\"fast f/stop\"

[timmyquest]

I keep hear you guys refering to fast F/stop being a lower f/stop.

I take it because the lower the number the larger the apatrue the more light comes in in a shorter time thus allowing a faster shutter speed thus allowing a sharper image.

Do i have this correct?

[fwhitesides]

That's mostly right. However, the wider the aperture, the more there is a tendency for the image not to be as sharp. This has a lot to do with the physics of light, which I won't get into here.

Basically, the smaller the aperture, the sharper the resulting image will be and the deeper the depth-of-field will be. A wider (larger) aperture doesn't equal a soft image per se, but it will often mean a less sharp image when using a cheap lens. A really good lens will be sharp at wide apertures, but smaller apertures pretty much always give sharper images, if not by much.

A lot folks like the faster lenses to allow for high shutter speeds for either action photos or available light exposures. I'm in the latter camp.

Hope this helps. You should probably pick up a introductory photography book. Most of them explain all of these concepts, which are critical for making photographs the way you see them in your viewfinder or your mind. Also, I'd get a book that focuses (no pun intended) strictly on lighting.

[timmyquest]

fwhitesides wrote:
This has a lot to do with the physics of light, which I won't get into here.



No need, that actually makes alot of sence to me.

I'm sorry if i'm annoying you guys with all these questions, just trying to soak up as much as i can...books dont always help me learn, my mind works in differnt ways and its best sometimes if i just explain where my head is "headed" (take that pun)

[DAMphyne]

I believe you'll find that the faster lens will auto-focus faster also. More light, quicker focus.

[timmyquest]

damphyne wrote:
I believe you'll find that the faster lens will auto-focus faster also. More light, quicker focus.

I'm not saying your wrong, i'm pretty new to this.

But isnt the apature open all the way until you take the picture...thus making your statement false?

[ohenry]

timmyquest wrote:
I keep hear you guys refering to fast F/stop being a lower f/stop.

I take it because the lower the number the larger the apatrue the more light comes in in a shorter time thus allowing a faster shutter speed thus allowing a sharper image.

Do i have this correct?



You have to consider shutter speed, aperature, and film speed when you look at exposure. Assuming a constant ISO, as you decrease lens aperature, you increase lens speed accordingly (to maintain the same exposure).

f/stops are actually inverse numbers. Consider 1/1 the largest opening....f/4 = 1/4, f/8 = 1/8 and so forth...so the larger the number, the smaller the shutter opening. Each f/stop (1, 1.4, 2, 2.8, 4, 5.6, 8, etc )and each shutter speed (1, 1/2, 1/4, 1/8, 1/16, 1/30, 1/60, 1/125, 1/250) and so on, each change the amount of light that reaches the sensor/film by double (or 1/2 depending on which way you go).

A larger aperature admits more light, thus the shutter can close quicker (higher shutter speed) and still maintain the proper exposure -- hence, larger f/stop lenses are considered "fast" lenses

[DaveG]

timmyquest wrote:
I keep hear you guys refering to fast F/stop being a lower f/stop.

I take it because the lower the number the larger the apatrue the more light comes in in a shorter time thus allowing a faster shutter speed thus allowing a sharper image.

Do i have this correct?



Fast, in photographic jargon usually means that either a) light passes through the lens
more efficiently or b) the film is more sensitive to light.

For example an f2 lens is faster than an f2.8 lens. With lenses you will pay a premium
for speed (i.e to be fast) Those monster lenses that you see in the end zone of football
games are often 300mm f2.8’s. You can easily buy a 300 f4 for a little bit more than 1/4
of the price of a f2.8! For example a Canon 300 f4 is about $1100 while the 2.8 is around
$3900US. And this is for ONE stop. But it lets a sports photographer have one more
stop of shutterspeed to freeze action or one less stop of ISO to improve quality. In any
case it’s a decision that many pro’s make.

Speed that you’ll pay a premium for at one focal length (300 mm f2.8 ) is sneered at at
others. You couldn’t GIVE away a 50 mm f2.8 for example. The cheapest lens Canon
makes is a 50 mm f1.8, and that’s about $80US or less! Long lenses tend to be slow so a
600 f4 is a speed demon and $7000US, while that 300 f4 is run of the mill.

Film has its speed too. ISO 400 film is faster than ISO 100 and so forth. You pay a
premium for faster film too, which I’ve never quite understood since you want to use the
slowest film, or ISO setting on digitall cameras, as you can get by with.

[timmyquest]

ohenry wrote:
timmyquest wrote:
I keep hear you guys refering to fast F/stop being a lower f/stop.

I take it because the lower the number the larger the apatrue the more light comes in in a shorter time thus allowing a faster shutter speed thus allowing a sharper image.

Do i have this correct?



You have to consider shutter speed, aperature, and film speed when you look at exposure. Assuming a constant ISO, as you decrease lens aperature, you increase lens speed accordingly (to maintain the same exposure).

f/stops are actually inverse numbers. Consider 1/1 the largest opening....f/4 = 1/4, f/8 = 1/8 and so forth...so the larger the number, the smaller the shutter opening. Each f/stop (1, 1.4, 2, 2.8, 4, 5.6, 8, etc )and each shutter speed (1, 1/2, 1/4, 1/8, 1/16, 1/30, 1/60, 1/125, 1/250) and so on, each change the amount of light that reaches the sensor/film by double (or 1/2 depending on which way you go).

A larger aperature admits more light, thus the shutter can close quicker (higher shutter speed) and still maintain the proper exposure -- hence, larger f/stop lenses are considered "fast" lenses

Unless i'm misunderstanding what you said...

acording to this article: http://www.uscoles.com/fstop.htm

your not right :-\

***EDIT***

lol, my bad...i was reading it wrong.

[CyberDyneSystems]

timmyquest wrote:
damphyne wrote:
I believe you'll find that the faster lens will auto-focus faster also. More light, quicker focus.

I'm not saying your wrong, i'm pretty new to this.

But isnt the apature open all the way until you take the picture...thus making your statement false?


...but all lenses have a MAXIMUM aperure opening.. this is the aperture listed in the name of the lens (ie: 70-200mm f/2.8) The f/2.8 is the maximum aperure,. or the most light this lens will let in.

So when auto focusing,. the lenses with the larger (lower number) maximum aperture will focus faster than a lens with say a max aperure of f/6.3,. becuase,. as you say,. even if the f/2.8 lens is "stopped down to f/6.3 as well,. it will still take full advantage of the f/2.8 while autofocusing.

Other things in manufacturing will contribute to AF speed of course,., but all other things being equal,. it is a rule of thumb that the lens with the larger max aperture will focus faster on a given camera.

[timmyquest]

I got ya

[defordphoto]

Wanna see how f-stop can effect focus? You know when you're trying to read something...small print and you squint your eyes so you then can bring it into focus and read it?

Same concept as closing down your lens (higher f-stop number) for a larger DOF.