Photography notes

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Digital

On digital ISO

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

tl;dr:

• higher ISO means more gain during readout of the sensor.

• Which can be seen as more sensitivity,

since it's just amplification, it increases the amount of signal as much as the noise

...but it's a little more interesting than that due to the pragmatics of photography (and some subtleties of electronics)

• You generally want to set it as low as sensible for a situation, or leave it on auto

ISO in film indicates the grain size in photo rolls (see ISO 5800). This amounts to: Smaller grains bring out finer detail but require more light to react for the same amount of image; larger grains is coarser but more sensitive.

ISO in digital photography (see also ISO 12232) is different. It refers to the amplification used during the (at this stage still analog) readout of sensor rows - basically, what gain to use before feeding the signal to the ADC. ...okay, but what does that do? What does it amount to in practice?

Given a sensor with an image currently in it, the only change it would really make to the readout is brightness, not signal to noise, or quality in any way.

In that sense, it has no direct effect on the amount of light accumulated in the sensor. However, since it is one of the physical parameters the camera chooses (alongside aperture and shutter time), it can choose to trade off one for another.

For example, in the dark, a camera on full auto is likely to choose a wide open aperture (for the most light), and then choose a higher ISO if that means the shutter time can be lowered to not introduce too much motion blur from shaking hands.

There are other such tradeoffs, e.g. controlled via modes -- for example, portrait mode tries to open the aperture so the background is blurred, sports mode aiming for short shutter time so you get minimal motion blur, but at the cost of noise, and more. But most of these are explained mostly in terms of the aperture/shutter tradeoff, and ISO choice is relatively unrelated, and can be explained as "as low as is sensible for the light level".

Still, you can play with it.

Note that the physical parameters are chosen with the sensor in mind - to not saturate its cells (over-exposure) {{comment|(also other constraints, like avoiding underexposure, signal falling into the noise floor, and in general also tries to use much of storage range it has (also to avoid unnecessary quantization, though this is less important.)

As such, when you force a high ISO (i.e. high gain) but leave everything else auto, you will effectively force a camera to choose a lower shutter time and/or smaller aperture.

Which means less actual light being used to form the image, which implies lower signal-to-noise (because a noise floor is basically a constant). The noise is usually still relatively low, but the noise can become noticeable e.g. in lower-light conditions.

Similarly, when you force a low ISO, the camera must plan for more light coming in, often meaning a longer exposure time. On a tripod this can mean nicely low-noise images, while in your hands it typically means shaky-hand blur.

In a practical sense: when you have a lot of light, somewhat low ISO gives an image with less noise. When you have little light, high ISO lets you bring out what's there, with inevitable noise. Leaving it on auto tends to do something sensible.

Infrared

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Infrared and digital sensors

Bare optical camera sensor's construction typically means they have sensitivity that runs off into UV on one end and into IR on the other.

Note that with just how large the range of infrared is, that's only near-infrared, and then only a smallish part of that.

For reference, our eyes see ~700nm (~red) to ~400nm (~violet) (what we call visible light), while CCD and CMOS image sensors might see perhaps 1000nm to ~350nm.

The amount and shape of that overall sensitivity varies, but tl;dr: they look slightly into near-IR (also slightly into UV-A on the other end).

This is mostly a bug rather than a feature, so there is usually specifically a filter to remove that sensitivity.

Changing that sensitivity

Broadly speaking, there are two things we might call "infrared filters": IR-cut and IR-pass, which do exactly opposite things of each other.

• IR-cut filters are the thing already mentioned above: on optical image sensors, these will remove most of that IR response

because it turns out to be easier to add a separate infrared-cut filter than to design the sensor itself to be more selective(verify)

usually a glass filter right on top of the camera image sensor, because you'd want it always and built in

they look transparent, with a bluish tint from most angles (because they also remove a little visible red)

cuts a good range above some point, or rather transition, often somewhere around 740...800nm

Since it's a transition, bright near-IR might still be visible, particularly for closer wavelengths. For example, IR remote controls are often in the 840..940nm range, and send short but intense pulses, which tends to still be (barely) visible. But there aren't a lot of bright near-IR sources that aren't also bright in visible light, so this smaller leftover sensitivity just doesn't matter much.

• IR-pass, visible-cut filters

often look near-black

cut everything below a transition, somewhere around 720..850nm range

using these on a camera that has an IR-cut will give you very little signal (it's much like an audio highpass and lowpass set to about the same frequency - you'll have very little response left)

but on a camera with IR sensitivity, this lets you view mostly IR without much of the optical

Notes:

• DIY versus non-DIY NIR cameras

image sensors are usually made for color, which means each pixel has its own color filter. Even if you remove the IR filter, each of these may also filter out some IR

as such, even though you can modify a many optical sensors to detect IR, they'll never do so quite as well as a specific-purpose one, but it's close enough for creative purposes

There are also NIR cameras, which are functionally much like converted handhelds, but a little more refined than most DIYing, because they select sensors that look further into NIR (verify)

• If your camera's IR-cut is a physically separate filter, rather than a coating, you can removing that to get back the sensitivity that the sensor itself has

doing so leaves you with something that looks mostly like regular optical, but stronger infrared sources will look like a pink-ish white.

White-ish because IR above 800nm or so passes through all of the bayes filter color filters similar

pink-ish mostly because infrared is next to red so the red and it passes it more, so red tends to dominate.

This is why IR photographers may use a color filter to reduce visible red -- basically so that the red pixels pick up mostly IR, not visible-red-and-IR.

Which is just as false-color as before but looks a little more contrasty.

These images may then be color corrected to appear more neutral, which tends to come out as white-and-blueish

• visible-and-IR mixes will look somewhat fuzzy, because IR has its own focal point, so in some situations you would the visible-cut, IR-pass filter

• You can't make a thermal camera with such DIYing - those sensors are significantly different.

Thermographic cameras often sensitive to a larger range, like 14000nm to 1000 nm, the point being that they focus more on mid-infrared than near-infrared

In photography, base cameras and lens filters give you options like:

cutting IR and some red - similar to a regular blue filter

passing only IR

passing IR plus blue - happens to be useful for crop analysis(verify)

passing IR plus all visible

passing everything (including the little UV)

For some DIY, like FTIR projects,

note that there are ready-made solutions, such as the Raspberry's NoIR camera

If you want to do this yourself, you may want to remove a webcam's IR-cut filter - and possibly put in an IR-pass filter.

• In webcams the IR-cut tends to be a glass in the screwable lense

which may just be removable

• For DSLRs the IR-cut this is typically a layer on top of the sensor

which can be much harder to deal with

two IR-cut filters from webcams. Looking through them looks blue, they reflect red.

While in DSLRs the IR-cut is typically one of a few layers mounted on top of the sensor (so that not every lens has to have it), in webcams, the IR-cut filter may well be on the back of the lens assembly

See also:

• http://dpfwiw.com/ir.htm#ir_filters

• https://kolarivision.com/articles/choosing-a-filter/