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Remember Kodak Technical Pan, Ilford Pan-F, Kodachrome 25? If you can't, you might be surprised to know that these low sensitivity films are equivalent to ISO settings on digital cameras or 25 and 50. If you aren't surprised then you have probably used films as 'slow' as these in the past. Maybe you still do! And maybe you are frustrated that, in general, digital cameras can't be made to work like these slow films. ISO 100 is generally the lowest ISO speed you should expect on a digital camera and many won't even let you go that low, with ISO 200 being the minimum. A notable recent exception is the new [URL="http://dpnow.com/8417.html"]Nikon D4[/URL] which offers a minimum ISO 50 setting.
Ultra low speed film used to be praised and derided in equal measure. If you wanted to eliminate grain and take photos in very bright conditions then ISO 25 and 50 films were great. But under more typical or low light conditions these low sensitivity films would be a pain in the neck, forcing you to use large apertures and slow shutter speeds. We're so used to cranking up the ISO speed with our digital cameras but in the days of film changing your ISO used to mean swapping out the film or using a second camera.
So, surely, with super low noise, high sensitivity digital cameras today, how could we miss extra low ISO settings? Well, we do - sometimes. I have been caught short on more than one occasion when the ambient brightness of the scene I was photographing was too much, forcing me to stop down the lens further than was ideal. It's less of a problem if you have a pro or semi-pro spec. camera that can take your shutter speed up to 1/8000th of a second, but even at 1/4000th on a bright sunny day you can face over exposure problems at ISO 200 in some circumstances. The subject itself might be very reflective, like a white building, or a light coloured flower. Or you might be aiming to use a wide aperture to blur the background. You might also want to reduce the brightness to force a slow shutter speed for motion blur effect. If you do find yourself in one of these situations you might have to resort to a neutral density filter or maybe a polarising filter. It's no surprise that we hear more and more about so-called 'Big Stop' extra dark neutral density filters.
Why are modern digital cameras so averse to low ISO settings? The sweet spot for sensitivity, noise and dynamic range in modern digital still camera sensors is around ISO 100 or 200 and has tended towards the 200 mark in recent years. This may be because sensor manufacturers are trying very hard to improve ultra-high ISO performance, sacrificing a bit of performance at the low sensitivity end. Rather like the megapixel race, there is definitely an ISO race going on. Just a few years back, ISO 320O was considered a luxury and even then it might not actually be very usable. Today ISO 12800 is quite common. The new [URL="http://dpnow.com/8417.html"]Nikon D4[/URL] has an ISO boost setting that hits a remarkable ISO 204800. That's 4 EVs more sensitive than ISO 12800!
So why don't camera manufacturers offer 'boost' settings that let you lower the ISO? Some do, like Nikon and the D4 again, but it's rare. The problem is that you risk saturating the sensor. Each photosite on the sensor surface builds electrical charge in proportion to the amount of light or number of photons it receives. But there is a limit to the charge capacity of a photosite. At its simplest, the limit is exceeded but the brightness value for that photosite stops at the threshold and so no further information is recorded at that pixel - it's simply pure white. At its worst, the charge leaks into neighbouring photosites and corrupts the values there, creating colour distortions that could be exhibited as noise, poor contrast, colour bleed, etc.
One way around this, which is supported in some compact digital cameras like the Nikon P7000 series, Canon G9/G10/G11, etc., and the Olympus XZ-1, is to integrate a neutral density filter that can be switched in when required. But this is impractical with system cameras because of the large size of the sensor and interchangeability of lenses.
I have [URL="http://dpnow.com/forum2/blog.php?b=80"]reflected in the past[/URL] that we're much more used to slow lenses than we used to be. It's quite normal for a standard kit lens today to have a maximum aperture range of f/3.5-5.6, when in the 'old' days f/1.8 was pretty much standard and even a cheapie option was f/2.8.
But fast lenses are making a come-back. More affordable f/2, f/1.8 and even f/1.4 lenses are appearing, especially for system cameras with relatively small sensors. The problem is of course is that these cameras often have a relatively high minimum ISO and usually don't offer super-fast maximum shutter speeds.
Camera manufacturers are beginning to realise that some photographers want less pixels in exchange for better quality pixels. I wonder if we will see a similar recognition that slow ISO options could be a deal-maker too?
I have 3 lenses that don't take front filters so can't even use ND filters.
Yes, obviously, anyone who lives in a bright climate has periodic day dreams, as their display turns red and flashes 4000 1.4 at them several times a day, of a sensor designer running rapidly out of a swamp with a huge alligator at his heels while we all jeer and pelt him with scratched ND filers and step-up rings: but that's not the issue.
We can survive stupid base ISOs and endless scrabbling to keep light out of the camera, intensely irritating as it is.
What is a problem is companies using sensor architectures originally perfected during the Napoleonic wars, disguised by huge lashings of NR and sharpening so that people are lucky if they only lose one eyebrow instead of gaining an extra four, all due to a bonkers marketing battle to produce ever higher ISO numbers so that people who couldn't shoot their way out of a paper bag can blame their inability to take a decent portrait of their grandchildren on not having this year's hot poop camera instead of their refusal to learn basic photographic techniques such as capturing the reversing of direction, and as a result turning out cameras with no RAW latitude at base ISO.
If you want to produce something that can fondly be called art instead of snaps, sooner or later you are going to want to pull up the shadows. And if the sensor is a cheapo item optimised only for rendering something faintly related to a picture at ISO-Something-Ridiculous, that isn't going to be possible.
So instead of testing the camera at ISOs that are really only going to be used in anger, as opposed to for bragging rights, by a handful of indoor sports shooters, take a shot at base ISO in raw and pull up the shadows a few stops. That actually tells us if the camera is any good or not!