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Amount of Light APS-C vs Full Frame

Discussion in 'Beginner's Corner' started by GrahamWorley, Sep 30, 2017.

  1. GrahamWorley

    GrahamWorley New Member

    Having played with both a Pentax KP (24Mp APSC) and a K1(36Mp Full Frame) I was surprised to find that at high ISO (6400+) I was getting significantly better images out of the crop sensor. Having read numerous times that full frame sensors got more light than crop sensors I got to thinking what on earth was going on. The basis of the observation was two images shot with KP 35mm f4 and K1 50mm f4 . I assumed that any lens (ignoring loss in the glass) should transmit the same amount of light towards the sensor if at the same f stop as another. Given a 1.5x crop factor there is 2.25x less surface area on a crop sensor so we only get roughly half the light collected by the 35mm as the rest is effectively wasted. Any sensor pixel should be sensitive to the total amount of light falling on it. Given the two cameras mentioned and an arbitrary amount of light X the light per pixel in the crop and full frame should follow as X/2.25 x 1/24,000 and X * 1/36,000. Thus per pixel the full frame gets 48/36 times as much light. Now log to the base two of that is 0.41 which would be the number of stops difference of light exposure. Firstly can someone confirm that my sums are correct. If they are then why on earth is there such a hype about full frame, the amount of extra light here is negligible. Furthermore it appears that the sensor in the KP is a bit newer and a lot better (perhaps some fancy processing too) at handling low light. So why did I want the K1..........both are great cameras I have to say...but you would have expected the opposite observation.
  2. PeteRob

    PeteRob Well-Known Member

    The broad thrust of your expectation is right but the performance at high ISO is technology driven and low noise at high pixel density is something that has been improved upon quite a bit.
    Roger Hicks likes this.
  3. cliveva

    cliveva Well-Known Member

    when you increase ISO it is the effects of the processor that effect the image,(through amplification) rather than the light gatheing abilities of the sensor. The dynamic range is better in FF, which is why I use FF.
  4. Terrywoodenpic

    Terrywoodenpic Well-Known Member

    If the larger sensor had the same pixel density and was identical in everything except different in size, the light gathering ability would be identical.
    However sensors are not the same Pixel sizes, densities. micro lenses, front or back illumination and many other construction details and firmware choices ensure that they are all very different.
    In terms of real results it is usually impossible to tell which camera took which photograph.
    When printed at normal sizes it is usually impossible to know if a shot were taken on a crop or full frame sensor.
    Differences are usually only apparent at extreme enlargement or in extreme conditions.
    Even then not every guess would be right.
    The general wisdom that a good big one will always beat a good little one it getting harder to prove. And for some subject matter is not true at all.
  5. GrahamWorley

    GrahamWorley New Member

    Hmm. Isn't the dynamic range a product of the amount of bits the camera reads out from the ADC on the sensor less the effect of noise. Both the Pentax models listed have 14bit readout implying the theoretical maximum dynamic range is 2 raised to the power 14. In practice this would be lowered by noise but I wouldn't expect the size of the sensor to affect the dynamic range. As Terry pointed out pixel density is the issue, if this is the same then we would assume equal noise performance. I guess in the past we've not seen equal pixel densities, but with full frame sensors now pushing 50megapixel the density matches that of a 24megapixel crop sensor. Thus I would suggest it is likely that the difference in performance would narrow or even vanish.
  6. PeteRob

    PeteRob Well-Known Member

    Dynamic range is less. Some Sony made sensors claim 14 stops but 12 - 13 is more typical for 14 bit sensor at base ISO.

    If you view a similarly framed image taken on crop sensor and full frame then you are enlarging the results from the smaller sensor. For wide angle shots using the full area of the frame this favours full frame cameras especially if making big prints. With telephotos and subjects that need cropping this favours the crop-sensor. There is also a tendency to use budget lenses on crop sensor cameras and big enlargements will challenge them.

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