You do not understand f-stop at all by dtmateojr

Started Jul 27, 2014 | Discussions thread
bobn2
bobn2 Forum Pro • Posts: 62,713
On the cited blogs
10

1. http://dtmateojr.wordpress.com/2014/02/28/understand-your-lens-part-3/ — Concentrate on understanding the effect of focal length on light intensity because a lot of people tend to ignore this bit.

OK, you're attempting to explain f-number. Ultimately a sloppily written and poorly conceived article, but not actually wrong - just that it doesn't add anything to this discussion. The f-number is the focal length divided by the aperture. We knew that.

2. http://dtmateojr.wordpress.com/2014/03/08/debunking-the-myth-of-full-frame-superiority/ — If there is one thing that you’d.want to fully understand here, make it the “thought experiment” on dividing a full frame sensor which also explains how shutter curtains work.

(from that article)

Anyway, as the myth goes, a full frame sensor has less noise compared to a smaller sensor because it can gather more light. The basic premise is that all things being equal (lens, sensor, processor, incident light), the size of the sensor is the single most significant factor that affects noise because of its superior light gathering capacity. As I have mentioned in my previous post, this is not true. In fact, in reality, a full frame sensor gathers less light than a smaller sensor and I will expain this later.

This is total baloney, so let's look at the 'explanation'. It's too wordy and unfocussed to repeat here (you can g back to your article if you think I'm misrepresenting you), but the essence is that since the FF 'needs' a longer focal length, then the light at the sensor coming from the aperture will be attenuated by the inverse square law and therefore the FF sensor will receive less light. Now, the obvious endpoint of this argument is that smaller sensors are better and the smaller they are, the better they are. Therefore, if it's true we'll find that phonecams are the best performers of all cameras. Yet people use Four Thirds and APS-C sensors and get better results than phonecams. So, lets take a sanity check. Your argument doesn't accord with reality, so it is simply wrong. Why does the inverse square law operate - simply because the light is spread over an ever larger spherical shell as it travels outwards from the source. So, imagine that we had a light collector at 50mm from the principal plane of a lens and another at 100mm, with both lenses having the same aperture diameter. To collect the same light the 100mm collector must have four times the area since it is positioned at a 'shell' with twice the radius and therefore four times the area. Now, this relationship between aperture, focal length and sensor size is precisely what we have for two cameras with a relative crop factor of 2, 'equivalent' focal lengths and where the f-number of the larger system is double the f-number of the smaller. This article makes a lot of other fallacious points including your bogus 'thought experiment' about covering up half of the sensor of a D800, where apparently each half retains the noise characteristics of the whole sensor. Again, demonstrably not true, at least if you allow the half images to be displayed same size as the full image, which is the only sensible way of comparing imaging systems with different size sensors.

3.http://dtmateojr.wordpress.com/2014/06/10/debunking-the-myth-of-full-frame-superiority-part-2/ — This is a good counter-argument to the fact that no two digital sensors are exactly the same even if they are of the same type.

This article claims to debunk DPR's suggestion that the Sigma 18-35/1.8 APS-C zoom is 'equivalent' to an f/2.8 FF zoom. It depends on two arguments. One, an f-number is always an f-number. Well, yes, but why would we use f-number to compare the effect of lenses on different sensor-size systems. This is not ever explained. The second argument is film. Film manufacturers made the same emulsion for different formats and it always had the same sensitivity, apparently this 'busts' the myth. This is a simple non-sequitur. The manufacture of emulsion in different formats leads in no way to the conclusion that the Sigma is not equivalent to an f/2.8 lens on FF.

4. http://dtmateojr.wordpress.com/2014/05/19/megapixel-hallucinations/ — Some full frame protagonists insist on comparing ENLARGED APS-C images to full frame equivalents in terms of noise. Of course an enlarged APS-C photo will, for the lack of a better word, enlarge everything including noise.

This article is aimed at DxOmark's practice of normalising their data to the same effective image size. It's argued that this is intrinsically wrong without saying why (in fact it is suggested, rather disrespectfully, that DxOmark only does it because they are in the business of 'ranking cameras'). The real question is what are the sensible conditions under which to compare cameras, and to me at least the same final image size one seems entirely reasonable. If you were to tell an APS-C buyer that their camera was OK but thy had to restrict it to making images 0.7 times the linear size of those form an FF camera, they would think you were talking nonsense (for good reason). There is no direct correlation between sensor size and output image size, people buy cameras to make images of the size that they want to make, not dictated by the size of the camera sensor. In any case, how is an image comparable with a much smaller one?

5.http://dtmateojr.wordpress.com/2014/04/21/rain-can-teach-us-photography/ — explains what happens in a sensor and why PIXEL size and NOT sensor size matters in greater detail.

The crux of this argument is the following text:

Again, sensor size does not affect exposure (rain level analogy). A f-stop is a f-stop (rain gauge analogy) and is not affected by lens focal length or sensor size. Therefore a smaller sensel will have the same exposure as a larger sensel. However, a larger sensel gathers more light therefore it will have lesser noise. This is why a 12Mp full frame has better noise performance vs a 12Mp APS-C sensor. This is also why a 12Mp full frame Nikon D700 has way better noise performance vs a 36Mp full frame D800 by virtue of the larger sensels.

Unfortunately for your argument, this is not true, the D800 has way better noise performance than the D700, but don't let facts stand in your way.

This is also why a 16Mp APS-C D7000 has the SAME noise profile as a full frame 36Mp D800.

Also not true when you view the images the same size.

And thus, we arrive at the following conclusions:

1. SENSOR size has no effect on exposure.

Well, that one is trivially true from the definition of exposure. It has no impact on the other arguments at all, however.

2. SENSOR size has no effect on noise.

You are arguing from a false antecedent , twice (it isn't true that a 12MP FF has better noise performance than a 12MP APS-C sensor because 'a larger sensel gathers more light therefore it will have lower noise' and it also isn't true that the D700 has better noise performance than a D800).

3. SENSEL size ultimately affects noise.

the same thing again. in fact, you can demonstrate the falsity of this argument simply by taking the DxOmark comparison of the two cameras (D700, D800).

By the way, in one of your articles you call the people who have a different opinion than yours 'clowns'. You'd need to be very sure that you could substantiate your point of view in the face of yours before applying such an insult, even if it were necessary at all. Frankly, given the quality of the arguments you have put forward above, your confidence is misplaced.

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Bob

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