Differences in f stop?

[Photoguy]

Can anyone explain to me how much MORE light a 1.8 prime lens lets in as opposed to a 2.8 constant aperture? And how much more light a 1.4 prime lens lets in than a 1.8 prime?

I've got the kit lens that came with my Canon...18-55mm 3.5-5.6. I'm looking at getting a 50mm 1.8 prime to allow for low light handholding and bokeh. (did I spell that right?) I'm also probably buying a constant aperture Tamron 17-50mm 2.8. The 17-50 is much sharper than my kit lens. But I usually shoot wide angle and was thinkig maybe going with the Sigma 10-20mm 4-5.6 instead of the Tamron, since I already have that range covered by my kit lens. So I'm torn between the constant aperture and near "L" glass quality of the sharpness I hear this lens gives, and the wider angle of the Sigma. Can I handhold the 50mm prime 1.8 in lower light and get good results? Will it let in enough light? I usually shoot with a tripod but in the darker rides a tripod isn't feasible.

Thoughts? Opinions?

 

[JoeDif]

Can anyone explain to me how much MORE light a 1.8 prime lens lets in as opposed to a 2.8 constant aperture? And how much more light a 1.4 prime lens lets in than a 1.8 prime?

Thoughts? Opinions?

I'm not positive but I believe the f1.8 will give you an additional 1 and 1/3 stops over the f2.8 and the f1.4 will give you an additional 2/3 over the f1.8

 

[ukcatfan]

http://en.wikipedia.org/wiki/F_stop

 

[boBQuincy]

Although Tamron's 17-50 is a very sharp lens Canon's 2nd generation 18-55 kit lens (the one with IS) actually outdoes the Tamron in sharpness in some ranges!

(according to the tests at photozone.de)

As for the build quality and feel, well that's another story. Still, for image quality and price I can't imagine not getting the 18-55 with a camera for the little extra it costs.

As far as I can see Joe has the numbers correct. The 50 f/1.8 is another inexpensive lens that everyone should have (unless they have the f/1.4).

 

[Spectro is #1]

The term PRIME only refers to the fact that it is a fixed focal length lens and is not related to the aperture opening. That is why a prime lens only has one number like 50mm f/1.8 and a multi focal will look like this 18-105mm f/3.5-5.6.

 

[GrillMouster]

Can anyone explain to me how much MORE light a 1.8 prime lens lets in as opposed to a 2.8 constant aperture? And how much more light a 1.4 prime lens lets in than a 1.8 prime?

Every time you increase the aperture by one stop, the amount of light doubles. So, f/2 allows double the light of f//2.8 and f/1.4 doubles the light of f/2 (f/1.4 allows four times the light as f/2.8). f/1.8 is about 1/3 stop in between f/2 and f/1.4, so so it allows about 2/3 more light than f/2. f/1.8 is about 2/3 stops from f/1.4, so f/1.4 allows about 4/3 (1 1/3) more light than f/1.8. Confused yet?

 

[JoeDif]

Every time you increase the aperture by one stop, the amount of light doubles. So, f/2 allows double the light of f//2.8 and f/1.4 doubles the light of f/2 (f/1.4 allows four times the light as f/2.8). f/1.8 is about 1/3 stop in between f/2 and f/1.4, so so it allows about 2/3 more light than f/2. f/1.8 is about 2/3 stops from f/1.4, so f/1.4 allows about 4/3 (1 1/3) more light than f/1.8. Confused yet?

Who's on first?

 

[GrillMouster]

Who's on first?

I don't know. Third base!

 

[jann1033]

another thing to check is, how good is the lens at the lowest f stop? if you have to go up a few stops to get a good photo, really what good are the lower stops since you'll never use them. ( IE i have read the tamron is best over f4, so unless you don't mind softness how much would you use it lower? don't have the lens so only from reviews, bob atkins and fredmiranda in a couple comments)

 

[boBQuincy]

Every time you increase the aperture by one stop, the amount of light doubles. So, f/2 allows double the light of f//2.8 and f/1.4 doubles the light of f/2 (f/1.4 allows four times the light as f/2.8). f/1.8 is about 1/3 stop in between f/2 and f/1.4, so so it allows about 2/3 more light than f/2. f/1.8 is about 2/3 stops from f/1.4, so f/1.4 allows about 4/3 (1 1/3) more light than f/1.8. Confused yet?

If we think of the f number as the square root of the aperture area (which it is) it becomes more intuitive. The area halves as the light gathering ability halves so the area sequence is 1, 2, 4, 8, 16, etc. This make the square roots 1, 1.4, 2, 2.8, 4, etc, the f stops we all know and love!

This doesn't really help does it? And YesDear is gonna' call me a nerd again, isn't he? It's not my fault, I'm an engineer!

 

[annnewjerz]

This doesn't really help does it? And YesDear is gonna' call me a nerd again, isn't he? It's not my fault, I'm an engineer!

Since he hasn't chimed in yet, I'll take care of it for him.

 

[YesDear]

Nerd!

But a correct nerd!

 

[boBQuincy]

Nerd!

But a correct nerd!

With degrees in engineering *and* information technology I suppose I fit that title as well as anyone ever did! I figure it's like Gary Oldman said in "Fifth Element", "never be ashamed of who you are".

 

[YesDear]

Only a true NERD would quote "The Fifth Element" to make a point about being a nerd!!! LOL

You go Bob!