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

[PaulWog]

*edit: I see someone already made a comment on page 1 about this.

Were you reading about t-stops?

T-stops are the net amount of light that will reach your sensor. Your lens, as well as your camera will influence the amount of light that is lost. For example, an f1.4 lens may have a t-stop number of 1.6 or even slightly worse.

This doesn't necessarily change for a longer or shorter lens.

Here is a great video by Matt Granger:

 

[thereyougo!2]

Surely in practical terms it will depend what is in your field of view. If I have a scene that has a dark foreground with the 16mm and using matrix metering it will meter differently to an 85mm field of view that is primarily in one part of the scene captured by the 16mm. To get the same exposure reading you'd need to move from the position you wee in with one to get the same amount in view. Or am I talking bullshine?

 

[PaulWog]

Surely in practical terms it will depend what is in your field of view. If I have a scene that has a dark foreground with the 16mm and using matrix metering it will meter differently to an 85mm field of view that is primarily in one part of the scene captured by the 16mm. To get the same exposure reading you'd need to move from the position you wee in with one to get the same amount in view. Or am I talking bullshine?

You're talking about exposure (a combination of ISO, aperture, and shutter speed). OP is asking about just aperture.

Aperture is the amount of light that is allowed through the lens. <-- the single thing the OP is asking about
ISO is the sensitivity of the sensor to the light.
Shutter speed is the length of time the sensor collects the light.

Exposure is the combination of all three, which results in an image (which is what you are talking about).

* To cover my bases, this is a forum, so I expect someone to get bent out of shape about wording (regarding ISO).

 

[thereyougo!2]

Surely in practical terms it will depend what is in your field of view. If I have a scene that has a dark foreground with the 16mm and using matrix metering it will meter differently to an 85mm field of view that is primarily in one part of the scene captured by the 16mm. To get the same exposure reading you'd need to move from the position you wee in with one to get the same amount in view. Or am I talking bullshine?

You're talking about exposure (a combination of ISO, aperture, and shutter speed). OP is asking about just aperture.

Aperture is the amount of light that is allowed through the lens. <-- the single thing the OP is asking about
ISO is the sensitivity of the sensor to the light.
Shutter speed is the length of time the sensor collects the light.

Exposure is the combination of all three, which results in an image (which is what you are talking about).

* To cover my bases, this is a forum, so I expect someone to get bent out of shape about wording (regarding ISO).

Not in his later reply:

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?

 

[Tim Tucker]

I think the blog is one of many where a case of equivalence gone mad has been diagnosed.

All this rubbish about equivalence and crop factors/total light collected has nothing to do with practical photography. A brief explanation of where it all originates:

It is in the mathematics of how we predict the behaviour of light, (and we can do it very accurately), that we cannot predict if an actual photon will behave or create a random event that you'd call noise on the sensor. But we can take a number of photons, (say 1,000,000), and predict exactly what percentage will create randoms events that we call noise.

That is all we can do. If you shine x amount of light on a sensor then y percent will generate the random events you see as noise.

So if you compare sensor performance it makes sense to do it at a point where they both produce the same amount of noise. And as noise is a percentage of total light it follows that if the percentage is the same then the total light hitting the sensor is the same.

You will of course be giving each sensor different exposures because exposure is the intensity of the light hitting each pixel, (light/mm sq), and not the total amount on the sensor.

The system of focal length/aperture diameter/f-stop/exposure/subject distance/field of view/dof is one where all of the above have a very symmetrical relationship to each other. Change one of them and another will change by an exact amount. Hold exposure constant and the relationships all behave in a nice predictable pattern. But if you hold total light as constant and make exposure variable then the pattern of relationships changes.

Now the system of practical photography is based on exposure being constant, it allows us to measure the intensity of light, (light per sq mm) because it's the light per sq/mm reacts with the 'pixels' on your chosen media. This produces the image which we control by knowing the exposure, or light per sq/mm, and not total light hitting the sensor.

All these 'crop factor' multiplies and equivalent f-stops are based on holding total light constant, and to do that exposure must vary across formats. You base your system of photography and understanding how to produce repeatable and predictable results on a system where exposure must vary, the very thing you must keep constant.

Which is complete idiocy.

EDIT: These symmetrical relationships work out extremely well. F-stop is the ratio between focal length in mm and the diameter of the effective aperture in mm. So for a 100mm lens at f2 the diameter of the effective aperture would be 100/2 or 50mm. A 50mm lens at f2 would have an effective aperture with a diameter of 25mm. There is a direct relationship between focal length and the area of the opening we call aperture. It is in the maths that this relationship exactly cancels out the extra light that a wide angle lens gathers from it's wider field of view. Therefore f2 on any lens on any format gives the same intensity of light (light/sq mm). It is the way the whole system is designed, around exposure being constant.

However there is also a symmetry that exists when you impose the conditions that total light and field of view remain constant. If you hold those two constant then you will find that to produce the same photo with the same dof across different formats then it is the diameter of the effective aperture in mm that remains constant and not exposure. Exposure must then vary across formats.

Now toss in the 'exposure triangle' where exposure is now suddenly variable and a 28mm f2.8 lens becomes a 35mm f4 or whatever, (even though it's not what they printed on the lens itself. There's not even a link to a 'Tony' youtube on the lens, so it must be part of the lies they're telling us... ). Of course this system of equivalence falls flat on it's face with any other type of media than DSLR and if you're not trying to take exactly the same photo as you would with the camera you don't have. Also to determine accurate exposure you must remember to divide your new corrected f-stop by the crop factor again, (or not multiplying by it in the first place. After all f-stop is a measure of exposure and never has been a measure of dof as that's also controlled by other factors).

Now I've used many different formats and I must admit that I've never used smaller formats to get the exact same results I would've if I'd used the camera I didn't bring with me and was not holding in my hands. Each format is a compromise and the bias between portability/hand held usage against image quality is slightly different in each. It was this way in film and always has been, you always played to the strengths of the format you were using and not the one you left at home.