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So (without limitations of the viewfinder for example) looking through the viewfinder creates a brighter image than normal view would be.

The lens would then work like a funnel(collects light from large area) or magnifying glass(creates bright spot as well).

I know the difference of t-stop vs f-stop and also know that f/1.0 is not a magical aperture value or something like that btw.

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Well, sort of. Think about the sun shining through a lens — it's immediately apparent that the focused spot of light is brighter than the unfocused.

However, the catch is that your "real view" also goes through a lens which focuses the light: your eye. So, in a sense, the real comparison is simply "Is there a lens which is brighter than the human eye?" — and the human eye's aperture is somewhere on the slower edge of fast lenses. No matter how you measure, it's certainly slower than a f/1.4 lens.

But, I think party what you're asking is if a lens can effectively act as an all-optical night-vision device. The catch is that the eye isn't much slower than that f/1.4 lens... probably not much more than 2 stops. That means that the fastest lens possible doesn't really gain that much.

Overall, at least in the context of photography, I think this turns out to be less exciting than it might seem at first. That's because the effect of a an aperture faster than the human eye is only part of the overall equation. We can also make sensors that have higher gain than even the night-adjusted human eye, but most crucially, we can use long exposure to integrate over a much longer time than our own vision does in low light. So, overall, it's really, really easy to produce an image which has an exposure much higher than the scene appears naturally.

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    \$\begingroup\$ If you're viewing an image projected onto a focusing screen (like when using an SLR) then the aperture of your eye is irrelevant to whether what's seen on the focus screen is brighter than the view with the naked eye, since your eye aperture applies a constant brightening factor to both. If you're talking about holding a lens up to your eye, than that's an optical system with two apertures and is therefore only as fast as the smallest aperture. \$\endgroup\$
    – Matt Grum
    Commented Feb 28, 2013 at 16:11
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    \$\begingroup\$ @MattGrum That is more or less what I meant indeed. So putting an f/0.5 lens (just an example) to your eye will not result in a brighter image than without the lens? \$\endgroup\$
    – FW.
    Commented Feb 28, 2013 at 16:13
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    \$\begingroup\$ @FW. It's not possible without some reduction in field of view / change of focal length. Otherwise you'd be able to screw something on the front of an f/2.0 lens to make it an f/1.4 lens. The Metabones Speed Booster only works because it screws on the back of the lens. \$\endgroup\$
    – Matt Grum
    Commented Feb 28, 2013 at 16:19
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    \$\begingroup\$ @MattGrum: As you alluded to, it depends on FoV. When I hold my 16X60 binoculars to my eyes, the stars and other objects in the night sky are much brighter than they are to my naked eye. My 4.5" reflecting telescope does this even more so. This is because they are taking the light falling on a much larger surface area than the pupil of my eye and concentrating that light onto the pupil of my eye. With a telephoto lens, the effective aperture is all that needs to be larger than the recording medium or eye pupil to concentrate more light per cm^2 than is falling on the objective lens. \$\endgroup\$
    – Michael C
    Commented Feb 28, 2013 at 17:03
  • \$\begingroup\$ In theory, it is possible to place additional elements on the front of a lens. If the addition has a larger surface area than the original objective by a factor greater than the additional magnification needed to concentrate the light collected through the aperture, it would result in a net brightness gain. It just isn't practical or cost effective to do this externally, but that is the essence of what a classical telephoto lens design is. \$\endgroup\$
    – Michael C
    Commented Feb 28, 2013 at 17:11
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I'd like to disagree with the other answers - we'll maybe just question them.

Take your magnifying idea. You are not making the sun brighter! You are just focusing the caught by the lens into a small point, making it appear brighter. Light gets "lost" through every surface it passes through or bounces off of. You may change the appearance of it's luminosity, but certainly not enhance it.

This is not a scientific argument from my part, merely an opinion..

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  • \$\begingroup\$ It should be a scientific one - specifically the 1st law of thermodynamics. You can't get more energy out of a system than you put in... :) \$\endgroup\$
    – James Snell
    Commented Mar 1, 2013 at 14:20
  • \$\begingroup\$ Light CAN be lost at each material interface. That does not necessarily mean it IS lost. Light is generally lost at disjoint material interfaces due to the different refractive indexes, which tends to cause some light to reflect. Multicoating helps cancel out reflected light, which prevents a loss in contrast, but not necessarily a loss in light overall. However, recently, both Canon and Nikon have started using nanocotings. With a nanocoating, the material interface is "softened", such that reflection never actually occurs because there is no hard interface. So, 99.95% of light is preserved! \$\endgroup\$
    – jrista
    Commented Mar 1, 2013 at 23:32
  • \$\begingroup\$ (Well, preserved at each interface...total light lost these days is less than 0.1%). The primary sources of light loss in a digital camera are: 1-The IR Cut and AA filters, 2-The CFA, 3-Photodiode Q.E. For any given pixel in a bayer-type sensor, as little as 10% of the light actually reaches the photodiode and frees an electron. :O \$\endgroup\$
    – jrista
    Commented Mar 1, 2013 at 23:35
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    \$\begingroup\$ "More light in the same place" is pretty much the definition of brighter. :) \$\endgroup\$
    – mattdm
    Commented Mar 2, 2013 at 3:17
  • \$\begingroup\$ @JamesSnell: You are not creating light to make the image brighter, you are redirecting it. By focusing almost all of the light falling on a large objective onto a much smaller image plane or exit pupil, you are concentrating all of that light on one small spot, at the expense of the surrounding area that receives none of the light it otherwise would if the optical system (in this case a lens) were not redirecting it. There is the same amount of energy in the system, it has just been redirected. \$\endgroup\$
    – Michael C
    Commented Mar 2, 2013 at 6:22
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This is physically impossible. Without active amplification, the luminance of the image can not be more than the luminance of the subject. Otherwise you would be violating the second law of thermodynamics. If you try to focus the sun rays on a black body, it can get really hot, but not hotter than the surface of the sun itself. For a mathematical proof, read this about the conservation of etendue, and remember that luminance is just luminous flux per unit of etendue.

Edit: The brightness can be optically increased if you use the word “brightness” in the astronomical sense: the “brightness” of a star is the illuminance it delivers to Earth. This is a definition that makes good sense for pointlike sources, like stars.

However, when talking about photography (other than astrophotography), we usually deal with extended scenes rather than pointlike sources. Then the proper notion of “brightness” is the luminance rather than the illuminance. And the luminance cannot be increased.

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  • \$\begingroup\$ Sure it can. Luminance is a measure of the intensity of light per unit area. If we collect light from a large area and redirect it to a much smaller area the luminance will be increased. We're not increasing the total number of photons, but we are increasing the number of photons per mm². \$\endgroup\$
    – Michael C
    Commented Oct 31, 2017 at 20:08
  • \$\begingroup\$ @MichaelClark: No. What you are defining is called illuminance And of course you can increase it by concentrating the light into a smaller area. What I am talking about is luminance, which is the photometric quantity most relevant to the question. And no, you cannot increase it unless you use some sort of amplification (like, e.g., a video camera and a monitor). Please, see the Wikipedia articles I linked to. \$\endgroup\$
    – Edgar Bonet
    Commented Oct 31, 2017 at 20:35
  • \$\begingroup\$ Extended scenes can be brightened in exactly the same way that point sources can. That's how something such as the Metabones Speedbooster works: light from a larger area is concentrated on a smaller area. \$\endgroup\$
    – Michael C
    Commented Oct 31, 2017 at 21:15
  • \$\begingroup\$ @MichaelClark: – sigh – The Speedbooster increases the illuminance on the sensor, not the luminance. Please stop mixing up the two notions, otherwise any discussion on the topic is pointless. \$\endgroup\$
    – Edgar Bonet
    Commented Oct 31, 2017 at 21:46
  • \$\begingroup\$ The question above makes no mention of luminance or illuminance. What it does ask is if looking through the viewfinder can be perceived by the human eye as brighter than the original scene. Your answer introduced an insistence of restricting the question to that of luminance rather than illuminance, which by your own definition in your first comment above is what the question actually asks. Shoot me for saying luminance above when I should have used illuminance, but the question as asked is clearly about illuminance, not luminance. \$\endgroup\$
    – Michael C
    Commented Oct 31, 2017 at 21:59
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This should be possible since the eye samples light from a relatively small surface area where as a lens samples light from a much larger area. The bigger problem is the huge discrepancy between the sensitivity of the eye and the sensitivity of sensors. I can already take photos with my Canon 5D Mark iii with fairly short shutters (sub 1/3 second, sometimes even 1/10 second) that have more color and contrast than what I actually see in the scene with my naked eye and I have really good night vision.

As far as something to help the naked eye, the biggest problem would be the necessary size of the front element I think. To double the amount of light, the lens would have to be very large since you are also going to have light lost in the lens system itself.

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Theoretically, yes. The human eye reportedly only opens as far as f/3.2, and there are many lenses faster than this. The Canon 50mm f/1.0 for example was marketed as being "faster than the human eye", although the f/3.2 figure suggests it shares that award with most prime lenses.

The biggest obstacle is designing a reflex mirror, pentaprism and focus screen that will accept the full light cone from a fast lens without clipping it and effectively narrowing the aperture. Consumer DSLR laser cut screens cut off at about f/2.5. Old fashioned ground glass screens don't have this limitation but aren't as bright generally.

note there's a crucial difference between looking through the viewfinder of an SLR and holding a lens up to the eye. In the first case you are viewing an image projected onto a screen, whereas in the second case you are forming a multi-lens system that includes the human eye.

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