Faster lens are brighter...BUT HOW? :O


devilry

New Member
Feb 16, 2006
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#2
Can anyone explain this phenomena?
To understand this, its very simple. You take 2 lens:

a) Your kit lens 18-55mm F3.5-5.6.
b) 50mm F1.8

1st Step: You mount ur kit lens 18-55mm F3.5-5.6 onto ur camera.
2nd Step: Zoom ur kit lens all the way to 55mm. The apeture of the lens is now at F5.6.
3rd Step: Look through ur camera viewfinder. You should find that it looks quite dark.
4th Step: Remove ur kit lens 18-55mm F3.5-5.6 & mount ur 50mm F1.8 lens
5th Step: Look through ur camera viewfinder. You should find that it looks much brighter.

This is the meaning of Faster lens are brighter.

Faster Lens = the apeture of the lens is very big to allow tonnes of light to fall through = brighter view in the viewfinder.

Now you get it?
 

sinned79

Senior Member
Jun 18, 2009
10,868
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www.aboutlove.sg
#3
this will speed up your understanding.



peep at the hole and imagine you are light entering this hole.

small hole = ?
big hole = ?
 

JJLoke

New Member
Aug 2, 2009
177
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#7
F stop = focal length / aperture diameter
 

Oct 13, 2009
61
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#9
This is on true if same exposure time is applied.
 

#11
To understand this, its very simple. You take 2 lens:

a) Your kit lens 18-55mm F3.5-5.6.
b) 50mm F1.8

1st Step: You mount ur kit lens 18-55mm F3.5-5.6 onto ur camera.
2nd Step: Zoom ur kit lens all the way to 55mm. The apeture of the lens is now at F5.6.
3rd Step: Look through ur camera viewfinder. You should find that it looks quite dark.
4th Step: Remove ur kit lens 18-55mm F3.5-5.6 & mount ur 50mm F1.8 lens
5th Step: Look through ur camera viewfinder. You should find that it looks much brighter.

This is the meaning of Faster lens are brighter.

Faster Lens = the apeture of the lens is very big to allow tonnes of light to fall through = brighter view in the viewfinder.

Now you get it?
mm so would it be the same if the 50 f1.8 is stopped down to f5.6 as with the kit lens at 55mm, f5.6?
 

Jamesf

New Member
Apr 1, 2008
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Island East
#15
mm so would it be the same if the 50 f1.8 is stopped down to f5.6 as with the kit lens at 55mm, f5.6?
The view finder is still as bright as f/1.8 for the 50mm prime eventhough you stop down to f/5.6. It will only affect the picture you took, not the amount of light entering lens for view finder or focusing. Correct me if i'm wrong.
 

CamInit

New Member
Nov 3, 2009
756
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#16
The view finder is still as bright as f/1.8 for the 50mm prime eventhough you stop down to f/5.6. It will only affect the picture you took, not the amount of light entering lens for view finder or focusing. Correct me if i'm wrong.
I don't think so. T-stops (not f-stops) for lens of different constructions are different. The number and type of lens elements will affect the amount of light reaching the view finder. Maybe not visually very significant, but it will be there.
 

ST1100

Senior Member
Jun 18, 2003
1,785
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Singapore, Bedok
#17
small hole = ?
big hole = ?
Sorry, but this is incorrect. Well, not entirely correct.

Consider a 300/2.8 versus a 50/1.0. By the bighole/smallhole logic, which would be brighter? The 300/2.8 has a physical aperture of 107mm. The 50/1 has an aperture of 50mm. So the bigger hole (107mm) is brighter? No. The 50/1 is brighter in the viewfinder, and by a full 3 stops. Again, a 300/2,8 and a 50/2.8 have the same brightness, despite the huge difference in physical aperture.

The viewfinder is brighter when the f-stop number is (numerically) smaller. This is independent of the actual physical size of the aperture.

The f-number is a *relationship* between the external aperture and the focal length. This relationship, or ratio, determines the brightness of the image, and counter-intuitively, is independent of aperture size. A f2.8 bright lens means that every point in the output image has a brightness of f2.8, no matter if the lens is 16/2.8 or 400/2.8.

So why is a faster lens brighter? It has to do with the amount of light (physical aperture size) per unit angle of view, which varies with focal length. A long lens, say 400mm, needs a lot of surface glass (143mm!) to collect enough light for f2.8 from a very small angle of view, while a wide angle lens, collecting light from more directions, need only 7.5mm of physical glass surface to achieve the same f2.8 brightness. This is best explained with diagrams, but i don't have one. Maybe you should just google it.
 

sinned79

Senior Member
Jun 18, 2009
10,868
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Singapore
www.aboutlove.sg
#18
Sorry, but this is incorrect. Well, not entirely correct.

Consider a 300/2.8 versus a 50/1.0. By the bighole/smallhole logic, which would be brighter? The 300/2.8 has a physical aperture of 107mm. The 50/1 has an aperture of 50mm. So the bigger hole (107mm) is brighter? No. The 50/1 is brighter in the viewfinder, and by a full 3 stops. Again, a 300/2,8 and a 50/2.8 have the same brightness, despite the huge difference in physical aperture.

The viewfinder is brighter when the f-stop number is (numerically) smaller. This is independent of the actual physical size of the aperture.

The f-number is a *relationship* between the external aperture and the focal length. This relationship, or ratio, determines the brightness of the image, and counter-intuitively, is independent of aperture size. A f2.8 bright lens means that every point in the output image has a brightness of f2.8, no matter if the lens is 16/2.8 or 400/2.8.

So why is a faster lens brighter? It has to do with the amount of light (physical aperture size) per unit angle of view, which varies with focal length. A long lens, say 400mm, needs a lot of surface glass (143mm!) to collect enough light for f2.8 from a very small angle of view, while a wide angle lens, collecting light from more directions, need only 7.5mm of physical glass surface to achieve the same f2.8 brightness. This is best explained with diagrams, but i don't have one. Maybe you should just google it.
i think at this stage i dun want to go too technical. hence i give a very brief general idea for TS to ponder.

small hole = ? is actually to let TS thinks about it. small hole means less light and small hole means small aperture (eg f16 and above).

big hole =? is actually to let TS thinks about it. big hole means more light and big hole means wide aperture (eg f1.2,1.4 etc).

I wun want to bring in the focal length + aperture comparison, cos that will makes TS confuse further. So I am only talking about just aperture.
 

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ST1100

Senior Member
Jun 18, 2003
1,785
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0
Singapore, Bedok
#19
i think at this stage i dun want to go too technical. hence i give a very brief general idea for TS to ponder.

small hole = ? is actually to let TS thinks about it. small hole means less light and small hole means small aperture (eg f16 and above).

big hole =? is actually to let TS thinks about it. big hole means more light and big hole means wide aperture (eg f1.2,1.4 etc).

I wun want to bring in the focal length + aperture comparison, cos that will makes TS confuse further. So I am only talking about just aperture.
Understood, hopefully TS starts thinking about it a little deeper on his own.
 

giantcanopy

Senior Member
Feb 11, 2007
6,232
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0
SG
#20
The view finder is still as bright as f/1.8 for the 50mm prime eventhough you stop down to f/5.6. It will only affect the picture you took, not the amount of light entering lens for view finder or focusing. Correct me if i'm wrong.
yes for most of the slrs, the lens has a lever that the camera can mechanically/electronically stop down the lens aperture only while taking the photo or depressing the dof ( or the liveview i think at least for my canon )

In that way, you get the brightest possible image from the viewfinder to ease in shooting / composition.

I don't think so. T-stops (not f-stops) for lens of different constructions are different. The number and type of lens elements will affect the amount of light reaching the view finder. Maybe not visually very significant, but it will be there.
I think Jamesf was referring to the stop down aperture happening during the snap

Ryan
 

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