Fast Lens: What makes a "fast lens". Does this refer to the speed at which the lens can lock focus on a subject? Regardless of the brand, is a fast lens one that has the lager...

Jerry Bruzek

What makes a "fast lens". Does this refer to the speed at which the lens can lock focus on a subject?
Regardless of the brand, is a fast lens one that has the lager aperture?
For example, if I have both a 70-200mm f/2.8 and a 100-400mm f/5.6 lens and I try to catch a bird in flight using a focal length of 200mm and set my apurture (manual mode) to 5.6, will both of these lenses have the same "speed of focus"?
Maybe I am way off base here, which is why I am calling on your expertise.

Thank you!

CHG_CANON Loc: the Windy City

A "fast" lens is photographer slang for a wide maximum aperture, preferably a fixed maximum aperture, say f/2.8 or wider, where f/2 would be an even better minimum 'wide' value to warrant this "fast" tag. Think f/1, f/1.2, f/1.4 for this "fast" slang application, although f/2 for 200mm is a pretty sweet lens.

You can find longer discussions, with examples, from a simple google: what is a fast lens

R.G. Loc: Scotland

A wider aperture allows a faster shutter speed, just as a more sensitive film (higher ASA) allowed faster shutter speeds - which is why it was called fast film.

Jerry Bruzek

Thanks, Chicago Canon Guy. I knew you would be able to help me! Trying to figure out why my rf 800mm f/11 has a challenge locking focus on birds in flight. Sounds like that is just the trade off for all that length.

You da Man!

Jerry Bruzek

R.G. wrote:

A wider aperture allows a faster shutter speed, just as a more sensitive film (higher ASA) allowed faster shutter speeds - which is why it was called fast film.

That makes sense and really helps.
Thank you!

Jerry Bruzek

Jerry Bruzek wrote:

Thanks, Chicago Canon Guy. I knew you would be able to help me! Trying to figure out why my rf 800mm f/11 has a challenge locking focus on birds in flight. Sounds like that is just the trade off for all that length.

You da Man!

Now I just need to learn how to do a proper "quote reply".

therwol Loc: USA

CHG_CANON wrote:

A "fast" lens is photographer slang for a wide maximum aperture, preferably a fixed maximum aperture, say f/2.8 or wider, where f/2 would be an even better minimum 'wide' value to warrant this "fast" tag. Think f/1, f/1.2, f/1.4 for this "fast" slang application, although f/2 for 200mm is a pretty sweet lens.

You can find longer discussions, with examples, from a simple google: what is a fast lens

With digital cameras, increasing the ISO can compensate for a "slower" lens in low light. Faster lenses allow you to shoot with shallow depth of field, sometimes desirable.

BebuLamar

Jerry Bruzek wrote:

Thanks, Chicago Canon Guy. I knew you would be able to help me! Trying to figure out why my rf 800mm f/11 has a challenge locking focus on birds in flight. Sounds like that is just the trade off for all that length.

You da Man!

So when they say a lens is "Fast" they don't mean that it would auto focus fast but rather the lens has large aperture and transmit more light. However, a lens with larger aperture can help in auto focus speed but it's not the only factor. How fast the lens can focus also depends on whether its built in AF motor is fast or it has to rely on the camera AF motor, how heavy the focusing mechanism is etc...

User ID

Jerry Bruzek wrote:

Thanks, Chicago Canon Guy. I knew you would be able to help me! Trying to figure out why my rf 800mm f/11 has a challenge locking focus on birds in flight. Sounds like that is just the trade off for all that length.

You da Man!

Different bodies vary in their facility for AF tracking with challenging lenses like that.

BTW that term "fast" dates waaaay back to
when an f/6.3 lens was faster than an f/11 lens and would reduce a typical 20 second exposure to only 3 seconds. Portrait sitters benefited greatly from the faster exposure.

User ID

Slow lenses required a bracket to steady the head of a portrait sitter for half a minute. See photo below.

billnikon Loc: Pennsylvania/Ohio/Florida/Maui/Oregon/Vermont

Jerry Bruzek wrote:

Thanks, Chicago Canon Guy. I knew you would be able to help me! Trying to figure out why my rf 800mm f/11 has a challenge locking focus on birds in flight. Sounds like that is just the trade off for all that length.

You da Man!

Slower lenses track slower, faster lenes track faster. Yes, it is that simple.
The Canon 800mm f11 produces great images in bright light. But again, the faster the lens, the quicker it acquires focus and locks on and tracks. You should be fine from about 10AM to about 5 PM in full sun.
Good luck and keep on shooting until the end.

agillot

Maybe we should call it a brighter lens ???

Canisdirus

Jerry Bruzek wrote:

Thanks, Chicago Canon Guy. I knew you would be able to help me! Trying to figure out why my rf 800mm f/11 has a challenge locking focus on birds in flight. Sounds like that is just the trade off for all that length.

You da Man!

In your lens' case the trade off is the diameter of the front element.

RolandDieter

You mentioned shooting birds in flight. For you the "fast" issue would be shutter speed so you can avoid blur which, if its not too great, might merely look like soft focus. To avoid blurring means that when you shoot birds in flight you want a faster lens. As you would most likely be using a telephoto, an f/2.8 may be the fastest you could get.

Canisdirus

800mm prime is a marketing dream...even at f/11.

But I don't think af speed is the problem usually.

There is a reason why historically pro's went and still do go with 600mm f/4's.

Ever try to track a bird at 800mm? Holy smokes...good luck...you'll get some...and you will surely miss some.

So, you end up shooting birds from farther away...to at least have a better chance at tracking in flight.
Which negates the reason for the 800mm in the first place. Same with 1000mm lenses...worse naturally.

At f/11...you need a ton of light to kick up that shutter speed...and you still have a tracking challenge.

Now for slow moving targets...deer and such...just fine.

Birding...sure...but you will work your arse off getting it.