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Thread: Need Advice?

  1. #21
    Member jas1984's Avatar
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    Default Re: Need Advice?

    Quote Originally Posted by FrogmanTan View Post
    thanks dare devil123....

    anyone, do you happen to know what is the different between 50mm f/1.8G and 50mm f/1.8D?
    Hope This help..

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  2. #22

    Default Re: Need Advice?

    hi jas...

    a bit confuse....but nevermind, is not english but is my technical understanding for this, probably really have to try than will know

  3. #23

    Default Re: Need Advice?

    Quote Originally Posted by jas1984 View Post
    So 50mm is not 50mm on the D3100 and is 75mm.
    Somehow I find that some photographers like to use the terminology of focal length loosely when related to crop factors. A 50mm lens will always have a focal length of 50mm regardless of which sensor it is used with. Even when you use it against a medium format sensor, it is still 50mm. I would also like to bring in one of the close thread where there is a heated debate over Depth of Field (the frustum within a perspective where things will be kept in so-called focus based on the size of the circle of confusion). DOF is determined by the lens perspective(relative to focal length), aperture size and circle of confusion. Sensor is not part of the equation as I understood.

    Focal Length is one of the lens property. It is not sensor related and therefore likewise crop factor is not part of the equation too. What is often discussed of so called focal length together with crop factor is nothing more than an APPARENT FOCAL LENGTH relative to 35mm negative film standard. Due to legacy reason, it seems that 35mm is often used as standard to discuss on the size of the image cast on the sensor and hence crop factor relative to 35mm negative film size(22mm by 16mm for camera aperture).

    For Nikon DSLR, Full Frame sensors are (almost) equivalent of 35mm film dimension. DX are smaller by a factor of 1.5. Canon DSLR also have 1.6 crop factors for their APS-C sensors.


    Using the simplified lens image in courtesy of Wikipedia above, you can imagine the lens in the middle is your complex arrangement of lens that you owned. In actual fact, there is little difference between the real lens and this simplified one if you regard the real lens setup has a blackbox. Now assume the inverted "Real image" size you observed as a green arrow is the actual image, fully extended to the height of the DX sensor. Also assume that it is formed using a 50mm lens. When you use a FX sensor, the image is smaller by a factor of 1.5. That 1.5 is the crop factor because DX is like cropping your image in your photoshop. Excess lights when using FX lens on DX body are basically projected out of the sensor into the black surfaces. In order to get back the same size of the green arrow in the FX sensor when produce out as an image, we need to use a 75mm lens with a much tighter field of view(fov) or perspective. Having a tighter fov, the projected image will be bigger on the FX sensor and if you match against the DX sensor, it will not fit in at all.

    Therefore we often says that DX sensor having a 1.5 crop factor when using a 50mm lens has a FOV that is equivalent to the FOV of a 75mm lens used on a FX sensor. I hope it makes it clearer for those confused between the loosely used focal length terminology with respect to sensor size.

    As a side note, IMHO, the DOF given in the DOFMaster is not an absolute value. If you read up on CoC in http://en.wikipedia.org/wiki/Circle_of_confusion, you will understand that this value is subjective to the resolution power of an audience eyes. I have met people that have exceptionally good resolution power that they can make out words as small as the lines height on a A4 paper at a distance while even with my corrected eyes using specs, I cannot make out the words. This cannot be fixed for me even if the most accurate pair of prescribed lens. So for people with very good eyes, the DOF of the very same lens can be wider than someone with bad resolution because the CoC is smaller. So please just take the DOFMaster as a guide and don't be too stagnant about it.
    Last edited by David Kwok; 2nd October 2011 at 01:17 AM.
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  4. #24
    Member jas1984's Avatar
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    Default Re: Need Advice?

    David thanks for showing me some light.. have read through what you have wrote but still a bit confused on the CoC part.. is it possible for you to explain it in a more simple way?

    Am i right to say if only i take the FX sensor as a standard.. having a 35mm lens mount on a crop sensor body the fov will be like having a 52.5mm fov on a fx sensor?

    So by saying 50mm is not 50mm on the D3100 and is 75mm is wrong and it should be the fov of a 50mm lens on a crop sensor have a equivalent fov of 75mm on a fx sensor due to the 1.5 crop factor? Do correct me if i am wrong again.. many thanks.. so sorry if i have share something wrong..
    Last edited by jas1984; 2nd October 2011 at 02:16 AM.
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  5. #25

  6. #26

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    Quote Originally Posted by David Kwok
    There is a choice between wide angle that you would like and blur background(bokeh) that you would like.
    Do note that bokeh does not mean background blur...
    Alpha

  7. #27

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    Quote Originally Posted by Rashkae

    Do note that bokeh does not mean background blur...
    Yes you are right. Bokeh refers to the quality of the blurry effect caused by oof. The word itself doesn't means blur indeed. Thanks for correcting.
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  8. #28

    Default Re: Need Advice?

    Quote Originally Posted by jas1984 View Post
    David thanks for showing me some light.. have read through what you have wrote but still a bit confused on the CoC part.. is it possible for you to explain it in a more simple way?
    Did you read the wiki link I have given about CoC in the earlier post ? http://en.wikipedia.org/wiki/Circle_of_confusion
    Read it if you have not, it has a rather extensive explanation on CoC and how it is determined in digital sensors, I doubt I can explain better than it, unless you are confused in the explanation given. Then you let me know which part do you not understand.

    Quote Originally Posted by jas1984 View Post
    Am i right to say if only i take the FX sensor as a standard.. having a 35mm lens mount on a crop sensor body the fov will be like having a 52.5mm fov on a fx sensor?

    So by saying 50mm is not 50mm on the D3100 and is 75mm is wrong and it should be the fov of a 50mm lens on a crop sensor have a equivalent fov of 75mm on a fx sensor due to the 1.5 crop factor? Do correct me if i am wrong again.. many thanks.. so sorry if i have share something wrong..
    Yes, you are quite right this time round. The equivalence as so to speak is not the focal length. It should be the field of field exhibited by the lens of a particular focal length that is being compared. The FOV of a 50mm lens on a DX sensor compared against the FOV of a 75mm lens on a FX sensor.
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  9. #29
    Member jas1984's Avatar
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    Default Re: Need Advice?

    Quote Originally Posted by David Kwok View Post
    Did you read the wiki link I have given about CoC in the earlier post ? http://en.wikipedia.org/wiki/Circle_of_confusion
    Read it if you have not, it has a rather extensive explanation on CoC and how it is determined in digital sensors, I doubt I can explain better than it, unless you are confused in the explanation given. Then you let me know which part do you not understand.
    Yes, I have read up on the link in your earlier post but still cannot understand...



    Quote Originally Posted by David Kwok View Post
    Yes, you are quite right this time round. The equivalence as so to speak is not the focal length. It should be the field of View exhibited by the lens of a particular focal length that is being compared. The FOV of a 50mm lens on a DX sensor compared against the FOV of a 75mm lens on a FX sensor.
    Thanks for the clarification...
    Last edited by jas1984; 2nd October 2011 at 02:04 PM.
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  10. #30
    Member jas1984's Avatar
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    Default Re: Need Advice?

    Quote Originally Posted by eleveninth View Post
    Thanks..
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  11. #31

    Default Re: Need Advice?

    Quote Originally Posted by jas1984 View Post
    Yes, I have read up on the link in your earlier post but still cannot understand...
    Due to lens design, imperfect manufacturing process, as well as inherent light properties, a point of light projecting from one side of the lens do not necessarily get projected as a focused point on the screen(sensor or film). Often the point of light will look similar to a small circle projected on the screen. In fact, it doesn't look like a circle all the time, it should form the shape of the aperture, but for simplicity sake, we assume it is a circle. There are a lot of light properties that contribute to the formation of this small circle. It can be light diffraction, colour aberration, imperfection of the lens surface that failed to project all light rays into a single point, inconsistency in the density of the lens that causes different refractive index for different portion of the lens, bubbles in the lens, and so forth..

    Hence the question here is how small should this small circle be so that it will look sharp and focus to a human eye. Each human eye's acuity is different and hence what one may observed as a circle might just be a point to another. This is the circle of confusion(CoC) that we are referring to. Using 0.03mm as suggested for a full frame (35mm) sensor, it will means any projection of focused light rays that form a circle of diameter larger than 0.03mm is considered out of focus and can be observed by a human eye as a circle. Anything below of that diameter will be too small for the acuity of a human eye to differentiate between a circle or a point and hence considered in focus.

    I hope this makes it clear for you
    Last edited by David Kwok; 2nd October 2011 at 10:49 PM.
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  12. #32
    Member jas1984's Avatar
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    Default Re: Need Advice?

    Quote Originally Posted by David Kwok View Post
    Due to lens design, imperfect manufacturing process, as well as inherent light properties, a point of light projecting from one side of the lens do not necessarily get projected as a focused point on the screen(sensor or film). Often the point of light will look similar to a small circle projected on the screen. In fact, it doesn't look like a circle all the time, it should form the shape of the aperture, but for simplicity sake, we assume it is a circle. There are a lot of light properties that contribute to the formation of this small circle. It can be light diffraction, colour aberration, imperfection of the lens surface that failed to project all light rays into a single point, inconsistency in the density of the lens that causes different refractive index for different portion of the lens, bubbles in the lens, and so forth..

    Hence the question here is how small should this small circle be so that it will look sharp and focus to a human eye. Each human eye's acuity is different and hence what one may observed as a circle might just be a point to another. This is the circle of confusion(CoC) that we are referring to. Using 0.03mm as suggested for a full frame (35mm) sensor, it will means any projection of focused light rays that form a circle of diameter larger than 0.03mm is considered out of focus and can be observed by a human eye as a circle. Anything below of that diameter will be too small for the acuity of a human eye to differentiate between a circle or a point and hence considered in focus.

    I hope this makes it clear for you
    Many thanks David.. Thanks for going through all the trouble to explain it to me... really appreciate..
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