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I see that Sony FDR-AX100's lens has opens at f/2.8 while Sony FDR-AX33's lens opens at f/1.8

I am often doing photos (Canon 6D) or videos in places where a f/1.8 lens would be a great advantage over a f/2.8 lens. (And things are usually moving so filming with 6D and such an aperture is not an option.)

But what I read on the Internet tends to say that "FDR-AX100 is better than FDR-AX33 in low light because it has a bigger sensor."

So maybe I haven't tried enough device yet and ask the wrong question but to what extent is a bigger sensor better for low light than a wider opening ? Will FDR-AX100 really be better where a wider aperture the f/2.8 could be used ?

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These are video cameras, but the basic principle is the same as with still cameras: the FDR-AX100 has a much larger sensor. It is a 1" type, while the FDR-AX33 has 1/2.3" type sensor. (These formats are common in still cameras too, with the 1/2.3" size being prevalent in compact point and shoots and 1" becoming more common in higher-end compacts and a few interchangeable-lens systems).

The 1/2.3" sensor measures about 6.17mm × 4.55mm, for a sensor area of about 28.50mm². A 1" sensor is about 13.20mm × 8.80mm, or 116mm², four times the area. In photography, we normally think of metering, exposure, and light-gathering ability in terms of "stops", where each stop is a doubling or halving — so, 4× is two doublings, or, a two-stop advantage.

Meanwhile, going from f/1.8 to f/2.8 is about one and a third stops. So, even with the faster lens, the larger sensor has an advantage.

Of course, that's assuming the underlying technology is basically the same. Sensors can also vary in fundamental sensitivity and noise levels as well, so this is only a general guideline.

Oh, and also: if you view at the same size (which presumably you would, with video), the difference in sensor size also affects apparent depth of field, with the effect approximately equal to the ratio of the sensor's width or height — so in this case, in terms of depth of field, f/1.8 on the smaller-sensored camera will give approximately the same depth of field as f/3.6 on the larger-sensored one, assuming the same framing. (Or to put that another way, if you want more shallow depth of field, you can get more from the FDR-AX100, even though the lens is slower. With either camera, assuming the light is good, you can stop down for more depth of field — more of the scene in focus.)

If the FDR-AX33 had an f/1.4 lens, both of these factors would just about exactly cancel each other out. (Since f/1.4 to f/2.8 is two whole stops.)

  • I am getting a bit confused in the depth of field parts, and never sure if a want more or less depth of field (in terms of vocabulary), but say it another way, in the case of videos, I do not want "bokeh". So far, and after trying some online calculator, I have the feeling that the smaller sensor may be better for this. Am right or is it the other way around? – TTT May 11 '15 at 14:40
  • Another point I am worried about now is that I'm use to shoot videos with Sony RX-100 M2, which I'm looking to upgrade. But it is an ultracompact with a 1'' with f/1.8 aperture, so both AX33 and AX100 would be a loss in f-stop, right ? – TTT May 11 '15 at 14:46
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    @TTT right, in this case, the smaller sensor will have a slight bit more in focus for the same framing, at the widest possible aperture. However, if you have enough light and stop down the aperture on the 1"-sensor camera to f/3.6 it will be the same. Since the sensor is 4× larger, though, that two-stop loss of light is theoretically roughly exactly compensated for and you should see about the same level of noise. So, in theory, there's no disadvantage to the larger sensor in any case. – mattdm May 11 '15 at 14:51
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    No, the other way around. The RX100m2 has both the larger sensor and the faster lens and is therefore theoretically better than either video camera. Note, though, that these are only theoretical factors and there may be others (including the technology that went into the sensor itself) which may also play in. (A smaller sensor from today may beat a larger one from five years ago in many areas — or it might not.) – mattdm May 11 '15 at 14:58
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    @TTT I have no idea about 4k video. That question is probably better for video.stackexchange.com – mattdm May 12 '15 at 13:25

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