[Frank29]

I know we all lust after these F1.4 and 1.8 lenses, but I have a silly question I haven't been able to find the answer to:

If you have two lenses, say an 50mm F1.4, and an 18-50mm kit lens with a F4.5 maximum aperture, and you set both at 50mm and F8. Is the same amount of light going to the sensor from each lens, or is the 'faster' lens allowing more light at F8 than the slow lens?

In other words, is F8 at a given focal length, the same sized hole, regardless of the maximum aperture of the lens? Or does the faster lens still let more light through, because it has a larger front element?

Something I've been pondering, mainly because I have realized that shooting group portraits you can't get away with wide open apertures any way, so the value of the fast lenses seem over-rated to me.

Frank

[Aaron28]

you set both at 50mm and F8. Is the same amount of light going to the sensor from each lens

yes! same amount at same f stop...…

'fast' is how wide the lens can be......f1.4 allows more light than f1.8.....so a 50/f1.4 is 'faster' than a 50/f1.8.......but the 50/f1.4 set at f1.8 allows the same light as a 50/f1.8 at f1.8.....

[termy]

mmmmm.

To my understanding, f8.0 means f8.0, be it on this lens or that lens or any other lens, or be it at 50mm or 85mm etc.

f8.0 will let in the same amount of light (am assuming same shutter speed), be it on whatever lens.

I certainly hope i am not incorrect in that understanding of it.

[swanlefitte]

Edit. I posted on iq. Not transmission.
Lensrentals just did testing on this.
It turns out that not quite but there is truth in the idea.
Lens Rentals | Blog
"Conclusions:
I think it’s pretty obvious, at least for lenses shot at 35mm. If you plan on shooting stopped down, a good f/1.8 or f/2 prime is nearly, but not absolutely, as good as the more expensive f/1.4 primes. "

[bkpix]

Yes, they are the same, at least in theory. One practical difference, though, is that faster lenses -- especially f2.8 and faster -- are often better in quality than slower lenses. That means that sometimes the faster lens will have less distortion and greater across-the-frame sharpness, for example, than a slower lens, even at the same f-stop.

There may also be minor variation in the amount of light transmitted by any two lenses set at f8, depending on the quality of the glass and the number of elements in the lenses.

[tokyoscape]

you should get the relatively same amount of light between lenses at the sam f-stop.
As to the overrate comment to the faster lenses, Faster lenses usually (but not always) are those with the better optic. So you pay for the better quality glass. And we use faster lenses for lowlight handheld shooting too.

[Kozlok]

Actually, the "same light" question get's a bit tricky. The T-stop indicates how much light is actually getting to the sensor. A lens with a lot of elements and coatings that absorb light will have a "slower" T-Stop at the same f-stop compared to a simpler lens with no coatings. More info here: F-Stops vs T-Stops - What are they? and when do you use them? - DIY Photography

It is approximately true, though. F-stop is a reasonable indicator and they should be about the same. The "hole" (aperture), is the same diameter for all lenses for a given focal length and f-stop.

Now onto the third topic, about how fast lenses are not valuable. A "good" lens is better at any aperture than a "worse" lens. Mostly, really good lenses are also at least a little bit fast. The 50/1.8 will probably be sharper and give more pleasing photos than any zoom lens at 50mm. Whether it is "worth it" is a purely aesthetic and budget based question. Of course the 50mm f2.8 macro will be sharper than the 50/1.8 also, so this case the "slower" lens is "better", though there are other compromises when you start using macro lenses for every day shooting (focus hunting being the worst one).

The important thing is to buy a lens that accomplishes the goals you have for photography. If you have a 1080 monitor and don't pixel peep and don't print large, then no, you won't ever see the difference in "better" lenses. If you look at your shots on a 4k monitor and pixel peep and print very large, then yes, you will see the difference.

[jpipg]

Yeah they are all the same. f8 isn’t always the same size hole, because it depends on the focal length, but f8 on a $5 28mm lens is going to let in the same amount of light as f8 on a $5000 300mm lens. Of course a $5000 lens will generally have better image quality performance than a $5 lens irrelevant of the aperture used.

I suppose it might be worth mentioning that in terms of extreme accuracy, say 10ths or even 100ths of a stop, lenses do vary in their light transmission due to coatings, dust, number and composition of elements and such minutiae. Also as lenses, or more accurately, aperture mechanisms, age, they become less accurate and will vary a little from lens to lens.

[grog85361]

An "f-number" or f-stop, is the ratio of the lens' focal length to the diameter of the physical aperture opening. So, f8 is f8, for any local length, for any lens.

[UncleVanya]

As stated above the T stops could be slightly different but the f stop is telling you the same info for both lenses. Imagine you had an old handheld meter, if the exposure want roughly the same for all lenses at the same f stop then you would need to set a maximum aperture before taking an exposure reading which isn't the case.

[StiffLegged]

I would have said there’s no useful difference between the prime and the zoom at 50mm f/8 but I’m not so sure some immensely technical, tremendously obscure argument will now take dozens of posts to prove us all wrong. Tony Northrup, I just know you’re out there...

[mcgregni]

The more I read this the more I find the terms "fast" and "slow" to be very strange ways to describe maximum aperture openings!

[swanlefitte]

I would have said there’s no useful difference between the prime and the zoom at 50mm f/8 but I’m not so sure some immensely technical, tremendously obscure argument will now take dozens of posts to prove us all wrong. Tony Northrup, I just know you’re out there...

No not Tony, Roger in the link I posted earlier.
"The main takeaway from this is that zooms are less interchangeable than primes, copy variation is much more noticeable. If you make a general statement about zooms (like I did above for the Canon 24-70s sharpness) be very aware there will be a lot of copies that behave differently.

This is how zooms are. If you ask me to find a zoom that has no field tilt or centering change throughout the range, I’d tell you it’s not worth me testing a hundred copies in the faint hope of finding one."😁

[Paul the Sunman]

Lenses are never at their best wide open. So, irrespective of the quality of the lenses, an f/1.4 is likely to be "better" (contrast, edge sharpness, centre sharpness, vignetting) at f/2.8 than an f/2.8 lens wide open. This may no longer be true once they are both stopped down to f/4.

[kiwi_jono]

If the lenses have the same light transmission efficiency and using the same aperture then the same amount of light will reach the sensor.
f-stop is different to T-stop (which takes into account lens transmission efficiency).
On the other hand the Depth Of Field (with the same camera, same focal length, same aperture setting - but different lenses) should be the same.