Still struggling with night shots

Discussion in 'Digital Photography' started by jwt, Aug 13, 2007.

  1. jwt macrumors 6502

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    #1
    Title says it all. I'm struggling to understand why my tiny Canon Powershot lets more light in than my Digital Rebel XTi. Below are some shots taken with my equipment. All shots were taken in Time Value mode at ISO100, 1/8 sec., and no flash.

    First shot: Canon Powershot A70. f=5.41mm, f/2.8. This shot resembles room lighting.

    Second shot: Canon Digital Rebel XTi with 17-85 IS. f=24mm, f/4.0.

    Third shot: Canon Digital Rebel XTi with 18-55 kit lens. f=24mm, f/3.5.

    I can't understand why the dSLR shots are so dark when they should be letting roughly 12 times the light in that the Powershot does. Let's do some math.

    Powershot: pi*[(5.42/2)/2.8]^2 = 2.9.
    Rebel XTi w/ kit lens: pi*[(24/2)/3.5]^2 = 37.
    37/2.9=12.6 times the light

    Can someone please explain?
     

    Attached Files:

  2. swiftaw macrumors 603

    swiftaw

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    #2
    The f stop numbers on Point and Shoot cameras do not correspond directly to those on SLR cameras. f/2.8 on a fixed lens camera is roughly equivalent to f/11 on an SLR

    What were the exposure times for those shots?
     
  3. ChrisA macrumors G4

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    #3
    What!?!?!

    F/2.8 is f/2.8 is f/2.8 Well okthere might ba a 10% error but never does f/2.8 equal f/11
     
  4. swiftaw macrumors 603

    swiftaw

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    #4
    Sorry, I misread what Bryan Peterson wrote in Understanding Exposure. He was saying that f/2.8 on a single lens camera gives you the equivalent depth of field as f/11 on an SLR, not that they were actually equivalent. My bad.
     
  5. ChrisA macrumors G4

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    #5
    The results are exactly as expected. Why would you think the SLR lets in more light? You are thinking because it is physically larger? Well so is the sensor so the light is spread "thinner".

    That's why no one cares what the diameter of the aperture. All they care about is the ratio of the diameter to the focal lenght. That's all that matters. a 5mm lens at f/8 is the same as a 200mm lens at f/8
    Think about the inverse square law - light falls off with the square of the distance. Move the effective nodal point of the lens away from the sensor and you need more photons for the same illumination level. If you think in terms of ratios it evens out.

    To say it another way. Yes the 100mm f/8 lens has 20X more diameter then the 5mm f/8 lens but the lens is also 20X farther away fro the sensor. Light falls with the square of distance AND area depends on square of radius so the two squared terms cancel. So ALL f/8 lenses give the same exposure. That's why photographers for the last 100+ years think in "stops". not diameters.

    Now that we've covered that, your question is easy: The A75 has a faster lens than the Rebel. Why? cost. Lens elements cost goes up fast with diameter. An f/2.8 5mm lens is still tiny and cheap so Canon can afford it. Also the P&S needs the fast lens because it can't use a high ISO with such a small sensor.
     
  6. ChrisA macrumors G4

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    #6
    Right DOF is a function of focal length and aperture. As a rule of thumb DOF is inversely proportional to the size of the sensor or film frame. So if the P&S has a 8mm wide sensor and the dSLR a 24mm wide sensor the P&S will have three times the DOF for the same f-stop and if the zoom s set to get the same field of view.
     
  7. arogge macrumors 65816

    arogge

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    #7
  8. jwt thread starter macrumors 6502

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    #8
    Agreed, but I wanted to do the comparison with everything being as equal as possible, and the Powershot isn't capable of high ISO settings.
     
  9. jwt thread starter macrumors 6502

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    #9
    ChrisA,

    Excellent explanation. Thanks. So, if I understand you correctly, if I get a f/2.8 lens for the SLR, I should get the same level of exposure as I do with the smaller lens. Is this correct?
     
  10. arogge macrumors 65816

    arogge

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    #10
    Maybe, but what are you trying to do with the camera? The aperture should be set for depth-of-field first, and then the shutter speed should be adjusted for proper exposure.
     
  11. Sdashiki macrumors 68040

    Sdashiki

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    #11
    You do understand the doubling/halving of light?

    Double the shutter speed, half the light.

    Halve the f-stop, half the light.


    Its combinations of the two, taking ISO as a baseline of course, that create a proper exposure. Changing one requires changing the other to get the same exposure.

    example:

    125 @ f2.8 = 60 @ f5.6 = 250 @ f1.4

    all the same exposure, but depth of field and movement changes.


    night shots always require one of the following to get a proper exposure:

    slow/long shutter speed

    wide open aperture


    if you need to capture movement at night, youll need a flash or have to work with real nasty motion blur or invisibility even.

    if you need a greater DOF, youll need an even slower shutter to capture all the light needed for a small aperture.
     
  12. jwt thread starter macrumors 6502

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    #12
    I'm going to start traveling a lot, and I'd like to take hand held shots of cities at night without motion blur. ****, I'd like to be able to take shots indoors under dim lighting like my $200 P&S can do. I bought this dSLR intending to take shots I couldn't take before, but I'm finding I can't take shots I used to be able to with my P&S.
     
  13. wheezy macrumors 65816

    wheezy

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    #13
    What you need to do is get a faster lens, f/4.0 isn't fast enough for indoor shots without a flash unless you set the ISO to 1600/3200.

    Looking at your pictures, the PS at 2.8 is best, the kit at 3.5 is 2nd, 4.0 is 3rd in brightness. If you purchased a 2.8 or faster lens, you would get better, brighter shots from your SLR.

    I would recommend the 50mm f1.8, it's very fast, perfect for indoor shooting. And, it's anywhere from $70-$100 brand new where you look.

    For city shots at night you just want a tripod, trying to get a handheld shot with the aperture wide open won't give you enough aperture-sharpness that you would want. So for that, a slower shutter speed, slower aperture on a tripod works best.
     
  14. stcanard macrumors 65816

    stcanard

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    #14
    Yes -- everything is the same. 1/8 sec @ F/2.8 @ ISO 100 should be identical exposure no matter what camera and lens combination you are using. They've already done the math so you don't have to.

    Any difference is due to calibration errors in the camera, which does frequently happen on the ISO setting -- check http://dpreview.com they usually do tests in their detailed reviews to line up "reported" and "actual" ISO.
     
  15. arogge macrumors 65816

    arogge

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    #15
    You need a faster lens and maybe IS. The dSLR is obviously better for low-light conditions. It has less noise, higher ISO options, and better focusing. You should consider the 17-55 f/2.8 lens.
     
  16. jwt thread starter macrumors 6502

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    #16
    I'm not a pro, nor a trust fund baby. There's no way in hell I'm spending $1200 on a lens.
     
  17. GoCubsGo macrumors Nehalem

    GoCubsGo

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    #17
    Only trust fund babies spend $1200 on a lens?
     
  18. Abstract macrumors Penryn

    Abstract

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    #18
    Have you considered making the settings identical before doing a comparison? ;)
     
  19. jlcharles macrumors 6502

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    #19
  20. walangij macrumors 6502

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    #20

    I agree, if you can't afford the 17-55 f/2.8 IS the 17-50 is a great alternative with IQ on almost the same level. I used it in my travels this summer and the results were great, nice images even with 1600 ISO. I did find that the IS really is worth the extra money b/c there are some shots I couldn't get with the Tamron alone but wish I did (but I did find a way either by steadying myself, makeshift tripod from whatever I could get, actual tripod, ect.). Another great starter that most recommend is the 50 f/1.8, very cheap and should land you those handheld non-motion blur shots in cities albiet with higher ISO. Those 2 would be a great starter pair for less than $500 replacing the 17-85 IS (which you could resell for a nice sum -I'm assuming your test lens is yours).
     
  21. stcanard macrumors 65816

    stcanard

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    #21
    First off, as suggested by Abstract, try taking the shots with the same settings -- they should look the same. If you can't get the XTI to look as good as Powershot under the same conditions, you bought the wrong camera. Return the XTI and stick with the Powershot. End of story.

    Assuming you are able to get it to look the same, then you need to look to the DSLR's strengths:

    1) Shoot something at ISO 400 on both cameras, compare the noise.
    2) Shoot something at ISO 800 on both cameras, compare the noise (if you P&S can't do 800, keep it at 400 and compare to the XTI at 800 -- I'll bet the 800 still wins).

    At that point you've probably already gained 2 stops with the DSLR -- I'll bet an XTI at ISO 800 is cleaner then the P&S at 200.

    Then as suggested by walangij look at the 50mm f/1.8 -- it a $100 lens, and probably gains you another 1 - 2 stops over the widest aperture on the P&S -- you have to be careful about depth of field wide open, but the image quality is more than worth it.

    Now you've gained 4 stops -- what used to be 1/15s can be taken at 1/60 -- at 50mm that's the difference between blurred and clear.

    On my Nikon, with a $300 lens (55-200VR) I can take handheld shots down to 1/10 second if I'm careful. At ISO 800 that's a night time shot. This one is handheld 1/10s f/4.0 with that 55-200VR, no noise reduction applied (a quick shot through aperture, you wouldn't see that noise)

    f/1.8 on a 50mm will give you the ability to shoot the same kind of shot I do at f/4.0 with VR. This one was even done at 55mm focal length.

    [​IMG]
     
  22. arogge macrumors 65816

    arogge

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    #22
    There are cheaper alternatives from Tamron and Sigma, but not with IS. If you want to handhold shots with a shutter speed of less than 1/20 at the wide-angle, you need IS to avoid camera shake.

    You should also consider the Sigma 30 f/1.4 lens. It has a 50 mm field-of-view on a 1.6X-crop body, which is not very wide, but the f/1.4 aperture allows for faster shutter speeds.

    http://www.sigmaphoto.com/lenses/lenses_all_details.asp?id=3300&navigator=6
     
  23. jwt thread starter macrumors 6502

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    #23
    I did. See the following.

    Anyway, with the same settings the Powershot obviously lets more light in. However, once I increase ISO on the SLR, the shots taken are much much better than the Powershot. Even at ISO 50 and 100, the powershot is very grainy. At ISO 1600, the SLR isn't even noticeably noisy. I guess the SLR has a less sensitive detector at the same ISO as the powershot and requires some adjustment to take good shots.
     
  24. James L macrumors 6502a

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    #24
    Well, not really. In the original post, you had the P&S at f/2.8, and the DSLR at either f/3.5 or f/4.

    f/2.8 lets twice as much light in as f/4 does. See this scale:

    http://en.wikipedia.org/wiki/Image:Aperture_diagram.svg

    So, even though you had the settings such as ISO the same, the aperture was not the same. You had 200% more light coming in on the f/2.8 than on the f/4. If you set your P&S to f/4 and compared it to your DLSR at f/4, with all other settings the same, you would have a much more accurate comparison.

    p.s. $1200 is not super expensive for a great lens. SLR photography is a horribly expensive hobby, and one where you truly get what you pay for. Thankfully nice lenses in the $700 - $1000 range are available or my wife would leave me!
     
  25. Abstract macrumors Penryn

    Abstract

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    #25
    Without actually proving it, I'm going to just say that your cameras are fine, and you don't need to retest them. The difference in f-numbers you used say it all. If you really do want to do this test, then you're going to need to use the same ISO, same shutter speed, AND the same aperture size. You got 2 out of 3 correct, but the f-number has to be the same because this is what tells you how much light enters the lens and passes all the way down the barrel and hits the sensor in your camera.
     

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