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FORUMS Canon Cameras, Lenses & Accessories Canon EF and EF-S Lenses 
Thread started 08 May 2011 (Sunday) 12:06
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Beginners questions on F stops with different lenses

 
jwhittaker
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May 08, 2011 12:06 |  #1

For different lenses, does F5.6 let the same amount of light through?
Eg. I have a Canon 50mm F1.8, and a Canon 15-85mm lens.
If I have the Canon 50mm at F5.6, and the Canon 15-85 at F5.6 will the same amount of light get through....
OR...
Is a faster lens (the Canon 50mm) always going to let more light through at the same aperture as the 15-85. If it does let more light through, the speed would be quicker or the iso lower etc.... or in other words the 50mm would take better pictures indoors.

Thanks.


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Sorarse
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May 08, 2011 12:27 |  #2

F 5.6 will be the same whichever lens you use. So if you have a meter reading that says with 100 ISO set and a shutter speed of 1/2125s you need f/8 for a correct exposure, you can put any lens on, set f/8 and know that the exposure will be correct.

The reason this is so is because the f/ number is a ratio; focal length/diameter of opening.


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May 08, 2011 12:36 |  #3

Sorarse wrote in post #12370559 (external link)
The reason this is so is because the f/ number is a ratio; focal length/diameter of opening.

I can definitely see how this could be confusing though, since 1/8th of a 500mm lens is quite a lot bigger than 1/8th of a 10mm lens.


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ekinnyc
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May 08, 2011 12:44 |  #4

if you have a constant aperture lens, ie. 24-105 f/4, it retains the same aperture through the zoom range. a lens like the EF-S 18-55 3.5-5.6 has a maximum aperture of 3.5 at the 18mm end, but as you zoom to the 55mm end, in order to retain the ratio, max aperture increases to 5.6

fixed aperture lenses are bigger partly because optics must move to retain the same constant f-stop ratio


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jwhittaker
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May 08, 2011 13:35 as a reply to  @ ekinnyc's post |  #5

Thanks for answers.
The reason I asked is when I first bought my camera I got some brilliant indoor shots with the 50mm 1.8 of my relaltives (who live a long way away).
I looked at the shot info and it said F 4.0, which is within the range of my new 15-85.
But I kept reading that the 15-85 wasn't that good indoors.
From what you are saying I should have little difficulty recreating the shots at F 4.0 with the new lens (using same ISO and time values).
If they weren't so far away I would pop round and try to recreate them but can't do it just yet.
Admittedly some of the shots on the 50mm were at F2.0 so I can't have it all my own way with the new 15-85, but as least I know I can get close.
My own indoor tests (not exhaustive) aren't that great with the 15-85, but it is a different house with different lighting.


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May 08, 2011 13:37 |  #6

jwhittaker wrote in post #12370861 (external link)
Thanks for answers.
The reason I asked is when I first bought my camera I got some brilliant indoor shots with the 50mm 1.8 of my relaltives (who live a long way away).
I looked at the shot info and it said F 4.0, which is within the range of my new 15-85.
But I kept reading that the 15-85 wasn't that good indoors.
From what you are saying I should have little difficulty recreating the shots at F 4.0 with the new lens (using same ISO and time values).
If they weren't so far away I would pop round and try to recreate them but can't do it just yet.
Admittedly some of the shots on the 50mm were at F2.0 so I can't have it all my own way with the new 15-85, but as least I know I can get close.
My own indoor tests (not exhaustive) aren't that great with the 15-85, but it is a different house.

Depends if you used flash, if they were moving, and how bright it was indoors. Can you get away with F4? Sure, and sometimes you NEED F4. However, if you used the F1.8 you could have used 2 stops lower ISO.


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ekinnyc
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May 08, 2011 14:21 |  #7

if you want a lens that performs well in low light and that isnt prime, look into the tamron 17-50 f/2.8 (VC or non-VC)... its an absolutely fantastic lens, and can be had for about $350 on the forums here. you lose 35mm on the tele end, but you gain a 2.8 constant aperture, which is a bliss in low light situations. you have to decide what is more important.

the 50 is a decent lens, but its AF isnt great in low light, and at shooting at 1.8, you lose alot of DOF


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jwhittaker
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May 08, 2011 15:57 |  #8

ekinnyc wrote in post #12371017 (external link)
if you want a lens that performs well in low light and that isnt prime, look into the tamron 17-50 f/2.8 (VC or non-VC)... its an absolutely fantastic lens, and can be had for about $350 on the forums here. you lose 35mm on the tele end, but you gain a 2.8 constant aperture, which is a bliss in low light situations. you have to decide what is more important.

the 50 is a decent lens, but its AF isnt great in low light, and at shooting at 1.8, you lose alot of DOF

Thanks for tip, yes I did get some shots with the end of the nose in focus only!


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May 08, 2011 15:58 |  #9

Nominally, f/8 allows the same amount of light through. In reality, sometimes there can be noticable difference in the amount of light tranmitted to the focal plane, because each of the air-glass transitions within a lens is not 100% effecicient in spite of coatings used to improve that transmission efficiency. That is why the motion picture industry used lenses with a 'T-stop' rating, which indicated the relative transmission efficiency, so that as FL changed the cinematographer could make the lens transitions not noticeable to a viewer who would be distracted by significant brightness changes in the scene whenever different FL lenses were used.


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Beginners questions on F stops with different lenses
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