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Thread: Qn abt macro, magnification, sensor size and perspective........

  1. #1
    Senior Member zoossh's Avatar
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    Default Qn abt macro, magnification, sensor size and perspective........

    Have always been confused about this....

    1. What exactly does true macro photography with 1:1 magnification means? Am i right to say that....

    It means with for example that a 90mm 1:1 lens and a 36mm wide sensor, should show an insect that measures 36mm from head to tail (left to right) at a distance of 90mm from the insect to the optic centre, covering the entire width of the resultant image?
    ... which means that if the insect is 180mm from the optic centre, the frame will cover 72mm width, and the 36mm long insect will cover half the width of the frame?
    ... and if the minimum focusing distance is 270mm, the frame will cover 108mm width, and the 36mm long insect will cover a third of the width of the frame?

    and if my sensor width is 18mm with a conversion factor of 2, at 1:1 magnification factor of the 90mm lens,
    if the insect is 90mm away from the optic centre, the resultant image only capture 18mm centre portion of the 36mm long insect
    if the insect is 180mm away, then only will a life-size 36mm long insect fills up the width of the image on the 18mm sensor.
    if the insect is 270mm (3x90) away, the frame will cover 54mm (3x18) with the 36mm long insect covers 2/3 of the picture instead of 1/3 of the picture.
    essentially the change of sensor size with a conversion factor of 2, doubles the apparent magnification by 2 (subject fills up half the frame at 180mm distance at 36mm sensor size but fills up the whole frame at 180mm same distance with a 18mm sensor size), even though the actual magnification of the sensor size v.s the subject size is still 1:1.

    And with a reduction of sensor size by half, it will have a reduced angle of view and field of view, with a change of perspective from what a 36mm wide sensor will see at 90mm focal length, but this 18mm wide sensor using a 90mm lens instead will have the same perspective of what a 36mm wide sensor will see at 180mm focal length, with the same angle of view, requiring the same 36mm long subject to be at 180mm away instead of 90mm away to fill up the width of the image.

    If the angle of view is changed by the reduction of sensor size, the perspective of near and far objects also changes, so is it true that people say a 90mm focal length lens will retain the perspective relationship of a 90mm focal length even with a change of sensor size?



    2. does magnification factor always change with a zoom range macro lens? is it true that as long as a macro lens is zoom and not prime, it can't possibly have a constant magnification factor of 1:1. and hence true constant 1:1 magnification factor can only be achieved by a prime lens?



    3. how exactly does the framing differs between a 90mm normal lens and a 90mm macro 1:1 lens?
    if arbitrarily a 90mm macro 1:1 lens appears to cover the width of the same subject as a 90-270mm lens will do at 270mm focal length, would the perspective, angle of view and field of view be that of a 90mm or 270mm?
    Last edited by zoossh; 10th February 2007 at 10:06 AM.

  2. #2
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    Default Re: Qn abt macro, magnification, sensor size and perspective........

    1) yes, a 36mm long insect will fill up a 36mm full frame sensor from left to right. the working distance of a 90mm 1:1 is normally about 300mm (i dono anything about optic center)
    The same goes as a cropped 18mm, a 18mm insect will fill it up and a 36mm long insect will only appear in 1/2 if one shot it at 1:1. To get a full view of the 36mm insect, one will have to reduce the magnification to 1:2.

    2) I can't be sure, as as 1:1 macro zoom lens are rare (or do they even exist). But from an example of a Nikkor 70-180mm f/4.5-5.6D ED Micro, at 70mm it can only achieve 1:3.21, and at the tele end (180mm) it is able to achieve 1:1.33.
    So in this case you are right that zoom macro lens doesn't have a constant magnification.

    3) I can't ans as i do not own one yet.

  3. #3

    Default Re: Qn abt macro, magnification, sensor size and perspective........

    i think if you put it that way, with optical centre and what not, it is a little confusing. might be easier to talk about focusing distance instead.

    1) at 1:1 magnification, the object you are photographing is exactly the same size as its image on the film plane.

    looking at my tamron 90mm macro focus window, it says 1:1 (max magnification) at focusing distance of 29cm (minimum focusing distance). this makes sense, since the closer you move to an object, the larger the magnification. (notice that it's 29cm, not 90mm)

    so magnification depends both on focal length and on minimum focusing distance. a 180mm lens doesn't need to focus as closely as a 90mm to get 1:1 magnification. that's why they say 180mm macros give more working distance than 90mm macros.

    2) no idea

    3) a 90mm macro lens has the same field/angle of view as any other 90mm prime, or zoom set at 90mm. framing is thus exactly the same. the important difference, as i've said above, is that the macro lens can focus much closer, allowing much more magnification.

  4. #4

    Default Re: Qn abt macro, magnification, sensor size and perspective........

    1) bingo

    2) i thought the zooming only changes the working distance? 1:1 will still remain as it is... i may be wrong..

    3) how you compare perspective of 90mm normal lens and a 90mm macro?
    i think you should compare the perspective of 60mm micro 105micro 200mico oh! 90mm macro if you like

  5. #5

    Default Re: Qn abt macro, magnification, sensor size and perspective........

    1) Yes. The distance is 90mm for a simple 1 piece lens which we learn in Physics in secondary school so that total is 90mm for subject to optics centre + 90mm for optics centre to screen = 180mm. Apparently, camera lenses are much more sophisticated and the focal length and shooting distances are not as simple.

    I did a simple linear regression on the mininum shooting distance of macro lenses vs their focal lengths and found that an approximate equation after some rounding of numbers for easy remembering :

    Minimum shooting distance (for 1:1) = 2 x focal length + about 100mm.

    The r is very high at 0.99+, meaning extremely fitting straight line.

    Full frame @90mm and shoot from 28cm, you get the insect 36mm filling exactly the 36mm wide of the image sensor at 1:1 magnification.

    On a 2x crop factor camera :
    1:1 appears 2:1 (i.e. ½ of the 36mm insects on the 18mm gives the impression of it being a 72mm insect on the 36mm full frame).
    1:2 appears 1:1
    1:3 appears 2:3
    1:4 appears 1:2
    1:5 appears 2:5

    Only the angle of view is the same for 90mm on 18mm sensor and 180mm on 36mm sensor. The narrower field of view of a smaller 18mm sensor vs 36mm sensor is exactly offset by using a shorter 90mm lens which is wider then the 180mm.

    However, even though the angle of view is the same, the perspective is different for different focal lengths used because different focal lengths will compress and render the relative distances (distances between front and back objects) differently, creating a different perspective. A longer focal length will compress more and therefore the object behind will appear much closer to the front object than they would appear with a shorter focal length used. This means that although you are able to frame the main subject in the same position and size you want in the frame with different focal lengths at different shooting distances, everything else in the frame would appear different with different focal lengths used.

    2) On a full frame, the approximate shooting distance to get 1:1 :

    @180mm, 360mm + 100mm = 46cm
    @120mm, 240mm + 100mm = 34cm
    @105mm, 210mm +100mm = 31cm
    @90mm, 180mm + 100mm = 28cm
    @75mm, 150mm + 100mm = 25cm
    @50mm, 100mm + 100mm = 20cm

    To get constant 1:1 over the zoom range could be a technical possibility issue and/or a cost issue.


    3) Framing for the 90mm normal and 90mm macro lens is the same except the ability to focus at short shooting distance. Below a certain shooting distance, only the macro 90mm can achieve sharp focus properly.

    To get 1:1 magnification, shooting distance :
    @90mm, 180mm + 100mm = 28cm
    @270mm, 540mm + 100mm = 64cm

    The normal lens is not able to focus at such near distances and this is the practical problem.

    The 90mm Macro lens will of course take the perspective of a 90mm lens while the perspective of the 90-270mm lens will vary according to the actual focal length used.
    Last edited by Clockunder; 11th February 2007 at 11:05 AM.

  6. #6
    Senior Member zoossh's Avatar
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    Default Re: Qn abt macro, magnification, sensor size and perspective........

    Quote Originally Posted by Clockunder View Post
    The 90mm Macro lens will of course take the perspective of a 90mm lens while the perspective of the 90-270mm lens will vary according to the actual focal length used.
    hence in short, the 90mm macro v.s. normal 90mm magnify to the same extent on the same object from the same distance, and thus share the same angle of view, field of view and perspective. but becos the macro lens allow closer focusing distance, the distance changes and hence the magnification?

  7. #7

    Default Re: Qn abt macro, magnification, sensor size and perspective........

    Quote Originally Posted by zoossh View Post
    hence in short, the 90mm macro v.s. normal 90mm magnify to the same extent on the same object from the same distance, and thus share the same angle of view, field of view and perspective. but becos the macro lens allow closer focusing distance, the distance changes and hence the magnification?
    yep

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