Does 24mm 2.8 allow more light than a 50 1.8?

[nerwin]

I've always wondered if a wide angle lens with a maximum of aperture of f/2.8 actually allows more light in than say a 50mm with a f/1.8 aperture because of the focal length being so much wider it gathers more light than a longer focal length lens would.

I never really put much thought into this and want to know if there is any truth to this, is it just simply physics?

 

[fmw]

f numbers are f numbers. f2.8 on a 28mm lens produces the same exposure as f2.8 on a 50mm lens or a 300mm lens.

 

[pixmedic]

the aperture blades are behind the front element, so no matter how much light comes in through the front, only the amount the aperture is set at gets through. so, the aperture setting lets in the same amount of light no matter the focal length or size of lens.

 

[astroNikon]

I think he's asking about the mathematical equation in relation to aperture / focal length.
This will explain a bit better but give the same result as mentioned ==> Photography tips

 

[nerwin]

Ok...so what I was reading earlier was complete BS.

So it doesn't really matter then. If a 16mm lens and 85mm lens is both shot at f/4..the exposure would be the same?

 

[astroNikon]

Ok...so what I was reading earlier was complete BS.

So it doesn't really matter then. If a 16mm lens and 85mm lens is both shot at f/4..the exposure would be the same?

can you provide a link ?

 

[nerwin]

Ok...so what I was reading earlier was complete BS.

So it doesn't really matter then. If a 16mm lens and 85mm lens is both shot at f/4..the exposure would be the same?

can you provide a link ?

I have no idea where it was. I was googling this topic beforehand and I just coulden't get a straight answer. It was mixed results.

 

[pixmedic]

Ok...so what I was reading earlier was complete BS.

So it doesn't really matter then. If a 16mm lens and 85mm lens is both shot at f/4..the exposure would be the same?

yes.
f/4 is f/4 regardless of the lens.
other things might be affected by the specific lens, like bokeh, field of view, distortion....but not the light that is let in to the camera.

 

[Ysarex]

The whole point of f/stop values is to standardize exposure across all lenses.

Been watching those Youtube videos again.........rots holes in your brain.

Let's try a for instance: Let's photograph the same light source with both lenses. To make it easy let's use a standard that's always the same size and distance -- the moon. Everybody likes to photograph the moon. Let's use film because we can directly compare results. Take both shots of the moon one with your 24mm and then one with your 50mm. Place the two pieces of film on a light table and right away you'll notice that the image of the moon is bigger in the film from the 50mm lens. In fact it's four times bigger. That would seem to suggest that the 50mm is "gathering" more light. In this case the only real light is the moon and with the 50mm you get four times as much as with the 24mm.

I put the term "gather" in quotes. Use of that term may be a source of your confusion.

Joe

 

[480sparky]

F-stops Explained

 

[Big Mike]

The F number is a ratio between the focal length and the diameter of the entrance pupil.

So a longer focal length will require a larger physical opening to get the same F number. Amount of exposure is offset one for the other, so the same F number will give you the same exposure on any lens. (well, there will be minor differences, one lens to another but it's close enough).

 

[unpopular]

The entire point for aperture number is to standardize exposure across focal lengths by effectively changing the exit diameter so that they perform similarly regardless of focal length. While it is true that shorter lenses are faster at any given exit diameter, the aperture number provides an apples-to-apples measure of exit diameter relative to focal length.

Because F = f/d, d=f*F. So what we can say here is that for 50mm at F/1.4 the diameter would be 35.7mm. So for 28/1.4 the effective exit diameter is 20mm. A 20mm aperture on a 50mm lens has an f-ratio of 2.5. So as you can see, the shorter lens needs a smaller aperture to achieve the same f ratio relative to the longer lens. By adjusting the aperture, we compensate for focal length.

It is true that other factors affect transmission, and actually this can be pretty substantial - take for example the Samyang 35/1.4, which is off pretty significantly. In photography, this is not so much an issue because you typically do not need to precisely match DOF and exposure across multiple lenses.

In cinema, however, this is more problematic since your shutter time is often fixed (or in the least far less flexible). This is why cinema lenses are often measured by Transmission Number instead. T-stops reflect the actual degree of attenuation at any given diameter. So for a T-number of 2.0, half of the intensity that enters the lens will exit.

 

[KmH]

If a 16mm lens and 85mm lens is both shot at f/4..the exposure would be the same?

Take some photos, and judge for yourself.

 

[chuasam]

Ok...so what I was reading earlier was complete BS.

So it doesn't really matter then. If a 16mm lens and 85mm lens is both shot at f/4..the exposure would be the same?

yes

 

[unpopular]

Ok...so what I was reading earlier was complete BS.

So it doesn't really matter then. If a 16mm lens and 85mm lens is both shot at f/4..the exposure would be the same?

please tell me you didn't read this on a blog somewhere.