What does F stop have to do with lense speed

[Gaerek]

Speed Simply refers tho the size of minimum aperture. Think of it as the speed that it can let light in. For example

F1.2 is a large aperture and lets lots on light in (within the time the shutters open). Fast lens

F4.5 is a smaller aperture and does not let as much light in(within the time the shutters open). Slow Lens

But if the 1.2 has a low depth of field and you have to crank it to 4.5 to get a good shot then how is the 1.2 any better than the 4.5. I would think a 67mm filter size 4.5 would be better than a 52mm filter 1.2. Because you have increased depth of field in a 67 4.5 and the 67 lets in more light. What picture would you take with a depth of field reduced to 1.2?

I have two answers for this.

1) All lenses have a sweet spot where they are the sharpest. For slower lenses, this is typically a pretty small aperture, like f/9 or f/11. For faster lenses, its usually much lower, maybe around f/5.6 or even f/3.5 potentially. Lenses are typically soft near the aperture extremes. In your case, the slow lens is right at it's extreme aperture. The fast lens is closed down several stops past it's extreme aperture. The fast lens will be sharper.

2) Are you always going to be in a situation where the optimum aperture is f/4.5? Chances are, that f/1.2 is going to come in handy in certain circumstances. In other words, the fast lens is WAY more versatile than the slow lens.

 

[Neil S.]

"Fast" lenses, those with physically large such as f1.4, 1.2 and the holy grail, Leicha's f0.95 are special purposes lenses. They are really meant to help you acheive one or both of two purposes: To let in large amounts of light so that you can use higher shutter speeds in dim conditions (for instance an f1.4 lens will allow you to use a shutter speed as much as four times faster than many 'kit' lenses; this could be the difference between having to shoot at 1/15 of a second and 1/60th).

The second purpose is to permit very shallow depths of field as you've already alluded to. This is most often desirable in portrait work, where you may want the face in sharp focus and the rest of the subject softly focused, and the background completely out of focus.

Depth of field is also a function of focal length. At any given aperture the shorter the focal length (wider angle lens) the greater the depth of field a lens will have. Check out this on-line depth of field calculator and plug in different numbers to get a better sense of how much of a scene will be in focus based on lens choice, focal length, and aperture.

Good explanation!

Its unreal how much light a 1.2 lens can let in. I had no idea how much till I got mine.

I can shoot at ISO-100 in a dark room with just the light from my computer monitor, and still get descent shutter speeds.

With a kit lens it would probably be like 5sec+ lol.

Its too bad that the 85mm 1.2 has the downsides it does, otherwise it would be my fav lens.

 

[Christie Photo]

What does F stop have to do with lense speed?

f stop (the maximum f stop or a lens) is sometimes referred to as lens "speed." It's not a particularly good description. "Bright" would be a better word to use.

And, yes... shooting at maximum aperture often affords too shallow depth-of-field.

But it's quite popular these days, especially with budding photographers. It a judgement call when shooting in light insufficient to use a smaller aperture, requiring one to use a higher ISO, wider aperture or slower shutter.

Many factors enter into making a good decision, but for me the primary considerations are the subject of the photograph and how the image will be used.

-Pete

 

[OrionsByte]

Also take a look at this:

Canon Camera Museum | Camera Hall - Lenses (EF Mount)

It shows you all the elements of a Canon EF-S 17-55mm f/2.8 lens, and you can see that many of the elements are much smaller than the one at the front of the lens.

 

[Gaerek]

Well, first of all, don't confuse the filter size with the width of the aperture - they're not the same thing. When talking about aperture, it's always a ratio, not a precise measurement. A lens at f/4 is going to let in the same amount of light whether it's got a 52mm filter size or an 85mm filter size. Put differently, it's not the size of the lens that matters, it's the ratio of its aperture (which is the smallest element in the lens, which may not be the opening at the end) to its focal length.

A lot of photography is about trade-offs. An increase in aperture (smaller f/stops) will let more light in, but will decrease the depth of field. Faster shutter speeds will stop action in its tracks, but requires more light by either a wide aperture (and thus a shallower DOF) or higher ISO. Learning that "exposure triangle" is vital to learning photography.

I don't understand why a wider filter would not give more available light. I have not gotten any wide ones yet but this is what I am thinking about for future purchases so I know what to buy.

What happens to a hose that has a kink in it? It slows the water down. If a lens with a large front element has an element inside that is much smaller, that small element will be like a kink in the hose. It becomes the maximum aperture, even though the lens has a big element up front. However, typically, lenses with large front elements are faster lenses.

 

[PatrickCheung]

think about it this way. no matter how big your front element, all light has to travel through the aperture. a lens with a 77mm filter thread still sends its light through the aperture... which is a set size.

why don't you try attaching different lenses with different filter sizes on your camera, control the ammount of light in your testing area, set the iso and aperture, and keep those settings throughout the test. you should get similar shutter speeds....

when they say "fast" they mean how fast of a shutter speed you can get with the maximum aperture.

if you're shooting indoors with low light, a .2 difference (you said "1.2 and 1.4 make no sense to [you]") in f/stop can give you more options as to how to shoot... for example, you could lower your iso and shoot at a slower speed for less noisy shots... or raise your iso and shoot at a faster speed to freeze your subject. there's a good extra stop of light in that .2 stop difference.

and while you might end up stopping down to f/3 or w/e for your ideal shot... you might not be ABLE to in some situations (like indoor shooting)

why do people pay more for 2.8 or 1.4/1.2 lenses? well... you get more options. and if you can control apertures such as f/1.4 or f/1.2 then you can get soem pretty creative shots.

 

[Bryce]

I am glad I know what the elements are to know that the glass on the inside can be small. This post is teaching me alot rather quickly. I am going to have to go over the post over and over I think but man this is educational.
Now do I even want to bother getting a lense with a wide filter size or would that be a waste compared to the f stop or is there a happy median?

 

[Dao]

This 2 lenses are my Canon EF 50mm lens.
The one on the left is f/1.4 version and the one on the right is the f/1.8 mk1 version
Both lens were set at f/4 and their aperture size is about the same (of course, different shape since the f/1.8 version has less blades)
Since aperture is the ratio of focal length to effective aperture diameter and both lens are 50mm (focal length) with aperture set at f/4, therefore the physical aperture size should be the same. And it has nothing to do with filter size.

 

[Neil S.]

For canon owners(and maybe Nikon) there is a cool way to see apeture changes in action.

On the bottom left of the body, close to the mount, there is a little button.

If you set the minimum apeture(f/22 for example), and then look down the lens pressing it, you will see the apeture open and close.

For an ultrafast lens the change is massive, like from a tiny hole to the whole opening on the lens.

This will let you see what its actually doing when you take the shot.

The lens will always be at maximum apeture before the shot by the way, so the viewfinder is bright.

Anyone know if Nikon owners can do this?

 

[OrionsByte]

I am glad I know what the elements are to know that the glass on the inside can be small. This post is teaching me alot rather quickly. I am going to have to go over the post over and over I think but man this is educational.
Now do I even want to bother getting a lense with a wide filter size or would that be a waste compared to the f stop or is there a happy median?

I wouldn't worry about the filter size as a critical specification unless you actually have some filters you're going to attach to it. The more expensive lenses have elements that reduce distortion and other unsavory artifacts, as well as elements that maintain a large aperture. When you're shopping for a lens, the aperture and focal length are what you're going to want to pay attention to as far as how they apply to how you want to use the lens. The size of the lens is largely a function of those two specifications; a lens with a large focal length and a large aperture is going to be a beast of a lens.

So all that to say, don't shop based on size, shop based on specs.

 

[bhee321]

This 2 lenses are my Canon EF 50mm lens.
The one on the left is f/1.4 version and the one on the right is the f/1.8 mk1 version
Both lens were set at f/4 and their aperture size is about the same (of course, different shape since the f/1.8 version has less blades)
Since aperture is the ratio of focal length to effective aperture diameter and both lens are 50mm (focal length) with aperture set at f/4, therefore the physical aperture size should be the same. And it has nothing to do with filter size.

I think this comment more than the others will give the OP a better understanding of all of this. He had things confused thinking that the bigger the lens the more light enters.

 

[Neil S.]

This 2 lenses are my Canon EF 50mm lens.
The one on the left is f/1.4 version and the one on the right is the f/1.8 mk1 version
Both lens were set at f/4 and their aperture size is about the same (of course, different shape since the f/1.8 version has less blades)
Since aperture is the ratio of focal length to effective aperture diameter and both lens are 50mm (focal length) with aperture set at f/4, therefore the physical aperture size should be the same. And it has nothing to do with filter size.

Heres the 85mm 1.2 and the 35mm 1.4 to give you more of an idea.

The 85 1.2 the aperture takes up like the entire lens body.

1.

 

[kundalini]

For canon owners(and maybe Nikon) there is a cool way to see apeture changes in action.

It's called the Depth of Field Preview Button. On Nikons it is located so that you can easily depress it with your right middle finger.

 

[Neil S.]

For canon owners(and maybe Nikon) there is a cool way to see apeture changes in action.

It's called the Depth of Field Preview Button. On Nikons it is located so that you can easily depress it with your right middle finger.

Ahh ok. Thanks for the info.

Forgot what it was called.

I would recommend people learning photography use this to help them learn what exactly the aperture is doing with different f/stops.

I know it helped me. :thumbup:

 

[Neil S.]

This 2 lenses are my Canon EF 50mm lens.
The one on the left is f/1.4 version and the one on the right is the f/1.8 mk1 version
Both lens were set at f/4 and their aperture size is about the same (of course, different shape since the f/1.8 version has less blades)
Since aperture is the ratio of focal length to effective aperture diameter and both lens are 50mm (focal length) with aperture set at f/4, therefore the physical aperture size should be the same. And it has nothing to do with filter size.

How did you set them both at f/4 with them off the camera?

This is probably a stupid question....but I must ask it lol in the pursuit of knowledge. :mrgreen: