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Thread: Question on Perspective

  1. #1
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    Default Question on Perspective

    Hi guys, I understand that with wider lens, the pic's perspective will be more exaggerated.

    But if I use the same lens on a DSLR with crop factor, will the perspective be affected? After so thinking, I think the perspective of a lens shouldn't change with the cam's crop factor, just like the DOF.....am I right?

    Thank you very much!

  2. #2

    Default Re: Question on Perspective

    Quote Originally Posted by lastboltnut View Post
    Hi guys, I understand that with wider lens, the pic's perspective will be more exaggerated.

    But if I use the same lens on a DSLR with crop factor, will the perspective be affected? After so thinking, I think the perspective of a lens shouldn't change with the cam's crop factor, just like the DOF.....am I right?

    Thank you very much!
    The Field of View is narrower for DSLR with crop factor when compared to full frame.

    For e.g., a picture taken by a DSLR with a crop factor of 1.5 @18mm would appear to be taken at 27mm on a full frame.

    So the "perspective" is changed.

    Moreover, DOF is affected by sensor sizes because of the different magnification needed to enlarge the actual image on the sensor to a certain viewing picture size. (of course, at very short focal length, DOF is usually very large and any difference is not as apparent as at much longer focal length)

    These are all apparent in small point and shoot cameras when compared to full frame. For e.g., at just 6mm-9mm focal length (which is supposedly like ultra ultra wide on full frame) on PnS cameras which, just like a DSLR with a crop factor, are actually cameras with a crop factor of about 4-6x, the picture would appear to be taken at 35mm on the full frame and the front-to-back distances between subjects do not appear to be very much magnified when compared to what our eyes normally see (usually equivalent to about 50mm on full frame). If 6mm-8mm is used on a full frame, then the fron-to-back distances between subjects would appear extremely magnified.
    Last edited by Clockunder; 7th November 2006 at 02:18 AM.

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    Default Re: Question on Perspective

    Hi, I am not very sure, but what I understand is abit diff......for example, if I have an 18mm prime lens, a FF cam and a 1.5CF cam.

    I use the same lens to take a pic each on both cam, at the same location, same composition, same aperture. Then the pics will have the same DOF, same perspective (not sure here), just that the FOV is narrower for the 1.5CF cam. The object is not magnified to the same size in the original pics....And I tot I can obtain the same pic of the 1.5CF cam by using the FF cam pic and crop it to the same size and every thing in the pic will be the same.....can I say this?

    Thanks.

    Quote Originally Posted by Clockunder View Post
    The Field of View is narrower for DSLR with crop factor when compared to full frame.

    For e.g., a picture taken by a DSLR with a crop factor of 1.5 @18mm would appear to be taken at 27mm on a full frame.

    So the "perspective" is changed.

    Moreover, DOF is affected by sensor sizes because of the different magnification needed to enlarge the actual image on the sensor to a certain viewing picture size. (of course, at very short focal length, DOF is usually very large and any difference is not as apparent as at much longer focal length)

    These are all apparent in small point and shoot cameras when compared to full frame. For e.g., at just 6mm-9mm focal length (which is supposedly like ultra ultra wide on full frame) on PnS cameras which, just like a DSLR with a crop factor, are actually cameras with a crop factor of about 4-6x, the picture would appear to be taken at 35mm on the full frame and the front-to-back distances between subjects do not appear to be very much magnified when compared to what our eyes normally see (usually equivalent to about 50mm on full frame). If 6mm-8mm is used on a full frame, then the fron-to-back distances between subjects would appear extremely magnified.

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    Default Re: Question on Perspective

    Quote Originally Posted by lastboltnut View Post
    Hi, I am not very sure, but what I understand is abit diff......for example, if I have an 18mm prime lens, a FF cam and a 1.5CF cam.

    I use the same lens to take a pic each on both cam, at the same location, same composition, same aperture. Then the pics will have the same DOF, same perspective (not sure here), just that the FOV is narrower for the 1.5CF cam. The object is not magnified to the same size in the original pics....And I tot I can obtain the same pic of the 1.5CF cam by using the FF cam pic and crop it to the same size and every thing in the pic will be the same.....can I say this?

    Thanks.
    Correct!

    Regards,
    Arto.

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    Default Re: Question on Perspective

    O! Thank you bro! You are definitely helpful in many ways!

    Quote Originally Posted by Artosoft View Post
    Correct!

    Regards,
    Arto.

  6. #6

    Default Re: Question on Perspective

    Quote Originally Posted by lastboltnut View Post
    Hi, I am not very sure, but what I understand is abit diff......for example, if I have an 18mm prime lens, a FF cam and a 1.5CF cam.

    I use the same lens to take a pic each on both cam, at the same location, same composition, same aperture. Then the pics will have the same DOF, same perspective (not sure here), just that the FOV is narrower for the 1.5CF cam. The object is not magnified to the same size in the original pics....And I tot I can obtain the same pic of the 1.5CF cam by using the FF cam pic and crop it to the same size and every thing in the pic will be the same.....can I say this?

    Thanks.
    You're right that you can obtain the same picture of the 1.5CF cam by using the FF cam pic and crop it to the same size and every thing in the pic will be the same (except of course the subjects will appear relatively bigger in the frame for the 1.5CF). The image formed on the 1.5CF sensor is exactly the same as the one on full frame except for the crop. So as far as the original image is concerned, it's exactly the same.

    The difference comes in viewing the picture and this is where DOF is different because DOF is a perception issue about what appears to be in focus and what is not.

    So DOF is affected not only by the actual image image formed on the sensor but also by the magnification needed on the original image to make it into a certain picture viewing size and also by the picture viewing distance. Magnification of the original image to for e.g. 4R picture size :

    Full frame : 4.2x
    1.5CF sensor : 6.6x

    That is why the sensor size comes into the calculation and is incorporated into DOF calculators. Try using the same focal length , same subject distance and same aperture in the calculator below to see the difference in DOF with different sensor sizes. Smaller sensors, all things equal, have shallower DOF. (One may wonder about the anomaly between the expected much shallower DOF for the very small PnS sensors and the great DOF experienced in real life. This can be explained by the fact that small PnS sensors' shallow DOF is usually overwhelmed by the short focal length used) :

    http://photoinf.com/Tools/Don_Flemin...alculator.html

    This is usually explained with the term "Circle of Confusion".

    Take a look at the 2 pictures below and one would perceive that the 2nd picture (cropped picture to simulate taken at the same focal length, same light condition, same aperture, same distance etc. except on a smaller sensor size) has a shallower DOF even though it's the same picture. Notice the small leaves in the background (at the top left hand corner of both pictures or other places). In the original picture, the leaves which appears much smaller also looks much sharper than in the cropped picture. That's the effect of different magnification needed to achieve the same picture viewing size.

    Original picture (downsized to 800x533):


    Cropped and then downsized to 800x533 to have the same viewing size as the above picture :
    Last edited by Clockunder; 7th November 2006 at 04:36 PM.

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    Default Re: Question on Perspective

    Quote Originally Posted by Clockunder View Post
    The Field of View is narrower for DSLR with crop factor when compared to full frame.

    For e.g., a picture taken by a DSLR with a crop factor of 1.5 @18mm would appear to be taken at 27mm on a full frame.

    So the "perspective" is changed.
    I don't think this is true leh. A pic taken on a 1.5CF cam using 18mm will just be like taken on a FF cam, just that you have a internal black border. Therefore, perspective does not change.
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    Default Re: Question on Perspective

    Ok, I get what you mean. It's just the threshold of how capable our eye is able to detect sharpness (like COC of 0.2mm). Ok, that is if you resize the pic. If I dun then all will be the same.

    But in this case, perspective shouldn't change rite? Like if you take a pic of a box in ISO angle, and you measure the front edge of the box on the pic (for example 2") and when you measure the rear edge (for example 1"), then this ratio of 2":1" shouldn't change rite? So the perspective is the same rite?

    I understand that WA lens gives more exaggerated perspective, like for example of the box above, 2":1" may be with normal lens and WA lens may give 2":0.75".

    Hope you know what I am trying to say.....it's a bit messy here.

    TIA

    Quote Originally Posted by Clockunder View Post
    You're right that you can obtain the same picture of the 1.5CF cam by using the FF cam pic and crop it to the same size and every thing in the pic will be the same (except of course the subjects will appear relatively bigger in the frame for the 1.5CF). The image formed on the 1.5CF sensor is exactly the same as the one on full frame except for the crop. So as far as the original image is concerned, it's exactly the same.

    The difference comes in viewing the picture and this is where DOF is different because DOF is a perception issue about what appears to be in focus and what is not.

    So DOF is affected not only by the actual image image formed on the sensor but also by the magnification needed on the original image to make it into a certain picture viewing size and also by the picture viewing distance. Magnification of the original image to for e.g. 4R picture size :

    Full frame : 4.2x
    1.5CF sensor : 6.6x

    That is why the sensor size comes into the calculation and is incorporated into DOF calculators. Try using the same focal length , same subject distance and same aperture in the calculator below to see the difference in DOF with different sensor sizes. Smaller sensors, all things equal, have shallower DOF. (One may wonder about the anomaly between the expected much shallower DOF for the very small PnS sensors and the great DOF experienced in real life. This can be explained by the fact that small PnS sensors' shallow DOF is usually overwhelmed by the short focal length used) :

    http://photoinf.com/Tools/Don_Flemin...alculator.html

    This is usually explained with the term "Circle of Confusion".

    Take a look at the 2 pictures below and one would perceive that the 2nd picture (cropped picture to simulate taken at the same focal length, same light condition, same aperture, same distance etc. except on a smaller sensor size) has a shallower DOF even though it's the same picture. Notice the small leaves in the background (at the top left hand corner of both pictures or other places). In the original picture, the leaves which appears much smaller also looks much sharper than in the cropped picture. That's the effect of different magnification needed to achieve the same picture viewing size.

    Original picture (downsized to 800x533):


    Cropped and then downsized to 800x533 to have the same viewing size as the above picture :

  9. #9

    Default Re: Question on Perspective

    Quote Originally Posted by Splutter View Post
    I don't think this is true leh. A pic taken on a 1.5CF cam using 18mm will just be like taken on a FF cam, just that you have a internal black border. Therefore, perspective does not change.
    Are you referring to vignetting?

    Which picture are you saying has internal black border? The one on FF or the one on 1.5CF cam?

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    Default Re: Question on Perspective

    lastboltnut, you are right!

    Clockunder, what I meant was its just cutting away a internal border of the pic taken on the FF cam.
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  11. #11

    Default Re: Question on Perspective

    Quote Originally Posted by lastboltnut View Post
    Ok, I get what you mean. It's just the threshold of how capable our eye is able to detect sharpness (like COC of 0.2mm). Ok, that is if you resize the pic. If I dun then all will be the same.

    But in this case, perspective shouldn't change rite? Like if you take a pic of a box in ISO angle, and you measure the front edge of the box on the pic (for example 2") and when you measure the rear edge (for example 1"), then this ratio of 2":1" shouldn't change rite? So the perspective is the same rite?

    I understand that WA lens gives more exaggerated perspective, like for example of the box above, 2":1" may be with normal lens and WA lens may give 2":0.75".

    Hope you know what I am trying to say.....it's a bit messy here.

    TIA
    The original image on a FF is only 36mm x 24mm and the one on a 1.5CF is about 24mm x 16mm. So magnification is in almost all situations a must for viewing whether on screen or on print. Even if you don't do any resize on the image, the image is magnified many times when you view it on screen.

    For perception, if you're talking strictly about the exaggerated distortion, then I would say it's the same because it's the same image. But then distortion is not uniform across the WA lens and it seems to be much more exaggerated towards the edges especially when the subject is very near. If it's a cropped picture off the FF picture, then you may not get the extreme exaggerated distortion at the edges which you can achieve with a full frame if you had used a full frame lens on a 1.5CF camera at the same actual focal length. Because of this difference, there are some WA lenses designed specifically for APS sized sensors.

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    Default Re: Question on Perspective

    Ok Clockunder, I think I have got all the answer I intended for this thread. U are talking in a deeper manner (like considering the COC of the viewer and the WA edge distortion which are crop with APS sensor). I started with consideration of comparing only the center portion of the pic (for perspective) and with no re-sizing of the pic on DOF.

    Now all is clear! Thanks for your patience and explaination and even making pics to show. Also the discussion in a deeper manner, so that now I can think in that depth too.

    Thanks again!

    Quote Originally Posted by Clockunder View Post
    The original image on a FF is only 36mm x 24mm and the one on a 1.5CF is about 24mm x 16mm. So magnification is in almost all situations a must for viewing whether on screen or on print. Even if you don't do any resize on the image, the image is magnified many times when you view it on screen.

    For perception, if you're talking strictly about the exaggerated distortion, then I would say it's the same because it's the same image. But then distortion is not uniform across the WA lens and it seems to be much more exaggerated towards the edges especially when the subject is very near. If it's a cropped picture off the FF picture, then you may not get the extreme exaggerated distortion at the edges which you can achieve with a full frame if you had used a full frame lens on a 1.5CF camera at the same actual focal length. Because of this difference, there are some WA lenses designed specifically for APS sized sensors.

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    Default Re: Question on Perspective

    Hi Splutter, the black border or vignetting happens on FF cam when using DX lens....if its a normal lens, no prob.

    And I think Clockunder is also right in the perspective issue if you consider the edge distortion, which is usually more at the edge and thus more exaggerated perspective, but the edge are cropped in APS sensor, so it may look less in perspective.....if compare only center to center portion, then the perspective will be the same.


    Quote Originally Posted by Splutter View Post
    lastboltnut, you are right!

    Clockunder, what I meant was its just cutting away a internal border of the pic taken on the FF cam.

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    Default Re: Question on Perspective

    Sigh...nvm I think I'm severly misunderstood.
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    Default Re: Question on Perspective



    Ok, I guess a pic should help me express myself better. Essentially, what I meant was that if the same lens (FF-able lens) was used both on a FF and a 1.6CF cam at exact same distance and settings, the above 2 pics will result. As can be seen, there is no perspective change since crop factor literally crops the picture out from a full frame. The objects in the frame remain the same RELATIVE size to other objects in the frame. This has nothing got to do with magnification since you can view it at any size you want on the screen.
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