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Feb 2006
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Widening my approach

I made the decision to go with a wider/brighter lens, as I had hinted at earlier this month. My eye had been on either the 24mm f/1.8 or 20mm f/1.8 from SIGMA. I finally decided to go as wide as reasonable and it is the:

AF 20mm f/1.8 (D) EX Aspherical DG DF RF (ø82mm-filter)

It is a little bit more money than the 24mm and definitely has a bigger filter-ring, so that cost will be higher, also. That, coupled with the fact that I already have a 24mm f/2.8 ... going a bit wider seemed the better "capability" decision.

Again, from my earlier discussions, this lens allows for a much wider Depth of Field (DOF) at f/1.8 than the 50mm f/1.4, at the same distance.

Subject distance or Center of Focus (CoF) is at: 10-feet

Lens . . . Aperture . . . . DOF
50mm . . . f/1.4 . . . = 0.68 ft
50mm . . . f/1.7 . . . = 0.81 ft
50mm . . . f/1.8 . . . = 0.86 ft
50mm . . . f/2.8 . . . = 1.36 ft

20mm . . . f/1.8 . . . = 5.82 ft
20mm . . . f/2.8 . . . = 10.5 ft

24mm . . . f/1.8 . . . = 3.88 ft
24mm . . . f/2.8 . . . = 6.51 ft

Bump the whole shebang out to 20 feet!

Subject distance or Center of Focus (CoF) is at: 20-feet

Lens . . . Aperture . . . . DOF
50mm . . . f/1.4 . . . = 2.75 ft
50mm . . . f/1.7 . . . = 3.28 ft
50mm . . . f/1.8 . . . = 3.47 ft
50mm . . . f/2.8 . . . = 5.58 ft

20mm . . . f/1.8 . . . = 30.6 ft (that is, near: 13 ft, far: 43.6 ft)
20mm . . . f/2.8 . . . = 131.4 ft (that is, near: 10.8 ft, far: 142.1 ft)

24mm . . . f/1.8 . . . = 17.5 ft (that is, near: 14.5 ft, far: 32 ft)
24mm . . . f/2.8 . . . = 37 ft (that is, near: 12.5 ft, far: 49.5 ft)

As you can see, that 20mm at aperture f/1.8 is really going to be quite handy for getting those school plays and recitals in low light ... when you can get close, w/o obscuring the entire crowd, lost in an "outside the DOF" fog.

If you do not understand this presentation, that's okay ... as you really have to have a wide-aperture to truly appreciate it and what is happening. A 50mm lens is just not going to work close-up.

In fact, to get the same effective DOF the 20mm gives you @ f/1.8 at ten-feet away ... with the DT 50mm f/1.8 you would have to back up to 25-feet.

Doing the same thing at 20-feet (20mm @ f/1.8) ... the DT 50mm f/1.8 lens would have to be 57-feet away. The subject is getting pretty small at that point.

My UWA, the SIGMA AF 10-20mm f/4-5.6 DC EX is quite a bit darker, as you can tell ... nearly 3 f-stops worth! Zooms always are. It is quite excellent for outdoor work, but indoors ... it can take a beating with low light and the resultant "noise" at higher ISOs. The wide PRIME eliminates that to a large degree.

Anyway, that is what this lens brings to the party. Low light capability and ... some real room.

Below is a rough diagram to demonstrate the 20mm @ f/1.8 DOF at the two different subject (CoF) distance:

Attachment 46707

Let there be ENOUGH light, eh?
Last edited by DonSchap; 05-29-2009 at 10:06 AM.

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