Having trouble with wrapping my head around a technical concept

Discussion in 'The Watering Hole' started by Jock Elliott, May 28, 2014.

  1. Jock Elliott

    Jock Elliott SC All-Pro

    Jan 3, 2012
    Troy, NY
    I am thinking about photographing the night sky.

    On the one hand, you have a camera like the Panasonic LX7, with an f/1.4 lens -- a real light bucket, right? But a small sensor.

    On the other hand, you have the Sony A6000 with a big sensor but a kit lens with just an f/3.5 aperture.

    So, under the night skies, which one is more likely to be able to capture the grandeur of the night?

    Cheers, Jock
     
  2. bartjeej

    bartjeej SC Hall of Famer

    Nov 12, 2010
    bart
    If the larger sensor camera has a smaller aperture (higher f-number), it'll need higher ISO's to compensate (or a slower shutter speed, but we're ignoring that here). For these kind of purposes it's good to compare equivalent apertures. Just like with equivalent focal length, you simply multiply the f-stop by the crop factor and you see the light gathering potential of a certain lens/sensor combination.

    The Sony A6000 has a crop factor of 1.5, so an f/3.5 lens has a light gathering potential comparable to a full frame camera with an f/3.5*1.5=f/5.25 lens.
    The LX7 has a crop factor of 5.1, so at f/1.4 that's a light gathering potential similar to a full frame camera at f/1.4*5.1=f/7.14 lens.

    So the sensor/lens combination on the A6000 has an equivalent aperture that is 7.14 / 5.25 = 1.36 times larger than the LX7 (almost one stop of difference in light gathering potential). Add to this the fact that the Sony's sensor is more modern and therefore probably more efficient, and the Sony should give better results.
     
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  3. TraamisVOS

    TraamisVOS SC Hall of Famer

    Nov 29, 2010
    Melboune, Australia
    Don't forget that the night sky moves. So if you have to keep the shutter open longer, you're going to get star trails. If that's not what you want then the f/3.5 lens is out of the question. If you do want star trails then you're going to want to stop down whatever lens you've got anyway.
     
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  4. bartjeej

    bartjeej SC Hall of Famer

    Nov 12, 2010
    bart
    From my previous post follows that the Sony has such a large sensor size advantage that by the time it upped the ISO to give similar noise performance as the LX7, you're getting either a near stop brighter exposure, or a near stop faster shutter speed.

    For instance, the Panasonic could be at f/1.4, shutter speed of 1 second, and ISO 1,000. To give the same shutter speed at f/3.5, the Sony will have to up the ISO to 8,000, but crucially, its noise performance at ISO 8,000 is still better than the Panasonic at 1000!

    In fact, it's so much better (almost one stop, as I showed above), that you could increase the Sony's ISO to 16,000 and still get the same noise performance as the LX7! This higher ISO would either give you a brighter view of the night scene, or allow you to lower the shutter speed (to roughly 0.5 seconds). And all this is not even taking into account the presumed higher efficiency of the more modern Sony sensor.

    Please note that if you compare images at 100% (for instance in DPR's studio comparison tool), the A6000's higher pixel count will give the impression of more noise, but if you view the images at the same overall size (for instance a 20*30 inch print), the overall image quality will be as I described.
     
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  5. Jock Elliott

    Jock Elliott SC All-Pro

    Jan 3, 2012
    Troy, NY
    Thanks! Of course, it would be even better if the a6000 were married to a really fast lens!

    Cheers, Jock
     
  6. Luke

    Luke Super Moderator

    Nov 11, 2011
    Milwaukee, WI USA
    Luke
    And there's plenty of superb vintage f1.8 lenses (and enough of them are around or under $100) that can be adapted to the Sony mount if you're willing to focus manually.
     
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  7. Biro

    Biro SC All-Pro

    Aug 7, 2011
    Jersey Shore
    Steve
    What's more... when shooting the heavens, especially when using a tripod, manual focusing shouldn't be difficult.
     
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  8. Djarum

    Djarum SC All-Pro

    Jul 10, 2010
    Huntsville, AL
    Jason
    For me, I would need to test the dark noise at equivalent exposures. Also there is a Dynamic range component, which the larger sensor typically will win. The other thing to look at is lens quality, especially coma/astigmatism at the edge. Stars are unforgiving, and will show coma or astigmatism very easily. When I do my night sky shooting, I always shoot at ISO 200.

    The biggest challenge IMHO these days is finding a dark sky. Long exposures will show city skyglow quite easily, washing out many stars that might be in the photo.
     
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  9. Djarum

    Djarum SC All-Pro

    Jul 10, 2010
    Huntsville, AL
    Jason
    One wouldn't think, but I find it quite difficult with my PENs if there isn't a bright enough star in the sky to aim too. As long as a bright star is around, one can MF to the star and just leave it there.
     
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  10. bartjeej

    bartjeej SC Hall of Famer

    Nov 12, 2010
    bart
    shouldn't focusing on infinity do the trick? I know light years are technically speaking not infinity, but practically speaking it's pretty close, no?
     
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  11. KillRamsey

    KillRamsey SC Hall of Famer

    Jun 20, 2012
    Cambridge, MA
    Kyle

    These are what I was going to say... f1.8 is almost surely going to be out of the question anyway since the corners will be flaring out like crazy unless you stop down a little. And don't get your hopes up if you're within 100(?) miles of any bigger city. You'll just get street light glow washing it all out. Middle-of-nowhere Vermont was ok, Martha's Vinyard was a little better than VT, and the Andes of Peru were WAAAAY better.
     
  12. Briar

    Briar SC All-Pro

    Oct 27, 2010
    Scotland
    Karen
    Phew! Was browsing this thread on my phone (not signed in) and almost fell off my seat on reaching Luke's post. A photograph of a woman with a quote underneath stating words to the effect "come and get me guys"! I thought Luke had ditched the record scene for a shady way of making a few dollars! Bloomin' 'Eck!
     
  13. Luke

    Luke Super Moderator

    Nov 11, 2011
    Milwaukee, WI USA
    Luke
    I guess I don't want to have the 6th post a thread anymore.

    Although if anyone knows a shady way for me to make more money, then I can start buying more cameras...... :daz:
     
  14. Djarum

    Djarum SC All-Pro

    Jul 10, 2010
    Huntsville, AL
    Jason
    I don't know about Sony cameras, but infinity on micro four thirds cameras actually focused beyond infinity, if that makes any sense.