I've had this discussion with a few friends - oddly the professionals want to have it on their cameras but can't explain why. I think it makes them feel 'nearer' to film
I was taught many years ago at art school that ISO is the sensitivity of the film to light. More sensitive film needs less light to be exposed but the pay off is that it becomes grainy. The art (or science) of photography was the balancing of how much light the film needed (ISO) with how much light gets in (Aperture) and for how long (Shutter Speed).
The last two are physical things that haven't changed with digital photography, but now film has been replaced with a sensor, the sensitivity can't physically change.
When i crank up the ISO on my digital camera I get more grain in the picture, but grain is not an artefact of digital photography, so whats the point?
Are camera companies putting this traditional feature into modern cameras (and only the more expensive ones) to make old -school photographers feel at home?
OK, the higher ISOs allow shooting in lower light, but surely its just a software effect, like sticking it through Photoshop and pushing the levels.
Reply to this The "grain" you get then you crank up the ISO is noise from sensor. It's like turning up an amplifier on a sound system, you get hiss in the background that isn't in the signal - similarly when you turn up the ISO on your camera it puts colours into pixels that aren't there.
There are noise reduction techniques built into cameras and post processing software, but it is always at the detriment to fine detail. take a look at
this Canon DSLR review which illustrates the different outputs of the camera at differing ISOs nicely.
Reply to this But 'cranking up' is all done by software, surely - the actual sensor doesn't become physically more sensitive.
My whole point is that the sensor can't increase sensitivity, so Digital ISO shouldn't be considered in digital as it is in film...
Reply to this The sensor can effectively be seen as an amplifier, they have a set signal:noise characteristics. So the higher you set the gain (ie higher ISO) the higher the noise levels. You are right, the sensor doesn't get any more sensitive, but by adjusting the ISO you are adjusting the gain/amplification of the light that the sensor is seeing, which when turned up to high levels introduces noise.
Noise isn't added in by any software (quite the opposite actually), it is a property of the CCD/CMOS.
ISO as used in digital cameras is an equivalent sensitivity to light to that of film, it is probably measured (guessing) in lux/second (amount of light hitting the sensor divided by time) but that would probably scare people off!
You should do an experiment: take two consecutive shots with the camera on manual first a properly exposed shot at the highest ISO your camera can shoot at and then keep the same all the same settings (and conditions!) and then shoot at the lowest ISO your camera will do. Then put them both in photoshop and try and adjust the levels to make the photos look the same, you won't end up with the same shot! If you can shoot in RAW I would because there a lot more headroom in that format than .jpgs.
Reply to this I guess that by electrically boosting the signal on the receptor you are physically changing the picture before its recorded digitally.
When I bought my 7D the sales assistant told me that the ISO over 3200 was boosted by software, below that it is restricted to give similar operation to film.
Just seems to me that digital cameras are made to replicate film cameras to please the camera-buying public, whereas the whole format should be treated from the ground-up and invented from scratch.
There's probably features and ways of working that have been missed out on...possibly!
Reply to this Agreed, people seem to have a preconceived idea of what a camera should look like and therefore how it works. I'm pretty sure if there was a true clean slate design then we would probably end up with something different.
Sales assistants... they will say anything to sell something! 😉 I think what they might have been referring to is that your 7D has two extended ISO modes, H1 & H2, which are ISO6400 and 12800... I believe you have to access these from the custom functions menu? - could this be the "software" they were referring to?
I quite like film grain, and it is certainly better than the horrible flat, reddish/brown hue that compact cameras take at high ISOs these days.
Nice choice of camera BTW, and happy shooting!
Reply to this cheers, although i find it slightly annoying as i'm used to a pocket-sized Canon A650 with its little flip out screen and full manual controls. Just what you get used to isn't it!
The 7D is lovely for very lowlight HD filming - getting some lovely test shots around London in the evening. Want to push it higher onm the ISO and lengthen the shutter speed to try and get streaky car lights and glowing city skys on moving video, or something similar. I saw it on an expensive BBC doc the other day and thought... i can do that!
It does appear to be horribly grainy at high ISOs though...
I will investigate the H2 mode...thanks for that, happy shooting too!
Reply to this For your camera, ISOs less than 3200 are amplified electronically. Basically, each pixel in the sensor builds up a charge for a given amount of time. When the sensor dumps the data to the processor it first amplifies the analog voltages electronically before it converts the voltage into a digital signal. The ISO is the amount that this signal is amplified. When the sensor converts the signal to digital it will break up the amplified analog signal into 4096 levels of intensity (if I remember correctly). Once the signal is digital it is impossible to subdivide these 4096 levels of intensity. Lets say that the scene is really dark and the maximum intensity is 10 out of 4096, then there are only 10 levels of intensity which will result in blocky gradients and shades. If you had instead set the ISO higher, then the analog signal, which has an infinite range of intensities, could have given you 4096 levels of intensity where all 4096 levels are used in your image. The downside is increased noise due to the amplification.
Above 3200, the camera is just taking the digital signal and amplifying it. This runs into the same problem that happens if you pick the wrong ISO range.
So, this is a necessary setting and not one that is just to make traditional photographers more comfortable. The only way to get around this would be to make a sensor that has more than 4096 levels of intensity, however that really comes down to adding more circuits in the camera and therefore cost.
If you want to get car light streaks, you want a low ISO (which requires more photons to take an image) and slow shutter speed.
Reply to this I think you might be getting confused between colour depth and sensor sensitivity. The difference between light and dark in a picture is the dynamic range. This is to do with the exposure of the photo and is what the camera can resolve between highlights (all white) and lowlights (all black). The dynamic range of the sensor is generally measured in Electron Volts (eV) or f-stops. The number 4096 only relates if you are taking RAW shots and your camera has 12bit colour depth (2^12=4096 colours). Most of the new cameras have a 14bit colour depth (including the 7D), and therefore can see 16384 variants of any given colour. Of course this all for nothing when it is converted to a jpeg, as they are only 8bit, and have 256 gradients between light and dark.
Really dark or really light scenes don't get blocky gradients because of ISO, it is because of lack of dynamic range: ie there is no recorded change in colour for adjacent pixels, or an incorrectly exposed shot. Mostly the second option when a camera is in my hands...
The camera is always amplifying the amount of light it sees on the sensor to an equivelent ISO, it is just that it is not a linear relationship, so as soon as you get over a certain point (most would agree is iso1600), noise increases greatly.
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