Focal length and full sensor vrs.crop sensor

Discussion in 'Beyond the Basics' started by uplander, Mar 12, 2009.

  1. uplander

    uplander TPF Noob!

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    I've been thinking about this for some time. This statement is out there all the time, "A 400mm lens on a 1.6 crop factor sensor is equivalent to a 640mm on a full size sensor".

    I believe this is a true statement when you are comparing field of view which is expressed in angle. So what you have is a lens that projects the same size image but because the sensor sizes are different the smaller sensor captures a smaller portion of the image which results in a narrower FOV. The image isn't any different it's just that the smaller sensor captures less of it, hence the narrower FOV.

    I believe stating that a 400 mm lens on a 1.6 crop sensor becomes a 640mm lens is misleading. One tends to think it makes the lens more powerfull in magnification which isn't true. I would think the magnification is the same unless there is some difference in the distance from rear element to the sensor.

    So if that distance is the same ( I would tend to think it is ...I may be wrong though) Then the only difference is the amount of the image projected by the lens each respective sensor catches. So then the area of the image each sensor captures are exactly the same in image size and quality except the crop sensor records less of it ( IE the bordering edges)

    Hence I think that a 400mm lens is a 400mm lens despite what body it is on. Or in other words saying it is the equivalent of a 640mm on a 1.6 crop factor sensor is a bunch of hooey! A 400mm lens on a crop body does not get you any closer to your subject than a full size body

    Unless.....you start looking at the amount of pixels in each sensor or lets just say does having more pixels/cm2 will give you better resolution when you crop and in effect give you more magnification which is a whole another can of worms. So maybe we should just stay and debate the first part of this post.

    I got a feeling this should be interesting if you really put some thought into it.

    Discuss!!!:mrgreen:
     
    Last edited: Mar 12, 2009
  2. Jaszek

    Jaszek No longer a newbie, moving up!

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    The lens is the same (no duh!) it's just the field of view taht's different and aperture. Also if you look at P&S cameras the focal lenghts are like 6mm and the aperture is 2.8 since the sensor is so small. You also don't get such a shallow DOF on a P&S than on a FF body (I'm just using that example since there is a huge difference due to sensor size)
     
  3. AlexColeman

    AlexColeman TPF Noob!

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    I don't really see your point. Obviously it is the same lens, however those equivalencies are just to give one standard measurement. Thats how everyone knows 28 equiv. is a little wide, while 400 equiv. is tele.
     
  4. anubis404

    anubis404 TPF Noob!

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    I'm not quite sure what you are saying. A 400mm is in fact a 600mm equivalent on a 1.5x body. No one ever said the focal length became 600mm. Crop factor is almost like cropping the image and blowing it up without losing any quality. While not being the same as focal length, it looks very similar. If you don't believe me I'll show you pictures taken with a 50mm DX lens and a 50mm FX lens.
     
  5. uplander

    uplander TPF Noob!

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    Alex ..your missing it completely.. A 50mm on crop body gives the same DOF as on a full frame body ..It's just the FOV is different.


    A 28mm on a full frame body is a hell of a lot wider than on a crop body but the the DOF is the same. get it?

    The lens sees the same and in the same aspect regardless of the body. The crop body just cuts the edges off.
     
  6. uplander

    uplander TPF Noob!

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    But the image recorded is identical except the edges are cut off.

    And yes there is a difference, An image taken with a 400 mm lens has a different DOF then one taken on a 600mm.

    Put a subject in the foreground with either lens keeping the foreground subjects the same size and the background is dramatically different between the 400 and the 600.
     
  7. Dao

    Dao No longer a newbie, moving up!

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    This is what I believe.

    A 100mm focal length lens on a 1.6x crop, is only a 100mm focal length lens with a "Field of View" equivalent to a 160mm focal length lens on a 35mm film SLR camera or a full frame sensor DSLR camera. That is. nothing more.

    It should not be the same as you have a 160mm lens.

    Let take an exmaple.


    B----S--------------------------------C

    Where B is the background. S is the subject and C is the camera.
    "----" line is the distance.


    Take a photo (PHOTO1) with a 100mm lens in a crop body.

    Now, change the camera with a full frame sensor body. Take another photo. But this time, I want to take the photo (PHOTO2) so that it has the same Field of View of the 1st one (PHOTO1).
    In order to do that, I need to walk closer to the subject. So it become

    B----S-------------------------C


    As we all know, now the distance between "S" and "C" is shorter.


    Now when you compare PHOTO1 and PHOTO2. You will find the FOV on both photos are the same. The object size is the same. In other words, the crop body seems to have a better "zoom" power.

    Does that mean 100mm = 160mm in the crop body? I will say no since the ratio of the distance between "B-S" and "S-C" is different, therefore the perspective distortion characteristic is going to be different. Is it noticeable? I do not know since I only have crop body cameras, so I cannot do any test. But I think if distance of "B-S" is roughly equal to "S-C" in PHOTO1, then it may be noticeable.


    This is what I believe and please correct me if I am wrong.
     
  8. Dao

    Dao No longer a newbie, moving up!

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    Also, from the above example, the Angle of view is also different.
     
  9. anubis404

    anubis404 TPF Noob!

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    Exactly, which gives the effect of it having a longer focal length.

    I'm not talking about DOF, I'm talking about equivalence.
     
  10. Village Idiot

    Village Idiot No longer a newbie, moving up!

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    This is wrong.

    DOF calculator:
    Online Depth of Field Calculator

    try it out. Does the crop factor and everything for you.
     
  11. Steph

    Steph No longer a newbie, moving up!

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    Well, this comment about DOF is not true. It is not that simple and when you put more thought into it, it gets horribly complicated. The DOF is the zone of a picture which is acceptably sharp. That will depend on the size of the final print, the viewing distance and the circle of confusion diameter limit, which depends on the size of the sensor/film you use. So your 28mm lens won't produce the same DOF on a full frame body and on an APS-c sized sensor.
     
  12. Big Mike

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

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    Equivalence of what?

    The only think that really is equivalent (to something different) in this scenario is the FOV (as mentioned in the original post).

    The DOF, the perspective, the magnification...these things do not change because of the 'crop factor' of the camera.
     

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