# Front element size and light gathering



## plassbakkallen (Mar 31, 2011)

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 (Mar 31, 2011)

Because the intesity of the light is constant?


----------



## 480sparky (Mar 31, 2011)

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 (Apr 1, 2011)

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


----------



## Garbz (Apr 1, 2011)

Edsport said:


> 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 (Apr 1, 2011)

Edsport said:


> 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 (Apr 1, 2011)

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 (Apr 1, 2011)

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


----------



## 480sparky (Apr 1, 2011)

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 (Apr 2, 2011)

480sparky said:


> 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 (Apr 2, 2011)

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 (Apr 3, 2011)

dxqcanada said:


> 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 (Apr 3, 2011)

Garbz said:


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


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.




Forkie said:


> Edsport said:
> 
> 
> > 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 has this so called hole, it has to collect the light and direct it to a certain focal point which would be the hole.



480sparky said:


> 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.



dxqcanada said:


> 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"
> ...


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 (Apr 3, 2011)

Edsport said:


> ...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 (Apr 4, 2011)

Edsport said:


> 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.


----------



## Edsport (Apr 4, 2011)

SNBniko said:


> Edsport said:
> 
> 
> > ...snip...
> ...


 
The 12 inch funnel will collect more water than the 6. Set it up and test it and you'll see it's not false. We are talking about collecting rain correct? If your're not talking about collecting rain than you should state what the heck the funnel is collecting...


----------



## Crystal (Apr 4, 2011)

Well I wasn't confused until I started reading everyone's explanation..... Now my head hurts & I'm more confused than ever :meh:


----------



## 480sparky (Apr 4, 2011)

It's not how big the glass is out front.... it's the internal diameter of the lens (aperture) that is important.

That's why manufacturers tout the maximum aperture (f/1.8, f/3.5, etc....) as opposed to the diameter of the front glass.


----------



## SNBniko (Apr 4, 2011)

Edsport said:


> SNBniko said:
> 
> 
> > Edsport said:
> ...


 
I thought we were collecting rainwater in the cookie pans.  God dammit.  hahaha


----------



## SNBniko (Apr 4, 2011)

dxqcanada said:


> 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"
> ...


 
No mention of rainwater, but I'm not sure if it's implied.  I am so confused now.  The answer to the above question though, assuming all else is the same and the only variable is the large end funnel diameter, the answer is yes.  They will.  (and neglecting a whole boatload of other things like slip and friction.  ha)


----------



## 480sparky (Apr 4, 2011)

SNBniko said:


> I thought we were collecting rainwater in the cookie pans.  God dammit.  hahaha



No, we're using kitchen funnels as lens caps.


----------



## SNBniko (Apr 4, 2011)

Well in that case, cookie pans may work better... you know, non-stick and all.  ha.


----------



## Edsport (Apr 4, 2011)

SNBniko said:


> dxqcanada said:
> 
> 
> > Think of it in terms of two funnels.
> ...


 Post # 10 talked of collecting rain then his post # 12 mentioned water so it's easy to assume he was talking of collecting rain as well but now i think everyone in this thread is confused.


----------



## Edsport (Apr 4, 2011)

480sparky said:


> It's not how big the glass is out front.... it's the internal diameter of the lens (aperture) that is important.
> 
> That's why manufacturers tout the maximum aperture (f/1.8, f/3.5, etc....) as opposed to the diameter of the front glass.



It's a known fact that a telescope that has a bigger lens with the same aperture of one that has a smaller lens will collect more light so the diameter of the aperture isn't the only factor that lets a certain amount of light through to the sensor...


----------



## 480sparky (Apr 4, 2011)

Edsport said:


> It's a known fact that a telescope that has a bigger lens with the same aperture of one that has a smaller lens will collect more light so the diameter of the aperture isn't the only factor that lets a certain amount of light through to the sensor...



A telescope with a larger lens will usually have a larger aperture to go with it.  They may have the same f/number, the the measurement of the apertures are different.  Increasing only the front lens size and not changing the aperture will not increase the brightness of the image seen in the eyepiece.......... unless you also increase the diameter of the aperture as well.  F/numbers are not units of measure, they are the result of a mathematical equation.


----------



## Helen B (Apr 4, 2011)

The f-number (or strictly the T-stop) is the determining factor for image brightness. The diameter of the entrance pupil is the determining factor for light gathering (the focal length then comes into play to determine the image brightness, because of its effect on magnification).

For two lenses of the same focal length and entrance pupil diameter (and hence the same f-number) but different front element diameters, the light gathering ability and image brightness will be the same.

The diameter of the front element is determined by the relative positions of the entrance pupil and front element*, and by the angle of view. Ideally the entrance pupil should be fully visible through the front element over the entire angle of view of the lens. This means that the closer the front element is to the entrance pupil, the smaller it can be; and the narrower the angle of view the smaller the front element can be when compared to the entrance pupil. There are telephoto designs for which the entrance pupil almost fills the front element, and wide angle designs for which the front element is very large compared to the entrance pupil.

*There are cases where the front element is also the entrance pupil (simple lenses, telescopes), thus the size of the front element determines the entrance pupil diameter.

Best,
Helen


----------



## Edsport (Apr 5, 2011)

480sparky said:


> Edsport said:
> 
> 
> > It's a known fact that a telescope that has a bigger lens with the same aperture of one that has a smaller lens will collect more light so the diameter of the aperture isn't the only factor that lets a certain amount of light through to the sensor...
> ...


When talking telesecopes the lens is the aperture. If a telescope has 10 inch aperture that means it has a 10 inch lens so as you say a larger lens will usually have a larger aperture isn't quite right, a larger lens do mean a larger aperture because the lens is the aperture...

Copied and pasted below from Telescope Science: All About Telescope Aperture

Aperture is the size of the main optical lens of the telescope. It can be specified either in inches or millimeters. &#8220;Light Grasp&#8221; which is how much light an optical system can pull is a function of the area of the aperture, so that, &#8220;Light Grasp&#8221; goes up and down in squares. 
For example, the Light Grasp of a 50mm pair of binoculars is 51, that is each ocular is 51 times brighter than you can see with the naked eye. This is most apparent at night or in low light situations. A 100mm binocular will pull about 4 times more light with a light grasp value of 204.
Light grasp can be calculated by the formula (A/38.495) where A is the area of the objective lens in square millimeters.
Little jump in aperture are significant jumps in light grasp.
end of paste.

A telescope and binoculars lens size matters to collecting light so why wouldn't a camera lens that has a bigger diameter collect more light than a smaller lens? It should be a simple answer instead of a confusing one...


----------



## jake337 (Apr 5, 2011)

Edsport said:


> 480sparky said:
> 
> 
> > Edsport said:
> ...



Read helen b.'s statement above yours.  Because we are not using binoculars or telescopes.  What we use has a different design then both.


----------



## Edsport (Apr 5, 2011)

Oh a different design. OKKKK now i fully understand and here i was confused...


----------



## table1349 (Apr 5, 2011)

Edsport said:


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



Well you can, it just ends up looking like this.



:lmao::lmao::lmao:


----------



## dxqcanada (Apr 5, 2011)

Helen B said:


> The f-number (or strictly the T-stop) is the determining factor for image brightness. The diameter of the entrance pupil is the determining factor for light gathering (the focal length then comes into play to determine the image brightness, because of its effect on magnification).
> 
> For two lenses of the same focal length and entrance pupil diameter (and hence the same f-number) but different front element diameters, the light gathering ability and image brightness will be the same.
> 
> ...



I agree.

That's what I was trying to say with my funnel question ... though I don't think it worked :thumbup:


----------



## 480sparky (Apr 5, 2011)

gryphonslair99 said:


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


 

Pffft.


----------



## reedshots (Apr 5, 2011)

If you have a 12" funnel with a 1" small end and a 6" funnel with a 1" small end.
Take the 12" small end and put it in front of your left eye.
Take the 6" funnel small end put it in front of your right eye.

If they are equaly bright then it would stand to reason that a Camera front glass lens of 77mm F2.8 would not be any brighter than a 58mm F2.8 lens as the sensor sees it.  
The light intensity will be the same, but on the other hand if you use a wide angle (say) 10mm outside day time shot at F8 and use a 200mm at F8 same center target. (much reduced field of view) 
This is when you could say the wide angle lens sees more light there for you have to slow down the shutter speed with the 200mm, not because it is not getting the same intensity of light, but because it is not getting as much reflected light.


----------



## Garbz (Apr 6, 2011)

reedshots said:


> If they are equaly bright then it would stand to reason that a Camera front glass lens of 77mm F2.8 would not be any brighter than a 58mm F2.8 lens as the sensor sees it.


 
I can see where you were trying to go with this but it is a somewhat false example as when you look through the funnel there's no adjustment optics. As you look through a funnel nothing outside that 1" area is contributing to the image, however at widest aperture all of the 77mm or 58mm of the front element would usually contribute something to the image.


----------



## FunkyRider (Aug 23, 2011)

I've been puzzled by the same thing, but this website seems to explain it pretty clearly:

*Misconception 10: When a lens is used at a small aperture, only a small part of the front lens is used.*
When a lens is set to a small aperture, each image point utilizes a small part of the front element. That part has an area that approximately equals that of the lens entrance pupil. However, different image points depend on different parts of the front lens. The image center uses a small part around the optical axis, other areas of the image use portions that are displaced from the axis. In total a substantial area of the front lens is needed to illuminate the entire image. It is true that the very margins of the front element are often sidelined at small apertures, the more so as the focal length increases.

(Misconceptions in photographic optics)

Have a thought about it, hold a lens in your hand, when viewed from different angles in front, you will see the aperture opening via Different part of the front element, that's why bigger front element doesn't give you more light, it only spreads the light further apart. However, one good thing a large front element might provide is less Optical vignetting, because when viewed from far corners of the lens, front element does not block the aperture as much as a lens when smaller front element.


----------

