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Thread: APS-C Camera & its Crop Factors on Lenses.

  1. #1

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    APS-C Camera & its Crop Factors on Lenses.

    Nikon D7000, a DX with a crop factor of 1.5 plus a Nikon AF-S 10-24mm, a DX wide-angle Lens.
    1. Does it means, both are DX, the focal length still changed to 15-36mm ?
    2. What if this DX Lens is fixed to a FF camera ?

    Please help to verify the confusion !

    TQ.

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    Moderator Donald's Avatar
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    Re: APS-C camera & its crop factors on Lenses.

    Quote Originally Posted by Albert Sim View Post
    Nikon D7000, a DX with a crop factor of 1.5 plus a Nikon AF-S 10-24mm, a DX wide-angle Lens.
    1. Does it means, both are DX, the focal length still changed to 15-36mm ?
    2. What if this DX Lens is fixed to a FF camera ?

    Please help to verify the confusion !

    TQ.
    Albert

    The terminology relating to this matter can be confusing.

    The rated focal length of the lens is 10-24mm. This will be the true focal length range if the lens was fitted onto a full frame camera. (Please note that I do not know Nikon equipment, so must assume that this lens can be fitted to a full frame camera).

    If the lens is fitted to a camera with a smaller sensor, the the effect will be to increase the effective focal length range by a factor of 1.5; i.e. 15 - 36mm, when compared to the same focal range on a full frame camera body.

    Now, if the only camera body you use has a 1.5 crop factor sensor, the the comparison is, for all practical purposes, totally irrelevant. Your understanding of what the lens can do will be informed by the results you get with the body that you own. The fact that the lens would behave differently on a full frame body is, in many respects, irrelevant.

    For example, I use a Canon 70-200 f4 L on a Canon 40D body. I am very happy with the results. If I used the same lens on a full frame Canon body I would get different results. But since I do not have such a body, then I do not know, and am nor aware, of those differences. So, in that respect, the question is irrelevant to me.

    I think too many people get too confused and concerned about this issue. Any lens does what it does on the body you affix it to. If you like the results that's good. And that's all that matters.
    Last edited by Donald; 19th April 2012 at 03:26 PM.

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    Tringa's Avatar
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    Re: APS-C camera & its crop factors on Lenses.

    I agree. A DX lens set at 10mm on a camera with a crop factor of 1.5 will give a view equal to 15mm on a full frame camera.

    As far as I know DX lenses are designed to cover the crop factor sensor and therefore if used on a camera with a full frame sensor (ie 36mm x 24mm) the corners of photos will be dark.

    Dave

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    Re: APS-C camera & its crop factors on Lenses.

    From Ken Rockwell about this lens
    "If you use it on film or FX you'll get just a circular image in the center a10mm, marginal results at 15mm and perfectly fine results from 18-24mm."

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    Re: APS-C camera & its crop factors on Lenses.

    Quote Originally Posted by Donald View Post
    I think too many people get too confused and concerned about this issue. Any lens does what it does on the body you affix it to. If you like the results that's good. And that's all that matters.
    Thank you so much Donald,
    You give me the sense of relief. In facts, I am thinking of, for the past 1 or 2 months, to upgrade to a full-frame. Among my gears, there is this Nikon DX 10-24mm and one Sigma DC 18-50mm that made me worried. Well, I am ok now. As long as I am happy with the results, who cares. If not, just trade-in for a FX lens and solve the problem. Hahaaaa.

  6. #6

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    Re: APS-C camera & its crop factors on Lenses.

    However, Dave & Mars,
    I realy appreciate for your info given in replying my thread.

    Thank you so much guys

    Best Rgds,

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    Re: APS-C camera & its crop factors on Lenses.

    Donald,
    I think we need to be a little careful with statements like, "This will be the true focal length range if the lens was fitted onto a full frame camera." This implies that the focal length could be something else depending on the camera it's fitted to. This is, of course, incorrect. The focal length is what it say it is. Its effect will differ depending on the sensor size of the camera body it's used on.
    The trick is to take the sensor size and square it up, i.e. a 16 x 24 will 'square up' to 24 x 24, then take the diagonal which will be 34mm in this example. Therefore 34mm is the 'Standard' or 'Normal' focal length for that sensor. Less than that will be 'wide-angle' and more will be 'telephoto'.

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    Moderator Donald's Avatar
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    Re: APS-C camera & its crop factors on Lenses.

    You're quite correct Chris. In this instance I was trying to frame a response that I thought would help Albert understand an issue that he was trying to comprehend and the wording I used seemed more appropriate in the circumstances.

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    Moderator GrumpyDiver's Avatar
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    Re: APS-C camera & its crop factors on Lenses.

    There are two aspects to a lens design. One is the focal length, which does not vary at all, and the second is the image circle, which is diameter of the circle the lens projects.

    As an example, if you were able to mount a 50mm lens for a medium format camera on your D7000 it would give you the same result as putting DX 50mm lens on your camera; it would work like a short telephoto lens. If you put it on a full-frame DSLR, it would perform like a normal lens and if you put it on medium format camera, you would get a wide-angle view. However this is a bit of an illusion; the sensor on Nikon is about 15.8 x 23.6 mm, so if you took a piece that size out of each of the images you captured on each of these three formats, they would all be identical.

    Going the other way, i.e. mounting your DX lens on anything other your DX camera is not going to work particularly well, which is where the image circle comes in. One reason that full-frame and medium format camera lenses cost more is that they have to illuminate a larger area. For a DX camera, the image circle is around 28mm (the diagonal of the sensor), whereas the image circle for a full-frame sensor is around 43mm and a medium format camera (based on the 2-1/4 square format) is around 77mm.

    If you mount your DX lens on anything larger than DX sensor, the centre portion of the image will be quite good for the first 28mm, but then you will get into a circle that has terrible resolution and finally everything will be black.

    Hopefully this is clear, rather than confusing...

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    Re: APS-C camera & its crop factors on Lenses.

    Quote Originally Posted by GrumpyDiver View Post
    There are two aspects to a lens design. One is the focal length, which does not vary at all, and the second is the image circle, which is diameter of the circle the lens projects.

    As an example, if you were able to mount a 50mm lens for a medium format camera on your D7000 it would give you the same result as putting DX 50mm lens on your camera; it would work like a short telephoto lens. If you put it on a full-frame DSLR, it would perform like a normal lens and if you put it on medium format camera, you would get a wide-angle view. However this is a bit of an illusion; the sensor on Nikon is about 15.8 x 23.6 mm, so if you took a piece that size out of each of the images you captured on each of these three formats, they would all be identical.

    Going the other way, i.e. mounting your DX lens on anything other your DX camera is not going to work particularly well, which is where the image circle comes in. One reason that full-frame and medium format camera lenses cost more is that they have to illuminate a larger area. For a DX camera, the image circle is around 28mm (the diagonal of the sensor), whereas the image circle for a full-frame sensor is around 43mm and a medium format camera (based on the 2-1/4” square format) is around 77mm.

    If you mount your DX lens on anything larger than DX sensor, the centre portion of the image will be quite good for the first 28mm, but then you will get into a circle that has terrible resolution and finally everything will be black.

    Hopefully this is clear, rather than confusing...
    Sorry to be picky Manfred, but you cannot take the diagonal of an un-square sensor. You must square it up first otherwise the projected circle will not cover the sensor. This makes the 'standard' lens for a DX 34mm or a FX (or 35mm film) 51m.

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    Re: APS-C camera & its crop factors on Lenses.

    Quote Originally Posted by Albert Sim View Post
    Thank you so much Donald,
    You give me the sense of relief. In facts, I am thinking of, for the past 1 or 2 months, to upgrade to a full-frame. Among my gears, there is this Nikon DX 10-24mm and one Sigma DC 18-50mm that made me worried. Well, I am ok now. As long as I am happy with the results, who cares. If not, just trade-in for a FX lens and solve the problem. Hahaaaa.
    Don't laugh. That's what you're going to have to do if you go full-frame. Using a DX lens on an FX body is problematic. While the lens mounts and works just fine, it's designed for a smaller sensor. Putting it on an FX sensor will require the camera to crop the image down, so you won't be using the full resolution of the sensor, or you'll have an image with black corners/edges, since the lens can't completely cover the larger FX sensor.

    Chances are good, you're going to have to replace both those lenses. This is the huge "hidden" cost with moving from crop to full-frame. The other is that every lens you use will change its character. I had to relearn every single one of my lenses when I moved from the 50D to a 5Dii.
    Last edited by inkista; 19th April 2012 at 07:45 PM.

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    Moderator GrumpyDiver's Avatar
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    Re: APS-C camera & its crop factors on Lenses.

    Quote Originally Posted by krispix View Post
    Sorry to be picky Manfred, but you cannot take the diagonal of an un-square sensor. You must square it up first otherwise the projected circle will not cover the sensor. This makes the 'standard' lens for a DX 34mm or a FX (or 35mm film) 51m.
    Chris - I'm not sure where you are coming from. There is no reason to have to "square anything up".

    You have to calculate the diagonal based on the length and height of the sensor, as this is the maximum dimension that the lens has to cover. diagonal = square root (length squared + height squared). Sorry; kind of hard to type the Pythagorean theorem here, but that is how this dimension is calculated. Anyways, that dimension is the minimum diameter of the image circle.

    If you square things up, and I assume you mean take the largest dimension of the sensor as the basis of your square. I'm not sure where the squaring off comes from. You actually get an image circle that is far larger than what is required if you do that.

    I'd draw it up later, but haven't got any software on the machine I am working on right now to do that.

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    Moderator GrumpyDiver's Avatar
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    Re: APS-C camera & its crop factors on Lenses.

    Quote Originally Posted by inkista View Post
    Don't laugh. That's what you're going to have to do if you go full-frame. Using a DX lens on an FX body is problematic. While the lens mounts and works just fine, it's designed for a smaller sensor. Putting it on an FX sensor will require my camera to crop the image down, so you won't be using the full resolution of the sensor, or you'll have an image with black corners/edges, since the lens can't completely cover the larger FX sensor.

    Chances are good, you're going to have to replace both those lenses. This is the huge "hidden" cost with moving from crop to full-frame. The other is that every lens you use will change its character. I had to relearn every single one of my lenses when I moved from the 50D to a 5Dii.

    Or you could have done what I did, I bought mostly FX lenses over the past 3 years assuming that I would eventually convert to a FX. It would have been a waste of money for the DX alone, but I won't have to spend it again when my DX finally arrives. The way Nikon D800 shipments are going right now, that could still be some time....

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    Re: APS-C camera & its crop factors on Lenses.

    Actually, DX lenses will work quite well with the D800. True, you "only" get 16mp instead of the full 36mp but that seemed to be plenty for everyone until now.

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    Re: APS-C camera & its crop factors on Lenses.

    Quote Originally Posted by benm View Post
    Actually, DX lenses will work quite well with the D800. True, you "only" get 16mp instead of the full 36mp but that seemed to be plenty for everyone until now.
    Yeah, but if you specifically blew the $2700 to get 36MP and the wider FoV, it's kind of defeating the purpose, isn't it?

    I also think that getting only FX lenses makes sense only if you plan the move to FX within the year or you have no need for an ultrawide/wide angle. Otherwise, you're cutting yourself off from having anything suitable on the wide end on a DX sensor. If you buy good lenses, they hold value. You're unlikely to lose much selling them on. There's really no way to go cheap, good, and ultrawide on an FX camera (well, unless you like mustache distortion and manual lenses: Samyang 14/2.8).

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    Re: APS-C camera & its crop factors on Lenses.

    Quote Originally Posted by GrumpyDiver View Post
    Chris - I'm not sure where you are coming from. There is no reason to have to "square anything up".

    You have to calculate the diagonal based on the length and height of the sensor, as this is the maximum dimension that the lens has to cover. diagonal = square root (length squared + height squared). Sorry; kind of hard to type the Pythagorean theorem here, but that is how this dimension is calculated. Anyways, that dimension is the minimum diameter of the image circle.

    If you square things up, and I assume you mean take the largest dimension of the sensor as the basis of your square. I'm not sure where the squaring off comes from. You actually get an image circle that is far larger than what is required if you do that.

    I'd draw it up later, but haven't got any software on the machine I am working on right now to do that.
    OK Manfred,
    Believe what you like. I studied optics for 3 years and I'm pretty sure I haven't forgotten everything I learned.
    When you've done your sums (and you don't need a computer to do it), I'd be interested to see your results and workings.

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    Moderator GrumpyDiver's Avatar
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    Re: APS-C camera & its crop factors on Lenses.

    Quote Originally Posted by krispix View Post
    OK Manfred,
    Believe what you like. I studied optics for 3 years and I'm pretty sure I haven't forgotten everything I learned.
    When you've done your sums (and you don't need a computer to do it), I'd be interested to see your results and workings.
    I've drawn up our two versions of what an image circle is. I've never seen the version you talk about, but then the last time I studied optics was in 1st year engineering in university. The image I've drawn on the left is the only definition I've ever seen. From a design standpoint, one would obviously be larger than this minimum to take care of any minor alignment issues, lens performance issues (in a real lens you want to ensure that the level of drop off, vignetting and sharpness is acceptable).

    I'd like the see the math around the the squaring off things. It almost sounds like a rule of thumb for a factor of safety in design work.

    Somehow I can't seem to embed the image; here is the link.

    http://www.cambridgeincolour.com/for...chmentid=22368
    Last edited by GrumpyDiver; 20th April 2012 at 02:02 AM.

  18. #18
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    Re: APS-C camera & its crop factors on Lenses.

    Quote Originally Posted by GrumpyDiver View Post
    I've drawn up our two versions of what an image circle is. I've never seen the version you talk about, but then the last time I studied optics was in 1st year engineering in university. The image I've drawn on the left is the only definition I've ever seen. From a design standpoint, one would obviously be larger than this minimum to take care of any minor alignment issues, lens performance issues (in a real lens you want to ensure that the level of drop off, vignetting and sharpness is acceptable).

    I'd like the see the math around the the squaring off things. It almost sounds like a rule of thumb for a factor of safety in design work.

    Somehow I can't seem to embed the image; here is the link.

    http://www.cambridgeincolour.com/for...chmentid=22368
    Hi Manfred,
    Can I begin by apologising. It was late and I did what I fear many might have done, confused your calculation as the establishment of focal length rather than image circle. In my defence, the discussion you joined was about focal length and not image circle.
    I can see where you're coming from and both your illustrations are valid, but the first is only from a coverage point of view and the second covers both aspects. Whilst your contention is perfectly valid from the point of coverage alone, it does ignore the matter of perspective. We should not forget that this is a photographic forum and that should be at the forefront of our thinking.
    Short focal lengths (Wide Angle) will display an enhanced perspective. i.e. objects will appear further apart than they really are and the shorter the focal length the greater the effect. Similarly, long focal lengths (Telephoto) have the opposite effect of foreshortening the perspective and, again, the greater the focal length the greater the effect. In the middle of this is the 'Null', 'Standard', Normal' focal length where the perspective is approximately as perceived by the human eye and this is calculated by 'squaring' the image plane and calculating the focal length as you've shown in your second example. The image circle, whilst important, for practical reasons is not calculated in this way because of, as you rightly say, drop-off, image sharpness and 100 other factors.
    I think we are probably in agreement and once again I apologise for taking your argument along the wrong track.

  19. #19
    Moderator GrumpyDiver's Avatar
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    Re: APS-C camera & its crop factors on Lenses.

    Quote Originally Posted by krispix View Post
    Hi Manfred,
    Can I begin by apologising. It was late and I did what I fear many might have done, confused your calculation as the establishment of focal length rather than image circle. In my defence, the discussion you joined was about focal length and not image circle.
    I can see where you're coming from and both your illustrations are valid, but the first is only from a coverage point of view and the second covers both aspects. Whilst your contention is perfectly valid from the point of coverage alone, it does ignore the matter of perspective. We should not forget that this is a photographic forum and that should be at the forefront of our thinking.
    Short focal lengths (Wide Angle) will display an enhanced perspective. i.e. objects will appear further apart than they really are and the shorter the focal length the greater the effect. Similarly, long focal lengths (Telephoto) have the opposite effect of foreshortening the perspective and, again, the greater the focal length the greater the effect. In the middle of this is the 'Null', 'Standard', Normal' focal length where the perspective is approximately as perceived by the human eye and this is calculated by 'squaring' the image plane and calculating the focal length as you've shown in your second example. The image circle, whilst important, for practical reasons is not calculated in this way because of, as you rightly say, drop-off, image sharpness and 100 other factors.
    I think we are probably in agreement and once again I apologise for taking your argument along the wrong track.
    Chris - As I said, I suspect that definitions more than anything else is where we were effectively spliitting hairs. I've been known to be wrong at times...

    Regardless, I was trying to make two points.

    1. A lens of a given focal length will always have the same "view" regardless of the format of the sensor or film. If you shoot with the 50mm lens, the lens will always "see things" the same way regardless of format. If you shoot with the same lens on FX and DX format and view the images at the same size, the DX will look like it has been zoomed in as compared to the FX. If asl an experiment you shot two portraits with a 50mm lens from a tripod, and the DX gave you a 3/4 shot, the FX would result in a full-length view.

    If on the other hand, you looked at the actual images side by side, you would notice that the DX shot and the FX shot image of the subject would be the same size, but the FX would have more subject matter.

    2. You could shoot both these shots with an FX lens and get good results, but if you shot both with a DX lens, the DX image would look fine, but the FX shot (unless you set the camera to DX mode) would result in a shot that had blurry and heavily vignetted edges because of the image circle issue we have been discussing.

  20. #20
    rpcrowe's Avatar
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    Re: APS-C camera & its crop factors on Lenses.

    A 50mm lens (or any other focal length you pick) is the same focal length on a crop camera or a full frame.

    Perhaps this illustration will help:

    APS-C Camera & its Crop Factors on Lenses.

    This illustration is designed to explain the various crops on different DSLR cameras. While Nikon has only two different size sensors (full frame and a 1.5x crop) Canon has produced three sizes, (full frame, 1.3x crop and 1.6x crop).

    The image circle on the left shows the circle projected by a lens designed for a full frame camera and the image circle on the right shows that which would be projected by a lens designed for a crop camera.

    The red field of view is that captured by a full frame sensor, the green shows the field captured by a 1.3x sensor and the blue shows the field of view on the 1.6x sensor.

    You can see that the mountains within the images captured by the blue fields of view are exactly the same size in both images regardless of the sensor size. That is because a focal length is exactly the same whether it is used on a full frame or a crop camera.

    However, you will notice that the full frame "red" sensor will cover a greater area that the "blue" crop sensor. This is what becomes confusing and which makes some photographers think that lenses are of different focal lengths when used on crop and full frame cameras. They are not but, the full frame sensor will cover a wider area than the crop sensor.

    If you have a lens designed for a crop camera, it will project an image circle smaller than the image circle projected by the lens designed for the full frame camera. The image projected by a lens designed for a crop camera will not cover a full frame sensor.

    I don't know about Nikon but, the EF-S Canon lenses designed exclusively for 1.6x cameras cannot physically fit on a full frame camera. They fit into a camera deeper into the camera and there is a shorter distance between the back of the lens and the sensor than with the EF lenses designed for full frame cameras.

    To make this even more confusing, the Tokina 12-24mm and 11-16mm lenses are different animals altogether. Although, they are designed to use with 1.6x crop cameras, they are physically like the EF lenses in how deep they seat into the camera and they can physically fit on full frame cameras. However, the image circle that they project will not cover the entire full frame sensor when used at wider focal lengths and will vignette the image.

    Canon Camera/Lens Compatibility

    1. Canon EF lenses will physically fit and will work on all Canon DSLR cameras: full frame, 1.3x crop and 1.6x crop

    2. Canon EF-S lenses will only physically fit on 1.6x crop canon cameras. Note to muddy the waters even further, they will not physically fit on the original Canon 1.6x crop cameras: D30, D60 and 10D. They will only fit on 20D and later cameras as well as on all Rebels.

    3. Tokina 12-24mm and 11-16mm lenses will physically fit on all Canon DSLR cameras but, the image size projected by those lenses will not cover a full frame sensor at wider focal lengths. As an example, I have an old Canon D60 camera which will not accept lenses with EF-S mounts. It will, however, fit my 12-24mm Tokina and since the D60 is a 1.6x crop camera, I can use my 12-24mm Tokina at all focal lengths.
    Last edited by rpcrowe; 20th April 2012 at 02:16 PM.

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