Aperture and lens speed

[korreman]

Simple question that's been bothering me:

When comparing lenses, will lens speed be directly related to aperture? Can two lenses have the same maximum aperture size, while one is faster than the other?
I tried doing some math to figure it out, and I failed miserably.

 

[astroNikon]

Lenses will have markings like
18-35/2.8
or 18-105/4.5-5.6

the /2.8
and /4.5-5.6

is considered the lens "speed"
which is the maximum Aperture opening that the lens will do for the focal length

so .. for instance on the first one ...
18-35 .. the lens wil allow the aperture of f/2.8 to be used for the entire focal length from 18mm to 35mm

on the 2nd one .. at 18mm the maximum aperture can be f/4.5
at 105 the maximum aperture is 5.6
in between it changes from 4.5 to 5.6

of course, at any focal length the aperture could be selected all the way down to, say, f/22 (the smallest size of the aperture)


take a look at 50mm lenses
and you can find
f/1.2 - largest opening, all the way to f/22
f/1.4
f/1.8

of course, each of those lenses can go down to a f/22
but the "fastest" lens is the f/1.2

the smaller the number the larger the opening
and thus the more light (aka "faster") it lets in for that shutter speed.

 

[korreman]

Lenses will have markings like
18-35/2.8
or 18-105/4.5-5.6

the /2.8
and /4.5-5.6

is considered the lens "speed"
which is the maximum Aperture that the lens will do for the focal length

so .. for instance on the first one ...
18-35 .. the lens wil allow the aperture of f/2.8 to be used for the entire focal length from 18mm to 35mm

on the 2nd one .. at 18mm the maximum aperture can be f/4.5
at 105 the maximum aperture is 5.6
in between it changes from 4.5 to 5.6

of course, at any focal length the aperture could be selected all the way down to, say, f/22 (the smallest size of the aperture)

Oops, let me be more clear.
You have two lenses with the same maximum f/number, but different focal lengths. Can one be faster than the other?
f/number describes aperture size proportional to focal length, so it's not a unit; a 50mm/2.8 lens would have a larger aperture than a 20mm/2.8 lens. But at the same time, a 50mm lens would have a smaller angle than a 20mm lens, and therefore less light would be allowed to get to the aperture in the first place.
So, can one be faster than the other? Or do these factors cancel each other out? Or is there a more complex relationship?

 

[astroNikon]

So basically which is "faster"
a 20mm/2.8
a 24mm/2.8
a 50mm/2.8
85mm/2.8
etc

interesting question ... someone else is gonna have to chime in

 

[WayneF]

So basically which is "faster"
a 20mm/2.8
a 24mm/2.8
a 50mm/2.8
85mm/2.8
etc

interesting question ... someone else is gonna have to chime in

For all practical purposes, the exposure speed is the same. That is the meaning, and the purpose of marking them all f/2.8.


fstop number = focal length / effective aperture diameter. This is an equalizing procedure, so all f/2.8 is the same exposure.

A light meter will measure the light and read f/2.8..... and it does not care which f/2.8 lens you use.

In practice, the short lens will see wide angle, and the long lens will see telephoto, which can of course change the scene that the light meter sees to meter, but ignoring that different view, then f/2.8 is f/2.8



Technically, one lens can transmit slightly more light than another, and this measured performance is calibrated in what is called T-stops.

But modern lens coatings largely make this be very minor, and ignorable. Your camera meter looks through that lens anyway, and measures the result. If it says use f/2.8, use f/2.8.

 

[Vince.1551]

You are probably looking at the Transmission Value of a lens or light gathering ability. Best way to test this is to use a long Tele zoom to take a spot reading at say f2.8 if it allows and then use a 50mm and below lens to take the same spot reading at the same f-stop and then use an external light meter to do the same spot reading. Chances are the reading is different. Test on subdued lighting as this is where the difference is most pronounced.

 

[Overread]

As said the idea of aperture is to give you an unbiased unit of measurement.

So in the same light a shot taken with:

50mm at f2.8 ISO 100 1/150sec
and a
20mm at f2.8 ISO 100 1/150sec

Will give the same exposure at the camera. This is why you can use things like external light-meters to meter the light and they can give you the settings to use for a shot based on the ambient light exposure they read. It also means that you could take a shot with one camera to "test" the exposure and then repeat the shot with another camera (a method often used with things like large format photography where photographers would use instant polaroid shots to get a handle on the exposure and then they could set the right settings for the large format film).



So to all extents and purposes aperture values are equal - a 50mm f2.8 is as "fast" as a 300mm f2.8


Now there are a few differences however;
1) Real world differences in manufacture and tolerance mean there might be some slight variation although this shouldn't be a full stops difference, but it would explain why you might see slightly different histograms even if you were comparing the same focal lengths.

2) Effective aperture; Many modern lenses will change their effective aperture and sometimes focal length; as they focus closer to the camera. Macro lenses display this the most and your average f2.8 macro will be closer to around f5.6 at its closest focusing distance (Nikon cameras report this change for macro lenses - Canon ones do not even though the changes are similar for both brands and designs).
Note that the internal light meter in the camera reads the light as it comes through the lens; so effective aperture changes are already taken into account by the meter so you don't need to adjust for it when using the cameras built in meter.

 

[Derrel]

So...I always think about the old question: Which is heavier? A pound of gold, or a pound of feathers? I mean ONE of those identical units of measurement MUST be heavier! Right? I mean, am I right? Can I get an "Amen!"???

 

[astroNikon]

So...I always think about the old question: Which is heavier? A pound of gold, or a pound of feathers? I mean ONE of those identical units of measurement MUST be heavier! Right? I mean, am I right? Can I get an "Amen!"???

Give me a pound of gold and I'll get back to you on that. No need for the feathers, I know how much they weight

 

[korreman]

So, the f/number thing is to give an unbiased unit. I still might want to figure out how, but now I know the "what". Thanks!

 

[Designer]

So, the f/number thing is to give an unbiased unit. I still might want to figure out how, but now I know the "what". Thanks!

The "f" number is a ratio, and in theory all that have the same maximum "f" number should be the same "speed".

The correct way to write the ratio is 1:2.8 but most photographers tend to use the "shorthand" and drop the "1:" part of it. All aperture ratios can be expressed in the terms of the ratio i.e. 1:1.8, 1:2.8, 1:3.4 etc.

 

[KmH]

The f-stop is a ratio of the lens opening to the lens focal length.
As focal length increases the field-of-view (FoV) gets smaller and the amount of light the lens 'sees' also diminishes. Since the FoV changes, the total reflectance of the scene being imaged will change by some amount.

So the f-stop is expressed as a ratio of the lens aperture relative to the lens focal length, but the amount of light enering the lens can change with focal length.

A 50 mm lens set to f/2 has a lens aperture that is 1/2 as wide as the lens focal length, so the lens aperture is 25 mm wide.

A 100 mm lens set to f/2 has a lens aperture that is 1/2 as wide as the lens focal length, so the lens aperture is 50 mm.

A 180 mm lens set to f/2 has a lens aperture that is 1/2 as wide as the lens focal length, so the lens aperture is 90 mm.

 

[greybeard]

Can two lenses have the same maximum aperture size, while one is faster than the other?

Aperture is the main determining factor but not the only one. It is possible for two lenses with the same maximum aperture to have different light transmission characteristics because of lens construction, No.of elements, coating, etc.

 

[korreman]

It seems like I'm now getting contradicting information. I get that it's a ratio, and f/stop means the focal length divided by some number, usually a square root of 2^n. But now some say that the increased focal length means that less light gets in, without taking into account the larger aperture.
I might try again to calculate this myself and find the answer, though it'll take a hell and a half.

Try to think of this scenario: You have a simple two-dimensional camera, facing an infinite grey-card that is equally bright everywhere. Take glass and coating out of the equation. Your variables are f-stop and focal length. Your constants are sensor size and distance from lens to grey-card.

Now, will two different lenses at different focal lengths and the same aperture have the same speed?

 

[Vince.1551]

Don't just read the comments. Most information are 'plucked' from the internet. Do a practical test like I mentioned earlier. Keep a note of the conditions when you are testing. Oddly no one mentioned the testing conditions for their conclusions. I recommended testing at low lights coz I feel that is when most lenses faced challenges (keeping the camera model as a constant).
Good luck and have fun.