200Lens Aperturef/1.4Exposure Bias-1/3 EVMetering ModePatternLight SourceUnknownFlashNo FlashFocal Length50.00 mmRenderingNormalExposure ModeAutoWhite BalanceAutoScene Capture TypeStandardGain ControlNoneContrastNormalSaturationNormalSharpnessNormalSubject Distance RangeUnknown
>> Vincent !!8LCSE0Zp1mL
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CA it has a problem with (on my D50, will have to wait to see how the D300 handles it)

Its a strange sort of CA too, its really thin but really saturated. Its almost like Super Chromatic Aberration. Don't get me wrong, its way better than the Nikon 50mm f1.4 during daylight. The Nikon was horrible for big fuzzy purple edges.

Camera-Specific Properties:Equipment MakeNIKON CORPORATIONCamera ModelNIKON D50Camera SoftwareVer.1.00Maximum Lens Aperturef/1.4Sensing MethodOne-Chip Color AreaColor Filter Array Pattern670Focal Length (35mm Equiv)75 mmImage-Specific Properties:Horizontal Resolution240 dpiVertical Resolution240 dpiImage Created2008:10:21 15:20:00Exposure Time1/2000 secF-Numberf/1.8Exposure ProgramAperture PriorityISO Speed Rating200Lens Aperturef/1.8Exposure Bias-1/3 EVMetering ModePatternLight SourceUnknownFlashNo FlashFocal Length50.00 mmRenderingNormalExposure ModeAutoWhite BalanceAutoScene Capture TypeStandardGain ControlNoneContrastNormalSaturationNormalSharpnessNormalSubject Distance RangeUnknown
>> Vincent !!8LCSE0Zp1mL
And that is all.
I would love to try this lens on a D700 or D3, but I don't have access to one.
It seems to hold its sharpness / contrast right to the corner, but this is 1.5x crop after all.

Overall, i'm happy with it, might not be worth the premium it costs over the Nikon f1.8 to most people, but if you have extra money to throw around like I did, it is a definite improvement.