I`ve read the info on this but am still confused. I usually shoot 200 outside and 3-400 inside but am dead certain that I don`t really know why !!! Anyone good at explaining.

Don't know about good at explaining, but I'll give it a shot. Basically, this means the signal to noise ratio. What? To answer, think of turning up the volume when playing your car radio. If you turn up the volume, you get more music, but also more background noise and garbage. There comes a point when the noise drowns your top ten Barry Manilow medley and then you have to give up.

The same thing happens with digital cameras. The signal (= music) is amplified when you increase your ISO, but so is the noise. On cameras with very small sensors (compacts have sensors the size of a pinky's finger nail), noise is also created because the individual photosites (the millions of pixels recorded) heat up as they are very close together. The camera finally adds noise (grain) to a point that the image is unbearable - just like Barry Manilow.

We raise the ISO sensitivity (i.e., amplify the signal reception by the sensor) when we can't lengthen the exposure time (camera shake) or adjust the aperture (depth of field). Inversely, it's worth remembering that too sensitive an ISO setting leads to burnt out highlights.

When using flash, avoid going above 200. My other rule of thumb is dollar value of camera = maximum usable ISO. So, if you paid a $1000, you can safely shoot at that ISO setting and print good quality images. One notable exception is the $6000 Leica M9 that's absolutely crap over 1600 ISO, so I'm told.

Some DSLRs, notably mid-range Nikons, start at 200 ISO without expansion. Some compacts allow you to shoot at ISO 50.

There are also post-production noise reduction applications, all of which soften the image, which we generally tend to dislike.

Hope this helps. And in the interest of fairness, replace Barry Manilow with the Bay City Rovers or, if you're French, anyone, but Claude François is a good start.

Thanks that`s very clear. As I shoot mainly micro....probs with camera shake and depth of field are something I`m trying to improve on all the time and I hate using a tripod. ps, whats `softness`, is that blur or a different thing altogether? Regards Viv

I think the other thing to consider when choosing your ISO is what will you be using the image for. If its to create small scale images or to post on the internet (Photoblog for example) you may well get away with quite a high ISO before it becomes noticeable. If you want high quality art-prints or you'll be printing/viewing large scale images then you'll need to use a low ISO. Exactly how high or low you can go may not be that easy to determine though it does come with experience. It may therefore be worthwhile taking a number of test shots so that you can figure out the limits on the shot you want to take.

remember when u were learning to drive the motion is jerky and we are constantly wondering and using our brain consciously to turn or brake or accelerate or change gears. with time it becomes our second nature and the car becomes an extension of our feet.
we need not know the Otto cycle to know how the car works but knowing it would make us appreciate it.
similarly a camera is also an extension of our eyes and what we see . learning about the exposure triangle will allow us to appreciate the way we see through our camera.
one interesting view I had read when I was trying to understand ISO was to imagine a person sitting in a room with darkglasses on!

Softness vs. sharpening: basically, it means that when magnified at over 100% some details in an image look slightly blurry, especially in the corners on full-frame cameras and particularly in the centre (all cameras). It begins with the lens' resolving power (not resolution), i.e., its ability to transmit details to the sensor. Anyone with a telescope will know what resolving power is. For example, if Barry Manilow lived opposite me and I trained a telescope on his bedroom at night, I would see that he wears a corset when he undresses. With a telescope with higher resolving power, I could read the washing instructions label on the corset. That's also the difference between a soft image and a sharp one. A soft image isn't blurry really. It just looks "flat". A really sharp image "pops".

Centre and corner sharpness is impossible at the lens' maximum aperture ("wide open"). Diffraction also affects sharpness at the other end (e.g. f/22). When photographers talk about a lens' "sweet spot", they refer to an aperture (and focal range) where the lens produces its best - and sharpest - results, usually somewhere between f/5.6 and f/11, although there are exceptions. The Canon 100 mm f/2.8 L macro mk II L IS comes to mind.

People forget that digital cameras are non-digital - the lens and the sensor are analog mechanisms that record light imperfectly. Firstly, light is bent, diffracted, distorted and generally messed about with in the set of glass components comprising the lens. Secondly, the sensor records photons, but the Analogue to Digital (A/D) converter is required to produce electronic (digital) files. In any conversion there is loss - for example, in the signal to noise ratio above, some of the details are lost as noise, but also in the analogue to digital conversion too as sharpness.

Next comes the issue of lenses. Generally speaking, expensive lenses are sharper than cheap ones. Prime lenses are sharper than zooms. A macro lens is necessarily sharp. No one expects compact cameras to have sharp lenses, although there are some exceptions.

Images can (in fact, must) be sharpened in post-processing. There is the counter-intuitively named "unsharp mask" filter in Photoshop and other image editors (it's a historic throwback to a technique used in film processing). There are other filters and gizmos that selectively sharpen your image. Perhaps the best known now is the "clarity" slider in Lightroom that sharpens your midtones.

All digital images must be sharpened, but not to the same degree. Portraits have to be selectively sharpened (eyes, teeth, glossy lips), with the skin softened. The best way to age someone is to push the clarity slider to the right in LR.

The following image demonstrates the difference between soft and sharp. It comes from my revenant account. If you look at the blow-ups, you'll see how terrible a Sigma 50 mm f/1.4 lens is for softness at max aperture on a full-frame DSLR.



Hope this helps.

Helps? Immensely!!!

Again, please note from the above that a full-frame DSLR punishes poor quality lenses remorselessly. Corner sharpness is not so much of a problem with APS-C bodies. If you're taking concert shots with the musician clearly lit (and the background in darkness), you don't need to worry about this.

As mentioned above, the exposure triangle is a guide, not an absolute rule. In most shots, we strike a compromise between exposure, aperture and ISO for the circumstances. Sometimes we *add* noise because of the excessively "clean" look produced by modern cameras. (There's a grain slider in LR just for this...) One of the things that always disappoint me when I see a "sepia" shot intended to add a "ye olde worlde" look is the super sharp focus that was just not possible when sepia was used in film.

Here are two examples: one a RAW capture out of the camera and the other with some tonal adjustments and above all, a sharpening technique. They both come from my revenant account:

Thanks have just taken several sequential photos ranging from 100 iso to 320 iso, (maybe I shouldn`t have chosen a damsel fly which never sits still). The differerence I can see, but quantifying it or explaining, no way, just visually I like the lower iso. Softness I can see in your photo quite clearly the image is no longer crisp and as u say a good one pops out at you usually with no post processing. Many thanks for the help and I`m sure there is going to be other stuff I caun`t quite get my head around. Plus I still just twiddle knobs, buttons and dials until I see in the screen what I have in my head...no hope really!! )

I think twiddling knobs is one of the reasons why we all started and is certainly no reason to stop...