Light from f/2.8 lenses

So I just bought the new Tamron 24-70 f/2.8 VC Di lens. It is quite a substational lens compared to my Tamron 28-75 f/2.8. The glass itself is probably twice the size, width, and weight. My question is: With both lenses being f/2.8, why is one so much smaller? And, with f/2.8 being equal, does the larger lens let in more light, or is f/2.8 the same across all lenses, no matter what the size?

John E wrote in post #14482761
So I just bought the new Tamron 24-70 f/2.8 VC Di lens. It is quite a substational lens compared to my Tamron 28-75 f/2.8. The glass itself is probably twice the size, width, and weight. My question is: With both lenses being f/2.8, why is one so much smaller? And, with f/2.8 being equal, does the larger lens let in more light, or is f/2.8 the same across all lenses, no matter what the size?

Not sure if this is right... so take it with a grain of salt.

F-Stops are claculated the focal length of the lens (i.e. 50mm) divided by the "effective" aperture diameter. Apertures can be located nearly anywhere in the lens, from the front element to the rear. My understanding is that the effective aperture opening is always calculated as if it is located at the front element. However, it usually isn't.

I believe larger diameter glass can cause less cromatic aberations and can be sharper at the edges. Hence, there is a trade-off between the quality of the lens and cost of the lens, because larger glass is exponentially more difficult to produce within micron level tolerances.

I suppose glass size cannot be a diffinitive indicator of quality, but perhaps can be a rule of thumb. You have to consider quality of the glass, lens coatings, design and other variables.

Just realized I didn't answer your question completely. The aperture value of 2.8 means that they do in fact allow the same amount of light at 2.8 and any other aperture for that matter.

Furthermore, it doesn't matter the focal length. At a given f-stop, say 2.8 a 35mm and a 400mm will both allow in the same amount of light, even though the 400mm aperture diameter will be much larger. The equalizing factor is the field of view is much wider for the 35mm, so the cumulative amount of light is equal for both lenses at 2.8 (or any other f-stop).

John E wrote in post #14482761
So I just bought the new Tamron 24-70 f/2.8 VC Di lens. It is quite a substational lens compared to my Tamron 28-75 f/2.8. The glass itself is probably twice the size, width, and weight. My question is: With both lenses being f/2.8, why is one so much smaller? And, with f/2.8 being equal, does the larger lens let in more light, or is f/2.8 the same across all lenses, no matter what the size?

The same f-stop setting used on ANY lens should produce the same brightness at the film plane or digital sensor in a camera, assuming the scene that the lens is looking at is illuminated at the same level and has the same reflectivity characteristics.

For an experiment to prove this, one would want to be photographing something like an evenly lit large gray wall because ordinary scenes have all sorts of brightness levels in them.

Great! Thanks! It's always puzzled me how Tamron can make the 28-75 f/2.8 be so small and light compared to Canon's "brick" 24-70L f/2.8, when they both let in the same amount of light.

It's hard to make wide zoom lenses.

It's a huge difference to make a lens that zooms to 28 mm and to 24 mm.

One lens have 18 lens elements in 12 groups.
One have 16 lens elements in 14 groups.

A larger front element helps with vignetting, corner sharpness, barrel distortion, ... Note that I write "helps". It isn't automatically so that the lens with the largest front element always wins - it's a question of design skills, and number of lens elements, and number of "exotic" lens element shapes/materials too.

In theory, all lenses will project the same amount of light on the sensor if using same aperture. It reality, it isn't so. Depending on design, materials etc, there will be losses inside the lens, because of reflections and transmission.

Isn't the whole point of T stops for video the reason that F stops are not accurate enough?

John E wrote in post #14482949
Great! Thanks! It's always puzzled me how Tamron can make the 28-75 f/2.8 be so small and light compared to Canon's "brick" 24-70L f/2.8, when they both let in the same amount of light.

One treats the light with kindness and the other shoves it through a meat grinder.

Mis-statement in terms here. Different lenses do not transmit the same AMMOUNT of light. They transmit the same RATIO of light. 2.8 on your iPhone is not the same quantity of light as 2.8 on your 200. A different ammount most definatley will pass through it. However the ratio will be close.

Mark1 wrote in post #14485378
Mis-statement in terms here. Different lenses do not transmit the same AMMOUNT of light. They transmit the same RATIO of light. 2.8 on your iPhone is not the same quantity of light as 2.8 on your 200. A different ammount most definatley will pass through it. However the ratio will be close.

I have no idea what you're trying to say with the term "ratio".

Any lens set at a particular f-stop will illuminate the film plane or sensor surface with the same intensity of light. That's not saying that there are the same total number of photons going through the lens, as the field (angle) of view changes with different focal lengths. However with the same ISO value (or film speed), shutter speed, and f-stop setting the exposure will be the same (within manufacturing tolerances of the lens' f-stop settings).

In simplistic terms, the brightness of the image on the film will be the same for any focal length lens which has its aperture control set to the same f-stop value. If this were not true, light meters and the exposure settings we use would be useless.

I think one of the reasons the Tamron 24-70 is more compact than the canon 24-70 has to do with image quality more than maximum aperture. While the Tamron may perform really well in the center, the canon performs much better in the corners - especially on FF sensors. Keeping image corners sharp on wide lenses requires a lot of high quality glass - as seen in canons 24-70 2.8L.

I'm not trying to state the above as fact, just mentioning a possible contributing factor.

Here's two photos for comparison:
http://www.the-digital-picture.com …omp=0&FLIComp=0​&APIComp=0

SkipD wrote in post #14485485
I have no idea what you're trying to say with the term "ratio".

Any lens set at a particular f-stop will illuminate the film plane or sensor surface with the same intensity of light. That's not saying that there are the same total number of photons going through the lens, as the field (angle) of view changes with different focal lengths. However with the same ISO value (or film speed), shutter speed, and f-stop setting the exposure will be the same (within manufacturing tolerances of the lens' f-stop settings).

In simplistic terms, the brightness of the image on the film will be the same for any focal length lens which has its aperture control set to the same f-stop value. If this were not true, light meters and the exposure settings we use would be useless.

Unfortuanately, this seems to break down with Macro lenses, at least with Canon Macros!

SkipD wrote in post #14485485
I have no idea what you're trying to say with the term "ratio".

Any lens set at a particular f-stop will illuminate the film plane or sensor surface with the same intensity of light. That's not saying that there are the same total number of photons going through the lens, as the field (angle) of view changes with different focal lengths. However with the same ISO value (or film speed), shutter speed, and f-stop setting the exposure will be the same (within manufacturing tolerances of the lens' f-stop settings).

In simplistic terms, the brightness of the image on the film will be the same for any focal length lens which has its aperture control set to the same f-stop value. If this were not true, light meters and the exposure settings we use would be useless.

Because aperature is indeed a ratio. While the number we talk about is found with a mathmatical calculation including the lenth and width of the lens. In actual use, we are dealing with the ammount of available light (flash and ambient) vs how much makes it through the aperature. Remember 1 stop is half/double the light. You cant make a half with out a ratio of some kind.

I admit I was not complete in my reply. I was trying to clear it up without calling anybody out.

But....

And

The concept is fairly good. But when the wrong words are used it becomes wrong. It in fact is not the same amount as was stated. Aproximately the same intensity as you have stated is correct. (Differences is glass light transmition effeciency will come into play if you want to get technical. But in the real world it is close enough.) Small difference. But big enough to make it wrong.

Mark1 wrote in post #14486035
Because aperature is indeed a ratio. While the number we talk about is found with a mathmatical calculation including the lenth and width of the lens. In actual use, we are dealing with the ammount of available light (flash and ambient) vs how much makes it through the aperature. Remember 1 stop is half/double the light. You cant make a half with out a ratio of some kind.

I admit I was not complete in my reply. I was trying to clear it up without calling anybody out.

But....


And


The concept is fairly good. But when the wrong words are used it becomes wrong. It in fact is not the same amount as was stated. Aproximately the same intensity as you have stated is correct. (Differences is glass light transmition effeciency will come into play if you want to get technical. But in the real world it is close enough.) Small difference. But big enough to make it wrong.

Indeed, it is not!
First, that word has only one a, it's an aperture, not aperAture.

Second, aperture is a hole, an opening, and it's measured in millimeters.
F-number is a ratio (between the diameter of an aperture and focal length) and it's dimensionless.

As you said before: small difference, but big enough to make it wrong..

tonylong wrote in post #14485936
Unfortuanately, this seems to break down with Macro lenses, at least with Canon Macros!

If I Remember Correctly (not at all guaranteed) that is because focal length is calculated with the lens focused to infinity. As the lens is focused closer the effective FL changes, thus changing the f-stop ratio, although for a lens with a normal focus range the shift is small enough to be within tolerances (we can, after all, only set exposure to 1/3 stop precision). However, at the close distances of macro lenses the relationship breaks down.

It occurs to me that in the same way that in a constant maximum f-stop zoom lens the aperture diaphragm is cammed to the zoom ring, it should be possible to make a macro lens with the diaphragm cammed to the focusing barrel and I expect that is the case with good quality lenses - as opposed to when you use extension rings and lose light.