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Thread: Equivalent Focal Lengths

  1. #1

    Default Equivalent Focal Lengths

    Ive been trying to understand this whole cropped sensor and equivalent focal length thing but i fail to understand it...
    I have no idea how to calculate equivalent focal length and how it affects my camera... ok maybe a little but still its a maze to me.. yea. so it would be great if someone could explain to me what it all is... (:
    PS. I'm a nikon user so itd be great if you could use DX and FX to relate this to me (: thanks

  2. #2

    Default Re: Equivalent Focal Lengths

    Quote Originally Posted by Marcusluvsgreen View Post
    Ive been trying to understand this whole cropped sensor and equivalent focal length thing but i fail to understand it...
    I have no idea how to calculate equivalent focal length and how it affects my camera... ok maybe a little but still its a maze to me.. yea. so it would be great if someone could explain to me what it all is... (:
    PS. I'm a nikon user so itd be great if you could use DX and FX to relate this to me (: thanks
    Crop in here is exactly the same as when you crop a photo in photoshop or any other image manipulation software. You drag a same aspect ratio rectangle smaller than the original image and trim all the sides. That's what crop factor in camera bodies is about. DX sensors are 1.5 times smaller than FX sensors. FX sensors are "almost" equivalent of 35mm film size used in SLR. As I am informed, DX started as the digital sensor used in camera bodies, before moving to FX (back to the future - 35mm).

    Regardless how much you crop, focal length which is basically a lens parameter DOES NOT change. Focal length has nothing to do with the sensor size. Things didn't really get larger because you crop. It is larger because the per pixel size in the sensor got smaller. Meaning if you have 10MPs on the FX and 10MPs on the DX sensors, the DX sensor pixel is smaller. However when you view the images produced by these 2 different sized sensors, the presented pixel on screen is treated the same size. Hence it somewhat seems like you magnified a smaller view from the camera to the same image size on screen with higher resolution.

    Strictly speaking, you NEVER get equivalent focal length when you move in or zoom in between a transition from FX to DX sensor. At each focal length, you get different perspective, and at different focal lengths, the perspective distortion is different. So I personally don't consider how it is interchangeable in terms of saying 75mm at FX is the same as 50mm on DX. It's NOT TRUE. This statement is used very loosely to indicate the framing is similar in size. Meaning if you frame a photo frame at 75mm at FX, it will looks very similar to if you frame a photo frame at 50mm in DX.

    HOWEVER the images you get from 75mm at FX and 50mm at DX are NEVER THE SAME. Perspective distortion and other factors will made the result image DIFFERENT.
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  3. #3

    Default Re: Equivalent Focal Lengths

    Thanks David! That cleared all my doubts (: but im still unsure about 1 thing haha... FX lenses on a DX camera and DX lenses on a FX camera... i get that it will never be the same but how do you count "equivalent" focal lengths here in these 2 situations

  4. #4

    Default

    Do take note that even DX lenses gives you the 1.5 crop factor if used on the DX camera.

  5. #5
    Member konstrain's Avatar
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    Default Re: Equivalent Focal Lengths

    one way that i found out about the differences was to compare 2 photos shot on the same location, same scene, same lens but different body.
    however, it does takes a bit of effort to find these kind of photos online.

    to confirm what i have learnt online, i went ahead and got a friend who's carrying a full frame. we shot on the same location, mounted each camera bodies with the same lens on the same tripod and took 2 photos. it was then i realized what i shot on my own cropped body, differs from the one with full frame.

  6. #6

    Default Re: Equivalent Focal Lengths

    Quote Originally Posted by Marcusluvsgreen View Post
    Thanks David! That cleared all my doubts (: but im still unsure about 1 thing haha... FX lenses on a DX camera and DX lenses on a FX camera... i get that it will never be the same but how do you count "equivalent" focal lengths here in these 2 situations
    When you use a lens designed for DX in a FX camera, your camera may choose to automatically fall back to DX mode. Meaning it will only use a smaller rectangular portion of the whole sensor at the centre to record the lights. It means you get less resolution for the image outputted. If your camera doesn't fall back, or you choose not to, you will likelyget darken corners or completely black partial circular exposure because the lights projected by the lens will not fall completely on the sensor. The darken corners are known as fall offs.

    When you use a FX lens in a DX camera, you get to carry a lot of glass that are never useful on the DX sensor. You get to train up your biceps, as a benefit. The focal length retains as mentioned earlier. The frame you get for a 50mm FX lens on a DX body is near to 75mm, BUT like i mentioned earlier, it is NOT the same image as you would get on a FX body.

    I hopes this is clear enough.
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  7. #7
    Senior Member edutilos-'s Avatar
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    Default Re: Equivalent Focal Lengths

    Quote Originally Posted by David Kwok View Post
    HOWEVER the images you get from 75mm at FX and 50mm at DX are NEVER THE SAME. Perspective distortion and other factors will made the result image DIFFERENT.
    The perspective depends on the position of the vantage point. From my understanding, based on this, 75mm with FF and 50mm with CF (1.5x crop) will be the same.

    The perspective factor comes into play when someone tells you that you can zoom in and out with your legs to account for crop factor differences. But in your above example, I don't think it applies.
    Last edited by edutilos-; 28th February 2012 at 05:58 PM.

  8. #8
    Senior Member edutilos-'s Avatar
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    Default Re: Equivalent Focal Lengths

    Elaboration on my last post here for more in-depth clarification (I'm too lazy to type out a whole load of explanations when people have so kindly written up articles all over the Internet on this).

    Source: DPanswers: Crop factor explained

    Perspective

    Some people believe that “perspective” in photography is determined by the focal length of the lens used. They will argue that cer­tain focal lengths have a certain, inherent perspective or “look” that will be present irrespective of the size of the sensor. In other words, they will argue that a 135 mm lens will always give the “compressed” look one gets from using a telephoto lens, and a 28 mm lens will always give the “expanded” look of a wide-angle lens.

    This is wrong. There is nothing special about the focal length. Perspective in a photograph is deter­mined by one thing: the position of the photographer relative to the scene. (To be precise, perspective is determined by the distance from the front nodal point to the scene).

    The reason people believe that a specific focal length effect a certain look is because the narrow FOV of a telephoto lens forces the photographer to take up a position far away from the scene, and the large FOV of a wide-angle lens allows the photographer to move closer. It is these differences in position, and not the focal length as such, that effect the difference in perspective.

    It follows that as long as you stay in the same position, you get the same perspective no matter what focal length lens you put on the camera.

    Below is a demonstration of this. It shows two photographs of the same scene. The first is taken with a 135 mm lens, the second with a 28 mm lens. The reason they show the same perspective is because the 28 mm lens is used on a camera with a much smaller sensor. The second photograph can be considered to be a 7.18 mm by 5.32 mm crop from the middle of a 36 mm by 24 mm film frame.

  9. #9
    Senior Member edutilos-'s Avatar
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    Default Re: Equivalent Focal Lengths

    Quote Originally Posted by Marcusluvsgreen View Post
    Thanks David! That cleared all my doubts (: but im still unsure about 1 thing haha... FX lenses on a DX camera and DX lenses on a FX camera... i get that it will never be the same but how do you count "equivalent" focal lengths here in these 2 situations
    Your focal length doesn't change EVER, a focal length is actually tied to the LENS.

    From wikipedia: Focal length - Wikipedia, the free encyclopedia

    The focal length of an optical system is a measure of how strongly the system converges or diverges light.
    Maybe a quick history of photography. Hopefully it will help you understand the whole fiasco a bit better.For a period of time precluding the digital explosion, 35mm film was the most popular format. It was cheap, relatively light, and the cameras using it were widespread. The size of a 35mm film frame is what the size of a FX camera sensor is today. So everyone from the old days of 35mm film are used to linking focal lengths to a certain FOV (field of view), and that is precisely why FX cameras are termed "full frame". It all has to do with 35mm film.

    Now, a DX sensor is smaller than a FX sensor. David has already explained this. But everyone is used to 35mm film. So to keep things simple, the artificial concept of crop factor is introduced so that everyone can be happy and talk about things in the same way. David has already explained how the crop factor of 1.5x (or 1.6x for Canon APS-C) is derived, so I don't think there is a need to add on.

    DX or FX, your focal length is tied to the lens as I've already mentioned.

    So if you use a lens DESIGNED for use with a full frame sensor (termed a FX lens), say 24mm focal length, pairing this with a FF sensor yields 24 mm x 1 (crop factor) = 24mm perspective in 35mm film terms.

    If you use the same lens with a CF sensor, you will get a 24mm x 1.5 (crop factor) = 36mm perspective in 35mm film terms.

    Lenses designed for with a FF sensor can be used with no visible issues on FF or CF. If you understand David's post on how CF sensors actually take a "crop" because the resultant image screen (I suppose we have at one point time or another gone through the lesson of projection of images) is smaller in size, then one can say that the lens is designed to produce images to be projected onto a LARGE SCREEN (FF sensor). Naturally if you use the same lens on a SMALLER SCREEN, you just end up having part of the image spill out onto the wall (in this case your camera chamber) but there are no issues.

    Now, if you talk about DX lenses, designed for use with CF sensor... You can pair this with a FF sensor, but now, you have a lens designed to produce images to be projected onto a small screen. They couldn't care less if you used it with a larger screen, so you will get the vignetting and circles appearing in the image that David has mentioned. Projecting it onto a small screen is fine.
    Last edited by edutilos-; 28th February 2012 at 06:12 PM.

  10. #10

  11. #11

    Default Re: Equivalent Focal Lengths

    Quote Originally Posted by edutilos- View Post
    The perspective depends on the position of the vantage point. From my understanding, based on this, 75mm with FF and 50mm with CF (1.5x crop) will be the same.

    The perspective factor comes into play when someone tells you that you can zoom in and out with your legs to account for crop factor differences. But in your above example, I don't think it applies.
    I think you are right. I am wrong about this. Indeed perspective distortion shouldn't influence the image results. If not, while performing zooming from 50mm to 75mm, on the lens, the optical image would be changing within the cropped area, which isn't happening. I was mistaken about this portion. I come to realize it's the lens distortion in terms of pin-cushion/barrel that was affecting my judgement.
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