what does a 'slower lens' actually mean?

Discussion in 'Photography Equipment & Products' started by cgennoe, Nov 19, 2007.

  1. cgennoe

    cgennoe TPF Noob!

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    Hi there!

    I understand that this refers to aperture but what exactly does this mean?

    For example, a lens that can be stopped down to 2.8 over an indentical lens with a max aperture of f4 is understood to be 'faster' Can you explain? Thanks very much!
     
  2. monkeykoder

    monkeykoder TPF Noob!

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    The f4 lens lets less light in taking more time to properly expose the film thus it is slower.
     
  3. The Phototron

    The Phototron TPF Noob!

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    A max aperture of 2.8 lets in more light, which allows you to use a faster shutter speed than you would with a max aperture of 4, which lets in less light.
     
  4. fmw

    fmw No longer a newbie, moving up!

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    The maximum aperture of any lens is a ratio between focal length and the diamter of the front element. Faster lenses have wider maximum apertures. This provides more flexibility in terms of exposure and depth of field management. Slower lenses don't have this advantage but are usually lighter, smaller and less expensive because they are designed around a smaller front lens element.
     
  5. Mike Jordan

    Mike Jordan TPF Noob!

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    In English... A fast lens can create a properly exposed image with less light in the same amount of time as a slow lens. Or another way... A slow lens either needs more light or more time (or both) to create the same properly exposed image as the faster lens. (assuming they are using their widest appurture)

    It's like the ole hose and bucket analagy... you can fill a 5 gallon bucket with a 1" hose twice as fast as you can with a 1/2" hose. Or you can fill the 5 gallon bucket with the 1" hose in the same amount of time with half the water flow.

    Mike
     
  6. Helen B

    Helen B TPF Noob!

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    Sorry Fred, but that isn't generally true, not even approximately, unless you are referring to a single lens design - but it is almost impossible for a single design of a multi-element lens to be available in different maximum apertures because you can't simply make the elements bigger without altering the lens design.

    There is a relationship between the diameter of the front element and the maximum aperture, of course. The aperture is determined by the diameter of the entrance pupil - that is the appearance of the iris (the aperture blades) when you look into the front of the lens. The aperture is not determined by the physical diameter of the iris either, unless the iris is in front of the front element, in which case the iris is likely to be the entrance pupil.

    In the case of some telephoto lenses the entrance pupil will be the same size, or almost the same size as the front element, but in the case of most other lenses the entrance pupil will be smaller than the front element. If the iris is in front of the front element, then the entrance pupil could be the front element or the iris.

    Measure a few lenses, and see if the maximum f-number is the focal length divided by the diameter of the front element.

    Best,
    Helen

    Zeiss Distagon 35 mm f/3.5 for Contax 645, showing the relative sizes of the entrance pupil and the front element. The actual iris is shown by the gap between the faint grey lines just to the left of the exit pupil.


    [​IMG]
     
  7. fmw

    fmw No longer a newbie, moving up!

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    Sorry, Helen, I'll stay with my simple explanation. Faster lenses of a given focal length have larger front elements. That it why they are more expensive to manufacture and weigh and cost more. I'm not talking about the physics of refracting light. This isn't a scientific treatise. It is a comment based on seeing and using thousands of lenses over a half century of photography. Spare me the lecture and the drawings. When you have a 1/2 century of photography under your belt then you can haul the drawings out again.
     
  8. Helen B

    Helen B TPF Noob!

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    Fred,

    If you read what I actually wrote you will see that I do not dispute that the size of the front element and the aperture are related. You wrote that the aperture was a ratio of the focal length to the diameter of the front element. That is misleading, and incorrect. Had you written that they were related, I would not have commented.

    I'll have to wait another five years before I have as much experience as you, because I only have 45 years to your 50. I hope that I remain as careful as I am now to make my posts as accurate as I can.

    Best,
    Helen
     
  9. Alex_B

    Alex_B No longer a newbie, moving up!

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    Isn't it, that for lenses, which really consist of one lens only, Fred is right? Then there is only one lens which then is also the front glass. Of course the aperture blades will then be somewhere behind the front glass, but the actual aperture would be calculated by extrapolating aperture diameter to the position of the lens and then dividing f/extrapolated_diameter. For an aperture wide open, this would translate into f divided by the (front) lens diameter.

    just when you go to more complex lenses with many elements, then this relation will become more an approximate one since then there is more than one bottleneck than just the front glass.
     
  10. Helen B

    Helen B TPF Noob!

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    Alex,

    Yes, but I hoped that I had covered that, and other similar conditions, in my post:

    "...unless you are referring to a single lens design - but it is almost impossible for a single design of a multi-element lens to be available in different maximum apertures because you can't simply make the elements bigger without altering the lens design.

    ... If the iris is in front of the front element, then the entrance pupil could be the front element or the iris."


    Fred wrote: "It is a comment based on seeing and using thousands of lenses over a half century of photography. Spare me the lecture and the drawings."

    Here are two 28 mm f/2 lenses designed for full frame 35 mm. Note how the entrance pupil is the same diameter, though the two front elements are very obviously different diameters. You can also see that the aperture calculated from the front element diameter would be very different from that calculated from the entrance pupil diameter.

    [​IMG]

    Best,
    Helen
     
  11. Alex_B

    Alex_B No longer a newbie, moving up!

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    I was not trying to say your post was all wrong, and I did not claim that nothing of what I said was in your post. I just wanted to say something on my own, so overlap to other people's posts might occur from time to time.

    If people on here would only say things which had not been said before, then this forum would be a very quiet and boring place.
     
  12. Alex_B

    Alex_B No longer a newbie, moving up!

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    after all, not every reply starting with a but has to be a contradiction to what has been said. sometimes it just uses slightly different words or tone.
     

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