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’.

Speed Light Photography

Speed Light Photography – part 1

First things first, apologies for the gap in blog entries – I’ve been a bit “in absentia” of late for one reason or another.  I’ve got a few gear reviews to do between now and the end of the year, video tutorial ideas and requests are crawling out of the woodwork, and my ability to organise myself has become something of a crumbling edifice!

I blame the wife myself………………..

But I’ve come to the conclusion that for one reason or another I’ve become somewhat pigeon-holed as a wildlife/natural history photographer – going under the moniker of Wildlife in Pixels it’s hardly a big surprise is it..

But I cut my photographic teeth on studio product/pack shot and still life work – I loved it then and I still do.  And there’s NOTHING that teaches you more about light than studio work – it pays dividends in all aspects of photography, wildlife and landscape work are no exception.  Understanding how light behaves, when it’ll look good and when it’ll look like a bag of spanners is what helps capture mood and atmosphere in a shot.

The interaction between light and subject is what makes a great image, and I do wish photographers would understand this – sadly most don’t.

To this end I’ve begun to teach workshops that try to give those attending a flavor of the basic concepts of light by introducing them to the idea of using their speed lights to produce images they can do 365 days a year cum rain or shine – high speed flash, and simple product still life.

Both styles demand a high level of attention to detail in the way the light produced by the speed lights bends and wraps around the subject.  Full-blown studio lights have the benefit of modelling lights so that you can see this before you take the shot, but using speed lights means you have to imagine what the light is doing, so it’s level of difficulty begins high, but decreases with practical experience.

D4D5734 Edit Edit copy 598x900 Speed Light Photography

A basic 4 light setup with speed lights can produce some really soft and moody lighting with ease.

This Black Label shot went a bit bonkers in the final stages with the addition of smoke, but it gives you an idea of the subtlety of lighting that can be achieved with speed lights.

As for the setup, here’s a shot before I introduced the glass….

D302888 2 597x900 Speed Light Photography

Simple setup for the Black Label shot – note the well-appointed studio!

…featuring that most valuable of studio photographers tools, the Voice Activated Light Stand..!

Four SB800’s in all, the one on the right is running at 1/2 power and is fitted with an Interfit Strobies softbox and is double diffused using a Calumet 42″ frame (available here) and white diffuser – this constitutes the main light.

Just look at the size of the diffused disc on the face of that 42″ frame – all that from a poxy 2″x1″ flash head in less than 16″ – epic!

The SB800 on the left, fitted with another softbox is turned down to 1/64th power, and is there solely to illuminate the label where it wraps around the left edge of the bottle, and to get a second neck highlight. Although their is light emanating from it, its greatest effect is that of “bouncing” light from the right hand source back in to the bottle.

The V.A.L.S. is fitted with a third speed light that has a diffused snoot – note the expensive diffusion material and the highly engineered attachment method – kitchen towel and rubber band!  The sole purpose of this tiny soft light is to just help pull out the left side of the bottle cap from the intensely dark background towards the top of the shot.

The 4th SB800 is fitted with a 30 degree honeycomb and a “tits ‘n ass”; or TNA2 to be more correct; filter just to give a subtle warm graduation to the background.

Speaking of the background, this is a roll of high grade tracing paper – one of the most versatile materials any studio has, both as a front lit or back lit background, or as a diffusion material – just brilliant stuff, second only to Translum plastic, and a shed-load cheaper.

At the other end of the speed light photography spectrum is the most enjoyable and fascinating pastime of high speed liquid motion photography – a posh way of saying “making a mess”!

It doesn’t have to be too messy – just don’t do it on your best Axminster!

By utilising the IGBT (Isolated Gate Bipolar Transistor) circuitry given to us in speed lights we can deploy the very fast tube burn times, or flash durations, obtained at lower output power settings to our advantage.

Simple shots of water, both dyed and clear can produce some stunning captures:

D4D2271 Edit 511x900 Speed Light Photography

Streams of water captured back lit against a white background illuminated by two speed lights.

The background for this shot (above) is an A1 sized sheet of white foam board illuminated by a pair of SB910s.  The internal reflector angle is set to 35mm and the two speed lights are placed on stands about three feet from the background, just out of shot left and right, and aimed pretty much at the center of the board to facilitate a fairly even spread of light.

The power output settings for both speed lights is set to 1/16th which gives us 1/10,000th of a second flash duration.

Switching to tracing paper as a back lit background immediately puts us at a disadvantage in that it’ll cut the amount of light we see at the camera. But a back lit background always looks just that little bit better as it makes your lighting more easy to shape and control.

Doubling the speed light count behind the trace background to 4 now gives us the power in terms of guide number equal to your average studio light – but with full IGBT advantages.

Working a little closer to the background than we were with the white board/reflected light method we can very easily generate a smooth white field of 255RGB which will make our liquid splash shots really punchy:

D4D6364 Edit 2 599x900 Speed Light Photography

Working about 3 feet from a translucent background illuminated by 4 SB800’s gives us a much flatter white background, especially when deploying a 150mm or 180mm macro lens.

Shot with a 180mm macro lens at ISO 260 and f16 we have bags of depth of field on this shot.

Using 4x SB800s we can dial in the correct background exposure using the flash output power and camera ISO – we want a background that’s just on the verge of “blinkies”.  If we over expose too much for the background the light will wrap around the liquid edges too much, washing out the contrast and flaring – that’s something that muppet on Adorama TV doesn’t tell you!

Take a few shots holding the glass by the rim gives us a clean foot to the glass, so we can now go and make a nice composite in Photoshop:

D4D6364 Edit 900x900 Speed Light Photography

Composite of a couple of splash shots and a couple of “clean foot” images….

Happy sodding Valentines day for next year everyone……..yuck, but it’ll sell all day bloomin’ long!

A while ago I posted an entry on this blog about doing splash shots using a method I call “long flash short shutter” HERE.

All the shots on this entry have been taken using the “short flash long shutter” method.

This latter method is the more versatile one of the two because it has a more effective “motion freezing” power; the former method being speed-limited by the 1/8000th shutter speed – and it’s more costly on batteries!

BUT………there’s always one of those isn’t there…?

Short flash long shutter utilises the maximum X-synch speed or the camera.  This is the fastest speed we can use where the sensor is FULLY open, and it’s most commonly 1/250th sec.

Sussed the massive potential pitfall yet?

That’s right – AMBIENT LIGHT.

If any ambient light reaches the sensor during our 1/250th sec exposure time then WE WILL GET MOTION BLUR that will visually amount to the same sort of effect as slow synch, sharp image with under exposed blur trails.

So we need to make sure that the ambient light is low enough to render a totally black frame.

The “long flash short shutter” method works well in conditions of high ambient provided that the action can be frozen in 1/8000th sec.  If your camera only does 1/4000th sec then the method becomes somewhat less useful.

Freezing action depends on a number of things:

  • 1. Is the subject falling under gravity or rising against it?
  • 2. How far away is the subject?

A body falling under gravity is doing around 10mph after it’s fallen 2 feet from a dead start, and a car doing 100mph looks a lot slower when it’s 200 yards down the road than it does when it’s 20 yards away.

Similarly, if we have a cascade of liquid falling under gravity through the frame of our camera and (to avoid the jug or pouring vessel) the liquid has fallen 6 inches when it enters the top of the frame, and 30 inches when it vacates the bottom of the frame; we have to take a few things into consideration.

  • The liquid is faster at the bottom of the frame than at the top – think Angel Falls – the water pulls itself apart (that’s why the images can look so amazing).
  • If we shoot close with a short lens the speed differential across the frame will be the same BUT the overall speed will be a little more apparent than if we shoot with a longer lens from further away.

An SB910 has a 1/16th power output duration of 1/10000th sec and an SB800 1/10,900th at the same output setting (OEM-quoted values). With a 70mm lens close up this can make a subtle difference in image sharpness, but fit a 180mm and move further away from the subject to maintain composition, and the difference is non-existent.

If you are throwing liquid upwards against gravity, then it’s slowing down, and will eventually stop before falling back under the effects of gravity – quite often, 1/8000th is sufficient to freeze this sort of motion.

Both “long shutter short flash” and “short shutter long flash” are valid methods, each with their own pluses and minuses; but the method I always recommend people start with is the former “long shutter” method – it’s easier!

When a shot features a glass remember one thing – drinking glasses were invented by a race of photographer-hating beings! Glasses transmit, reflect and refract light through a full 360 degrees and you can really end up chasing your tail trying to find the source of an errant reflection if you don’t go about lighting it in the correct manner.

And if you put liquid in it then things can get a whole lot worse!

I’ll be doing some very specific workshops with Calumet in the near future that will be all about lighting glass and metal, gloss and matte surfaces, so keep your eye open if this sort of thing interests you – IT SHOULD ‘cos it’ll make you a better photographer….!

The simplest “proper” glass lighting method is what we call “bright field illumination” and guess what – that’s the method used in all the above liquid shots.

D4D6019 LDF 900x640 Speed Light Photography

Glass Photography – Bright Field & Dark Field illumination.

In the image above, I’ve photographed the same glass using the two ancient and venerable methods of glass photography – one is easy, the other a total pain in the ass; guess which is which!

I’m not going to go into this in detail here, that’ll be in a later post; but BRIGHT FIELD defines the outline of the glass with DARK lines, and DARK FIELD defines the glass white lines of WHITE or highlight.

If you guessed DARK FIELD is the pain the bum then you were right – you will see bits of your “studio” reflected in the glass you didn’t even know existed unless you get this absolutely spot on and 100% correct.

The nice thing about studio-style photography is that you have thinking time, without pressure from working with people, animals or weather and a constantly moving sun. You can start to work up a shot and then leave it over night, when you come back the next day and click the shutter everything is as you left it – unless you’ve had burglars.

You do develop a habit of needing more “grips” gear – you’ve NEVER got the right bit! But then again it’s far cheaper than the bad habit of tripod accumulation like my friend Malc is afflicted with!

Later Folks!

 

Please consider supporting this blog.

This blog really does need your support. All the information I put on these pages I do freely, but it does involve costs in both time and money.

If you find this post useful and informative please could you help by making a small donation – it would really help me out a lot – whatever you can afford would be gratefully received.

Your donation will help offset the costs of running this blog and so help me to bring you lots more useful and informative content.

Many thanks in advance.