Originally Posted by

**bensonga**
... I've often wondered, assuming equal optical and build quality, can there be much difference between a 50mm f1.4 and f1.2 lens?

I mean really....a .2 f-stop difference? There must be more to this than I know. If so, I would appreciate it if someone here could explain the technical differences to me.

It's a 30% difference in maximum light gathering power between the two lenses, and the f/1.2 lens allows an even shallower DoF rendering, if you like that sort of thing. Here are some DOFmaster numbers for you to think about, using Jim's Pentax K5 as a basis:

**50mm f/1.4**

Subject distance 10 ft

Depth of field

Near limit 9.67 ft

Far limit 10.4 ft

Total 0.68 ft

In front of subject 0.33 ft (48%)

Behind subject 0.35 ft (52%)

Subject distance 5 ft

Depth of field

Near limit 4.92 ft

Far limit 5.08 ft

Total 0.17 ft

In front of subject 0.08 ft (49%)

Behind subject 0.08 ft (51%)

Hyperfocal distance 290.2 ft

Circle of confusion 0.02 mm

**50mm f/1.2**

Subject distance 10 ft

Depth of field

Near limit 9.72 ft

Far limit 10.3 ft

Total 0.57 ft

In front of subject 0.28 ft (49%)

Behind subject 0.29 ft (51%)

Subject distance 5 ft

Depth of field

Near limit 4.93 ft

Far limit 5.07 ft

Total 0.14 ft

In front of subject 0.07 ft (49%)

Behind subject 0.07 ft (51%)

Hyperfocal distance 345 ft

Circle of confusion 0.02 mm

What's telling isn't just the shallower DoF capability, it's how much further out the hyperfocal focus point sits. That increases the amount of out of focus blur by more than just the small difference in light gathering capability might indicate.

Of course, such hyper-speed lenses tend to be absolute, top of the line, premium products of a lens manufacturer and they pay a lot of attention to tuning the rendering on them. I have both Nikkor 50mm f/1.4 AI-S and Nikkor 50mm f/1.2 AI-S lenses ... the difference in rendering between them is fairly noticeable.

G