Camera ISO Settings

The Truth About ISO

Back in the days of ‘wet photography’, we had rolls and sheets of film that carried various ISO/ASA/DIN numbers.

ISO stands for International Standards Organisation

ASA stands for American Standards Association

DIN – well, that’s ‘Deutsches Institut für Normung’ or German Institute for Standardisation

ISO and ASA were basically identical values, and DIN = (log10)ISO x10 +1, so ASA/ISO 100 equated to DIN 21….nope, I’m not going to say anything!

These numbers were the film ‘speed’ values.  Film speed was critical to exposure metering as it specified the film sensitivity to light.  Metering a scene properly at the correct ISO/ASA/DIN gave us an overall exposure value that ensured the film got the correct ‘dose’ of light from the shutter speed and aperture combination.

Low ISO/ASA/DIN values meant the film was LESS sensitive to light (SLOW FILM) and high values meant MORE sensitivity to light (FAST FILM).

Ilford Pan F was a very slow mono negative film at ASA 50, while Ilford HP5 was a fast 400 ASA mono negative film.

The other characteristic of film speed was ‘grain’.  Correctly exposed, Pan F was extremely fine grained, whereas correctly exposed HP5 was ‘visibly grainy’ on an 8×10 print.

Another Ilford mono negative film I used a lot was FP4.  The stated ASA for this film was 125ASA/ISO, but I always rated it (set the meter ASA speed dial) to 100ASA on my 35mm Canon A1 and F1 (yup, you read that right!) because they both slightly over-metered most scenes.

If we needed to shoot at 1/1000th and f8 but 100ASA only gave us 1/250th at f8 we would switch to 400ASA film – two stops greater sensitivity to light means we can take a shutter speed two stops shorter for the same aperture and thus get our required 1/1000th sec.

But, what if we were already set up with 400ASA film, but the meter (set at 400ASA) was only giving us 1/250th?

Prior to the release of films like Delta 1600/3200 we would put a fresh roll of 400ASA film in the camera and set the meter to a whopping 1600ASA! We would deliberately UNDER EXPOSE Ilford HP5 or Kodak Tri-X by 2 stops to give us our required 1/1000th at f8.

The two stops underexposed film would then be ‘push processed’, which basically meant it was given a longer time in the developer.  This ‘push processing’ always gave us a grainy image, because of the manner in which photographic chemistry worked.

And just to confuse you even more, very occasionally a situation might arise where we would over expose film and ‘pull process’ it – but that’s another story.

We are not here for a history lesson, but the point you need to understand is this – we had a camera body into which we inserted various sensitivities of film, and that sometimes those sensitivities were chemically manipulated in processing.

That Was Then, This Is Now!

ISO/ASA/DIN was SENSITIVITY of FILM.

It is NOT SENSITIVITY of your DSLR SENSOR….!!! Understand that once and for all!

The sensitivity of your sensor IS FIXED.

It is set in Silicon when the sensor is manufactured.  Just like the sensitivity of Kodak Tri-X Pan was ‘fixed’ at 400ASA/ISO when it was made at the factory.

How is the sensitivity of a digital sensor fixed?  By the SIZE of the individual PHOTOSITES on the sensor.

Larger photosites will gather more photons from a given exposure than small ones – it’s that simple.

The greater the number of photons captured means that the output signal from a larger photosite is GREATER than the output signal from a smaller photosite for the same exposure value (EV being a combination shutter speed and aperture/f number).

All sensors have a base level of noise – we can refer to this as the sensor ‘noise floor’.

This noise floor is an amalgamation of the noise floors of each photosite on the sensor.

But the noise floor of each photosite on the sensor is masked/obscured by the photosite signal output; therefore the greater the signal, the larger the signal to noise (S/N) ratio is said to be.

In general, larger photosites yield a higher S/N ratio than smaller ones given the same exposure.

This is why the Nikon D3 had such success being full frame but just over 12 megapixels, and it’s the reason that some of us don’t get overly excited about seeing more megapixels being crammed into our 36mm x 24mm sensors.

Anyway, the total output from a photosite contains both signal and noise floor, and the signal component can be thought of as ‘gain’ over the noise floor – natural gain.

As manufacturers put more megapixels on our sensors this natural gain DECREASES because the photosites get SMALLER – they have to in order to fit more of them into the finite sensor area.

Natural gain CAN be brought back in certain sensor designs by manipulating the design of the micro lenses that sit on top of the individual photosites. Re-design of these micro lenses to ‘suck in’ more tangential photons – rather like putting a funnel in a bottle to make filling it easier and more efficient.

There is a brilliantly simple illustration of how a sensor fits into the general scheme of things, courtesy of digital camera world:

%name Camera ISO Settings

The main item of note in this image is perhaps not quite so obvious, but it’s the boundary between the analogue and digital parts of the system.

We have 3 component arrays forward of this boundary:

  1. Mosaic Filter including Micro Lenses & Moire filter if fitted.
  2. Sensor Array of Photosites – these suck in photons and release proportional electrons/charge.
  3. Analogue Electronics – this holds the charge record of the photosite output.

Everything forward of the Analogue/Digital Converter – ADC – is just that, analogue! And the variety of attributes that a manufacturer puts on the sensor forward of this boundary can be thought of mostly as modifying/enhancing natural gain.

So What About My ISO Control Settings Andy?

All sensors have a BASE ISO. In other words they have an ISO sensitivity/speed rating just like film!  And as I said before THIS IS A FIXED VALUE.

The base ISO of a sensor photosite array can be defined as that ISO setting that yields the best dynamic range across the whole array, and it is the ISO setting that carries NO internal amplification.

Your chosen ISO setting has absolutely ZERO effect on what happens forward of the Analogue/Digital boundary – NONE.

So, all those idiots who tell you that ISO effects/governs exposure are WRONG – it has nothing to do with it for the simple reason that ISO effecting sensor sensitivity is a total misconception….end of!

Now I’ll bet that’s going to set off a whole raft of negative comments and arguments – and they will all be wrong, because they don’t know what they’re talking about!

The ‘digital side’ of the boundary is where all the ‘voodoo’ happens, and it’s where your ISO settings come into play.

At the end of an exposure the Analogue Digital Converter, or ADC, comes along and makes a ‘count’ of the contents of the ‘analogue electronics’ mosaic (as Digital Camera World like to call it – nice and unambiguous!).

Remember, it’s counting/measuring TOTAL OUTPUT from each photosite – and that comprises both signal and noise floor outputs.

iso1 900x900 Camera ISO Settings

If the exposure has been carried out at ‘base ISO’ then we have the maximum S/N ratio, as in column 1.

However, if we increase our ISO setting above ‘base’ then the total sensor array output looks like column 2.  We have in effect UNDER EXPOSED the shot, resulting in a reduced signal.  But we have the same value for the noise floor, so we have a lower S/N ratio.

In principal, the ADC cannot discriminate between noise floor and signal outputs, and so all it sees in one output value for each photosite.

At base ISO this isn’t a problem, but once we begin to shoot at ISO settings above base, under exposing in other words, the cameras internal image processors apply gain to boost the output values handed to it by the ADC.

Yes, this boosts the signal output, but it also amplifies the noise floor component of the signal at the same time – hence that perennial problem we all like to call ‘high ISO noise’.

So your ISO control behaves in exactly the same way as the ‘gain switch’ on a CB or long wave radio, or indeed the db gain on a microphone – ISO is just applied gain.

Things You Should Know

My first digital camera had a CCD (charge coupled device) sensor, it was made by Fuji and it cost a bloody fortune.

Cameras today for the most part use CMOS (complimentary metal oxide semi-conductor) sensors.

  • CCD sensors create high-quality, low-noise images.
  • CMOS sensors, traditionally, are more susceptible to noise.
  • Because each photosite on a CMOS sensor has a series of transistors located next to it, the light sensitivity of a CMOS chip tends to be lower. Many of the photons striking the sensory photosite array hit the transistors instead of the photosites.  This is where the newer micro lens designs come in handy.
  • A CMOS sensor consumes less power. CCD sensors can consume up to 100 times more power than an equivalent CMOS sensor.
  • CMOS chips can be produced easily, making them cheaper to manufacture than CCD sensors.

Basic CMOS tech has changed very little over the years – by that I’m referring to the actual ‘sensing’ bit of the sensor.  Yes, the individual photosites are now manufactured with more precision and consistency, but the basic methodology is pretty much ‘same as it ever was’.

But what HAS changed are the bits they stick in front of it – most notably micro-lens design; and the stuff that goes behind it, the ADC and image processors (IPs).

The ADC used to be 12 bit, now they are 14 bit on most digital cameras, and even 16 bit on some.  Increasing the bit depth accuracy in the ADC means it can detect smaller variations in output signal values between adjacent photosites.

As long as the ‘bits’ that come after the ADC can handle these extended values then the result can extend the cameras dynamic range.

But the ADC and IPs are firmware based in their operation, and so when you turn your ISO above base you are relying on a set of algorithms to handle the business of compensating for your under exposure.

All this takes place AFTER the shutter has closed – so again, ISO settings have less than nothing to do with the exposure of the image; said exposure has been made and finished with before any ISO applied gain occurs.

For a camera to be revolutionary in terms of high ISO image quality it must deliver a lower noise floor than its predecessor whilst maintaining or bettering its predecessors low ISO performance in terms of noise and dynamic range.

This where Nikon have screwed their own pooch with the D5. At ISOs below 3200 it has poorer IQ and narrower dynamic range than either the D4 or 4S.  Perhaps some of this problem could be due to the sensor photosite pitch (diameter) of 6.45 microns compared to the D4/4S of 7.30 microns – but I think it’s mostly due to poor ADC and S/N firmware; which of course can be corrected in the future.

Can I Get More Photons Onto My Sensor Andy?

You can get more photons onto your sensor by changing to a lens that lets in more light.

You might now by thinking that I mean switching glass based on a lower f-number or f-stop.

If so you’re half right.  I’m actually talking about t-stops.

The f-number of a lens is basically an expression of the relationship between maximum aperture diameter and focal length, and is an indication of the amount of light the lens lets in.

T-stops are slightly different. They are a direct indicator of how much light is transmitted by the lens – in other words how much light is actually being allowed to leave the rear element.

We could have two lenses of identical focal length and f-number, but one contains 17 lens elements and the other only 13. Assuming the glass and any coatings are of equal quality then the lens with fewer elements will have a higher transmission value and therefore lower T-number.

As an example, the Canon 85mm f1.2 actually has a t-number of 1.4, and so it’s letting in pretty much HALF a stop less light than you might think it is.

In Conclusion

I’ve deliberately not embellished this post with lots of images taken at high ISO – I’ve posted and published enough of those in the past.

I’ve given you this information so that you can digest it and hopefully understand more about how your camera works and what’s going on.  Only by understanding how something works can you deploy or use it to your best advantage.

I regularly take, market and sell images taken at ISO speeds that a lot of folk wouldn’t go anywhere near – even when they are using the same camera as me.

The sole reason I opt for high ISO settings is to obtain very fast shutter speeds with big glass in order to freeze action, especially of subjects close to the camera.  You can freeze very little action with a 500mm lens using speeds in the hundredths of a second.

Picture buyers love frozen high speed action and they don’t mind some noise if the shot is a bit special. Noise doesn’t look anywhere near as severe in a print as it does on your monitor either, so high ISO values are nothing to shy away from – especially if to do so would be at the expense of the ‘shot of a lifetime’.

Twilight & Astro Landscape Photography

Twilight & Astro Landscape Photography

Everyone likes a nice moody sunset, but great images await those camera operators that start shooting after most folks have started packing their gear away and heading home.

For me, twilight is where the fun starts.

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The rock stack lying off the boulder-strewn beach of Porth Saint, Rhoscolyn Head, Anglesey.

The low light levels on a scene once ‘civil daylight’ has ended mean you get awesome light with lower contrast shadows, subtle skies, and nice long shutter speeds for dreamy water effects without needing expensive 10 stop ND filters.

However, that awesome light vanishes very quickly, so you have to be ready!  I waited nearly 90 minutes for the shot above.

But that time was spent doing ‘dry runs’ and rehearsals – once the composition was set how I wanted it, the foreground was outside of DoF, so I knew I needed to shoot a focus stack as well as an exposure blend…mmmm….yummy!

Once we have made the long transition from civil daylight end to astronomical daylight end the fun really begins though.

Astro Landscape Photography

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The Milky Way over the derelict buildings of Magpie Mine in Derbyshire.

Astro landscape photography, or wide field astro as it’s sometimes known, is not as difficult as a lot of photographers imagine.

But astro landscape photography IS very demanding of your familiarity with your gear, and will require some expenditure on additional bits of kit if disappointment is to be avoided.

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D4D6747 Edit Twilight & Astro Landscape PhotographyD4D6754 Edit Twilight & Astro Landscape PhotographyHere’s the kit I usually venture out at night with:

Dew Heater Band (A).

An essential bit of kit for astro landscape photography – it’s amazing how rapidly a lot of lenses, especially super-wides like the Nikon 14-24 f2.8 encounter a problem with dew at night.  This will in effect fog the front element, starting at its centre and if left unchecked it can spread across the entire face of the lens.

Heating the lens front sufficiently to keep its temperature above the dew point for your current location and time will prevent a ruined session – don’t leave home without one!

This dew heater is powered by a battery (C) via a dew heater controller (D) with is basically a simple rotary rheostat which controls the level of current driving the heater band.

I use mine at about 75% of ‘full chat’ and it seems to work just fine.

A final note on dew heater bands – these are designed for use by those strange folk who spend hours behind telescopes.  They tape the bands in place and leave them there.  As photographers we need to add or remove them as needed.  The bands can prove fragile, need I say more?

Yes, it pays to carry a spare, and it pays to treat them with care and not just throw them in the camera bag – I’m on band number 3 with number 4 in reserve!

Intervalometer (B).

You will need to shoot a long exposure of you scene foreground, slightly re-focuused closer to you, at a much lower ISO, and perhaps at a slightly narrower aperture; this shot might well be 20 minutes long or more and with long exposure NR engaged to produce a black subtraction.

Yes, a lockable cable release and the timer on your watch will do the job, hence (F) and (G) in case (B) stops working!

But an intervalometer will make this easier – as long as you’ve read the instructions..doh!

If you want to shoot star trails the external intervalometer is vastly superior to your cameras built in one.  That’s because the in-camera intervalometer on nearly all cameras except the Nikon D810A is limited to a 30 second shutter speed.

An hours worth of star rotation is barely enough:

Final 600x900 Twilight & Astro Landscape Photography

 

But at 30 seconds shutter speed you will end up with 120 frames at fairly high ISO.

Far better to shoot at ‘bulb’ with a 5 minute exposure and lower ISO – then you’ll only have 12 frames – your computer with thank you for the lower number when it comes to stacking the shots in Photoshop.

There is also another problem, for certain marks of Nikon cameras.  The D800E that I use has a stupid cap on continuous shooting.  The much touted method of setting the shutter to 30 seconds and putting the camera in continuous low speed shooting mode and locking the cable release button down does NOT work – it only allows you to take 100 frames then the camera just STOPS taking pictures.

But if you use an external intervalometer set to a 30 second exposure, continuous and just drop the camera in BULB and Single Shot then the D800E and its like will sit there and fill your cards up with frames.

Other Essentials.

Micro fibre cloths, bin liners and gaffer tape (B,I and J).

After a couple of hours of full darkness your gear (and I mean all of it) will most likely be wet with dew, especially here in the UK.  Micro fibre cloths are great for getting the majority of this dampness off your camera gear when you put it away for the trip home.

Bin liners are great for keeping any passing rain shower off your camera gear when its set up – just drop one (opened of course) over your camera and tape it to the tripod legs with a bit of gaffer tape. Leave the dew heater ON.

Also, stick the battery supply in one – rain water and 13 volts DC at 4000MAh don’t mix well.

Photopills on your iPhone (G) is incredibly useful for showing you where the Milky Way is during that extended period between civil and astronomical daylight end.  Being able to see it in relationship to your scene with the Night Augmented Reality feature cetainly makes shot composition somewhat easier.

Head Lamp (H) – preferably one which has a red light mode.  Red light does not kill off your carefully tuned night vision when you need to see some camera setting control lever or button.

Accurate GPS positioner (K).  Not entirely an ‘essential’ but it’s mighty useful for all sorts of reasons, especially when forward planning a shot, or getting to a set position in the dark.

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The Milky Way towering over the National Coastwatch Institution (NCI) station at Rhoscolyn on Anglesey.

 

I love taking someone who’s never seen the Milky Way out at night to capture it with their own equipment – the constant stream of ‘WOWS’ makes me all warm ‘n fuzzy!  This year has seen me take more folk out than ever; and even though we are going to loose the galactic centre in the next few weeks the opportunities for night photography get better as the nights grow longer.

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The Milky Way over the derelict buildings of Magpie Mine in Derbyshire.

So if you want to get out there with me then just give me shout at tution@wildlifeinpixels.net

The Milky Way will still be a prominent feature in the sky until October, and will be in a more westerly position, so lots of great bays on the North Wales & Anglesey coast will come into their own as locations.

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The Milky Way over the Afon Glaslyn Valley looking towards Beddgelert and Porthmadog. The patchy green colour of the sky is cause by a large amount of airglow, another natural phenomenon that very few people actually see.

And just look at that star detail:

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Over the next few weeks I’m going to be putting together a training video title on processing astro landscape photography images, and if the next new moon phase at the end of this month comes with favourable weather I’m going to try and supplement these with a couple of practical shooting videos – so fingers crossed.

Night Sky Imaging

Night Sky Photography – A Brief Introduction

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I really get a massive buzz from photographing the night sky – PROPERLY.

By properly I mean using your equipment to the best of its ability, and using correct techniques in terms of both ‘shooting’ and post processing.

The majority of images within the vast plethora of night sky images on Google etc, and methods described, are to be frank PANTS!

Those 800 pixel long-edge jpegs hide a multitude of shooting and processing sins – such as HUGE amounts of sensor noise and the biggest sin of all – elongated stars.

Top quality full resolution imagery of the night sky demands pin-prick stars, not trails that look like blown out sausages – unless of course, you are wanting them for visual effect.

Pin sharp stars require extremely precise MANUAL FOCUS in conjunction with a shutter speed that is short enough to arrest the perceived movement of the night sky across the cameras field of view.

They also demand that the lens is ‘shot’ pretty much wide open in terms of aperture – this allows the sensor to ‘see and gather’ as many photons of light from each point-source (star) in the night sky.

So we are in the situation where we have to use manual focus and exposure with f2.8 as an approximate working aperture – and high ISO values, because of the demand for a relatively fast shutter speed.

And when it comes to our shutter speed the much-vaunted ‘500 Rule’ needs to be consigned to the waste bin – it’s just not a good enough standard to work to, especially considering modern high megapixel count sensors such as Nikon’s D800E/D810/D810A and Canons 5DS.

Leaving the shutter open for just 10 seconds using a 14mm lens will elongate stars EVER SO SLIGHTLY – so the ‘500 Rule’ speed of 500/14 = 35.71 seconds is just going to make a total hash of things.

In the shot below; a crop from the image top left; I’ve used a 10 second exposure, but in preference I’ll use 5 seconds if I can get away with it:

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Nikon D800E,14-24 f2.8@14mm,10 seconds exposure,f2.8,ISO 6400
RAW, Unprocessed, Full Resolution Crop

WOW….look at all that noise…well, it’s not going to be there for long folks; and NO, I won’t make it vanish with any Noise Reduction functions or plugins either!

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5 consecutive frames put through Starry Landscape Stacker – now we have something we can work with!

Download Starry Landscape Stacker from the App Store:
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Huge amounts of ‘noise’ can be eradicated using Median Stacking within Photoshop, but Mac users can circumnavigate the ‘agro’ of layer alignment and layer masking by using this great ‘app’ Starry Landscape Stacker – which does all the ‘heavy lifting’ for you.  Click the link above to download it from the App Store.  Just ignore any daft iTunes pop-ups and click ‘View in Mac App Store’!

I have a demonstration of Median Stacking on my YouTube channel:

This video is best viewed on YouTube in full screen mode.

In a manner of speaking, the ‘shooting aspect’ of Milky Way/Night Sky/Wide-field Astro is pretty straight forward.  You are working in between some very hard constraints with little margin for error.

  • The Earths rotation makes the stars track across our frame – so this dictates our shutter speed for any given focal length of lens – shorter focal length = longer shutter speed.
  • Sensor Megapixel count – more megs = shorter shutter speed.
  • We NEED to shoot with a ‘wide open’ aperture, so our ISO speed takes over as our general exposure control.
  • Focusing – this always seems to be the big ‘sticking point’ for most folk – and despite what you read to the contrary, you can’t reliably use the ‘hyperfocal’ method with wide open apertures – it especially will not work with wide-angle zoom lenses!
  • The Earths ‘seasonal tilt’ dictates what we can and can’t see from a particular latitude; and in conjunction with time of day, dictates the direction and orientation of a particular astral object such as the Milky Way.
  • Light pollution can mask even the cameras ability to record all the stars, and it effects the overall scene luminance level.
  • The position and phase of the moon – a full moon frequently throws far too much light into the entire sky – my advice is to stay at home!
  • A moon in between its last quarter and new moon is frequently diagonally opposite the Milky Way, and can be useful for illuminating your foreground.

And there are quite a few other considerations to take into account, like dew point and relative humidity – and of course, the bloody clouds!

The point I’m trying to make is that these shots take PLANNING.

Using applications and utilities like Stellarium and Photographers Ephemeris in conjunction with Google Earth has always been a great way of planning shots.  But for me, the best planning aid is Photopills – especially because of its augmented reality feature.  This allows you to pre-visualise your shot from your current location, and it will compute the dates and times that the shot is ‘on’.

Download Photopills from the App Store:

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But it won’t stop the clouds from rolling in!

Even with the very best planning the weather conditions can ruin the whole thing!

I’m hoping that before the end of the year I’ll have a full training video finished about shooting perfect ‘wide field astro’ images – it’ll cover planning as well as BOTH shooting AND processing.

I will show you how to:

  • Effectively use Google Earth in conjunction with Stellarium and Photopills for forward planning.
  • The easiest way to ensure perfect focus on those stars – every time.
  • How to shoot for improved foreground.
  • When, and when NOT to deploy LONG EXPOSURE noise reduction in camera – black frame shooting.
  • How to process RAW files in Lightroom for correct colour balance.
  • How to properly use both Median Stacking in Photoshop and Starry Landscape Stacker to reduce ISO noise.
  • And much more!

One really useful FREE facility on the net is the Light Pollution Map website – I suggest using the latest 2015 VIIRIS overlay and the Bing Map Hybrid mode in order to get a rough idea of your foreground and the background light pollution effecting your chosen location.

Don’t forget – if you shoot vertical (portrait?) with a 14mm lens, the top part of the frame can be slightly behind you!