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Thread: This is really bugging me

  1. #1
    Member Ron Evers's Avatar
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    This is really bugging me

    Rafael's post about effective focal length shots thread has my brain twisting once again.

    My M4/3 G1 has a crop factor of 2x so I think that means it has a reduced field of view compared to a 35mm film camera. I do not see it meaning a 50mm lens will have the magnification of a 100mm lens only the view of one . The way I see it, a 50 is a 50 but with a reduced field of view on a crop factor body. The adapters we use set the lens to focus on the sensor as though it was film i.e. the same offset as it would have on a film camera. The camera body is not changing the focal length of the lens. Am I incorrect in my thinking?
    Panasonic G1 & GF1 bodies, Panasonic 14-45, 45-200mm, various Minolta, Pentax, Canon FD, M39 & M42 mount lenses.

  2. #2
    gcogger
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    Re: This is really bugging me

    Correct, a camera cannot change the focal length of a lens. The focal length alone, however, tells you exactly nothing about the image recorded on the memory card, unless you also know the sensor size and resolution. It's the combination of the 2 that gives you an image on the memory card.

    The magnification (referring to the image size at the sensor plane) is only of passing interest since, until the sensor does it's 'thing', you haven't captured an image. (It describes what's happening half way through the image capture process).

  3. #3
    Member Ron Evers's Avatar
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    Re: This is really bugging me

    Quote Originally Posted by gcogger View Post
    Correct, a camera cannot change the focal length of a lens. The focal length alone, however, tells you exactly nothing about the image recorded on the memory card, unless you also know the sensor size and resolution. It's the combination of the 2 that gives you an image on the memory card.

    The magnification (referring to the image size at the sensor plane) is only of passing interest since, until the sensor does it's 'thing', you haven't captured an image. (It describes what's happening half way through the image capture process).
    I am not sure what it is you are telling me, is it that the image is magnified by the crop factor? If so, I do not understand.
    Panasonic G1 & GF1 bodies, Panasonic 14-45, 45-200mm, various Minolta, Pentax, Canon FD, M39 & M42 mount lenses.

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    Re: This is really bugging me

    Quote Originally Posted by Ron Evers View Post
    My M4/3 G1 has a crop factor of 2x so I think that means it has a reduced field of view compared to a 35mm film camera. I do not see it meaning a 50mm lens will have the magnification of a 100mm lens only the view of one . The way I see it, a 50 is a 50 but with a reduced field of view on a crop factor body. The adapters we use set the lens to focus on the sensor as though it was film i.e. the same offset as it would have on a film camera. The camera body is not changing the focal length of the lens. Am I incorrect in my thinking?
    Ron, magnification is a contentious issue but it boils down to the fact that as long as we talk effective magnification it's all the same. You are correct in saying that technically the 50mm lens doesn't change from one camera to the next - focal length is a lens attribute, not a camera attribute, and it remains the same. That same lens on a 2x crop factor camera will only look at the center section of the image, which effectively looks like you've magnified it (a zoom lens magnifies objects so the effective FOV change acts like magnification). Put that same 50mm lens on a medium format camera, and you'll see a larger image circle (though likely with heavy vignetting).

    As you said, the adapters merely place it at the correct distance of the rear focusing element to the focusing plane (i.e. the sensor/film) for which the lens was designed.
    -Dragos
    Panasonic GH1/G1, Canon FTb(n)/F-1, Mamiya C330F/RB67 Pro SD, Chamonix 45N-2, Nikon F5 + Assorted Lenses

  5. #5
    gcogger
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    Re: This is really bugging me

    Quote Originally Posted by Ron Evers View Post
    I am not sure what it is you are telling me, is it that the image is magnified by the crop factor? If so, I do not understand.
    I'm wary of saying the image is 'magnified' unless we all understand the same thing by the word 'magnify'. Really, many of the disagreements on these issues come down to people understanding different things for the same words...

    So instead, I'll give a concrete example.
    Let's say you have a full frame camera (with a 4:3 ratio sensor to keep things simple) and a 4/3 camera (with a sensor half the size of the full frame). Both sensors are 10MP. Now, so that we are discussing the effects of focal length and sensor size alone, assume you have a perfect lens (no abberations, perfect sharpness), and a perfect sensor (records those 10MP without noise, dynamic range problems etc.). Assume also there is no camera shake and no other things degrade the image.

    In this case, a photo taken from the same position with a 50mm lens at f/2 on the 4/3 camera, and with a 100mm lens at f/4 on the full frame camera will record exactly the same image onto your memory card. It will be impossible to tell them apart by looking at the 2 images.

    Of course, it's when lenses and sensors are not perfect that things get more interesting. Also, if the full frame camera has more pixels (which is usually the case) then it will resolve more detail.

  6. #6
    Member Ron Evers's Avatar
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    Re: This is really bugging me

    Thanks for taking the time to formulate that explanation gcogger.

    However, I must confess to having a difficult time getting my head around it. You have added another variable with aperture into the equation. I thought, maybe wrongfully, that the image circle was constant regardless of aperture. And, I still cannot see how the images would be the same in your example.

    Do you know of any link that graphically explains this?
    Panasonic G1 & GF1 bodies, Panasonic 14-45, 45-200mm, various Minolta, Pentax, Canon FD, M39 & M42 mount lenses.

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    Re: This is really bugging me

    Ron, I think you you can understand all this, most often there is just this initial problem grasping a problem. I can try:

    Starting simple.
    Take a lens, let's say a 150mm lens made for an old good 6x6 camera. Let's agree the image circle is real big, well covering any sensor we put behind the lens, and that we use the lens wide open whatever f-stop that may be.

    Now hold the lens in your hand, move it to your other hand, turn it upside down and finally place it on the table gently resting upon something that holds it steady. Can we agree the lens is the same all the time? Of course we can. there is nothing magically happening to the lens, regardless how we hold it, or were we place it. We could even place it in front of a camera held at place by an adapter. Any camera.

    Placing a lit candle in the front of the lens gives us a nice and shiny subject. Focus the lens at minimum and at a certain place the candle is projected sharp and nice against a white paper held up at the opposite side of the lens. OK?

    Now let's cut that paper in small pieces, in the same size as different sensors are! Take the 24x36mm piece of paper and hold it behind the lens where the candle is in focus. Let's say we can see the candle burning, the complete width of the candle and the top of it. Now take your APS-C sized (approx 15.5 x 23.2mm) piece of paper and replace the 24x36 piece with it. We get the same image, we just don't see as much of it as we did a few moments ago. OK.

    Finally we can take the 4/3 sized piece, the tiny 13x17.3mm piece of paper. Now we can predict that we'll see even less of the candle, maybe just the flame?

    The magnification is the same in every one of the images we have projected. As we haven't switched lens that is expected.

    In other words, you were right about everything you said in your first post.

    From there you can play with all this in any direction. For example, the great, nice image we got of the candle when projected onto the 24x36mm piece of paper, how could we get the same with the small 4/3-sized paper? The answer now comes naturally, we should exchange the lens to a 75mm. So, with a 75mm lens we get the same as a FF shooter would get with a 150mm lens mounted. If 24x36, or FF, is the baseline we can talk about 75mm for 4/3 and 150mm for FF as equivalent lenses in this given situation, or maybe express it as the 75mm lens has a 150mm effective length for us 4/3 users. I don't like the later expression at all as it may make people think of magnification.

    Two things;
    a)
    The small image we get on the 4/3-sized paper has to be blown up more than the image on the 24x36 piece of paper if we want prints the same size from both cameras. This means 4/3 lenses has to be sharper than FF lenses.

    b)
    For everything to be equivalent we have to mix in the aperture value. This has nothing to do with the image circle but the DOF. I don't think this is what you really asked about but I can make another equally amateurish try to explain that if needed. (In reality we can often see the image circle vary by the aperture, but for now all lenses we have been thinking of covers everything, incl a Hasselblad sensor.)

    All the above was probably already clear to you, I'm probably just exercising my English, Cheers,

    Jonas

  8. #8
    gcogger
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    Re: This is really bugging me

    Quote Originally Posted by Ron Evers View Post
    Thanks for taking the time to formulate that explanation gcogger.

    However, I must confess to having a difficult time getting my head around it. You have added another variable with aperture into the equation. I thought, maybe wrongfully, that the image circle was constant regardless of aperture. And, I still cannot see how the images would be the same in your example.

    Do you know of any link that graphically explains this?
    Quote Originally Posted by Jonas View Post
    ...
    b)
    For everything to be equivalent we have to mix in the aperture value. This has nothing to do with the image circle but the DOF.Jonas
    Yes that's exactly it - the aperture needs to change if you want to keep the DOF the same (otherwise the images will not be identical). Sorry Ron, I forgot to explain why I was changing the aperture. It's difficult to explain why the captured images would be the same without some diagrams, but I'll have a go

    Let's just think in one dimension at the moment - the height of an object. Say a 50mm lens is 'looking' at an object - it projects an image of the object, at the sensor, that is 10mm high. A 100mm lens will, in this case, project an image 20mm high.

    Assuming the sensors are 12MP (4000x3000 pixels, to make the maths easy) - the 4/3 sensor is 13.5mm high, so our mythical 4:3 ratio 'full frame' sensor will be 27mm high.

    The 50mm on the 4/3 camera will therefore project an image 10mm high on a 13.5mm height sensor - about 3/4 of the frame height. The object will be recorded by ~2250 pixels, occupying 3/4 of the height of the frame.

    The 100mm on the 'full frame' camera will project an image 20mm high on a 27mm height sensor - again, about 3/4 of the frame height. The object will (as before) be recorded by ~2250 pixels, occupying 3/4 of the height of the frame.

    Obviously, the same calculation applies for the width of an object. The data recorded in the image file will therefore be the same in both cases.

    Note that the size of the image circle is not relevant to any of this (as long as it is big enough to cover the sensor) and neither is the aperture - the size of the image projected at the sensor is determined just by the focal length. The reason we have to adjust the aperture to achieve the same DOF is a little more complex. If you are wondering why it's different in the 2 cases, it has to do with the fact that DOF is determined by how sharp something looks in the final print, not at the sensor.

    Also, as Jonas mentioned, this shows that any lens aberrations or softness are twice as visible on the 4/3 sensor. An edge blurred to 0.1mm at the sensor will cover ~23 pixels on the 4/3 sensor, but only ~11 on the 'full frame'. This is one of the reasons full frame sensors are nice Offsetting this is the fact that the 4/3 sensor only sees the center 50% of the image circle, where aberrations tend to be lower. As a result, using the same lens, a full frame camera tends to be better in the centre but may be worse at the edges than a 4/3 camera, which is more even across the frame.

    OK, I'll stop now before everyone nods off (Oh dear, too late...)

    Graeme
    Last edited by gcogger; 5th June 2010 at 13:56. Reason: Added my name to avoid people having to call me gcogger :)

  9. #9
    Member Ron Evers's Avatar
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    Re: This is really bugging me

    Well Jonas & Graeme you have been very patient with me. I truly appreciate your efforts to educate me. Well done, both of you. I am a little slow in my comprehension & have poor memory ever since I had open heart surgery last September. I am pretty fit otherwise.

    I think this thread is a candidate for a sticky to help other confused souls like me.
    Panasonic G1 & GF1 bodies, Panasonic 14-45, 45-200mm, various Minolta, Pentax, Canon FD, M39 & M42 mount lenses.

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