Low light sports question

Discussion in 'Digital Discussion & Q&A' started by mk04447, Aug 26, 2018.

  1. mk04447

    mk04447 TPF Noob!

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    Hello new to the forum. I've been shooting quite a bit of high school football and I've been experimenting with underexposing my images in camera to further decrease ISO. Underexposed by -0.3 has been causing some weird results and I am curious if I'm missing something.

    I'm shooting a D750 with a Sigma 70-200mm F 2.8. I have field access; I'm fine at 1/800, F 2.8, and roughly 6400 ISO, but I wanted to run at F 4 for faster focus. That pushed me up to 12800 which is ahh ahh grainy; usable, but not great. -0.3 got my ISO down, but my photos are appearing over exposed in places and are harder to edit. Doesn't make sense to me. LR can't seem to dehaze enough.

    Not a pro by any means, internet taught hobbyist would be a good description. Is this a viable strategy for high ISO management? Shouldn't I be able to make up a stop or two in LR? Am I thinking of EC correctly? Any suggestions? Thank you.


     
  2. ACS64

    ACS64 TPF Noob!

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    Well I'll try to at least start some discussion for you. Based on https://improvephotography.com/34818/iso-invariance/ the Nikon D750 has an ISO invariant sensor so it should respond to having the luminance lifted in PP of the raw files. That said -.3 EV comp is only a 1/3 stop under exposure I would think that the difference in the files would be almost imperceptible. Did you mean -3 instead of -.3? That said most examples I've seen of lifting luminance have lifted only from base 200 or so to 3200. It may be that 12800 is a bridge to far.

    That said I'd like to know the theory on why f4 would provide faster focusing than f2.8. Slightly greater DOF (6ft @ 100ft) but faster?

    A. C.
     
  3. mk04447

    mk04447 TPF Noob!

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    I've experimented with F 4 and it does allow the AF to acquire more quickly. This makes sense optically, with closer subjects, more of the sensor area is in focus due to the wider DOF; this creates a bigger target area for the processor. The camera doesn't know what it's looking at, it's all digital garbage to the processor; the manufacturer has come up with some algorithm and taught the camera to look for the digital pattern that means the photo is in focus. Here more of the image is in focus and this allows the math to work itself out more quickly. Try focusing on a close object at 1.4 vs. 22, it works.
     
  4. ACS64

    ACS64 TPF Noob!

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    I suppose if the camera stops the lens down before it focuses that would be true but mine have always seemed to focus before stopping down. The greater DOF would make focus a less critical.
     
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  5. mk04447

    mk04447 TPF Noob!

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    You don't sound like you're in manual. That may be true with the settings you're using.
     
  6. weepete

    weepete TPF Supporters Supporting Member

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    Nope, have a look through a lens and hit your focus button then press the shutter release. You'll see that unless you are using an old MF lens the aperture only closes when the shutter button is pressed, not when focus is achieved.

    Could you provide us with an example image? it'd be better to look at one uneditied and then one edited. Stopping down from a correct exposure shouldn't be causing overexposure, even DR should be increasing slightly with an ISO reduction.
     
  7. mk04447

    mk04447 TPF Noob!

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    I'm not sure how an image would help? Try shooting a game with 2.8 and another at 4. If you shoot 1000 photos at each game, there will be more usable "in focus" shots the day you used 4, because of the DOF.
     
  8. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    Yes....but the camera won't focus any faster, which seems to be what you were implying.

    Every modern camera that I've seen, focuses with the aperture fully open...which gives it the most light to work with.
     
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  9. astroNikon

    astroNikon 'ya all Bananas I tell 'ya Supporting Member

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    With the D750, and DSLRs in general, when you press focus the lens is then put to wide open, which would be f/2.8 no matter what you have it set for. There is a secondary mirror that points down, which allows the focus AF sensor in the lower part of the body (not part of the main sensor) to create the focus and drive the lens. Then the secondary mirror and primary mirror move out of the way for the image to be taken.

    this page describes how focusing works on a DSLR with a schematic showing the location of the AF sensor at the bottom of the body. ==> How Phase Detection Autofocus Works - Photography Life
     
  10. astroNikon

    astroNikon 'ya all Bananas I tell 'ya Supporting Member

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    I also use a D750 and used for low light sports and indoor sports.

    With sports you have to have a certain shutter speed to stop of action, or lower a bit and balance out the slight blur with the Aperture and ISO.

    With f/4 you will have to have a higher ISO to compensate for it.
    So stick with f/2.8.

    Exposure Compensation, in Manual Exposure I think just adjusts your ISO a bit. @Derrel would know more about it.

    But there are points where Shutter, ISO, Aperture just isn't enough to get a quality photo ... unless you add more light. On football fields the fields are well lit. I've seen the ones in my areas where if the subject is near the sidelines there's a significant light dropoff, and if the Metering Area is too big then it can cause weird results.

    BUT .. I think you are having focusing issues. Maybe at f/2.8 you are not focused on the subject you want and thus your depth of field will have the main subject slightly out of focus.

    What focus mode are you in ? and have you checked your focus points in the back LCD to make sure you are hitting the subject directly ?

    Can you post some examples with full EXIF ?
     
  11. weepete

    weepete TPF Supporters Supporting Member

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    It would help because it's much easier to diagnose an issue when I look at an example, especially as your explaination does not make sense to me. So either I'm missing some information or your issue is not where you think it is. Either way you could go through pages and pages of text trying to troubleshoot and explain but you'll have a much better chance of getting a fix if you post an image with intact exif data we can have a look.

    And yes, I've done some sports shooting so know the script.
     
  12. ACS64

    ACS64 TPF Noob!

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    I haven't shot field sports except in daylight but I've shot moving aircraft with an E-620 and a 70-300 lens. Even in its day that combo was not the fastest focusing system in the world but using f11 or f16 produced more in-focus shots than f5.6, diffraction be damned. That was not because the system focused faster at f11 than at f5.6 but rather that the increased DOF covered the inadequacies of the focus system.

    A. C.
     
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