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Aperture on lenses - Clarification requested.

Discussion in 'Photography Forums' started by RimBlock, Aug 19, 2005.

  1. RimBlock

    RimBlock
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    Well as another P&S to DSLR convert I have found myself a little confused over the 'F' number on lenses.

    Seeing as my main area of interest is protraits (full length) I have been advised to get a new lens from the stock 18mm-55mm on the Canon 350.

    From reading around I have found that the suggested size would be around 100mm allowing good full face without being in the models face.

    What is now confusing me is the lens specs of say the 28mm-135mm F3.5-5.6

    Ok I get aperture and the fact that the lower the F number the wider the aperture but when reading the specs for the lens it goes on with a minimum aperture of f/22 - f/36.

    Ok so is the F3.5-5.6 the aperture range of the lens over the full focal length or is it the minimum aperture for the top and bottom focal length or something else and where does the f/22 - f/36 come into it.

    Most of the books I have bought do not go into very much detail on new lenses and do not cover these points.

    Any clarification would be most helpful.

    Cheers
    RB
     
  2. MPK

    MPK
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    Well, I'm not an expert but to my understanding this lens has a maximum aperture value of F3.5 at a focal length of 28mm and a max of F5.6 at 135mm. Not sure what exactly happens inbetween, e.g. when you zoom to 50mm, but the max aperture value will somewhere inbetween 3.5 and 5.6. On the other hand you have of course the minimum aperture value, which is F22 at 28mm and F36 at 135mm.

    If you're looking at an ideal focal length of 100mm for portraits (don't know if that's optimal) then you first need to take into account that the 350D is not a full frame camera and has a focal length multiplier of 1.6x, i.e. if you're at 100mm in fact you have 160mm focal length as compared to a normal 35mm camera. This lens is therefore really a 45mm-216mm lens. Hence, if you want actual 100mm you need a focal length of around 63mm with the 350D. For portraits you want a lens with a low max. aperture value around that focal length for a narrow depth of field. The lens you mention will have around F4.0 around that. Canon has some L lenses with 2.8, but they are expensive. Also check out the new 24-105mm 4.0L that Canon is bringing out.
     
  3. RimBlock

    RimBlock
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    Ok,

    Thanks, that makes it a lot clearer.

    So at 28mm it has a min - max aperture of F22 -> F3.5 and at 135mm it has F36 - F5.6

    I can pick up one of these for around £300 and thought it would be a better bet than the 55mm - 200mm F4.5 - 5.6 cannon lens. It also has the image stabiliser.

    Compared to the EF 85mm f/1.2L USM at £1,200 and being that this is for a hobby rather than for a paying job I have to put a limit on my expenses. I am also looking at possibly another £1,000 on lighing for a dissused large bedroom (minus furniture) as a studio.

    Luckly the only thing that is saving me money is the models as my fiancee (who used to be a model in her native Taiwan) and her friend (who is currently studying in London) have both agreed to be my guinea pigs.... sorry, practice models :D

    Oh and have just found this Buying a lens link which looks promising so far.

    Thanks
    RB
     
  4. ASH1

    ASH1
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    If you have a 350D, you may want to consider the EF-S 60mm f/2.8 macro.

    I've not long bought one of these, and it makes an excellent portrait lens as it becomes a 96mm with the 1.6x factor taken into account.

    It's a very sharp lens, and if you had the cash back vouches book with your camera, you can get £35 back. :thumbsup:
     
  5. mattym

    mattym
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    another corking lens is the 50mm 1.8 prime, its around £60, which is an absolute bargain, and its so sharp! as soon as the sale stuff i have is gone then im getting one.
     
  6. rdhir

    rdhir
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    If you are buying a lens for portrait photography...

    I use a Nikon 85mm/F1.4 and a Nikon D70. I'm not saying this to start a Canon v Nikon, but to point out something important that's not discussed in the thread.

    Generally in portrait you want to focus on the subject. That means blurring the rest of the photograph. To do this you want a narrow depth of field such that the subject is sharp, but background is blurred. The way you do that is by having a wide aperture. So generally a 3.6/5.6 is not IMHO up to the mark. This is because you are looking at a zoom. Find a fixed length 85-100 its not that important, even with the focal length adjusters, but you can always try it out in a camera shop. See how far away you have to be to get the full length portrait, and remember the further away the background is the better. Here is an example. I know its not full length but it show the idea. It was taken with the above equipment at f1.8,1/8000s at the weekend.

    http://www.dhirs.net/images/GaetonSling.jpg

    Not sure if you can see it on the reduced image, but my focusing was less than perfect. There is some blurring round the edges as I was a little too tight.

    Cheers


    Rajiv
     
  7. RimBlock

    RimBlock
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    Thanks for the advice all.

    It also brings up another question. Can macro lenses be used for other things and if so why are they denoted as macro lenses. What is so special about them ?.

    I am off to Singapore and Thailand next month for around a month and wanted a good camera for a bit of scenery photography as well. Hopefully the 28mm-135mm will enable both especially with the inbuilt image stabiliser as my hands tend to shake sometimes (should give up the tobacco :( ).

    After I get back I may well go for a prime lens and possibly splash out on an L series lens.

    I now have my remote shutter control (wired), mini tripod and 420ex speedlite. Not gonna be much room in the suitcase for clothes at this rate :devil:

    Cheers
    RB
     
  8. kenlynch

    kenlynch
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    Macro lenses can be used for other things, the Canon 100mm for example has a reputation as a great portrait lens. The designation macro lens means that it is capable of creating an image on the sensor that is "life-size" or bigger.

    To clarify, say I wanted to take a picture of an insect and that insect was about 2cm, then on the 350D using a macro lens I could focus in and get it to fill the frame (the 350D sensor being just over 2cm wide).

    It is often confused with how close you can get to the subject and still have it in focus, but that is not the case, although you are usually close in on your subject when using a macro lens.
     
  9. ASH1

    ASH1
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    Here's a link for more info on the EF-S 60mm.
    http://www.canon.co.uk/for_home/pro...cro_lenses/ef-s_60mm_f2.8_macro_usm/index.asp

    I also have the 28-135 IS, it's a good walk about lens, but personally I would use my EF-S 60 for portraits. Until I can afford some L lenses that is. Now where did I put my lotto ticket. :D
     
  10. ancientgeek

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    The f number is simply the distance from the lens to the image divided by the (effective) lens diameter. For a single element lens it's pretty straightforward. And the amount of light that gets in is proportional to the area, hence a change from f2 to f4 represents a quarter of the light (half the diameter is quarter of the area). That's why there's f2.8 in between in the standard range of numbers.

    The opening in question for a multi-element lens is somewhere inside. For a 50mm f2 lens wide open, it would be 25mm diameter. at f22, the opening would be close to 2mm in diameter. To avoid vignetting, the front and rear elements are going to be somewhat bigger than 25mm across.

    If you've got a zoom lens, it's naturally going to have a bigger f number when you move to telephoto, unless the designers do something fairly clever.

    All those elements made out of different kinds of glass are to give the sharpest, flattest image possible, across the whole frame. For a normal lens, the subject is many times the focal length from the camera, and the designs are typically far from symetrical and don't need to cope with varying subject-lens-image geometery. But for a macro lens, the subject and image may vary down to equidistant from the lens, and the design is typically much more symetrical. The macro lens needs to stay sharp right out to infinity, and this means the designers sacrifice maximum aperture.

    A macro lens will often change its internal geometry at close focussing distances to achieve the highest resolution; it may automatically adjust the lens opening as you focus to keep constant exposure time so far as possible, and it may even alter its focal length (like a zoom lens) as you focus, so that it gives a larger reproduction range.
     
  11. dood

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    I use a 28-135 for its versatility and IS. However, for bokeh (narrow depth of field) especially with portraits you must consider the 85mm 1.8 and the 50mm 1.8 (best value for money lens you can buy) or if money is not an issue then the 50mm 1.4. 50mm is great for candid portraits in confined spaces, the 85 mm allows you to step back when you have the space. If you are going for L lenses then disregard what I've said, although I would think that the lenses I've suggested would be more in line with the camera you have chosen, budget wise.
     
  12. RimBlock

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    Well the 28 - 135mm came today and after attaching I took a quick shot of a bowl with leaves and stuff in, on manual and to my emense surprise the histogram showed the sort of curve I have been kicking myself trying to get.

    Ok I finally worked out how to sett the aperture on manual last night so was playing with the manual settings then but zooming into the bowl upto the max 135mm just surprised me that it seems to have worked. I'll look at the pic when I get home as the lens arrived as I was getting dressed so couldn't stop to play too much.

    The 60mm EF-S looks like the most likely option when I come back from holiday from what has been said here. The new lens will be used for everything else at the moment.

    Thanks for the advice guys.

    RB
     
  13. MPK

    MPK
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    Just bear in mind that EF-S lenses don't work with the more professional Canon cameras. It makes sense to buy lenses which you can keep if you decide to upgrade your camera at some point.
     

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