The three great lies.

Started May 30, 2012 | Discussions thread
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Louis_Dobson
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The three great lies.
May 30, 2012

The long and short of it is the larger the sensor, the more light it can gather. That means it works better when light is in short supply. When light is plentiful, then there is no advantage either way, but the smaller sensor can be part of a smaller system. It's that simple. Sadly however, these three great lies have sprung up:

"An MFT camera cannot match a dSLR for shooting performance".

That's utterly wrong in theory, not flapping a mirror about should produce a performance boost . Until recently though nobody has tried to make a fast shooting dSLR replacement, now they have, and it works very well. Just because your MFT camera doesn't shoot, clear the card, or focus very fast is nothing to do with MFT.

"A 75mm f1.8 lens is like a 150mm f1.8 lens on a 135 sensor"

It's always a 75mm f1.8 lens however you cut it, putting it on a different sensor does not change the lens. However, to have a 35mm camera giving indistinguishable results you would have to lift the ISO by two stops, and use a 150mm f3.6 lens. Then you would get the same angle of view, depth of field, exposure time and signal to noise ratio. If 75mm = 150mm (which it does not), then f1.8 = f3.6 (which it does not either). This is because the f number is a ratio of the focal length over the true aperture, the true aperture being the hole in the middle, and the measure of the total light the lens can gather, not the intensity.

"135 Full Frame has better IQ than MFT", or "MFT is an IQ compromise"

First you have to define what IQ means. It's a word people bandy around without thought. As far as I am concerned, if you can make a big print from two cameras and put them side by side and no rational person can see a difference, then the cameras have identical IQ. On that basis, at base ISO, all the major cameras of around 16MP (going up to 24MP), be they full frame or APS-C or MFT have the same IQ. But then you must ask if you can PP the images heavily and still have the same IQ. Here the OM-D, and possibly the GH2 with LR4, still keep up fine. There are a few cameras with Sony chips that allow more manipulation, but you have exceeded what is required. And finally, there is the question of how high you can crank the ISO before the image falls apart. Here, undoubtedly, the bigger the sensor the better. This is where FF gains - you can shoot a D4 at ISO800 before you see any deterioration at all, whereas an OM-D should be used at base ISO if possible. So, it slightly depends on your definitions, but on my basis MFT and FF have the same IQ, but FF does high ISO better.

And this is all exactly as you would expect. FF gathers more light. When light is in short supply that's an advantage, when there is plenty of light it isn't. That smaller sensor means the lenses are effectively longer, but it also means they are effectively narrower. And the smaller the sensor, the less mass in moving parts to shift, and the faster the cameras shoot. Strip out the mirrorbox, they shoot faster again. MFT cameras so not shoot slower, do not have worse IQ, do not have some wonder system for making lenses twice as long but still as bright, but MFT cameras need more light than cameras with bigger sensors to get a result.
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