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03-19-2010, 12:49 PM   #1
sealonsf's Avatar

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f stop question

Help me out, I'm not a very "technical" person (that's a nice way to say it!).

So here goes:

If my lens is a 18-55 for example, and is f/3.5-5.6.

At 18mm the f stop would be?

At 55mm the f stop would be?

I'm sure a dumb question, but I'm trying to figure out which lens I want to get to replace this lens and I want the fastest lens I can get at 18mm (some lenses are set at f/2.8 and I'm trying to determine if that will fit my needs the best over lenses that have a range).

So, if the lens is faster at 18mm, I'm good to go with a camera that has a range. But if the camera is the slowest at 18mm, I'm better off with the f/2.8 lens.

Thanks for your help!

03-19-2010, 01:14 PM   #2
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At 18mm, the fastest f-stop is f/3.5
At 55mm, the fastest f-stop is f/5.6.

At 55mm, the SLOWEST f-stop is a couple down than at 18mm. That's how this kind of zoom works.

Some zooms have constant aperture, like my Vivitar Series 1 Version 1 70-210mm which is f/3.5 throughout. But that's big and heavy. AF lenses like that are also expensive.

Before you buy another lens, use the HELL out of the 18-55 kit. It's a good tool.

Then (here's my mantra) ask yourself: 1) Where am I going? 2) What will get me there? 3) What can I afford that will make me happy?

Which boils down to: What do I want to do that I can't do with what I have? And, how much do I want to spend in order to do that?

Answer those, and you'll have a better idea of what's next. Have fun!
03-20-2010, 08:40 AM   #3
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Right. Notice that at each focal length, you have a *choice* of f-stops. The number just tell you the largest. At 18mm, the largest you can get is f/3.5. f/2.8 is not much larger than that - there is practically no point buying a new lens just to get that minimal improvement at 18mm. However, f/2,.8 is a *lot* better than the f/5.6 which is the best you can do at the 55mm end.

There is no such thing as a lens that is set at f/2.8. Any lens that says f/2.8 means that is the *largest* aperture you can get, but you can always select a small one if you like (that's what Av and various other exposure modes are for).

So I agree with RioRico - get to know what you have first, and maybe get a book on hotography to familiarize yourself with these basic concepts, before even thinking about buying anything else. Otherwise, there's an excellent chance you'd end up wasting money on something you don't need because you didn't understand well enough what the limitations of what you already have are.
03-24-2010, 05:20 AM   #4
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Thanks for your help!
I'll double that: Thanks for your help! Glad I read this post. The fog is beginning to clear.
One Stop Short

03-24-2010, 07:46 AM   #5
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I'm an ultra newbie, but I wouldn't hesitate to recommend replacing the the kit lens with a Sigma 17-70 F2.8-4.5 macro. Well it might not be that much faster, it is a huge step up from the kit lens.

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