Lens speeds

Discussion in 'Beyond the Basics' started by Xmetal, Jul 17, 2005.

  1. Xmetal

    Xmetal TPF Noob!

    Nov 22, 2004
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    Newcastle, Australia
    Can others edit my Photos:
    Photos OK to edit
    Lens Speed - What is it? All I know is that the faster the lens the more expensive it is and i'd like to know why. Is it because of different optics in the lenses themselves? are Prime lenses faster than Zoom lenses?
    Are there any scales or diagrams i can see that would give me a better idea?
    *Confused* :grumpy:

  2. Hertz van Rental

    Hertz van Rental We're supposed to post photos?

    Nov 8, 2004
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    Where am I now?
    The 'speed' of a lens is all to do with the size of the chunk of glass on the front.
    The larger the front element, the larger the maximum aperture, the more light it gathers, the faster the shutter speed you can shoot with in a given situation. Hence 'fast lens'.
    The big lens element on the front means more optical glass and the more exacting the manufacturing process in order to get high quality. Some of the other lens elements tend to be bigger as well. It all adds to the cost.
    25 years ago a fast lens was outrageously expensive. Computer controlled manufacture should mean that faster lenses are relatively cheaper these days - look how much contact lenses have come down in price in the past decade.
    But there is still a lot of raw material in there so they are more expensive.
    They sure look good but you rarely shoot at full aperture so is the 1 stop difference worth the price hike? Only you can decide,
    As for zooms - they are trying to be several lenses in one. As you work the zoom control various lens elements and groups of elements move around. Even in this day of CAD/CAM you get slight inaccuracies so zooms are never quite as good as a good prime. But they are a damn site better than they used to be and for general purpose work they are fine.
    The maximum aperture on a zoom changes depending upon the focal length set.
    On primes f8 on a 50mm lens has a hole approximately half the diameter of f8 on a 100mm lens.
    f-numbers are calculated by dividing aperture diameter into focal length. This gives a constant and ensures that the amount of light transmitted by f-8 on a 50mm is the same as the amount of light transmitted by f-8 on a 200mm lens, and on every other focal length.
    On a zoom if the diameter of the aperture stays the same you can see that the f-number must change as the focal length does. So the maximum aperture at 25mm must necessarily be different at 75mm.
    That is why zoom lenses have a range of max apertures marked on the front eg: f2.4 - 6.
    f2.4 would be the widest aperture at the shortest focal length and this would change to a max of f6 at the longest focal length.
    The hole size stays the same, it's the ratio that changes.
    And if you want to know the science behind all this, look up the Inverse Square Law.
  3. ThatCameraThingy

    ThatCameraThingy TPF Noob!

    Jun 11, 2005
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    Bloemfontein, South Africa

    let'see if I can help you out here. If i look at one of my lenses it says "28-105mm 1:4.5-5.6"

    This means that my lens will have a maximum(largest hole) apperture at 28mm (F4.5) and will have a maximum of F5.6 at 105mm.

    a fast lens ie. 28-105 1:2.8 would have a maximum apperture of f2.8 all round. this would mean that for the same ISO i would be able to select a faster shutter speed for the same amount of light.

    because it needs to have a larger 'hole' fast lenses are normally much larger than the normal lenses. The longer the zoom lens , the larger the front element must be. this uses more optics , stronger construction, etc etc and in the end more money from you the consumer.

    since prime lenses needs to work on one 'zoom' (say 50mm) setting only they can be made with front lens elements that are just big enough to allow say f2.8. this means that they are normally smaller than say a 28-70 F2.8 wich would include the 50mm.

    also , because the fast lens lets in more light it will also allow the autofocus system more light to focus with , and thus be quicker, and will also reward you with a brighter viewvinder image.

    hope all of this makes sense.

  4. wharrison

    wharrison TPF Noob!

    Jun 8, 2005
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    Northwestern Michigan


    I will just add to the information:

    1. There is a mathematical relationship between the lens aperture (opening) and the focal length of the lens. For example, an 50mm F/2.0 designates that the maximum aperture (lens opening) is 25mm. If you that same lens at an aperture of F/16, it indicates that the lens opening is approximately 3+mm; a 100mm F/4 lens would, therefore, have a maximum lens opening of 25mm;

    2. Depth of field tables are based upon two factors: (1) the aperture of the lens and the ratio of reproduction - which is a ratio between the area of the film, i.e. 1 x 1.5 inches or more precisely 24 X 36mm. Therefore, if you are photographing an area that is 10 times the size of the 35mm negative, it is expressed as a ratio of reproduction of 1:10; if you're film is covering the same area as the film, the ratio of reproduction is 1:1.

    Therefore, all lenses exhibit the same depth of field provided they are used at the same lens opening and at the same ratio of reproduction. Consult some of the older (and still better) books on photography, such as The Pentax Way or The Leica Way - each has depth of field tables in the back of the book.

    Depth of field is based upon the fact that our eyes do always "see" so well. The eye can "see" a circle as a dot if the diameter of that dot is less than 1/100 of an inch in diameter on a 7 X 10 or 8 X 10 print held approximately 10 inches away.

    In order for that "dot" or "circle of confusion" (its actual name) to be less than 1/100 of an inch on an 8 X 10 print held 10 inches away, that dot or "circle of confusion" has to be less than 1/800 of an inch on the film/negative.

    Therefore, accurate focusing is very important, especially under available light conditions. If you're focus is off, you will be actually enlarging an unsharp image on the negative/film, which will become progressively unsharp the greater the enlargement.

    3. Obviously, a 50mm F/1.0 lens will transmit nearly twice as much light as a 50mm F/2.0 lens (50mm vs 25mm). One of the reasons to purchased high speed lens is to gain greater transmission of light through the lens to your eye so that you can (hopefully) focus more accurately (and more rapidly) under available light conditions.

    4. That factor doesn't make the 50mm F/1.0 lens "better" or "superior" in terms of image quality, because that is dependent upon the type of glass employed in lens construction; the overall design and purpose of the lens, etc. The 50mm F/1.0 M (rangefinder lens) Noctilux by Leica/Leitz is a specially designed lens for available light photography; whereas a 50mm F/1.4 M (rangefinder lens) Summilux or a 50mm M F/2.0 (rangefinder lens) Summicron might be entirely suitable for most of your photographic endeavors.

    A few decades ago, when I attended the week long Honeywell Photographic Technical Seminar, the three lecturers knew that 50mm lenses on SLR camera had to be designed a little differently due to the fact of the greater distance between the lens flange and the film plane - in order to allow for the movement of the mirror. They were also aware of the fact that this "retro-focus" design lead to a slight "barrel" distortation whereby straight lines at the edge of a photograph would be slightly curved.

    So they did a test with several 50mm lens including those made by Leica (rangefinder and SLR) nikon, pentax, canon, olympus, minolta, and a few others. They took photographs of a 20 X 24 test board with several straight lines along the edges; developed them; and enlarged them back up to the same size print (20 X 24 inches) with a Leica/Leitz Focomat IIC enlarger with Leitz enlarging lenses.

    The only lens that reproduced straight lines as straight lines was, of course, the Leica 50mm Summicron (rangefinder), The 50mm Summicron on the Leicaflex SL II (then in current production) exhibited just a slight curve near the center of the outer edges. As the 50mm lenses went downhill in quality, each exhibited more and more barrel distortion (curved, not straight lines).

    Obviously, the type of glass used, the quality of manufacture, the design of the lens, etc. all contribute to the cost of the lens.

    All such lenses worth consideration? That answer depends upon the standards you wish to employ in your photography, the type of photography that you do, i.e. what lens openings you generally use, etc.; and many other aesthetic and real factors, including one of the most important - your budget.

    When I was working at the better camera shop in Peoria, IL - decades ago - our customers would often consider the purchase of a 135mm lens as their next choice of photographic equipment. Although we carried independent lenses, I steered my customers away from them, simply because of the better quality glass and overall construction of the lens.

    Often, I would go to the back of the store and turn off the light in that section of the camera shop; load up two Canon cameras with different 135mm lenses. One would be the independent lens and the other would be the lens made by Canon. Even though the "indie" lens was a F/2.8 the Canon F/3.5 transmitted more light. And it was so noted, by the customer under these "low" light conditions. So with a difference of about $15.00 to $25.00, the Canon lens was the far better buy over the Vivitar.

    5. There is another factor to consider, namely the focusing accuracy of SLR and Rangefinder cameras. In both cases, the accuracy is dependent upon the viewfinder magnification and the base length of the rangefinder. For example, the Leica M3 has a physical rangefinder base of 69.25mm, i.e. the distance that the "range finder" mechanism are separated. Since the M3 has viewfinder image that is almost life size, actually it is 0.92, the measuring base is 63.71mm (The measuring base is determine by multiply the viewfinder image times the rangefinder base or 0.92 X 69.25 = 63.71

    In an SLR camera, the viewfinder image size can usually range from 0.92 to 0.94 - I tried to verify one or two of the older SLR film cameras, but couldn't find the necessary details or confirmation. So, we will settle on 0.92 for the purposes of our discussion.

    Well, now that we have a viewfinder image size, where do we find the "measuring" or rangefinder base of a SLR (film) camera. Well, the measuring base of an SLR camera is determined by dividing the aperture of the widest lens opening by the focal length of the lens.

    So with a 50mm F/2.0 lens, the measuring base is 25mm; Using the formula, viewfinder image size times the measuring base the figures are 25mm X 0.92 or 23mm. With a 100 F/2.0 lens, the measuring base increases; again using the formula viewfinder image times the focal length of the lens divided by the maximum aperture or 0.92 X 100/2 or 0.92 X 50mm = 46mm. By the time a 135mm telephoto lens is employed, the focusing measuring base of an SLR will begin to match that of the Leica M-3.

    However, with the use of wide angle lenses, the focusing accuracy of an SLR rapidly diminishes. For example, a 35mm F/2.0 lens has a measuring base of 17.5 mm. Multiply that by the viewfinder image size of 0.92 and you'll obtain a measuring base of only 16,1

    Not very accurate!!!

    Of course, we can put it in more simple terms. When you put a wide angle lens on an SLR camera, your lens is including a greater area on the same area of film. In order to accomplish that wider angle, the image size is reduced and looks further away. Consequently, fine detail is made smaller and more difficult to accurately distinguish, especially under low light conditions.

    Not what does this last discussion have to do with your basic question?

    Well, now that you have a a better idea of the problems of focusing accuracy with SLR cameras, wide angle lenses, and available light conditions, you should opt for the purchase of (good quality) high speed lenses to insure greater focusing accuracy under certain lighting and shooting conditions.

    Well, now you know more than you ever thought possible - thanks to a conscientious, Librarian and photographer! ;>)

    Best wishes in your photographic endeavors.


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