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Old Oct 12, 2010, 12:36 PM   #1
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Default Aperture confusion

Can someone explain aperture to me with a heavy emphasis on the technical 'why' it does what it does.

What I think I understand so far:

1/ A lower f-number is a wider aperture (or larger hole)
- OK

2/ A wider aperture lets in more light so the image is brighter
- OK, makes sense

3/ A wider aperture gives a shallower DoF
- This is where I'm struggling a bit. I've drawn my thinking below - that the wider aperture allows light from a wider 'cone' to reach each pixel which gives a shallower DoF


I think this is the right way to think about it but suggests that the area over which focus is perfect is zero in all cases - it's just that it's tolerably blurred over a further distance when you use a smaller aperture?

4/ Zooming/Macros
When you zoom, the camera has to reduce the aperture to avoid vignetting. Getting this slightly wrong is why some cameras show vignetting when zoomed. Makes some sense. This can be useful as it therefore increases the DoF which gets more of the object in focus but it would also work just as well if I reduced the aperture myself.

If I've got that all backwards then please shout.
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Old Oct 12, 2010, 1:13 PM   #2
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You have the first three entries right.

As for what zoom lenses do with aperture - you can find zooms that do all kinds of things. Depends on how they are designed, how well they are manufactured, ... The safest thing is to make no assumptions.
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Old Oct 12, 2010, 2:04 PM   #3
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Quote:
Originally Posted by MartinSykes View Post
Can someone explain aperture to me with a heavy emphasis on the technical 'why' it does what it does.

What I think I understand so far:

1/ A lower f-number is a wider aperture (or larger hole)
- OK.
Right.

Quote:
Originally Posted by MartinSykes View Post
2/ A wider aperture lets in more light so the image is brighter
- OK, makes sense.
Right.

Quote:
Originally Posted by MartinSykes View Post
3/ A wider aperture gives a shallower DoF
- This is where I'm struggling a bit. I've drawn my thinking below - that the wider aperture allows light from a wider 'cone' to reach each pixel which gives a shallower DoF
...
I think this is the right way to think about it but suggests that the area over which focus is perfect is zero in all cases - it's just that it's tolerably blurred over a further distance when you use a smaller aperture?
That's not bad, though I don't think I'd word it that way. Try this one: http://en.wikipedia.org/wiki/Depth_of_field#Effect_of_lens_aperture

Quote:
Originally Posted by MartinSykes View Post
4/ Zooming/Macros
When you zoom, the camera has to reduce the aperture to avoid vignetting. Getting this slightly wrong is why some cameras show vignetting when zoomed. Makes some sense. This can be useful as it therefore increases the DoF which gets more of the object in focus but it would also work just as well if I reduced the aperture myself.
No. There are two seperate, yet related concepts at work here. Aperture is the size of the opening in the diaphragm. F-Number is the ratio of the focal length to the diameter of the aperture. For instance, a lens with a focal length of 100mm and a maximum aperture of 50mm has an f-number of f/2.0 (100mm / 50mm = 2.0.) As you reduce the aperture to, say, 25mm, the f-number increases to f/4.0. That's why the f-number is frequently expressed as the denominator, with the actual focal length (or just "f") as the numerator.

The terms "Aperture" and "F-Number" are frequently used interchangeably, but when you start delving into the concepts a little deeper, you need to recognize the distinction.

In the case of lenses with changeable focal length (zoom lenses), the maximum size of the aperture is limited to some number smaller than the actual barrel diameter, but the focal length can change. So a lens with a maximum aperture size of 25mm, but has a focal length that ranges from 100mm to 200mm, the maximum f-munber will change with the focal length. At 100mm, the f-number would be f/4.0, but at 200mm, the maximum f-number would be f/8.0. Note that this would be a 100-200/4.0-8.0 lens, and there aren't a lot of those around. That's because the diaphragm is positioned in the lens where the light path is already constricted, so the actual diameter of the diaphragm isn't its effective diameter in the optical system. That's why there are 70-300/4.0-5.6 lemses, where the maximum effective diameter of the diaphragm is 17.5mm at a focal length of 70mm, but 54mm at 300mm.
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Old Oct 12, 2010, 2:37 PM   #4
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The f-number does cause a lot of confusion. I find it easier to understand if I write it as f/number, this being the correct mathematical relationship. 'f' is the focal length of the lens, so the actual aperture changes with focal length for a given number. F/8 is a different size hole for a 400mm lens than it is for a 50mm lens. The effective aperture is just what the formula indicates: for the 400mm lens it 50mm, and for the 50mm lens it is 6.25mm (at f/8 for both lenses) Note that I used the term effective. That would be the size of the front opening, if that was where the aperture was located. If you look at the maximum aperture of a lens, and its focal length, you will get an idea of how large the objective lens diameter is. (usually a little bigger, but it can't be smaller) As you can see, the aperture, at a fixed f/number, changes with changes in focal length. This is the major reason you don't see too many fixed aperture zoom lenses - they get big, fast.
The actual aperture, in a camera, is closer to the camera end of the lens, and is placed near a focal node, in order to keep the size small. It could be only 1/4 the size of the effective aperture, leaving us a hole of 12.5mm for the 400mm lens and only 1.56mm for the 50mm lens.
Vignetting is, I believe due more to the lens having to create a flat image. The natural focal plane of a lens isn't a plane, but a curved surface. Getting it to focus on something like a brick wall, where the farther the bricks are from the centerline of the lens, the farther they are from that natural focal point, means bending the light more at the edges than the center. Difficult enough to do with a fixed focal length, worse with a zoom.

brian

I see TCav beat me to the punch. The reason we use the f/number so much, is that it is a representation of the amount of light the lens is going to let through. F/4 gives the same level of light on the sensor,for both the above lenses, making things easier, in conjunction with shutter speed, to determine exposure level.

Last edited by VTphotog; Oct 12, 2010 at 2:46 PM.
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Old Oct 12, 2010, 3:31 PM   #5
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Thanks guys. I think I get it. So in aperture priority mode it is not actually the size of the aperture that we are selecting directly but the ratio of focal length to the aperture. So as we zoom, if we can keep the ratio constant then we will keep the brightness of the image constant and the camera will do the necessary maths to adjust the physical aperture as necessary which is one less job for the photographer.
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Old Oct 12, 2010, 8:16 PM   #6
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It is normally the lens itself that does the changing. Even older, manual lenses, could be set to a given f/number, which would stay the same while zooming. Fancy things, lenses.

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Old Oct 15, 2010, 1:37 AM   #7
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A point of clarification (I hope): You see an aperture value (f-stop) written as f/n. It is indeed a fraction, the ratio of focal length (f) to aperture/diaphragm diameter (n), with (f) always normalized to 1. Just as 1/2 is a bigger fraction than 1/4, so f/2 is a larger aperture than f/4.

And the aperture is measured at infinity focus. At closer focus, (n) effectively increases -- the diaphragm is further from the frame (film or sensor) so less light reaches it. (Exception: a very very few macro lenses are built with adjusting apertures that remain constant with focus.) There are formulae for this:
M = (TE - FL) / FL where M is magnification, TE is total extension, and FL is focal length; and

E = A * (M+1) where E is effective aperture, A is nominal (marked) aperture, and M is magnification.
Let's take a standard 50mm lens at infinity focus. Both the focal length and built-in extension are 50mm. Now put that on 150mm of extension (tubes and/or bellows). TE is 50+150 = 200mm. Magnification is (200-50)/50 = 3x. Now set the lens to f/8. Effective aperture is 8*(3+1) = f/32.

The moral: magnification eats light.
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