Crop Factor, Low Light and Aperture with m4/3 lenses?

Started Jun 15, 2014 | Discussions thread
alcelc Forum Pro • Posts: 11,573
Re: Understanding the Basics and how digital are or are not exactly ..

Franka T.L. wrote:

Well nor exactly fellow thru with standards ... that is well we shall get to that later alright

Now FOA, there is no such thing as croo factor, that term is grossly misused by people who only want to think of measuring a lens coverage as per 35mm film ( still capture ) as somewhat a holy grail .. so they say you have 2.0 crop factor on 4/3 sensor, But wait that camera do not crop anything when they capture, its just having a different size sensor. Just as in old days when I put my Hasselblad Zeiss 80mm standard on my Nikon FM2, I do not say its getting a crop factor of so and so. I say its an 80mm lens and well that's the same for any digital DSLR or mirrorless. Lens focal length and aperture is a PHYSICAL property of the lens and it does not change whether you use it for film or digital, 35mmm or 4/3 .. the only time one can say crop factor perhaps ( somewhat ) is when one shoot on a digital platform and set for in camera JPEG that's not native to the sensor aspect ratio. Say you set the M4/3 camera to shoot 16:9 HD format JPEG, then the camera indeed is cropping from the top and bottom of the said sensor ( which is 3:4 aspect ratio sized ). I term these misleading issue " Equivalent Coverage " as that's what it is. So a 25mm lens for 4/3 is having a equivalent coverage as a 50mm lens on 35mm still film capture. Mind that there' some caveat .. when we say equivalent ( or when they talk crop factor ) its measured on the long axis length wide and the coverage measured in degrees of measurement ( go back to high school math ) But if you tally the Math you can tell the factor for equivalent 4/3 lens coverage when used in Portrait format is in fact more like 1.9 as the 4/3 sensor is of course 4:3 aspect ratio when 35mm film is 2:3

So for your said M4/3 prospective needs you are looking at a M4/3 body with a 25mm lens and there's several, Panny has a 1.4 and Oly has an 1.8 both are speedy enough I shall say. And the standard lens can easily be complemented by equally good fast long focal / short Tele Macro like the Panny 45mm Macro / Oly 60mm Macro / Oly 45mm/1.8 or Zooms of various speed. Less choice on the wide end but they are there. And speaking of aperture, again they are physical to the lens and independent of the format of the said recording medium. So a said scene , being composed for exposure at ISO 100 needing f/4.0 at 1/125 shutter will still need that same exposure at ISO 100 when using your good old trusty Fujifilm Astia or whether you are using that new M4/3 set at ISO 100 ( effective, see below ) and no matter what kind of lens that might be.

One thing though, where we old film guys might need a new understanding is the speed of the medium , aka ISO setting for the film ( which is in build when we load the film ) and for these digital platform. Now here's a little fact that the Mfr probably do not really want to show around. The reality with almost all of todays digital platform is that they almost all cheat on the ISO setting . Not like film days when ISO had a rigid requirment for Film, Mfr to place a Ei / ISO setting on the film. The film must be able to reproduce that exposure set for a set light intensity to reproduce a set result to say it had that ISO. So when we load a ISO 1600 film we know we can safely use any measure to expose the film so long our exposure is based on calculating the medium to have an ISO 1600 sensitivity.

Well, no so for Digital platform. Their setting generally are over inflated. Say for the typical M4/3 camera, on a less than sunny day when we might want to use ISO 400 or ISO 800 so we can safely handheld and still use f/6.3 for that DOF. So we expose for that and WOW, that exposure just do not seems right .. because what , brasue almost all of the M4/3 bodies ( whichever model and brand ) had lablr their ISO in what they fashion as so. So ISO 400 on their camera is more like just having ISO 250 and ISO 800 is more like ISO 500 or even less. And this is also non linear Say if you take that same body and set the camera to ISO 3200, in most case you are only getting ISO 1200 and not 2000 ( as a linear rule would do ). So for anyone migrating from film to digital, there is a need to learn how the specific body ISO acually perform as ISO goes. The reason behind that is complex but mostly that had to do with the in camera JPEG which apply a tone mapping and the camera is usually configured to not to lost the highlight while boosting the shadow and then apply NR to made the exposure. Its not something magical, just the way it is. And for people who are more akin to shoot RAW or expose for control instead for in camera then one must realize this bit and work along it, and this is digital all of them, not just M4/3

So with old days and low light goes, most of us actually choose a fast lens not becasue of the speed but for the wide liberty to place exposure. And we trust our film to give that speed they labelled ( I swear on Ilford HP4 ) And we can do pretty much the same digitally, only for the fact that we need to work out our own effective ISO for the said body we choose. So if you have a particular setting for in camera JPEG you would want to test out to made sure that ISO 400 on that body is really how ISO 400 work for you, or might be you need to tune it up or down another notch or so. I shoot RAW, and do not particularly use Mfr's own processing and I've found DxO Mark's sensor review data of effective ISO pretty accurate ( as when I measure the exposure against my good old trusty calibrated light meter ).

There are other things to consider when switching from film to digital , but somehow we need to have a starting point right .. and getting the basic exposure right and in control is perhaps the first thing on the agenda ( other than actually going out and buying the gear in question )

Dear Franka, in very short can I say: assume the ISO is the same for a certain FF and MFT, then, for a particular scene if the FF can properly expose at 50mm FL, ISO 100, f/2.8 and 1/60", a MFT could have the same equivalent coverage (FOV) and same exposure at 25mm FL, ISO 100, f/2.8 and 1/60". The only difference is FOD?

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