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Let's say I have two lenses of 50mm, one with maximum aperture f/1.8 and another with f/1.4.

I understand that when using maximum aperture, for the latter lens more light will get to the film or photo sensor. That seems logical.

However, someone has told me that if in both lenses I will use f/1.8 aperture, still the latter lens will have more light. So, using same aperture shutter time will be still shorter for the latter lens. That sounds illogical to me.

Can you explain what makes lens brighter?

  • Hasn't this exact question been asked here before? – Michael C Oct 27 '15 at 0:51
  • Please point me to a duplicate if so. – dzieciou Oct 27 '15 at 4:20
  • Your 1.4 lens will not be brighter, but it will most likely be sharper than the other at F/1.8. Follow Michael Clark's link for more detail. – Matthieu Moy Oct 27 '15 at 6:55
  • The 1.4 lens will be brighter during focusing, so it will be able to focus in lower light. Even if it is stopped down to f/1.8 or whatever - it is fully opened during focusing. – TFuto Oct 28 '15 at 13:18
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However, someone has told me that if in both lenses I will use f/1.8 aperture, still the latter lens will have more light.

It sounds illogical because it's wrong. At f/1.8, both lenses will let in (approximately) the same amount of light.

I say "approximately" because the f number is derived only from the ratio of the focal length to the size of the aperture. In the real world, other factors, like the amount of light absorbed by the glass also play in — for cases where it's important to know precisely, there's a thing called "t-stop". That's primarily useful in video, where you might need to keep exposure perfectly consistent when switching lenses. In still photography, it's usually just ignored.

Most importantly, the t-stop isn't related to the maximum (widest) aperture of the lens — that is, it's not at all guaranteed that the lens with the widest maximum aperture will have the highest transmission. In fact, since such lenses often have more glass in order to compensate for artifacts at wider apertures, the transmission might even be less.

So, to answer the basic question: the f-stop — the size of the aperture — is the primary factor that makes a lens brighter or not, and different lenses at the same f-stop should give very close to the same results. Transmission is the other major factor, but outside of special cases, it's not considered a significant one. It isn't even generally reported in lens reviews (although you can find measurement of it at DxOMark, which specializes in numeric analysis of measurable aspects of lenses and sensors).

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    Everything you say is true. There's another factor though, and that's the accuracy of the lens aperture. Just because you set the camera for F/1.8 doesn't mean that's exactly what you'll get. – Mark Ransom Oct 27 '15 at 21:08
  • @MarkRansom Excellent point. And it it's not just accuracy — it's precision of the label as well. I don't expect that every lens which says 50mm f/2.8 on the barrel is actually exactly a focal length of 50.0000mm with an aperture of 17.85714mm. – mattdm Oct 27 '15 at 21:12
  • Hmm, I did a small experiment and compared what shutter speed my camera body selected for two lens, 50mm f/1.4 and 50mm f/4.0, both shot at f/5.6. First lens was resulting with shutter speed of 0.8 times shorter (1/50s) than the latter (1/40s) for the same scene. Doesn't that contradicts with what you said? – dzieciou Nov 1 '15 at 7:41
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    @dzieciou No, I do not think it's a contradiction. Assuming the framing and lighting are identical, I'd say it's just an example of the t-stop caveats in action. The f/4 is a macro lens, right? I wouldn't be surprised if the difference is more about precision and accuracy of labeling, as Mark Ransom points out. Are the resulting images identically framed? In any case, the difference is just under a third of a stop — which is generally a good rule-of-thumb limit for "does this difference matter"? – mattdm Nov 1 '15 at 10:41
  • @mattdm, that was zoom lens 28-74mm with focal set to 50mm. I haven't thought it might be caused by errors in labeling. Thanks for reminding me that. – dzieciou Nov 3 '15 at 7:12
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The big difference between the two lenses will not be when the picture is taken. At that point both lenses will allow the same amount of light through within the limits of the accuracy of their aperture settings and transmission ratios.

The big difference will be when the lenses are focused prior to stopping down. The wider aperture lens will allow more light to get to the focus array for auto focus or to the view screen for manual focus and will allow faster and more accurate focus.

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    +1, although the difference between f/1.4 and f/1.8 as in the example in the question won't be dramatic (or possibly even noticeable). – mattdm Oct 27 '15 at 6:49
  • Point well taken, although it does depend on the view screen in place for manual focusing and otherwise on the particulars of the AF system. With AF the wider baseline of an f/1.4 vs. f/1.8 lens will make a difference if the focus point has a wide enough baseline to take advantage of the wider angled edge rays. – Michael C Oct 27 '15 at 10:09
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The brightness of the image projected by the lens is a function of its working diameter (aperture) and focal length. The focal ratio (f/#) takes both into account. Any lens functioning at the same f/# as another, delivers the same image brightness.

The camera lens operates like a funnel. The larger its working diameter, the more light it will capture. We turn to the geometry of circles to explain how this works. Before we do, consider that giant astronomical telescopes are fitted with a lens many meters in diameter. This allows them to gather light that is super feeble.

Now the camera lens is fitted with an adjustable opening we call an iris. The camera iris mimics the human eye in that the colored portion involuntary changes the diameter of the pupil to accommodate for scene brightness. By the way, the pigmented iris is named for the Greek goddess of the rainbow.

Actually, it is the surface area of the opening that does the deed. As we open up the aperture, the diameter of the entrance increases as does the surface area. The factor, diameter to surface area is 1.4. Multiply any circle diameter by 1.4 and you construct a revised diameter that doubles the surface area. Thus the number set: 1 – 1.4 – 2 – 2.8 – 4 – 5.6 – 8 – 11 – 16 – 22 – 32. Note each value going right is its neighbor multiplied by 1.4 and then rounded. This is the mysterious f/number set in full stop increments. Each full stop doubles or halves the surface area (capture area).

Now the rest of the story: While the working diameter is the key to image brightness, there is another equally influencing factor. This is focal length. The focal length reveals the power of the camera lens to magnify. Longer focal lengths yield a larger image but at a price. Each doubting of the focal length decreases image brightness by ¼. In other words, a 50mm delivers say 100 units of light; a 100mm only delivers 25 units of light. This is because the higher magnification causes the same amount of light to be spread out over 4x more surface area. The camera baffles (blocks) the larger image only presenting the central portion to film or digital chip.

Consider that the two factors of working diameter (aperture diameter) and focal length comingle to yield the final brightness factor of the lens. This is chaotic because there is a hodgepodge of different lens diameters and focal lengths. There are a zillion combinations, each likely to deliver a different image brightness. We need a system that is universal, one that takes into account the two key factors and allows us to predict image brightness regardless of focal length or aperture diameter.

Focal ratio to the rescue: We need a ratio because a ratio is dimensionless. The f/number system (focal ratio) resolves this difficulty. We divide the focal length by working diameter (aperture). This is the focal ratio. Any lens operating at the same focal ratio (f/#) as any other, regardless of focal length or working diameter, delivers the same image brightness when imaging the same scene. Different systems have been tried, but we keep returning to the focal ratio (f/#) to set our cameras.

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