Help with 35mm Equivalent

Discussion in 'Photography Beginners' Forum' started by RPDean, Feb 23, 2010.

  1. RPDean

    RPDean TPF Noob!

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    Ok so i feel like a bit numpty but here goes...

    I’m trying to learn as much as possible about photography and it would be great if some experts here would be able to help me out/give advice
    Right now I’m (trying) to teach myself about 35mm equivalent – what it means, how you calculate it, why do you need it etc. I’m kind of struggling but getting there.

    Basically I’d like someone to tell me if what I’m understanding about this is on the right track or completely wrong. So here it is..


    When a focal length of a lens is described as being 35mm equivalent, it is telling you what that focal length would be on a 35mm film as opposed to a smaller image sensor. Basically its telling you what angle of view the focal length would give if it were on a full frame 35mm film. The focal length does not change but the field of view does due to the difference in image sensor size.


    Is this totally wrong or on the right track? :confused: its probably so simple..any help would be greatly appreciated.
    also why is 35mm equivalent used? is it just to tell a 35mm film user what angle of view he would get on his camera from a particular lens??
     
  2. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    Bingo.

    Because 35mm film cameras were the most popular cameras for the last three decades, give or take.
     
  3. c.cloudwalker

    c.cloudwalker TPF Noob!

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    That's the short answer. The longer one would be that on full frame bodies the focal length of a lens is the same as on a 35 mm film camera. But it is different with crop bodies.

    For example a Canon crop body has a ratio of 1.6 so a 50mm lens on one of those is actually equivalent to an 80mm lens (film equivalent.) Some people who use/need long lenses actually prefer crop bodies for that reason. Buy a 300mm and you actually have a 480mm. A worthwhile difference.
     
  4. SushiWarrior

    SushiWarrior TPF Noob!

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    An important thing to remember is that a 50mm lens is not "a 75mm" on a 1.5 crop or anything - the picture will look the same as another 50mm, just smaller. The main factor is how it "flattens" objects and perspective - with wide angles it still has a very distorted look as to how things look close/far away.
     
  5. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    No, the focal length isn't different, it's the field of view that changes.
     
  6. kkamin

    kkamin TPF Noob!

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    I don't think this is right, or maybe I'm not understanding correctly. But, a 50mm lens on a 1.6x crop sensor will have a field of view of an 80mm lens used on a full frame camera. Just imagine taking a photo with a FF camera with a 50mm lens. If you want to know what the same photo would look like taken with a 1.6x crop sensor, draw a rectangle 40% the size of the photo in the center--that would be your image.

    If you use a 32mm on a FF, it'll look wide. If you use that on a crop sensor, it'll look "normal".
     
  7. RPDean

    RPDean TPF Noob!

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    Thanks for all the replys..just wanted to know that i was understanding it correctly.
     
  8. fokker

    fokker No longer a newbie, moving up!

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    These explanations are both right, and good as well.
     
  9. Dwig

    Dwig TPF Noob!

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    Actually, the statement in the original quote about perspective is rather poorly worded, but its seems to be making an incorrect point. Perspective is not affected by a lens' focal length. Perspective is solely controlled by the relative distances between the camera and the various objects in the picture (foreground, background, ...). The only influence focal length has comes from the resulting field on view on some given format and that FOV's influence on the photographer's choice of shooting position.

    Modestly long "portrait" lenses are considered such not because the give a particular perspective. They are considered "portrait" lenses because when you shoot from a distance that produces a pleasing perspective of someone's face (size of nose compared to size of the more distant eyes and the even more distant ears ...) the lens produces the cropping that is considered appropriate for a portrait. Hence, when you use such a lens its FOV leads you to shoot from an appropriate distance. Still, its the shooting distance and not the focal length that creates the perspective. A 50mm lens on a DX format camera (1.5x crop factor) and a 75mm lens on an FX format camera (1.0x crop factor or "full frame") yield the same pespective when shooting the same subject from the same distance and will yield the same framing at that distance.
     
  10. Derrel

    Derrel Mr. Rain Cloud

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    ^^^:thumbup::thumbup::thumbup:
     
  11. SushiWarrior

    SushiWarrior TPF Noob!

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  12. Dwig

    Dwig TPF Noob!

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    And it is either completely off topic in this thread or, if you think it is on topic you misunderstand the optics and physics involved.

    The focal length references used in the Wikipedia article apply to only one format. If you use a different format, the perspectives in each image would be associated with different focal lengths since it would require a different FL lens to achieve the same field of view at the same distance. Again, it is NOT the lens focal length that produces the perspective. The perspective difference in the 3 images stolen from Wikipedia is the result of the change in camera position and not the change in lens. The lens change was only done so that the size of the foreground object remained roughly the same in each picture.
     

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