so basic of a question - fast lens?

a_spaceman · Oct 30, 2008

very silly and i guess basic question.
i read here and there od people talking about fast and slow lenses. what does that mean?
thank you in advance for enlighting a beginner!

hankejp · Oct 30, 2008

a_spaceman said:
very silly and i guess basic question.
i read here and there od people talking about fast and slow lenses. what does that mean?
thank you in advance for enlighting a beginner!

Just shooting from the hip but they may be either talking about the aperature for the shutter speed.

tenlientl · Oct 30, 2008

when a lens has a wider aperture, it means you 'can' use a faster shutter speed without having to sacrifice the amount of light that gets through.

maybe the 2nd part isnt really the reason, but yes, wider aperture

Dao · Oct 30, 2008

Yes, usually it is reference to the max aperture a lens is capable of.

For example, "EF 70-200mm F/2.8L IS" is faster than "EF 70-200mm F/4L IS" lens.

In other words, EF 70-200mm F/2.8L lens has a wider maximum aperture than the EF 70-200mm F/4L version.

With the wider aperture, the fast lens can use a faster shutter speed to obtain the same exposure with the slower lens with slower shutter speed (narrower aperture).

Pugs · Oct 30, 2008

If you think of the lens like a hose, light as water, and the "perfect" exposure as a bucket. The question is how long does it take to fill the bucket to get that "perfect" exposure. If the water (light) is flowing at a constant speed, the you can fill the bucket "faster" by getting a bigger hose.

SO, the lens maximum f/stop number is how "big" of a hose the lens is. Things are a little reversed so stay with me here. The smaller the f/stop number is, the LARGER the aperture opening is. So this means an f/1.2 is a LARGER aperture than an f/22.

With this in mind, if a lens is called a 50mm f/1.8, then it will be a wider "hose" (aperture) than a 50mm f/3.5. Lenses are typically identified by their focal length (50mm in this example) and their maximum aperture (f/1.8 and f/3.5 in this example).

Therefore, the 50mm f/1.8 is "faster" lens than the 50mm f/3.5 because it lets more light through meaning that you can leave the "water" running for less time. Basically, you can use a faster shutter speed to get the same amount of light if the lens is set at its maximum aperture.

Does this analogy make sense or did I completely throw a handful of dirt in the bucket making the water muddier?

a_spaceman · Oct 30, 2008

thank you all for the help! that was useful
just one more question for concreteness' sake

what is the difference if i take the same photo first with a slower shutter speed and less aperture and then with faster shutter and higher aperture?
as the bigger aperture allows faster shutter it will be easier to get sharp images without a tripod or of moving objects. that's where faster lenses' benefit lies, yeh?

god tonight i can't put two words in a row. sleeping deprivation. i apologize for that.

Pugs · Oct 30, 2008

Smaller f/stop number (wider aperture) means shallower depth of field. Larger f/stop number (smaller aperture) means deeper depth of field.

And yes, faster shutter speeds do mitigate camera shake quite a bit.

a_spaceman · Oct 30, 2008

so, and this will hopefully be the very last question, if i want to adjust the depth of field i have to consider the shutter speed as well while taking the photo?
i mean, does the different aperture make a big difference?
thanks a lot!

Pugs · Oct 30, 2008

If you want to adjust the depth of field and get a good exposure, you need to balance the amount of light coming in by adjusting the aperture and the shutter-speed.

Wider aperture balanced with faster shutter speeds = shallower depth of field.

Narrower aperture balanced with slower shutter speeds = deeper depth of field.

And YES, depth of field makes a HUGE difference. Just poke around at some of the pics posted in the galleries here and notice the difference between a portrait with nice bokeh (blurred out background) and a portrait with the subject AND the background in perfect focus.

Or pay attention to the difference between landscapes in which the focus is utterly soft for most of it but has one element that isn't the subject is in focus.

Play with it in your camera and you'll see a tremendous difference.

Turnerea · Oct 30, 2008

If the situation calls for manipulating the DOF, then you can shoot in aperture prioroty mode. This allows you to pick an aperture (thereby "picking" a DOF), and the camera meter will choose the correct shutter speed.

djacobox372 · Oct 30, 2008

Also: most lenses have poor sharpness at there maximum aperture--even the expensive ones!

Generally f8 and above produces ideal image quality, below that you are sacrificing quality for speed--this is true with any lens, just less noticable with higher quality/faster lenses.

tsaraleksi · Oct 30, 2008

djacobox372 said:
Also: most lenses have poor sharpness at there maximum aperture--even the expensive ones!

Generally f8 and above produces ideal image quality, below that you are sacrificing quality for speed--this is true with any lens, just less noticable with higher quality/faster lenses.

The best lenses (with regards to sharpness) have "good to great" sharpness at their maximum aperture and excellent sharpness stopped down. This is true in only a select few lenses, however-- in my experience a couple of examples include the Canon 35 and 85L primes, 70-200 2.8L zoom, and the Nikon 300 2.8. Super-fast lenses and some telephoto primes are at their sharpest between 2.8 and 4, rather than all the way down to 8.

NateWagner · Oct 31, 2008

right, I believe they are usually best stopped down about 2 stops from their maximum aperture. so, for a 2.8 that would be 5.6 I do believe, rather than at about 8 for slower consumer lenses

Do'Urden's Eyes · Oct 31, 2008

pretty n00b question here.. i understand what a stop is ie. stopping down the lens, exposing a stop more or less. but how MUCH is ONE STOP. i always undertood it as one unit of adjustment, like if i went from F2.8 to F3.2, or if i went from 1/200 to 1/250. these are all adjacent to each other when i adjust aperture or shutter speed. is there more to it then that?