Front element size and light gathering

[plassbakkallen]

Hi! First of all, I know this topic has been up before, and believe me, I have spent a lot of time searching and googling, but I cannot find an answer. My main question is:

When all else is equal (aperture, etc.), why doesn't a lens with a larger front element gather more light than one with a smaller?

Consider the following: I am photographing a flashlight that projects a cone of light towards my camera. All else equal, the lens with the larger front element would capture a greater proportion of the light cone coming from the flashlight, which, in my mind, would result in a brighter image.

I just cannot wrap my head around this, and I would be so grateful if someone could just explain this to me. Thanks for your time!

 

[Bitter Jeweler]

Because the intesity of the light is constant?

 

[480sparky]

If the front element is larger, then more light will fall upon it.

However, if the aperture is the same, it will only allow a certain amount of light to enter.


Think of it this way. I have a window on my house, and the side of my house is 10'x20'. Increasing the side of my house to 20'x40' is not going to get more light through the window.

 

[Edsport]

I'm confused with this also. A telescope that has a bigger lens lets in more light so why not a camera lens...

 

[Garbz]

I'm confused with this also. A telescope that has a bigger lens lets in more light so why not a camera lens...

A telescope which has a bigger lens does not have the same aperture as a telescope with a smaller lens. Conversely camera lenses magnify light through an iris (what is commonly referred to as aperture). The actual aperture number of a camera lens is the relationship between the front element and the iris opening, and not just the size of the element.

Thus a bigger telescope with a bigger mirror has a bigger aperture, but a bigger camera lens front element doesn't necessarily have to have a bigger aperture. That said if you do look at large aperture lenses such as the Canon 85mm f/1.2, you'll notice that large aperture lenses for the same sensor size typically have large front and rear elements, AND large irises too.

Canon EF 85mm f/1.2 L II USM Lens Review

 

[Forkie]

I'm confused with this also. A telescope that has a bigger lens lets in more light so why not a camera lens...

As other's are saying, the camera lens has an aperture (a hole which is smaller than the front element) between the front element and the sensor which only lets a certain amount of that light through. A telescope doesn't have this hole in the way, so all the light goes directly from the mirror to your eye (which stands in as the sensor in this case).

 

[480sparky]

F/stops, in camera optics, is the mathematical relationship between the focal length of the lens and it's internal diameter. The f/stop numbers we're all familiar with (f/2, f/4, f/8) are the results of the equation f=lens focal length/lens aperture.

A 50mm lens that measures a maximum of 25mm inside is 50/25, or f/2. Stop the blades down so they measure 12mm, and it becomes an f/4 (50/12) lens. Stop the blades down to 6mm and you have an f/8 (50/8) lens.

If increasing the size of the front element does not change the focal length of the lens, then the same amount of light will pass through. Assuming I'm photographing a scene that is evenly lit, it would make no difference in exposure what the size of the front element is on the lens. And changing lenses will not affect exposure, either....... using a lens with a 3" front element and using, say, f/11 at 1/125th to produce a proper exposure then changing to another lens with a 4" front element will still require f/11 and 1/125th.

 

[reedshots]

The lens does not collect light as in a (night vision scope) it merely directs the available light to the sensor.

 

[480sparky]

Next time it rains, put an 8x8 baking pan outside next to a 9x12 cake pan.

When it's done raining, measure how deep the water is in each pan. I'll bet they're the same, despite the 9x12 having more water in it.

 

[Garbz]

Next time it rains, put an 8x8 baking pan outside next to a 9x12 cake pan.

When it's done raining, measure how deep the water is in each pan. I'll bet they're the same, despite the 9x12 having more water in it.

A confusing analogy. I'm not sure what you're trying to show, but I think this example would say that if you have two different area sensor sizes you need differently sized glass to capture the same amount of light?

 

[dxqcanada]

Think of it in terms of two funnels.

One funnel has a wide opening of 12" and the small opening of 1"
The second funnel has a wide opening of 6" and a small opening of 1"

Will both let the same amount of water through ?

 

[Garbz]

Will both let the same amount of water through ?

That's even worse, because now the answer will depend on the rate of the water.

 

[Edsport]

I'm confused with this also. A telescope that has a bigger lens lets in more light so why not a camera lens...

A telescope which has a bigger lens does not have the same aperture as a telescope with a smaller lens. Conversely camera lenses magnify light through an iris (what is commonly referred to as aperture). The actual aperture number of a camera lens is the relationship between the front element and the iris opening, and not just the size of the element.

Thus a bigger telescope with a bigger mirror has a bigger aperture, but a bigger camera lens front element doesn't necessarily have to have a bigger aperture. That said if you do look at large aperture lenses such as the Canon 85mm f/1.2, you'll notice that large aperture lenses for the same sensor size typically have large front and rear elements, AND large irises too.

Canon EF 85mm f/1.2 L II USM Lens Review

Check the photo i got from Meade website and you'll see that there are bigger lenses that has the same F stop so a "bigger telescope with a bigger mirror" don't mean it has a bigger aperture

perture.

I'm confused with this also. A telescope that has a bigger lens lets in more light so why not a camera lens...

As other's are saying, the camera lens has an aperture (a hole which is smaller than the front element) between the front element and the sensor which only lets a certain amount of that light through. A telescope doesn't have this hole in the way, so all the light goes directly from the mirror to your eye (which stands in as the sensor in this case).

A telescope has this so called hole, it has to collect the light and direct it to a certain focal point which would be the hole.

Next time it rains, put an 8x8 baking pan outside next to a 9x12 cake pan.

When it's done raining, measure how deep the water is in each pan. I'll bet they're the same, despite the 9x12 having more water in it.

You can measure the depth but if weigh the rain it will weigh different, the 9x12 collected more.

Think of it in terms of two funnels.

One funnel has a wide opening of 12" and the small opening of 1"
The second funnel has a wide opening of 6" and a small opening of 1"

Will both let the same amount of water through ?

More will go through the 12 inch one...

If it's hard to see the photo i posted above you can see it here. http://www.meade.com/product_pages/lx200_series/compare.php

 

[SNBniko]

...snip...

Think of it in terms of two funnels.

One funnel has a wide opening of 12" and the small opening of 1"
The second funnel has a wide opening of 6" and a small opening of 1"

Will both let the same amount of water through ?

More will go through the 12 inch one...

...snip...

False. You're assuming the there's more water in the 12" one, when that may or may not be the case. If there is equal water placed in both, the rate of water through the funnels will be equal. This is also assuming that the length of the little ends are the same, else the longer end will flow slower due to friction and potential turbulence.

Actually if you think about it, the water may flow faster through the 6" one. Smaller diameter, but same volume means the 'smaller' funnel will have to be taller. Taller = more of the water's weight from above pushing it out of the funnel.

And so says the chemical engineering student. /geek moment, apologies for derailing the thread.

 

[Garbz]

Check the photo i got from Meade website and you'll see that there are bigger lenses that has the same F stop so a "bigger telescope with a bigger mirror" don't mean it has a bigger aperture.

You've added an extra variable into the discussion. Look closely and you'll see each of the telescopes listed have a different focal length. The larger scope also has a longer focal length leaving the F stop the same.
The original question asked was why does a lens with a larger element all other things being equal not let in more light.

In a telescope if you keep the focal length identical but increase the size of element (aperture) you DO let more light in as there's no regulating qualities in a simple mirror reflex system. The F stop is a function of the size of the entrance pupil and focal length only. In a camera this is the same but the entrance pupil is not the diameter of the front element but rather something far more complicated involving the iris in the lens.