Nikon AF-S NIKKOR 24-70 MM 1:2.8 G ED W /7100

Discussion in 'Nikon' started by marcantel, Feb 26, 2017.

  1. Booked a vacation in May 2017 in central Colorado, Divide near Pikes Peak, elevation bout 9100 ft..I want to get some shots of the Milky Way. According to a web site Dark Finder the noise pollution is in the center of the scale.I think it'll be pretty dark.My question is the Nikon lens 24-70 2.8 a good choice as I'm Shooting D7100 Dx format which is actually a 36-105. F 2.8 @ 30 sec 2500 ISO I'm reading is a good starting exposure.
     
  2. I'd be guessing you were not quite wide enough. The easiest way to check is to go outside tonight, if it's clear', and see for yourself! If you want some kind of horizon foreground, craggy peaks etc, I'm pretty sure you'll not get most of the Milky Way in frame, even in portrait orientation.

    Numbers wise, to get 90 deg, vertically ie Horizon to Zenith, you'd need 18mm in FX terms, so about 12mm DX.

    The 24-70mm on DX will give you about 53 > 20 degrees.

    What other glass do you have?

    I often use
     
    Last edited: Feb 27, 2017
  3. Unfortunately the DX format is a bit limiting when it comes to choice of good wideangle lenses. <p> Personally I'd be packing my Tokina 11-20mm f/2.8 zoom along with something a bit longer for such a trip. Have you thought about renting a lens?
     
  4. I'd go wide, and I'd stack (partly because it's easier to remove light pollution if you don't have to worry about saturating channels). And if you're stacking, you might be able to use a longer lens. Bear in mind that stars (whose light doesn't spread out with a longer lens, if you're in focus) respond to physical aperture and area sources respond to relative aperture - so if you want more stars to be visible, 70mm f/2.8 trumps (say) 20mm f/1.8. If you're mostly after the band of colour, though, you may be fine. I've recently picked up a couple of light pollution reducers (IDAS make them in camera lens sizes) - and a 200mm f/4 as my best option for using the 52mm LPS-V4, short of putting it in the back of my 200 f/2 - but I've not had the chance to do much research yet. They won't play with a wide angle, though. Likewise I've found the Skywatcher Star Adventurer to be worth a look, but I've yet to have time and clear weather coincide.
     
  5. Well yes, but then you'd have to use a Canon camera. :)
     
  6. Andrew, how many angels can you get on the head of a pin, and how many point sources does it take to make an area source?<p>So a single star gets brighter with increasing physical aperture, but a galaxy only gets brighter with increasing relative aperture? - Interesting!
    <p>I suspect in real life, rather than theory, that as long as the diffraction Airy disk or lens PSF doesn't exceed the bounds of an individual photosite, that it doesn't really matter what the physical aperture is.
     
    Last edited: Feb 27, 2017
  7. RJ: It's about light gathering area. With something that gets (visibly) bigger when magnified, the light spreads out, so a flat surface looks the same at f/2.8 no matter the focal length (which is why we use relative aperture in the exposure triangle). If the light isn't spread out, more absolute aperture means more light is gathered. That's why I can see a lot more stars through my 10" dobsonian reflector (roughly f/5, minus a bit for the secondary mirror) than I can through my 70-200 f/2.8, and why you'll often see more stars at the long, slow end of a variable aperture zoom (as I proved long ago with a 28-300 f/3.5-6.3). It won't affect the milky way itself, since you ARE magnifying an area of many stars (and nebulae, and dust clouds...) but it'll affect how bright the foreground stars are, if you care.<br />
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    To counter that argument, if you don't want star trails, you'll need a shorter exposure with a longer lens (because the earth today's faster relative to the field of view of a longer lens). Hence stack and/or track, and bring a torch to freeze the foreground. Oh, the other reason to stack: it's always at the end of a several-minute exposure that someone drives by with headlights on full beam, or someone kicks your tripod!<br />
    <br />
    Disclaimer: I'm a very, very amateur astronomer, and I'm sure you'll get much more informed advice from others, especially if they don't live under London's orange sky glow. But I hope the thoughts help. (Other tips: live view focus on a bright star, don't bother when the moon is up unless it's very new or that's what you're actually shooting, and a friend with a moderately - I use 50-200mW - bright laser pointer is the best way to see where you're aiming. Just make sure you circle the beam around your target so it doesn't stay accidentally fixed on a plane.)
     
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  9. I find the 70- 2.8 perfect for every thing else,but never had an opportunity like this one coming up ,I mean we'll be in the middle of no where at a cabin with a nice 180 deck.
     
  10. I maybe can rent the perfect lens.I have read what and how others have did it and see if I can get the same Eq.
     
  11. I know the theory Andrew, but it's always puzzled me how this gels with the equally valid theory that any luminous area can be thought of as a collection of countless point sources.<p>Perhaps it has something to do with adjacent incoherent sources being out of phase and randomly cancelling each others waveforms. I've asked physicists about the dichotomy and they don't seem to even see an issue.<p>However, we don't deal with perfect lenses, so there's always going to be a spread of any "point" image, and if stars were to be imaged as a singularity, then they'd easily slip down the cracks between photosites (or halide crystals).<p>It's a phenomenon worthy of further investigation IMHO.
     
    Last edited: Feb 28, 2017
  12. You can think of a luminous area like that (calculus and photo,net, who knew?) - and as you zoom in with a longer lens, you're looking at fewer and fewer of those (still countless) sources, so they get dimmer. But if you're at the same f-stop with your longer lens, you're also gathering more light from them because your physical aperture is correspondingly bigger, so everything cancels out. Pointed at a star, all the light from the star is hitting the same few sensor sites (if it's in focus) irrespective of the focal length, but a bigger lens (longer but with the same f-stop) still gathers more of the star's light and contributes it to the same few pixels. If you defocus the star and blur it by proportionately more with a longer lens, you'll get back to the "normal" behaviour of light proportional to f-stop. There's nothing special about stars (don't tell an astronomer) except that they're very bright and very far away, so you can see them over the background and they don't magnify detectably with any sensible lens. Except for the sun, obviously, but I'm not taking responsibility for the burned sensors coming from that experiment.<br />
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    If you have lots of stars in view, that's different - they're likely spread over an area in the sensor. When you zoom in, the area they're spread over gets (detectably) larger, so the light spreads out.

    I think you're over-thinking it. Incoherent light sources sum by accumulated intensity (where the intensity of a coherent source is the square of its amplitude, and two coherent sources interfering constructively have an intensity which is the square of their summed amplitudes - which is why lasers are bright). https://www.physicsforums.com/threads/adding-intensities-for-incoherent-waves.58037/ might help, but to be honest I last tried to get this straight in my head about 25 years ago, and I'm probably not going to do a wonderful job of explaining it. :) (I'm a software engineer - we don't have to deal with this stuff.)

    Yes, the "point" won't likely hit only a single photo site, there is some spread. But the spread shouldn't be getting bigger as the lens gets longer - I believe it's proportional to relative aperture (the diffraction is smaller for a bigger physical aperture, but the distance the diffracted light travels is also increased), but I'm not an optical physicist. So the star should still be the same size with your bigger lens, give or take. It has always occurred to me that with a really sharp lens at a big aperture, hitting photon sites without an AA filter ought to cause stars to appear as random colours, depending on which bit of bayer sensor gets hit - or the debayering may just have to assume everything is white, which is equally inaccurate. I don't think I have a lens that could cause this problem, but I think it's a theoretical possibility.<br />
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    In other news, you know how annoying it is when you write a large post and the web site logs you out half way through? And no, I'm not going to enable auto password fill on my browser (which wouldn't solve it anyway - I'd have lost part of my post if I'd not had the presence of mind to copy to the clipboard), partly because my employers are not going to be happy if I risk auto-fill on a corporate site.
     
  13. Andrew, optical aberrations scale with focal length, so for a given "quality" of lens, then the point-spread will definitely increase with focal length.<p>If there was any prospect of a clear night in the next 10 years I might be inclined to set up some practical tests. Instead the current miserable weather keeps me bantering here rather than getting out and taking pictures.
    <p>
    BTW, there's a mathematical theory that states no matter how much you magnify an infinite matrix, it always contains an infinite number of intersections. But mathematicians eh!<p>"What's the difference between a theoretical physicist and a pure mathematician?" - "The physicist will look at your shoes while they're talking to you."
     
    Last edited: Feb 28, 2017
  14. Aberrations do (mostly), but I'm not sure diffraction does, because diffraction reduces as the aperture increases. I believe the effect of physical aperture and focal length cancel out, but I could be wrong, and haven't checked the maths. https://luminous-landscape.com/do-sensors-out-resolve-lenses/ does talk just in terms of f-stops, though. I'm sympathetic to the clear night requirements, along with having some work projects done. I'll probably have free time just as summer comes around and there's too much light in the sky. Sigh.<br />
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    Marcantel: for what it's worth, I'm sure you can capture something decent with the 24-70, so I'd not sweat it too much when it comes to hiring alternatives unless that's the only reason you're going there. It does seem a slightly funny lens to use on a DX camera (partly because of how big it is), but I've not historically been one for mid-range zooms anyway, so I'm in no position to comment!
     
  15. f/2.8 is good but 24mm may not be wide enough. If your wide angle is f/3.5, it can be usable as well, just takes a bit longer. Yes it is very dark on the CO peaks, do bring a red flash light so you can see something as you fiddle around with your gear.
     
  16. No, this is not a good choice at all. Nowhere near wide enough. Probably the best choice is a Tokina 11-16mm f2.8, or maybe the Tokina 11-20mm f2.8. Tokina also makes a 14-20mm f2 which is even faster, but again barely wide enough at 14mm. Sigma is coming out with a 14mm f1.8 and that will almost certainly be my next lens. There is a Rokinon/Samyang 14mm f2.8 that is popular for this, but on DX it too is barely wide enough (but would work.)


    Kent in SD
     

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