What makes a lense "fast" ?

I'm very new here but I was looking to learn a couple of things about lenses..

I tried to use the search function but I can't seem to find anything useful about this...

What makes a lense "fast" (or "slow")?
I thought that the camera shutter speed was what controled the "speed" of a shot?

I have taken about 600 shots with the stock lense on the rebel XT (18-55) and I find that my photos are never as crisp as I want them to be. I have read that this particular lense is a little on the "slow" side

I feel like it is my skill that is limiting my photographs, but maybe it's because my lense is "slow"? I'm not sure..

My current/primary goal is to learn how to take better photographs of my reef aquarium. This involves a lot of closeup work.

Any info/advice in regards to this would be wonderful!

Thanks guys. You have an amazing community here.

the speed refers to the maximum aperature. IE f4 is slower than f1.4. f2.8, f2.0, f1.8, f 1.4,f 1.2 are all pretty much considered fast.

There has been many gallery quality pic taken with the kit lens. Mostly a fast lens will help background blur, bokeh, and low light shooting.

"Fast" relates to the size of the aperture (opening) of the lens. A fast lens has a wider maximum aperture (smaller f-number). It's described as "fast" because the wider the aperture, the more light the lens will allow to hit the sensor and, therefore, a faster the allowable shutter speed that will result in accurate exposure.

"Fast" just refers to the maximum aperture being "large" meaning a large opening through the lens. "Fast" is a relative term that usually means and aperture of f2.8 or larger. In this case "larger" means a lower number such as f2.0, f1.8, f1.4 or f1.2. Do a search on "aperture" if you need more understanding of what that means.
"Slow" generally means a lens who's maximum aperture is relatively small, f3.5 or smaller. "Smaller" in this case means f4.0, f5.6, f8.0, etc.
I think what you are referring to is lens "softness." This refers to the resolving power of the lens. The opposite is "sharp" or "tack sharp." You'll see those terms throughout this forum. The kit lenses are generally considered to be somewhat soft, particularly at their largest apertures.
Another term you will hear is "stop down." This refers to changing the aperture to a higher value (smaller opening) which will result in a "sharper" photo and more depth of field.
Sorry for all the quotes, but do some searches on those terms and you'll get a better understanding of these subjects.

Fast lenses are intended to work well with fast cameras, i.e., very short duration shutter operation...so then one gains lottsa speed. ISO is also good albeit much grainier than fast lenses.

Ahhh okay

It's starting to make more sense now. Great!

So with a "fast" lense.. the aperture is large enough so I can have a faster shutter speed without the photo being too dim...

What is the largest aperture on the stock lense anyways? The lowest f-number i can seem to bring it to is 3.5... (Is that even the f-value or am I reading something else...


Sorry I'm such a newb. Appreciate the help! Thanks =)

thefedz wrote in post #2276725
Ahhh okay

It's starting to make more sense now. Great!

So with a "fast" lense.. the aperture is large enough so I can have a faster shutter speed without the photo being too dim...

What is the largest aperture on the stock lense anyways? The lowest f-number i can seem to bring it to is 3.5... (Is that even the f-value or am I reading something else...


Sorry I'm such a newb. Appreciate the help! Thanks =)

You just answered your own question.

F-stop = aperture.

To confuse you some more the number f/1.4 is larger then f/4 . This is because the the f-stop is really a fraction. F/1.4 can also be written as 1:1.4 or 1/1.4 and f/4 as 1:4 or 1/4 and f=focal length.

thefedz wrote in post #2276725
Ahhh okay

It's starting to make more sense now. Great!

So with a "fast" lense.. the aperture is large enough so I can have a faster shutter speed without the photo being too dim...

What is the largest aperture on the stock lense anyways? The lowest f-number i can seem to bring it to is 3.5... (Is that even the f-value or am I reading something else...

Sorry I'm such a newb. Appreciate the help! Thanks =)

The kit lens is a variable aperture lens, f/3.5-5.6. This means that at 18mm it is f/3.5 and at 55mm it is f/5.6.

One thing you might want to consider is picking up the book Understanding Exposure by Bryan Peterson. It is regarded as the best book for learning the basics about photography. I picked up another book of his based on this book and find it invaluable.

With the kit lens if you want sharper pictures then you should start by closing the aperture (i.e. higher F number) down to about F8 in the display.

The image quality really starts getting better around here but obviously with the smaller aperture you need a decent amount of light. If you do not have enough light then you will need a slower shutter speed to let more in which is where you can run into problems of not holding the camera steady enough or something moving in the composition of the picture.

The third aspect is your ISO speed. The lower the speed (i.e. 100) the more detail will be captured but your shutter speed will also be slower. Up the ISO number (i.e. 1600) less detail will be captured (the picture will be grainier) but you can use a faster shutter speed because you are not asking for as much detail. Good news is Canon is renowned for its high ISO performance so don't be afraid to use higher ISO's when you need to.

You basically need to balance the three against each other and learn what you can use in different situations.

Sorry if I have oversimplified anything but this is just the simple way that I remember learning each and will do until you get more practice.

Thanks guys. Some really good info for me here.

thefedz wrote in post #2276828
Thanks guys. Some really good info for me here.

Stick around, you'll learn new stuff every day. There are some fantastic photogs on here that have a lot of great advice to share.

thefedz wrote in post #2276669
I'm very new here but I was looking to learn a couple of things about lenses..

I tried to use the search function but I can't seem to find anything useful about this...

What makes a lense "fast" (or "slow")?...

<hillbilly accent> a lens is fast if it has a "Hemi!!"</hillbilly accent>

The lower the f (aperture) number, the more light the lens sucks in. Also, lower f number gives you a faster shutter speed, thats what is meant by a "fast" lens.

a lens is fast if it goes home with you on the first date.

some people talk about lenses being "sharp" - that refers to lenses that look cool.

as the angryhampster said, "stick around, you'll learn new stuff every day." Or not.