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:
Let there be ENOUGH light, eh?
Last edited by DonSchap; 05-29-2009 at 09:06 AM.
- BFA, Digital Photography
A Photographer Is Forever
Look, I did not create the optical laws of the Universe ... I simply learned to deal with them.
Remember: It is usually the GLASS, not the camera (except for moving to Full Frame), that gives you the most improvement in your photography.