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    Canon Rumours

    I was reading an article on the Canon rumors site:

    "The recent sale for the EOS M suggests Canon is dumping large inventory of the camera for a successor. I also think it’s partially to get people to finally buy into the system and that Canon is going to put more muscle behind the product line.

    According to a source at Northlight Images, Canon is going to introduce a “basic” EOS M 18mp successor first, and follow it up with a 20mp APS-C dual pixel model afterwards. The latter camera is said to be aimed at the “full frame Canon shooter”, and will have an optional viewfinder. This goes along with what we’ve previously been told.

    Also in development is a focal length reducer for EF lenses, this will be announced with the 20mp EOS M camera."

    What is a 'focal length reducer' please?
    The camera works just fine, it's the idiot staring through the viewfinder that need's help!

    #2
    Re: Canon Rumours

    Very interesting. With the new firmware Canon has made a very good camera that much better.

    Tom

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      #3
      Re: Canon Rumours

      Originally posted by JaKS_Foto View Post

      What is a 'focal length reducer' please?[/I]
      As ever Google is your friend - I think. If you can understand even a smidgin of the techno babble on those pages a precis would be welcome!

      From what I can make out it is similar to the opposite of an extender. Then it gets complicated.

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        #4
        Re: Canon Rumours

        These things are available from third parties already (I assume DrJon will enlighten us). Effectively your 80mm f/1.8 becomes a 50 f/0.7 ... or something like that.

        See http://www.metabones.com/buy-speed-booster for a real example.

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          #5
          Re: Canon Rumours

          Oh gee, I guess I'm on the spot now... okay I'll give it a shot (off the top of my head, but having read it I think it's okay)...

          An Extender basically takes the centre part of the image circle coming out the back of a lens and spreads it out, so only the centre bit now covers the sensor and the rest misses it and is lost. Thus giving magnification but lowering light levels (for example 50,000 photons might have landed on 5 pixels before but with a 1.4x extender they will be spread over 10, so each gets half as many). This effectively increases your focal length as you just see the centre of the image but also increases the f-stop as the light level goes down. This is cool as your lens gets longer without having to buy a new one (and carry it around). The price is the optics won't be perfect and the lost light.

          A Focal Reducer does the opposite and squeezes an image circle down to a smaller size. So in our 1.4x example the 50,000 photons now land on 2.5 pixels worth of area. This only works if the lens has an image circle bigger than the sensor. So an EF lens onto EOS-M (which uses a standard Canon crop size sensor) will work, EF-S onto EOS-M won't as there's no light out that far from the sensor to squeeze onto it. This effectively reduces the focal length - you're seeing what you would with a wider lens, plus reduces the f-stop. This is kinda cool as you get to use all the light the big FF lens captures (so using all that glass you're carrying around, not just the central bits) and you get a lens that has a field of view about equivalent to the focal length that's written on it.

          I'll explain that last sentence in more detail...
          Lets say you have a Full Frame (EF) 50mm f1.4 lens and you attach it to your EOS-M via the adapter. Normally you get a field of view equivalent to an 80mm lens and the light gathering of a f1.4 lens (except the total light landing on the sensor is less than on FF so your overall image noise will be worse - see the P.S. for more). If you could attach a 1.4x reducer between it and the camera you get a (80/1.4=) 57mm f1.0 lens, so a focal length about what is written on the lens but an extra stop of light gathering (plus you now have just about the same amount of photons hitting the sensor per second as the FF camera with that lens). Canon could do a 1.6x extender to exactly even it out, but optics designers do like factors of two so 1.4x is perhaps more likely (plus keeps the f-stops sensible).

          The down sides with focal reducers are that once again you are using the outside parts of the lens which presumably aren't as good as the "sweet spot" you were using (with an EF lens on a Crop sensor) and as with the extender it's more optical elements in the path.

          They've been used on Astronomical telescopes for ages, I also think a number of the Olympus 4/3 lenses are actually bigger lenses with focal reducers, which is why they tend to be big and heavy plus a stop faster than you'd expect.

          Does that make sense to people?

          John

          P.S. (The more complicated bits I mostly left out) It's a real change in focal ratio as the diameter of the entrance pupil is changing (you only count the bit that hits the sensor), getting bigger or smaller and f-ratio is focal length divided by that. However (I probably should have left this bit out of the main part) the f-ratio is complicated as it's relative to lens focal length and sensor size, hence a lens with the same field-of-view and f-ratio puts a lot more total light on a FF sensor than a crop sensor.

          I'll try that another way... the 50mm f1.4 lens has an entrance pupil (at f1.4) of 35.7mm (50/1.3). An 80mm f1.4 lens has a 57mm entrance pupil. If we use a crop sensor with the 50mm lens it only uses an image circle made from the centre 22.3mm of the entrance pupil, so although you get the field of view of an 80mm lens the area of light you are capturing is a lot less than an 80mm f1.4 FF lens.

          P.P.S. The manufacturer just frigs the ISO settings to give an amount of amplification that makes the exposure maths work out, so a small sensor will generally use less photons to end up with a certain brightness in the final image (as it was amplified more). ISO really has no absolute meaning. This means in our P.S. example above the 50mm lens on the crop body is still giving you f1.4 for exposure, but you get a lot less light over the sensor's whole area than the FF camera's sensor does. That's why the crop camera sees our lens with the reducer as a f1.0 lens but the FF camera sees it as f1.4 while both sensors get hit by the same amount of photons per sec.
          Last edited by DrJon; 09-07-2013, 08:57.

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            #6
            Re: Canon Rumours

            Thank you John,

            I had to read that twice but it makes sense. Appreciate the reply
            The camera works just fine, it's the idiot staring through the viewfinder that need's help!

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