Aperture and lens speed

[korreman]

Well, thanks for the tip, but I don't really have the equipment to test it. I only have a Nikon d5200 with a kit lens.

 

[Vince.1551]

Try borrowing lenses or rent the lenses if the results of your test is really that important. ;-)

 

[korreman]

Try borrowing lenses or rent the lenses if the results of your test is really that important. ;-)

If it was that important, I wouldn't be asking in a photography forums beginner section.

 

[Vince.1551]

I have the same thoughts

 

[gsgary]

If you drop them from a height they will hit the floor at the same time

 

[KmH]

As mentioned above, if you meter a scene with the same lens aperture but using 2 different focal lengths, the FoV changes which also changes how much REFLECTED light there is the lens. The light meter in a DSLR camera can only measure reflected light. It cannot measure incident light or display strobe light (flash).

Each full stop of lens aperture increases or reduces the lens aperture AREA, not the diameter. sufficient to let in 2x more or 1/2 as much light when using the same focal length.

Doubling the AREA of the lens is a function of the square root of 2 - 1.4142.
Here are some of the classic full stop steps
f/1 x 1.414 = f/1.414 (rounded to f/1.4)
f/1.4 x 1.414 = f/2
f/2 x 1.414 = f/2.8
f/1.8 x 1.414 = f/4
and so on.

Note: Most DSLR cameras are set by default to show 1/3 stop steps, but most have options to display 1/2 step or full stop steps.
f/1.8 is 1/3 of a stop larger lens aperture than f/2 is.
f/1.2 is a full stop larger lens aperture than f/1.8 is. (f/1.2 x 1.414 = f/1.697 (f/1.8 rounded)

 

[WayneF]

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.

This repetition gets tiresome.

One stops "fstop number" is the square root of 2 times the previous full stop. f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, etc.

But that number is: fstop = focal length / aperture diameter, and it means constant speed.

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

Depends on your definition of aperture.

If you mean the same diameter, no, not the same speed. Short lens will be faster (at same diameter). f/stop number will vary with diameter ratio (if assuming you mean diameter).

If you mean the same fstop number, yes, same speed. That this the purpose and definition of fstop number. Same speed. How hard is this?

The fstop number is the same speed. That is the entire idea of it.

 

[vintagesnaps]

Ask a simple question... lol When using a longer lens the light has farther to travel; if you would shine a small penlight/flashlight thru say, a paper towel cardboard tube, and thru a toilet paper cardboard tube, the light would fall off going thru the longer tube (and not having tried this experiment if you used a stronger brighter flashlight I think you'd need longer tubes). Just shine a flashlight and you can see how the light spreads out and falls off as it has to travel farther.

So if the opening (aperture) is larger, more light can get thru; if the opening is smaller, less light. So yes, the same aperture on two lenses of different lengths would allow a different amount of light to reach the surface where the image is being recorded (sensor/film plane). For a scientific answer I guess you'd have to go ask a physicist! or look up some info. related to light and distance, refraction etc.

 

[WayneF]

So yes, the same aperture on two lenses of different lengths would allow a different amount of light to reach the surface where the image is being recorded (sensor/film plane).

For same aperture diameter, yes. For the same fstop number, no.

The focal length factor is not about the path length in the lens. It is about magnification of the field of view. Illumination per unit of area.

A short lens (wide angle) gathers a lot of light from a very wide view, and concentrates all of it onto the camera sensor area.

A long lens (telephoto) gathers much less light from a much smaller view, onto same sensor area. Less light collected from a smaller area.

But fstop = focal length / aperture diameter equalizes these, giving constant exposure at equal fstops. That's why we bother with fstop numbers.

At f/4, a 200 mm lens has aperture diameter 200/4 = 50mm diameter.
At f/4, a 50 mm lens has aperture diameter 50/4 = 12.5 mm diameter.

Exposure depends on the Area of that aperture. Area of a circle is Pi r squared. One stop is 2x light, and 2x area is 2 Pi r squared. This makes 2x exposure of next fstop number increment be a factor of square root of 2 (of the fstop number itself).
But for both lens to be equal f/4 exposure, the 200mm lens has an aperture diameter 4x greater than the 50 mm lens. And the SAME fstop number.

Photographic Tables, Aperture f/stop, Shutter Speed, ISO and EV goes into these details.

 

[greybeard34 7]

f stop is the ratio of the physical size of the aperture to the focal length, right?

 

[WayneF]

f stop is the ratio of the physical size of the aperture to the focal length, right?

Yes, but NOT the physical diameter, but instead the apparent "effective" diameter as seen through the magnification of the front lens elements.... as seen by the subject.

See the picture at top of page at Photographic Tables, Aperture f/stop, Shutter Speed, ISO and EV

 

[sk66]

The change in the FOV cancels out the change in the opening size... as you suspected. For the purposes of choosing a lens and exposure they are the same speed.

But that doesn't mean two different lenses are "actually" the same speed because there may be slight differences in the efficiency of the lens designs. We have no easy way of discerning that as it's not reported to us. And IME I've never seen a difference significant enough to affect an exposure. I really don't see any point in worrying about it. The truth of the matter is that "apertures," as well as ISO and SS's are all "approximations." And sometimes they are outright lies (i.e. a macro lens that doesn't report an aperture change when in macro mode).

*"aperture" means the "apparent" or "effective" size of the opening and not necessarily the actual physical size of the opening... but there's not much point in worrying about that either.

The actual efficiency of transmission for a lens is known as a T-stop. A T-stop is the F-stop adjusted for the efficiency of the design... Only cinema lenses use/report T-stops AFAIK.

 

[table1349]

Man, did this thread ever turn left at the fork.

 

[korreman]

Man, did this thread ever turn left at the fork.

View attachment 77406

Oops, sorry about that.

The majority says that the fact that the f-stop is a ratio between aperture size and focal length makes all lenses equally fast at the same f-stop, regardless of focal length. Also, the majority says that other factors might make lenses slower (coating, glass elements, etc.), but it's less than a full stop. And then there's something about effective aperture in macro lenses that I'll figure out if I ever decide to get one.

Well, this was a rather... interesting... discussion. Thanks for the input!

 

[Derrel]

f/stop is a simple, mathematical formula. For practical purposes, f/2.8 from a 35mm lens is equal to f/2.8 from a 50mm or 85mm lens.

However, the simple, mathematical calculation of f/stop does NOT necessarily reflect the T-stop, or the actual transmission of light. Cine lenses are marked in T-stops, so that the **exact** amount of light coming through to the film or video sensor can be set accurately.

On some lenses, especially ones with a LOT of air-to-glass surfaces, there can easily be a loss of effective f/stop, meaning the T-stop for an f/2.8 setting, on a multi-element zoom lens lens might easily be f/3, or even f/3.2.

Also, very strict testing equipment CAN SHOW that many lenses have markings that have a bit of...fudge factor...

Some very simple lenses, like 5- and 6- and 7-element telephotos can have very effective light transmission; my 135 f/2.0 AF-D Nikkor for example is my "brightest" lens...it seems extraordinarily effective at wide f/stops like 2 and 2.2 and 2.5...it makes a BRIGHT image...it's old...for all I know, maybe the aperture stop-down mechanism is wonky...