Does Lens Diameter Matter...???

For instance on my 24-105L (77mm Filter Size) at 30mm, f/4 compared to my Sigma 30mm (62mm Filter Size) at 30mm, f/4... Is there more light coming into the Canon or is f/4 the same amount of light regardless of the diameter... I'm not even sure why I'm asking, just curious...

f/4 is f/4 no matter the diameter of the lens.

Theoretically, they should be the same, but in reality, there will be some variation and either one could actually be a little brighter than the other.

Same goes for focal length. 30mm is not always 30mm.

Velorium wrote in post #10910427
f/4 is f/4 no matter the diameter of the lens.

I thought so... Thanks...

msowsun wrote in post #10910435
Theoretically...but in reality...

Love those answers...

Though obviously, the bigger the diameter, the more light is caught to be focused to the back of the lens. But to answer your question of "does it matter," it doesn't to us as consumers (aside from filter costs), just to the engineers of the lens.

Velorium wrote in post #10910538
Though obviously, the bigger the diameter, the more light is caught to be focused to the back of the lens. But to answer your question of "does it matter," it doesn't to us as consumers (aside from filter costs), just to the engineers of the lens.

The amount of light received only depends on the f number.
Diameter of lens does not always the same with the aperture.
It is usually bigger to avoid mechanical vignetting tho

actually the sigma gets more light than the canon.

assuming 100% transmissivity of the glass, at f4 they will get the same amount of light, regardless of size. f4 is purely a geometric property. However glass is not 100% transmissive, each air/glass interface produces some reflection, and the amount of glass produces some absorption. The coatings are designed to reduce the reflections at the transitions. Anyways the 24-105 has more elements, being both a zoom, and having IS, thus it looses more light than the sigma would.

This is not negligible, but probably doesn't amount to more than 1/3 of a stop in total.

cogliostro wrote in post #10910556
It is usually bigger to avoid mechanical vignetting tho

Correct, because more light enters the lens all around... The aperture mechanism just controls what hits the sensor.

I was wondering that myself. You can get telescopes with the same magnification, but larger diameters that gather more light. What's the difference between that and a lens?

Apollo.11 wrote in post #10910637
I was wondering that myself. You can get telescopes with the same magnification, but larger diameters that gather more light. What's the difference between that and a lens?

It's still technically a lens. In the case of that, it's just a bigger hole for your eye to look into. Ratios between glass elements are what matter more-so than the actual size of the pieces of glass when it comes to magnification. The size is only a factor in practicality for the usage.

Only for the filters you got..
..and not have...

mguffin wrote in post #10910397
For instance on my 24-105L (77mm Filter Size) at 30mm, f/4 compared to my Sigma 30mm (62mm Filter Size) at 30mm, f/4... Is there more light coming into the Canon or is f/4 the same amount of light regardless of the diameter... I'm not even sure why I'm asking, just curious...

Since this thread seems to take a drift:

Yes, F/4 is F/4, as some already said. And with regard to transmissiveness, as some said, that is also true, but not relevant for the aperture as such, as aperture just determines the relative size of the hole the light travels through, literally focal length divided by the aperture number, which gives you the diameter of this hole, and the relative brightness in f-stops as compared to the light outside.

The reason for the difference in filter size actually has to do mostly with the build of the lens. The Sigma 30 is a prime, with short build, and aperture fairly close to the front, with a fairly limited AoV, namely of about 45 degrees, as it is built as an APS-C standard lens. You need to measure this capture angle more or less from the aperture. This means that due to tthe short build, even though it is an F/1.4 lens (wide aperture), the rays captured don't need a very wide barrel in order to capture the light that the lens sees, and hence the filter size can stay relatively small.

Now, the 24-105 is a FF lens with a substantive build, and which goes down right to 24 mm, which impleas an AoV of roughly 75 degrees, and which has the aperture quite deeply recessed into the lens. In order to capture this AoV of 75 degrees, it will have to have a fairly wide barrel when considering its maximum aperture, because of its build length and because of the large AoV, which again implies a large filter size to avoid mechanical vignetting.

Of course, there also is the standardization issue: maybe the engineers could have gotten away with a 73.7 mm filter diameter, but it gets rather costly to have a filter made to exact required size for each lens, so they use a commonly accepted standard filter size a little larger, namely 77 mm (this is also true for the Sigma 30, BTW). A little larger is never harmful, a little smaller obviously may cause mechanical vignetting.

Transmissiveness becomes important for (professional) video shoots, where lighting is adjusted for optimum effect, and where you don't want slight changes in exposure due to the use of different lenses with different transmissiveness factors. Hence the use of T-stops (apertures corrected for transmissiveness of a specific lens) in film and video making. In photography that generally is automatically taken care of by TTL light metering.

HTH, kind regards, Wim

Thanks everyone...

mguffin wrote in post #10910397
For instance on my 24-105L (77mm Filter Size) at 30mm, f/4 compared to my Sigma 30mm (62mm Filter Size) at 30mm, f/4... Is there more light coming into the Canon or is f/4 the same amount of light regardless of the diameter... I'm not even sure why I'm asking, just curious...

Yes. Lens diameter does matter... but you might not reap all of the benefits of a larger objective lens from your camera/sensor depending on the body and conditions.

From a basic optics/physics standpoint, the maximum resolution of an image is dependent on the diameter of the objective lens. Larger diameter lenses also have larger light gathering power, but as others have stated, two lenses with the same f-number will still give the same exposure.

Ignoring sensor size (which admittedly, is an important factor), there are other reasons that cell phone cameras and ultra-compact cameras with their small lenses do not take photos with the same level of detail/resolution as large SLR cameras/lenses.