Are there any GOOD Digital Instructionals?

Started Sep 20, 2013 | Discussions thread
bobn2 Forum Pro • Posts: 54,937
Re: Cambridge in Color

MisterBG wrote:

bobn2 wrote:

Osvaldo Cristo wrote:

Cambridge in Color Tutorials is a very good start up point to digital photography technology. I recommend it strongly.


Been there already in this thread. It's very poor, has many gross errors and misleads many people.

Examples please...

Where to start...

In photography, the exposure settings of aperture, shutter speed and ISO speed are analogous to the width, time and quantity discussed above.

ISO is nothing like quantity. Reading this has misled people into thinking that increasing the ISO means you get more light.

ISO speed: controls the sensitivity of your camera's sensor to a given amount of light

No, it doesn't. The sensitivity of your camera's sensor never changes. ISO changes the relationship between exposure (as measured by the sensor) and image brightness, and doesn't change what the sensor does at all.

One can therefore use many combinations of the above three settings to achieve the same exposure.

No, only changing the f-number and shutter speed affects exposure.

The ISO speed determines how sensitive the camera is to incoming light. Similar to shutter speed, it also correlates 1:1 with how much the exposure increases or decreases.

No, it doesn't. Changing the ISO does not of itself changes the exposure. Only the f-number and shutter changes exposure. (see comments on the sensor above)

a lower ISO speed is almost always desirable, since higher ISO speeds dramatically increase image noise.

No they don't. Often higher ISO's reduce noise. What increases noise is low exposure.

When shooting in RAW mode under tricky lighting, sometimes it is useful to set a slight negative exposure compensation (0.3-0.5). This decreases the chance of clipped highlights, yet still allows one to increase the exposure afterwards.

You cannot change exposure after capture.

"Image noise" is the digital equivalent of film grain for analogue cameras. Alternatively, one can think of it as analogous to the subtle background hiss you may hear from your audio system at full volume.

Only a small part of digital camera noise arises from similar mechanisms to radio background hiss, the idea that it is similar leads to many mistakes, shared by McHugh, below.

Noise increases with the sensitivity setting in the camera

No, if anything it reduces (particularly the background hiss kind of noise he's talking about)

Some degree of noise is always present in any electronic device that transmits or receives a "signal." For televisions this signal is the broadcast data transmitted over cable or received at the antenna; for digital cameras, the signal is the light which hits the camera sensor. Even though noise is unavoidable, it can become so small relative to the signal that it appears to be nonexistent. Thesignal to noise ratio (SNR) is a useful and universal way of comparing the relative amounts of signal and noise for any electronic system.

The most important noise in cameras is the noise in the light itself, ignored in this discussion - misidentifying the noise source leads to plenty of mistakes later.

A camera's "ISO setting" or "ISO speed" is a standard which describes its absolute sensitivity to light.

'ISO' does nothing of the sort. Just read the standard to see that it doesn't.

Random noise is characterized by intensity and color fluctuations above and below the actual image intensity. There will always be some random noise at any exposure length and it is most influenced by ISO speed. The pattern of random noise changes even if the exposure settings are identical.

This last statement is simply and demonstrably false (unless he's just saying it's random)

For digital cameras, darker regions will contain more noise than the brighter regions

Brighter regions contain more noise, but they have a higher signal to noise ratio.

You can also see that increasing ISO speed always produces higher noise for a given camera

Wrong, reducing exposure (which might happen as a consequence of increasing ISO) produces higher noise (more specifically, a lower SNR)

The greater the area of a pixel in the camera sensor, the more light gathering ability it will have — thus producing a stronger signal. As a result, cameras with physically larger pixels will generally appear less noisy since the signal is larger relative to the noise.

This is a misleading simplification, the reasons probably too complex to argue here. The 'stronger signal' bit is hopelessly simplified. All you do changing the pixel size is determine how often the signal is sampled, not how 'strong' (whatever than means) it is.

This is why cameras with more megapixels packed into the same sized camera sensor will not necessarily produce a better looking image.

In general more megapixels produces a better looking image, viewed the same size (there are exceptions)

dynamic range is generally higher for digital SLR cameras compared to compact cameras (due to larger pixel sizes)

Not at all true. In general, smaller pixels (on the same size sensor) produce higher DR's. Digital SLR's produce higher DR vecause the collect more light, which is due to having large apertures with the same AOV - it has nothing to do with having 'larger pixel sizes'.

The black level is limited by how accurately each photosite can be measured, and is therefore limited in darkness by image noise. Therefore, dynamic range generally increases for lower ISO speeds and cameras with less measurement noise.

'Black level' is not set by image noise (although a high black level might be used to mask shadow noise). There is no causality in this statement. DR increases for lowere ISO's simply because exposure is (usually) increased and more light collected.

Most digital cameras use a 10 to 14-bit A/D converter, and so their theoretical maximum dynamic range is 10-14 stops. However, this high bit depth only helps minimize image posterization since total dynamic range is usually limited by noise levels.

Once the bit depth exceeds the DR (in stops) the extra bits do not minimise posterisation, the dithering caused by the noise does that.

As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance).

No, actually it doesn't. If the aperture is the same the DOF is the same. Of course, for the same aperture, with the same AOV therefore different FL, the f-number is different, which is probably what he meant, but f-numner is not aperture (I didn't raise that terminological confusion when it occurred in the section on camera controls, now we see the price)

This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject.

But he just said we're filling the frame with a subject the same size and distance, intoducing getting closer is just a confusion.

Larger sensor sizes can use smaller apertures before the diffraction airy disk becomes larger than the circle of confusion

He's already confused about apartures (see above). The simple truth he's not telling here is that diffraction is the same at the same DOF, whatever the sensor size.

Use the following calculator to estimate when diffraction begins to reduce sharpness.

No, don't. That calculator is complete nonsense, and applies to no observable reality.

The following diagrams show the size of the airy disk (theoretical maximum resolving ability) for two apertures against a grid representing pixel size:

Ths size of the Airy disc is not the 'theoretical maximum resolving ability'.

An important implication of the above results is that the diffraction-limited pixel size increases for larger sensors

There is no such thing as a 'diffraction limited pixel size'.

This factor may be critical when deciding on a new camera for your intended use, because more pixels may not necessarily provide more resolution (for your depth of field requirements). In fact, more pixels could even harm image quality by increasing noise and reducing dynamic range (next section).

There is no evidence whatever that this is true, in fact rather the reverse.

Larger sensors generally also have larger pixels (although this is not always the case), which give them the potential to produce lower image noise and have a higher dynamic range.

Not true. The larger DR of larger sensors derives from their ability to collect more light. For the same size sensor, more pixels will generally produce a larger DR.

Further, larger pixels receive a greater flux of photons during a given exposure time (at the same f-stop), so their light signal is much stronger. For a given amount of background noise, this produces a higher signal to noise ratio — and thus a smoother looking photo.

The size of the pixels does not affect the flux of photons, nor does it provide a stronger 'light signal'. As above, all it does is sample the signal that there is more frequently. The SNR is not higher (at the same measurement scale) and viewed the same size the picture is generally not smoother.

Depth of field is much shallower for larger format sensors

Only if you specify 'at the same f-number', which he hasn't.

In other words, if one were to use the smallest aperture before diffraction became significant, all sensor sizes would produce the same depth of field — even though the diffraction limited aperture will be different.

He's finally said the most important result but the bit about the 'diffraction limited aperture' is nonsense, This is something he has made up, a non-existant phenomenon.

That's just a few of his pages. Should I go on?

The site is riddled with nonsense, and a good part of the discussion that goes on on the forums here is clearing up the mistakes and misconceptions caused by that site.

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