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Old 18-04-2008, 05:35   #1 (permalink)
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Basic question on lenses

I have 2 lenses for my canon 400d and I'm wondering how you can tell what the aperture range is on the lenses. I have the standard 18-55mm lens but can't see anything on it relating to aperture. I also have the 75-300mm lens which from what I can see has on it 1:4-5.6. Does this mean the aperture range is maximum 4 at 75mm and 5.6 at 300mm? What then would be the minimum aperture?
I am trying to do natrual light portraits and have been told that a large aperture lens works best. Something around 1.8 or 2.8 is better and would give me a faster shutter speed. If this is the case what would the best lens be to use for portraits of children who move pretty fast!!
The pieces of puzzle are slowly coming together....
Thanks for your help everyone.
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Old 18-04-2008, 07:28   #2 (permalink)
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Re: Basic question on lenses

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Originally Posted by mickeymax View Post
I have the standard 18-55mm lens but can't see anything on it relating to aperture. I also have the 75-300mm lens which from what I can see has on it 1:4-5.6. Does this mean the aperture range is maximum 4 at 75mm and 5.6 at 300mm? What then would be the minimum aperture?
Yup, if this is your lens Canon UK - EF 75-300mm f/4.0-5.6 III then min is F/32-45

and Canon UK - EF-S 18-55mm f/3.5-5.6

Quote:
Originally Posted by mickeymax View Post
I am trying to do natrual light portraits and have been told that a large aperture lens works best. Something around 1.8 or 2.8 is better and would give me a faster shutter speed. If this is the case what would the best lens be to use for portraits of children who move pretty fast!!
The most cost effective lens is the 50mm, Canon UK - EF 50mm f/1.8 II
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Old 18-04-2008, 09:58   #3 (permalink)
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Re: Basic question on lenses

I don't claim to exactly understand how all this works but the range of aperture on a zoom lens works as follows...

The 'f' number is calculated as the focal length of the lens divided by the diameter of the aperture opening. This gives us the following formulae, where f = focal length of the lens, D = diameter of the aperture, and N = the 'f stop' number.

N = f/D
D = f/N

So on the 18-55mm f3.5-f5.6 lens this gives you the following:

At 18mm

D = 18 / 3.5 = 5.1mm

At 55mm

D = 55 / 5.6 = 9.9mm

The bit that I don't understand is why a lens doesn't have the same maximum aperture diameter (D) throughout its range but that probably has to do with the mechanics of how the lens is built and operates - maybe someone else can enlighten us on that bit.

This also explains why you get less depth of field the longer the focal length of a lens - the aperture diameter required to achieve the same f-stop gets larger and larger the further out you go. It also explains why some lenses are bigger than others. For example a f/2.8 300mm lens would need an aperture diameter of almost 11cm

Of course it is possible I have completely misunderstood all of this, it has been a long while since I studied optics, so if someone wants to come along and correct me then please do so!

Put simply, if you want a really blurred background use a wide aperture and a long focal length for a really large aperture diameter and minimum depth of field. If you want really sharp pictures with lots of depth of field use a short focal length and a small aperture for a really small diameter.

An 18mm lens at f/22 has an aperture diameter of 0.8mm!

Hope this helps. (And that I haven't got too many of the details wrong.)

Niall

Last edited by Larne; 18-04-2008 at 10:01. Reason: Corrected
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Old 18-04-2008, 10:31   #4 (permalink)
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Re: Basic question on lenses

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Originally Posted by Larne View Post
The bit that I don't understand is why a lens doesn't have the same maximum aperture diameter (D) throughout its range but that probably has to do with the mechanics of how the lens is built and operates - maybe someone else can enlighten us on that bit.
Some zoom lenses do just that. For instance, the 70-200 f2.8 is f2.8 throughout the range, as is the 18-55 f2.8. Snag is that for this constant aperture, the physical size of the lens is quite big and they are heavy. The optics that are found in the lenses with variable apertures throughout the range are much smaller and lighter, so these lenses tend to be more compact and lighter.
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Old 18-04-2008, 10:40   #5 (permalink)
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Re: Basic question on lenses

It wasn't so much the concept of a fixed maximum f-stop throughout the range of the lens but a fixed maximum aperture diameter.

Taking the example above, if f/5.6 at 55mm is an aperture diameter of 9.9mm then why when the lens is at 18mm can we only get a max aperture diameter of 5.1mm. If the lens could maintain the 9.9mm maximum aperture diameter throughout its entire range then the lens would do a maximum aperture of f/1.8 at 18mm (18 / 9.9).

That's the bit that I don't understand - as I said above, I can only assume it is in some way dictated by the mechanics of the zoom lens.
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Old 18-04-2008, 11:50   #6 (permalink)
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Re: Basic question on lenses

Yes, partly the mechanics of the lens but even more because of the internal optics that you'd need to eliminate aberrations and edge focusing issues at 18mm f/1.8. Most lenses - especially the less expensive ones - don't perform at their best until stopped down one or two stops from maximum aperture, so wider apertures, even if possible in terms of iris diameter, would give very poor results.
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Old 18-04-2008, 12:07   #7 (permalink)
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Re: Basic question on lenses

So if I read this right what you are saying is that because at a long focal length we are not expecting to get super sharp pictures (due to the limited depth of field) the manufacturers are happy to use more of the lens, allowing a larger aperture diameter. At shorter focal lengths where the longer depth of field would be more likely to show up limitations around the edge of the lens they reduce the maximum aperture diameter to compensate.

Makes sense. Thanks for that.

On a related question - on some of my photographys in the corners lines (such as tree branches) have a red line down one side and a green one down the other side. (Is this called Chromatic Abberation?) Would this be reduced by using a smaller aperture and is the kind of thing that the manufacturers are trying to avoid? Or is this caused by something completely different?
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Old 18-04-2008, 13:17   #8 (permalink)
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Re: Basic question on lenses

The angle at which light is entering the camera at long focal lengths is much less than at wide angle, so you don't get the same internal refraction and diffraction problems. So, all other things being equal, it's easier for the optics to produce sharp images across the whole of the frame at longer focal lengths.

Wide angle is a severe test of the optics, especially with digital sensors which tend to be more susceptible to aberrations than film because of the physical characteristics of the sensor itself.

Yes, the red and green edges are one form of chromatic aberration (CA), caused by light of different wavelengths not focusing in exactly the same place on the sensor. Another form of CA is 'purple fringing' which is very common when shooting something like tree branches against a bright sky - especially if the sky is overexposed.

CA is mainly down to the lens, but some sensors can introduce it too. Better lenses have less of it. The easiest way to get rid of red/green fringing is by using a RAW converter or image editor that has a CA reduction feature. Basically this shifts the red and green channels progressively towards the edges of the frame, so they are in alignment. I've used the CA correction in Bibble Pro and Lightroom, and they are extremely effective - it's easy to eliminate R/G fringing altogether. Purple fringing can be more tricky, and sometimes the only way is to selectively desaturate purples/magentas.
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