Why exactly we do not have a 40-150 1.8-5.6?

Discussion in 'Native Lenses' started by Klorenzo, Aug 4, 2015.

  1. Klorenzo

    Klorenzo Mu-43 All-Pro

    Mar 10, 2014
    Lorenzo
    I was thinking this: the 45/1.8 is an extremely small lens, with a 37mm filter thread. Then you have the small 40-150 with a 58mm filter thread.
    So you should have plenty of space in the front element to get the light for a 1.8 on the wide end of the lens.
    Given that no such beast exists, what does it happen exactly? Why/where is all that light lost?
    Or, in other words, why zooms require so much more light?
     
  2. tkbslc

    tkbslc Mu-43 Hall of Famer

    Mainly the reason is that in order to allow for the longer focal length, the lens body has to be too long for the shorter focal length. So when you shift the elements back far enough to cause a 40mm focal length, they are back towards the back of the lens. If you opened the aperture wide enough to fill the whole lens barrel, you would have massive vignetting like if you used a long narrow lens hood. So they have to use a smaller aperture to allow for a more friendly angle of attack, so to speak.

    Extremely crude drawing I spent 30 seconds on, but hopefully it makes the point. (Ovals are lens elements, lines are the angle of view, rectangle is the lens body) 150mm is at full extension and needs a narrow angle of view so full front element/aperture size can be used with no concern for lens body. In the middle image, we moved the focal length back to 40mm and kept the same aperture size. Elements have to shift backward to make this happen. Now we have the lens body in the angle of view as shown by red squiggles. So we have to reduce the size of the aperture and entrance pupil as in the one on the right to allow for the full angle of view without lens body interference.

    lens.
     
    Last edited: Aug 4, 2015
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  3. Klorenzo

    Klorenzo Mu-43 All-Pro

    Mar 10, 2014
    Lorenzo
    Thanks for the explanation and the drawing. I get what you mean but there are a few things that bother me.

    Let's take the small 40-150. When I am at 150 the barrel extends and we are at figure one. When I am at 40mm the barrel gets shorter and this should allow more light to get in. In your drawing this does not happen. EDIT: I know that the drawing is nowhere in the correct scale, I'm not criticizing it in itself, it just seems too simple.

    In this case you also start with a front element that has a little more then twice the surface of the 45/1.8 and you still loose about 2 stops, and this means getting, all considered, a little less then 1/8 of the entering light. A huge difference.

    Ok, when it collapses it's not as short as the 45: http://camerasize.com/compact/#521.93,521.95,ha,t

    And what about fixed barrel zooms?

    Last thing: it looks like a silly problem :) It seems strange that nobody ever tried to solve it just pushing all the rear block forward and refocusing in a different place. Maybe this already happens. Or maybe there were a few weird lenses where this was attempted.
     
  4. tkbslc

    tkbslc Mu-43 Hall of Famer

    Yes, I explained it very simply and used extremely crude drawings. Don't take them as perfectly and complete explanations.

    Remember that these cheap tele lenses are $100-300 lenses. You can likely get anything you want, but not at the price. If we introduce a very complex optical design with massive lens shift, then you've increased the cost and potential for optical flaws. Also consider that they have already used the large front element as an advantage to increase the aperture to 1.5 stops faster than the telephoto. It could be a straight 40-150mm f5.6. Regarding lens elements moving toward the front of the lens, they definitely do that. But even the dirt cheap Olympus 40-150mm has 13 lens elements. You simply can't compress those down to into the last half inch of the lens barrel.

    As for constant max aperture zooms, like an f2.8, same logic applies. 35mm f2.8 is about 1/3 the diameter of 100mm f2.8. 40mm f2.8 is just over 1/4 the diameter of 150mm f2.8. Even though the aperture opens up to 53mm for 150mm f2.8, we can't have a 40mm f0.75 end.
     
  5. b_rubenstein

    b_rubenstein Mu-43 All-Pro

    Mar 20, 2012
    Melbourne, FL
  6. Klorenzo

    Klorenzo Mu-43 All-Pro

    Mar 10, 2014
    Lorenzo
    I mean fixed length barrel, non-telescoping, like the 40-150 PRO, where they cannot use the "shorten the tube" trick. Probably they use the "push to the front" one.
     
  7. tkbslc

    tkbslc Mu-43 Hall of Famer

    Well, also, they aren't giving you a faster wide end like with the variable zooms. I would imagine with lenses like the 45-175, where you have all internal zooming that more significant element shift is going on, but also the power zoom helps facilitate that. Having the elements attached to a mechanical zoom ring vs a bunch of independent motors surely limits how much they can move, I would think.


    I was also thinking we might use compact cameras as an example. Take a look at the RX100. It's an F1.8-4.9. And the actual aperture size only varies about 1mm through the zoom range. But compact cameras have the ability to drastically change shape and size as you zoom in and out. And if you look into them as you do so, you will see massive amounts of movement with the elements. So again, I think it CAN be done, it is just a matter of cost, complexity and practicality.
     
  8. Klorenzo

    Klorenzo Mu-43 All-Pro

    Mar 10, 2014
    Lorenzo
    Is this a serious suggestion or are you somehow offended that I called it a silly problem?

    The problem is IMO silly, I expected a more complex reason, but this does not mean that it must/can have a simple solution.
     
  9. b_rubenstein

    b_rubenstein Mu-43 All-Pro

    Mar 20, 2012
    Melbourne, FL
    If one actually wanted to know about the subject, it's a serious suggestion. If, on the other hand, one wanted to have a shoot the breeze discussion (the true grist of internet forums), then no; ignoring that very few people have the background to work their way through that piece.
     
  10. gr6825

    gr6825 Mu-43 Veteran

    277
    Oct 10, 2012
    So you are mad nobody agrees with you about this topic you do not understand?
     
  11. piggsy

    piggsy Mu-43 All-Pro

    I think it comes down to a matter of other factors (what is the acceptable minimum focus at any length, how fast can it focus, how expensive a motor is required to this, what is the optical quality at either end, what magnification is achieved) and how expensive the total package would be to achieve 40/1.8 rather than 2.8.

    Rather than the kind of dry link from zeiss here's something that is similarly geeky but has more pictures (that you can mouse over ! :D) that illustrate how some of Nikon's lenses work - largely concerned with magnification and focal length but it is neat for visualising how they actually have to make the mechanical internals work.

    http://www.pierretoscani.com/echo_focal_length.html

    edit - ah I thought there was something else there to do with the compromises involved in making zoom lenses, it's on another page here -

    http://www.pierretoscani.com/echo_telezooms_english.html

    with flash animations of what needs to be moved inside the lens at different lengths under various different design styles.
     
    Last edited: Aug 4, 2015
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  12. David A

    David A Mu-43 All-Pro

    Sep 30, 2011
    Brisbane, Australia
    I think an f/1.8 to f/5.6 40-150mm zoom could be designed. The question is whether it could be designed in a size similar to the "small 40-150mm" as you called it or whether it would need to be physically larger. Then we get to questions like how much would it cost to make it, which has a direct impact on how much it would sell for. I don't think there's any doubt that it would have to be bigger than the small 40-150mm or that it would cost more. How much bigger and more expensive would it have to be before potential buyers are going to decide that they're better off just buying the current small 40-150 plus the current 45mm f/1-8 prime in order to get the extra speed at the 45mm end of the zoom range or, instead, that it's better to buy the f/2.8 PRO zoom for the extra speed at the 150mm end instead? I know that when I bought my 40-150mm f/2.8 PRO, the reason I did so was because I wanted most was the wider aperture at the 150mm end, not at the 40mm end because I already had the 45mm and 75mm primes.

    What that means is that there's going to be a limited market for the lens you suggest. No one who already owns the 45mm and small zoom would buy it because it offers them nothing unless it has some other feature like weather sealing which they need. No one who already owns the 45mm prime is going to buy it instead of the small zoom if it costs more than the small zoom and offers nothing that the small zoom doesn't . No one who is chasing a 150mm focal length is going to buy it if it costs more than the small zoom and is no faster than the small zoom at the 150mm end.

    Lens manufacturers need to sell lenses if they want to stay in business. I think the reason we don't have the lens you suggest is simply because it wouldn't sell enough copies to make offering one a good business decision.
     
    Last edited: Aug 5, 2015
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  13. piggsy

    piggsy Mu-43 All-Pro

    Yeah just from going over the links I posted earlier the internal focusing/internal zooming and related ease of weather sealing the 40-150 2.8 pro probably had a fair amount of impact on its performance and price point.
     
  14. Klorenzo

    Klorenzo Mu-43 All-Pro

    Mar 10, 2014
    Lorenzo
    I agree. With enough time I can get through that article, math is not the problem. The problem, as usual, is time. And if I ever should do it I'd probably start from a full book. That's why the link sounded a little contentious. It's not about "shoot the breeze" but to keep things simple where there is no need to do otherwise.

    I browsed through the article: I thought pages 9-10, then 21 could be the relevant part (exit pupil distance from the image plane). But none seems a factor big enough to explain about 7/8 of missing light (maybe is this calculation to be wrong). The article does not talk about zoom lenses so I have no idea where is the real difference from a fixed lens: lens diagrams look similar.
    The "hood effect" explanation also bothers me: I should get heavy vignetting more then uniform light decrease (EDIT: maybe do they limit the aperture until the level is even?)

    Just one last clarification: my question is not "can we build a 40-150 f1.2 lens?". Of course we can, maybe it's going to be as big as a car, fine. What I find strange is the speed difference between the 45 and the 40-150 considering the relative sizes.
     
    Last edited: Aug 5, 2015
  15. Klorenzo

    Klorenzo Mu-43 All-Pro

    Mar 10, 2014
    Lorenzo
  16. David A

    David A Mu-43 All-Pro

    Sep 30, 2011
    Brisbane, Australia
    OK, going back to that original question.

    The f stop number expresses a ratio between the focal length and the diameter of the lens opening the light passes through. The lens opening in question isn't the front element. The maximum size of the lens opening depends on the size of the internal elements and how they modify the path of light as it passes through the lens barrel. In a zoom, the internal elements tend to be a significantly smaller in diameter than the front element, and the path of the light passing through them is modified by the placement of each internal element as it passes down the barrel. That placement changes as you change focal length and the full diameter of each internal element isn't used at all focal lengths. The end result of that is that the diameter of the lens opening at the wide end of the zoom range, and throughout the zoom range for that matter, is a lot smaller than you would expect based on the diameter of the front element. So, despite the fact that the front element of the small zoom is much larger than the front element of the 45mm f/1.8. the diameter of the maximum lens opening the light passes through internally is a lot smaller than that of the opening the light passes through in the 45mm prime. You simply can't judge the size of the lens opening by looking at the size of the front element.

    Bottom line: the zoom isn't gathering a lot more light than the prime due to the greater size of its front element and then losing a lot of that light on the way through the lens. The size of the lens opening in the zoom is a lot smaller than that of the prime because of the way it has to be constructed if it's going to be the size it is and cover the focal length range it covers. It's simply not capable of capturing as much light as the prime even though it looks like it should capture more light than the prime due to the size of the front element.
     
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