Aperture question... this might be a bit stupid

[skullfunkerry]

Apologies in advance for any facepalming that might occur as a result of reading this post. I just want to run something past you all...

Since the aperture is measured as "focal length divided by...", two lenses at the same F stop will have apertures that measure differently if they have different focal lengths, yes?

So for instance a 13mm at F/2.0 would have a physical gap in the iris with a diameter of 6.5mm, and a 35mm at F/2.0 would have an actual diameter of 17.5mm?

So therefore, assuming that the lens bodies were constructed similarly, in the above scenario the 35mm lens would need a shorter exposure time, as more light will get through a 17.5mm hole...

The reason I'm asking: I have a couple of things I want to shoot, involving low light. They're stationary, so focal length doesn't really matter as I can zoom with my feet, but there are leaves and branches so I want the shortest shutter speed I can get away with... therefore I should use my 35mm lens rather than 13mm, yes?

Or is there an additional factor coming into play here that I haven't thought about?

 

[Gav.]

I don't think it's going to shorten the exposure time.

The 35mm lens has a larger diameter to compensate for the extra light needed for the extra length of the lens.

I understand the f number is a standard measure of light through the lens.

PS, I'm just a tool and could be totally wrong..

 

[droj]

Forget about focal length. And aperture diameter. f/2 is f/2.

 

[Don Fischer]

Boy that is a lot of figuring just to figure out which lens to use. Suggestion, take a shot with each and see which one you like!

 

[DemiLion]

Forget about focal length. And aperture diameter. f/2 is f/2.

Except when it is on a different sized sensor.

 

[skullfunkerry]

So yeah, basically I'm overthinking it all, aren't I? Thanks everyone!

And @Don Fischer - yes, that was my plan anyway! I was just wondering about the ins and outs of it all, as I tend to do with everything in my life

 

[myotis]

Apologies in advance for any facepalming that might occur as a result of reading this post. I just want to run something past you all...

Since the aperture is measured as "focal length divided by...", two lenses at the same F stop will have apertures that measure differently if they have different focal lengths, yes?

So for instance a 13mm at F/2.0 would have a physical gap in the iris with a diameter of 6.5mm, and a 35mm at F/2.0 would have an actual diameter of 17.5mm?

So therefore, assuming that the lens bodies were constructed similarly, in the above scenario the 35mm lens would need a shorter exposure time, as more light will get through a 17.5mm hole...

The reason I'm asking: I have a couple of things I want to shoot, involving low light. They're stationary, so focal length doesn't really matter as I can zoom with my feet, but there are leaves and branches so I want the shortest shutter speed I can get away with... therefore I should use my 35mm lens rather than 13mm, yes?

Or is there an additional factor coming into play here that I haven't thought about?

It's as @Gav and @droj. said. The further the light has to travel through the lens (i.e. the longer the focal length) the greater the reduction in the intensity of light reaching each photo site on the sensor (google "inverse square law"), So the longer the focal length the bigger the physical opening needed to keep a consistent and known amount of light reaching the sensor, across different focal lengths.

So regardless of focal length. an f2 aperture will always allow the same amount of light to reach the sensor, because it's a measure of the ratio between the focal length and the size of the lens opening.


There are some subtleties to this but most of the time these can be ignored, If interested google "t-stops"

 

[Suvv]

A longer focal length has a smaller field of view, it sees less of the scene.
It therefore needs a larger diameter hole to get the same amount of light.
The f number calculation takes that into account, if you use the same f number and the same shutter duration on lenses with different focal lengths you will get the same exposure.

 

[wiltw]

f/number is the Denominator of a fraction, and it is the absolute diameter (measured in millimeters) is the fractional part of the FL So f/2 on 100mm lens is 50mm in diameter, while f/2 on 50mm lens is 25mm in diameter, but lll f/numbers of the same value (e.g. f/2) transmit effectively the same intensity of light at the focal plane, regardless of FL and regardless of the format size the lens is used for. So you can take a shot with 50mm f/2, then switch to 100mm f/2, and the subject brightness as recorded by film will be unchanged.

At one time, the light transmission of the lens optics could differ even at the same f/number, so the T/stop came into being. But with modern lenses the T/stop is superfluous and has largely goine out of existence.