Need a new lens

Grumpy_one

I have a 20d with the 17-85 IS 4-5.6. I'm looking for something like a 50 1.8 or something ever faster. Mostly family and kids sports, some indoor sports with poor lighting. I'd like to have the 1.2, but don't have $1k to spend on a lens. Just how much faster is the 1.2 over the 1.8? I'm guessing the 1.8 will be fine for my needs. Thanks.

Tommyboy

For indoor sports on your crop-sensor camera, you might think about an 85mm f/1.8 or a 50mm f/1.4. Of course the 50mm f/1.8 trounces either on price, but both of these lenses are great glass and super fast for indoors and besides, are great portrait lenses.

Elaine

I just used the 85 1.8 in a gymnastics gym today. Other than being limited as far as where I could shoot from and my distance to my subject, I really liked how it performed, and a 50mm would have been too short in that setting. At about $315, it's not the cheapest but far from terribly expensive either.

Tee Why

If the focal length is ok, the 50mm f1.8 will probably be fine. If you want a touch faster speed and better AF and build, consider the 50mm f1.4 as well.

z_28

Grumpy_one wrote:

Just how much faster is the 1.2 over the 1.8?

About 3.5x faster

1.2 -> 1.4 -> 2.0 = 4x

ziggy53

z_28 wrote:

About 3.5x faster

1.2 -> 1.4 -> 2.0 = 4x

Not quite. The progression is:
f1
f1.4
f2
f2.8
f4
f5.6
f8
f11
f16
f22
f32
f45
f64
f90
f128

... with each increment doubling/halving either side.

To calculate the difference between fractional "f stops":

Divide the focal length by the aperture
Divide that number in half
Square that number
Multiply that number by Pi

This will yield the area of the aperture in mm.

A 50mm lens at f1.2 will yield 1363.5384781
A 50mm lens at f1.8 will yield 606.01710139


Dividing the results of 1363.5384781 by 606.01710139 shows that the f1.2 lens is gathering 2.25 times as much light as the f1.8 lens when they are both wide open (approximately).

pathfinder

The Ziggy Knows......

( thats from radio from years ago - "The Shadow Knows"....by a deep voice in an echo filled room)

gryphonslair99

ziggy53 wrote:

Not quite. The progression is:
f1
f1.4
f2
f2.8
f4
f5.6
f8
f11
f16
f22
f32
f45
f64
f90
f128

... with each increment doubling/halving either side.

To calculate the difference between fractional "f stops":

Divide the focal length by the aperture
Divide that number in half
Square that number
Multiply that number by Pi

This will yield the area of the aperture in mm.

A 50mm lens at f1.2 will yield 1363.5384781
A 50mm lens at f1.8 will yield 606.01710139


Dividing the results of 1363.5384781 by 606.01710139 shows that the f1.2 lens is gathering 2.25 times as much light as the f1.8 lens when they are both wide open (approximately).

Isn't there a rule that says no algebra on this forum?????? My head hurts now.

By the way Ziggy was that Apple Pi or Cherry Pi you multiplied by??

pathfinder

ziggy53 wrote:

Not quite. The progression is:
f1
f1.4
f2
f2.8
f4
f5.6
f8
f11
f16
f22
f32
f45
f64
f90
f128

Ziggy, that is the same distance in feet for a fixed light to change illumination by 1 fstop too. Moving a light from 2.0 feet to 2,8 feet cuts the light 1 stop. From 2.8 feet to 4 feet cuts it one more stop, und so weiter. ( and so forth)

pathfinder

Grumpy, get a 50 f1.4 or an 85 f1.8 depending on how wide/close need to be.

A 50 f1.4 is a very nice lens on a crop camera body, and a full frame body also - but for different uses.

ziggy53

pathfinder wrote:

Ziggy, that is the same distance in feet for a fixed light to change illumination by 1 fstop too. Moving a light from 2.0 feet to 2,8 feet cuts the light 1 stop. From 2.8 feet to 4 feet cuts it one more stop, und so weiter. ( and so forth)

True and handy!

jdryan3

pathfinder wrote:

Ziggy, that is the same distance in feet for a fixed light to change illumination by 1 fstop too. Moving a light from 2.0 feet to 2,8 feet cuts the light 1 stop. From 2.8 feet to 4 feet cuts it one more stop, und so weiter. ( and so forth)

So is there a symbol for "Foot Zoom" you can add to the formula? Average 6' man's stride at a normal walking pace is 1FZ, where 1FZ=0.8 meters?

z_28

ziggy53 wrote:

Not quite. The progression is:
f1
f1.4
f2
f2.8
f4
f5.6
f8
f11
f16
f22
f32
f45
f64
f90
f128

... with each increment doubling/halving either side.

To calculate the difference between fractional "f stops":

Divide the focal length by the aperture
Divide that number in half
Square that number
Multiply that number by Pi

This will yield the area of the aperture in mm.

A 50mm lens at f1.2 will yield 1363.5384781
A 50mm lens at f1.8 will yield 606.01710139


Dividing the results of 1363.5384781 by 606.01710139 shows that the f1.2 lens is gathering 2.25 times as much light as the f1.8 lens when they are both wide open (approximately).

I was in school so long time ago
But was very sure there is a step between 1.0 and 1.4
If not - I lived in darkness whole life :cry

ziggy53

z_28 wrote:

I was in school so long time ago
But was very sure there is a step between 1.0 and 1.4
If not - I lived in darkness whole life :cry

Each full f stop is based on a logarithmic progression of the square root of two. If you start with an aperture of f1, multiply it by 1.414 to get the next aperture of 1.4 (rounded 1.414).

Multiply f1.414 x 1.414 and you get f2.

Multiply f2 x 1.414 and you get f2.8.

Multiply f2.8 x 1.414 and you get f4.

... and so on.

claudermilk

z_28 wrote:

I was in school so long time ago
But was very sure there is a step between 1.0 and 1.4
If not - I lived in darkness whole life :cry

Sure there is, but that's half-stops and 1/3-stops.