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Thread: Focal length, Full Frame vs croped sensor

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    Francois's Avatar
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    Focal length, Full Frame vs croped sensor

    I'm a little bit confused about one difference between Full Frame (FX) sensors and cropped (DX) sensors...

    Does a FX camera with FX lens capture the same scene (at the same focal length) as a DX camera with a DX lens ? Before reading the article about sensor's size (http://www.cambridgeincolour.com/tut...ensor-size.htm), I thought it was the case.

    After reading this, which doesn't specifically discuss this point, I think I understand that an FX camera captures at 24 mm the same scene that a DX does at 16 mm (aprox.). Is it right, or FX 16 mm = DX 16 mm ?

    So a 16 mm FX lens on a DX camera would capture the same scene as a 36 mm (16*1.5*1.5) FX lens would on an FX camera ? (I hope I’m not too confusing)

    Thanks in advance for the enlightenment.

    P.S. I did some research on the forum, but didn't find this point discussed. Sorry if I missed it.

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    Moderator GrumpyDiver's Avatar
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    Re: Focal length, Full Frame vs croped sensor

    The difference between a FX and DX lens is something called the image circle, i.e. the amount of area that a lens will illuminate and is completely independent of the focal length of the lens. If you put a DX lens on a FX camera, you will only get a partial image, as it is designed to cover a crop frame sensor size and won't cover the full-size sensor. Going the other way; using an FX lens on a DX camera is fine because the lens is designed to project an image onto a full frame sensor, which is larger than the DX sensor.

    So you are correct. If you use a 16mm FX lens on a DX camera; the image you produce will be the same one that you get with a 24mm lens on a FX. Because of all of the confusion out there with different sensor sizes (FX and DX are Nikon designations), we often describe the effective focal length of a camera, based on its crop factor using the FX (35mm film) format as a base.

    DX has a crop factor of 1.5, Canon's crop frame DSLR cameras have a crop factor of 1.6; micro-four thirds (mFT) have a crop factor of 2; etc. There is also a small impact on depth of field (DoF). A DX camera shot at f/2 will have the same DoF or an FX camera shot at f/2.8.

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    Francois's Avatar
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    Re: Focal length, Full Frame vs croped sensor

    Thanks for the quick answer !

    My question is slightly different though, or I didn’t get your answer right. I’m aware of the magnification factor (x1,5 for Nikon APSC sensors) if you put a lens designed for an FX camera on a DX one. I’ll try my question otherwise, because I’m maybe mixing terms, so I’ll use an example.

    I have a D600 (FX) with a 70-200 f/2.8 and I take a shot @105mm.
    I have a D7000 (DX) with an 18-105 f/3.5-5.6 and I take the same shot @105mm

    Will my framing be wider with the D600, or will it be exactly the same in both cases ?

    I tried to simulate this on the Nikon’s lens simulator http://www.europe-nikon.com/en_GB/pr...nses/simulator
    My image is wider with the D600 on my scenario. But I’m not sure if the simulator is right, because if I then put the D600’s lens on the D7000 (same focal length), the result doesn’t change.

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    Re: Focal length, Full Frame vs croped sensor

    Good morning Francois,
    If the lens is suitable for FX(for the D600), for the same focal length setting (105mm in this case and NOT full frame equivalent - FFE), then the D600 would show more of the subject (wider frame) than for the D7000.
    The D7000 would show less of the subject - as if it were zoomed in (to FFE of 157.5mm).


    If the website automatically adjust to FFE, then the framing of the subject would be the same.

    I don't use Nikon and am not familiar with the site link and don't know how it works.
    Hopefully this helps.
    Graham

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    Moderator GrumpyDiver's Avatar
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    Re: Focal length, Full Frame vs croped sensor

    Quote Originally Posted by Francois View Post
    Thanks for the quick answer !

    My question is slightly different though, or I didn’t get your answer right. I’m aware of the magnification factor (x1,5 for Nikon APSC sensors) if you put a lens designed for an FX camera on a DX one. I’ll try my question otherwise, because I’m maybe mixing terms, so I’ll use an example.

    I have a D600 (FX) with a 70-200 f/2.8 and I take a shot @105mm.
    I have a D7000 (DX) with an 18-105 f/3.5-5.6 and I take the same shot @105mm

    Will my framing be wider with the D600, or will it be exactly the same in both cases ?

    I tried to simulate this on the Nikon’s lens simulator http://www.europe-nikon.com/en_GB/pr...nses/simulator
    My image is wider with the D600 on my scenario. But I’m not sure if the simulator is right, because if I then put the D600’s lens on the D7000 (same focal length), the result doesn’t change.
    Your shot with the 105mm setting D7000 will look the same if you took the same shot at 157.5mm on your D600.

    Your 105mm shot on your D600 will look the same using a focal length of 70mm on your D7000.


    A shot on a DX will always seem closer than when you shoot with a FX with the same lens (I shoot both a D90 and D800).

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    Re: Focal length, Full Frame vs croped sensor

    Thank you both Graham and Manfred for taking the time.

    @ Manfred : with the same lens, I get it. But here, it is about not using the same lens, but a DX lens on a DX body, (c.f. example). If what you describe is with each camera using the dedicated lens, then you say the same as Graham. If it's with the same lens, then it seems to me that you disagree. But I'm not sure which is what you said.

    @ Graham : I think I got your answer. An other example juste to check, "translated" in Canon
    With the 5D Mark III and the EF 24-70 f/2.8, I take a shot @ 24mm
    With the 60D and the EF-S 15-85 f/3.5-5.6, I take a shot @ 15mm
    I get the same image, right ? (Speaking of the framing only.)

    Other said, what's written (focal length) on the EF-S 15-85 is NOT Full-Frame Equivalent, right ?
    Last edited by Francois; 27th March 2013 at 04:03 PM.

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    Moderator GrumpyDiver's Avatar
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    Re: Focal length, Full Frame vs croped sensor

    Sorry Francois; Graham and I are saying exactly the same thing. The focal length of the lens marked on a lens does not change based on the type of camera format. A 50mm lens is still a 50mm lens, whether you use it on a DX, FX, medium format or view camera. The focal length is the focal length.

    A 50mm DX on a DX camera will not give you the same field of view (FoV) as a 50mm FX lens on a FX body. The only difference between a DX or FX lens is the image circle I mentioned in my original posting. A DX lens has been designed so that it only covers the area of a DX sized sensor, while a FX lens is designed to cover the area of a FX sensor.

    I'm not sure; but look at the CiC tutorial images; perhaps it will make things clearer.

    http://www.cambridgeincolour.com/tut...ensor-size.htm

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    Re: Focal length, Full Frame vs croped sensor

    This is not really a matter of magnification. It is a difference in field of view, which is a matter of trigonometry. A given lens will behave the same on any camera. However, the smaller the sensor, the smaller the arc that is captured on the sensor, and therefore, the smaller the arc that is captured in the scene. If s is the length of the sensor in any given dimension and f is the focal length, the field of view is:

    2 arctan(s/2f)

    In this case, you are making s smaller by the crop factor, which I think is 1.5 for Nikon. So regardless of the lens you put on the camera, the FOV will be 1/1.5 times the size on the DX as on the FX. Since that smaller FOV fills the entire sensor, it looks magnified by a factor of 1.5.

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    Re: Focal length, Full Frame vs croped sensor

    I usually explain it this way:

    A given lens always projects the same image no matter what the sensor size (the lens doesn't know how large or what shape the sensor is).

    It's simply that a smaller sensor can't capture the same area as does the large one because it's smaller (self evident?).

    The focused light rays that don't hit the sensor are absorbed by the black interior of the camera and are "wasted".

    In fact, this occurs no matter what the sensor size because all lenses project a circular shaped image, and sensors are usually rectangular. So part of the potential image from a lens is "wasted".

    Glenn

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    Re: Focal length, Full Frame vs croped sensor

    Ok, this time I think I get it. I'm a bit ashamed it took me so long, but I thank you all for taking the time for the explanation !

    And with the math [2 arctan(s/2f)], it makes the point easier for me to understand, and I have my answer : the Field Of View becomes wider with a bigger sensor (at same focal length)

    For more clarity, I should rename the thread "Field of view at same focal length, Full Frame vs croped sensor". Is it possible ? Or a forum moderator ?
    Last edited by Francois; 27th March 2013 at 08:31 PM.

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    Re: Focal length, Full Frame vs croped sensor

    Just to confuse you even more ...

    We talk about crop-factor cameras giving us a pseudo difference in focal lengths (compared to a full frame camera) with the theory in mind that it acts like a longer lens and thus gets us more detail than a wider lens cropped to the same field of view - but in reality it really just comes down to the pixel density of the photo receptors on the sensor (because in reality it's not multiplying anything - it's only "cropping" the image circle).

    Case in point ... take 2 cameras - the first being a Canon 1Ds3 - full frame - 21MP. The second a Canon 1D3 - 10.3MP - 1.3x "crop factor". If you take a shot with the 1D3, it's field of view will indeed make it look like it was taken with a longer focal length lens - but - if you were to take the same shot with the same lens on the full frame camera and then crop the image to that same field of view - then you'd have 2 shots that (on the surface) looked the same. So ...

    ... one might say that the 1D3 had the "long length" advantage because we had to throw away pixels from the 1Ds3 shot to make it look that same - but - the 1Ds3 actually had over twice as many pixels to start with and even when an image is cropped to look like the same field of view as a shot from the 1D3, it STILL has about 1/3 more pixels. So the camera that appeared to be at a disadvantage in a telephoto situation actually still has an advantage.

    So even though a camera like the 1D3 has a "crop factor", that doesn't NECESSARILY translate to any kind of advantage over any given full frame camera. Usually it does, but one ALWAYS has to take the respective megapixel counts into consideration.

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    Re: Focal length, Full Frame vs croped sensor

    I get your point. But with the same number of MP (for instance D4 and D7000 both 16 MP), you really get the cropped-sensor advantage (if you don't need high ISO and have a lens with a sharp center), right ?

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    Re: Focal length, Full Frame vs croped sensor

    Quote Originally Posted by Francois View Post
    I get your point. But with the same number of MP (for instance D4 and D7000 both 16 MP), you really get the cropped-sensor advantage (if you don't need high ISO and have a lens with a sharp center), right ?
    By that logic, a 16MP DMC-GH3 with it's m4/3 (17 x 15mm) sensor is better than either one and a 16MP DMC-FZ-60, with its diminutive 1/2.33" (6.08 x 4.56mm), is even better than a GH3!

    I'm just kidding . . .

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    Focal length, Full Frame vs croped sensor

    Quote Originally Posted by Francois View Post
    I get your point. But with the same number of MP (for instance D4 and D7000 both 16 MP), you really get the cropped-sensor advantage (if you don't need high ISO and have a lens with a sharp center), right ?
    I wouldn't say cropped-sensor ADVANTAGE - you will however get a free extra 50 odd percent on your focal length, but you'll also lose a similar amount at the wide-angle end.

    So whether its an advantage or disadvantage depends on what you're shooting.

    You definitely will get a smaller image in the viewfinder though. The other day I was playing with a 30D and I was actually quite shocked at how tiny the viewfinder image appeared.

    I suspect that not too many would want to go back to a crop after a FF camera.

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    Moderator GrumpyDiver's Avatar
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    Re: Focal length, Full Frame vs croped sensor

    Quote Originally Posted by Colin Southern View Post
    You definitely will get a smaller image in the viewfinder though. The other day I was playing with a 30D and I was actually quite shocked at how tiny the viewfinder image appeared.

    I suspect that not too many would want to go back to a crop after a FF camera.
    While it may sound strange to some; that was one of the main reasons I went FF. I never really adjusted to the tiny viewfinder on my crop-frame camera.

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    Re: Focal length, Full Frame vs croped sensor

    Of cours, the "advantage" is only for wildlife or similar photography, requiring powerful telephoto lenses.

    One of the reasons I'm asking all this is that I don't really need such long focal length, and need other FF capacities (ISO sensibility, shallow DOF). And my lenses would be much more versatile on a FF (my 35-70 f/2.8 becomes a 52-105 on my D7000, which I don't like much). But the investment is not so easy for me, so I'd like to be sure of where I go (plus I'll rent for a week-end, of course).

    The good news is, upgrading to a D600, I would get 50% more MP, so I wouldn't even lose the "advantage" of the D7000 for telephoto.

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    Re: Focal length, Full Frame vs croped sensor

    Quote Originally Posted by Francois View Post
    Of cours, the "advantage" is only for wildlife or similar photography, requiring powerful telephoto lenses.

    One of the reasons I'm asking all this is that I don't really need such long focal length, and need other FF capacities (ISO sensibility, shallow DOF). And my lenses would be much more versatile on a FF (my 35-70 f/2.8 becomes a 52-105 on my D7000, which I don't like much). But the investment is not so easy for me, so I'd like to be sure of where I go (plus I'll rent for a week-end, of course).

    The good news is, upgrading to a D600, I would get 50% more MP, so I wouldn't even lose the "advantage" of the D7000 for telephoto.
    It's always a compromise. Provided the MP counts are similar then the crop really will give you an effective focal length multplier, but it has several downsides as well. Keep in mind though, that if I post a photo here - even at a very generous 1200 x 800px resolution, I'm discarding around about 95% of the captured information - so you may not need the "advantage" of a crop-factor camera as much as first thought.

    Keep in mind too that you can add a tele-extender to many lenses that effectively null out the "crop factor" for a FF camera.

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