Explain f and lenses to me :)

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Dale_tem

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Dale
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f is the value of how much light is entering the lens, but is it in relation to the focal length?

Why do some lenses stay constant and others change over their length?

Is 18mm f/4 going to let in the same amount of light as a lens at 300mm f/4?

If I take a photo at 300mm f/5.6 and I take a photo at 500mm f/5.6, will the light levels be the same?

Is it just the focal length that effects the f value or do other factors enter in? If I have a 18-55mm lens at 55mm at f/5.6 and a 55-300mm lens at 55mm f/5.6, will the light levels be the same?

Thanks in advance :)
 
the f value is the only thing that affects how much light enters a lens in any given time. After that the shutter will also affect how much light reaches the sensor, and the ISO affects how sensitive the sensor is to thae light that reaches it.

The focal length has no affect on the amount of light.

200mm at f/4 is the same as 500mm at f/4 in terms of how much light is going through the lens once it opens
 
18mm f4 will let in the same amout of light a lens at 500mm f4,
and some lenses are designed to have a fixed f number say f2.8 or f4, say a 70-200mm and when you get a constant f number that when prices of lens increase. on other lenses say a kit 18-55mm f3.5-5.6 is made like it is to keep cost down, any lens at 5.6 at any focal length will produce the same light levels.
 
Dale_d3100 said:
f is the value of how much light is entering the lens, but is it in relation to the focal length?
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Yes. The f number is the RATIO of focal length to aperture. f/5.6 means the aperture is 5.6x smaller than the focal length.

Dale_d3100 said:
Why do some lenses stay constant and others change over their length?
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Design. It is harder to make a zoom with constant max aperture as when the lens gets longer the aperture has to get bigger. You therefore have harder constraints on the optical elements.

Dale_d3100 said:
Is 18mm f/4 going to let in the same amount of light as a lens at 300mm f/4?
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Effectively, yes. Whilst the hole is smaller in the 18mm, it is gathering light from a wider angle.

Dale_d3100 said:
If I take a photo at 300mm f/5.6 and I take a photo at 500mm f/5.6, will the light levels be the same?
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Yes.

Dale_d3100 said:
Is it just the focal length that effects the f value or do other factors enter in? If I have a 18-55mm lens at 55mm at f/5.6 and a 55-300mm lens at 55mm f/5.6, will the light levels be the same?
Click to expand...
Yes and yes.
 
joescrivens said:
The focal length has no affect on the amount of light.

200mm at f/4 is the same as 500mm at f/4 in terms of how much light is going through the lens once it opens
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Wellll, yes and no.......

The f-number for a given aperture is arrived at by dividing the focal length of the lens by the actual size of the aperture the light is passing through, so whilst f/4 on both the lenses in your example will give the same results the actual, physical size of the "hole" through which the light passes will be different.
 
Cool, thanks for clearing it up :)

I thought it was relative to the focal distance.

so 18/4 = 4.5, but with a much wider field of view more light can enter the lens.

400/4 = 100, but the much narrower field of view limits the amount of light entering the lens.

How bis is the aperture opening ? I gather its not 4.5mm and 100mm as that would be rather large on the larger lens. Is there an extra number in the formula somewhere?



So it all somehow balances out :)
 
Flash In The Pan said:
Wellll, yes and no.......

The f-number for a given aperture is arrived at by dividing the focal length of the lens by the actual size of the aperture the light is passing through, so whilst f/4 on both the lenses in your example will give the same results the actual, physical size of the "hole" through which the light passes will be different.
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never had you down for someone so pedantic.:)
 
Dale_d3100 said:
Cool, thanks for clearing it up :)

I thought it was relative to the focal distance.
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the actual physical amount that the aperture opens. i.e. if you measured the hole does. but the result is the same aperture. You don't need to get confused by how the focal length changes it.

Just know that if you shoot something at f/4 at 50mm and you shoot something at f/4 at 400mm the result in terms of exposure will be the same.

keep it simple!
 
Just to throw a spanner in the works ... :D

"T stop"

A lens with more elements will transmit less light to the film or sensor.

A while back a poster on here was doing some research into "t stop". Using auto ISO, I set my 200-400/4 at 300mm,set the aperture and shutter speed, and recorded the ISO needed on a subject. Switching to my 300/4 keeping the same aperture and shutter speed I again recorded the ISO needed on the same subject.

The results showed that there was around 2/3rds of a stop difference between the prime and the zoom, the latter recording a higher ISO.

Not surprising considering the 300/4 has 10 elements, and the 200-400 has 24, so a lot more glass to for the light to pass through.

It should be noted that "t stop" is more commonly used in cinematography rather than photography.
 
Martyn... said:
A while back a poster on here was doing some research into "t stop". Using auto ISO, I set my 200-400/4 at 300mm,set the aperture and shutter speed, and recorded the ISO needed on a subject. Switching to my 300/4 keeping the same aperture and shutter speed I again recorded the ISO needed on the same subject.

The results showed that there was around 2/3rds of a stop difference between the prime and the zoom, the latter recording a higher ISO.
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So, who will be the first to bring out an individual lens exposure compensation feature a la microadjust.
 
arad85 said:
So, who will be the first to bring out an individual lens exposure compensation feature a la microadjust.
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In real terms the difference is negligible, and until the chap asked for people with zoom / prime to carry out some tests, it was something I had not considered.

With cameras now capable of producing good results at high ISO settings it would not have any noticeable effect on images.

The only comment I would add is that it obviously effects auto focus as the more light that gets through the better, so in dull light I am better off with the prime than the zoom.
 
Dale_d3100 said:
I thought it was relative to the focal distance.

so 18/4 = 4.5, but with a much wider field of view more light can enter the lens.

400/4 = 100, but the much narrower field of view limits the amount of light entering the lens.

How bis is the aperture opening ? I gather its not 4.5mm and 100mm as that would be rather large on the larger lens. Is there an extra number in the formula somewhere?
Click to expand...
That's exactly right.

There isn't an extra number in the formula, and 400mm f/4 DOES require an aperture that's 100mm across. Have you ever handled a 400mm f/4 lens? They are quite big. Here's one.

302921_10150301569361794_162459801793_8093046_1142392370_n.jpg
 
Where abouts is the aperture in the lens? I believe it to be at the back of the lens near the body, but it doesn't seem wide enough at that point to have a 10mm diameter.
 
Dale_d3100 said:
Where abouts is the aperture in the lens? I believe it to be at the back of the lens near the body, but it doesn't seem wide enough at that point to have a 10mm diameter.
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The aperture that's implied by the f-number is actually something called the entrance pupil, which is the image of the actual physical aperture opening as seen through the front of the lens. It's a bit difficult to get your head around. This Wikipedia article does as good a job as I've seen of explaining it.
 
Dale_d3100 said:
Where abouts is the aperture in the lens? I believe it to be at the back of the lens near the body, but it doesn't seem wide enough at that point to have a 10mm diameter.
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That's because formula focal length/aperture diameter only works for single lens design.

For multiple lens design, it gets much more complicated. But in terms of light it still the same - A loupe of 50mm focal length and 25mm diameter diameter lets in the same ammount of light as 50mm F2 lens attached to your camera.
 
Martyn... said:
In real terms the difference is negligible, and until the chap asked for people with zoom / prime to carry out some tests, it was something I had not considered.

With cameras now capable of producing good results at high ISO settings it would not have any noticeable effect on images.
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I was thinking of making sure the metering is exposing correctly. A per-lens exposure compensation would mean you would get exposure lifted to a nominal "correct" point without the user needing to remember to change exposure compensation when changing lenses.
 
StewartR said:
The aperture that's implied by the f-number is actually something called the entrance pupil, which is the image of the actual physical aperture opening as seen through the front of the lens. It's a bit difficult to get your head around. This Wikipedia article does as good a job as I've seen of explaining it.
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I think that makes more sense. I thought a f/2.8 at 400mm would be TOOO BIG and didn't make sense.

The aperture is recorded at a virtual point inside the lens based on the field of view and can move forwards or backwards based on the focal length. Its the lenses in the lens that modify this image through the physical aperture and on to the sensor
 
arad85 said:
I was thinking of making sure the metering is exposing correctly. A per-lens exposure compensation would mean you would get exposure lifted to a nominal "correct" point without the user needing to remember to change exposure compensation when changing lenses.
Click to expand...

The camera does that anyway as it meters the actual light transmitted to it's sensors through whichever lens is attached.
 
Dale_d3100 said:
I think that makes more sense. I thought a f/2.8 at 400mm would be TOOO BIG and didn't make sense.
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You've never handled a 400mm f/2.8, have you? They're HUGE. I've just measured the front element of one and it's 151mm in diameter.

By contrast, the calculation of 400/2.8 gives the size of the entrance pupil as 143mm. I accept that, depending on the details of the lens design, a 143mm entrance pupil might not necessarily require a front element that's that big - but in practice it does seem to.
 
Martyn... said:
The camera does that anyway as it meters the actual light transmitted to it's sensors through whichever lens is attached.
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Doh! Very true ;)
 
StewartR said:
You've never handled a 400mm f/2.8, have you? They're HUGE. I've just measured the front element of one and it's 151mm in diameter.

By contrast, the calculation of 400/2.8 gives the size of the entrance pupil as 143mm. I accept that, depending on the details of the lens design, a 143mm entrance pupil might not necessarily require a front element that's that big - but in practice it does seem to.
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I always expect the front glass to be bigger then the aperture, but I thought the aperture was near the back, so 143mm at the back of the lens would be huge!
 
Dale_d3100 said:
I always expect the front glass to be bigger then the aperture, but I thought the aperture was near the back, so 143mm at the back of the lens would be huge!
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I think what Stewart is saying is that as you look at the front of the lens, the aperture would appear to be 143mm in diameter... The link to the wiki page on the entrance pupil has two photos of a 50mm f/1.8 lens - one stopped down, the other wide open, and the caption says:

A camera lens adjusted for large and small aperture. The entrance pupil is the image of the physical aperture, as seen through the front of the lens. The size and location may differ from those of the physical aperture, due to magnification by the lens.
Click to expand...
 
Catdaddy said:
I think what Stewart is saying is that as you look at the front of the lens, the aperture would appear to be 143mm in diameter... The link to the wiki page on the entrance pupil has two photos of a 50mm f/1.8 lens - one stopped down, the other wide open, and the caption says:
Click to expand...

143mm is the 400 divided by 2.8.

The caption says that if you look through the front of the lens and measure the aperture opening, that is the f value. That is not actually how big the aperture is as the view has been enlarged. The physical aperture will be a different size.

So if Stewart sets the aperture wide open and measures the aperture it would appear to be 143mm. if he however took an angle grinder to the lens and measured the actual aperture, it might be only 50mm.

Yay I get it :)
 
Me too - I never knew any of that before!

That being the case, on the expensive glass, I guess there must be another set of enlarging elements between the zoom elements and the physical aperture to ensure that the entrance pupil's f number remains constant throughout a zoom's range... I must admit, I've never given it much thought before, but now I'm intrigued...
 
lol

I like to understand things :)
 
Catdaddy said:
Me too - I never knew any of that before!

That being the case, on the expensive glass, I guess there must be another set of enlarging elements between the zoom elements and the physical aperture to ensure that the entrance pupil's f number remains constant throughout a zoom's range... I must admit, I've never given it much thought before, but now I'm intrigued...
Click to expand...

take a look here for some explanation of how that works

http://www.talkphotography.co.uk/forums/showthread.php?t=254794
 
Cool... thanks Joe
 
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