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Thread: EF-S lenses "true" focal length?

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    Member tmd77's Avatar
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    EF-S lenses "true" focal length?

    ok so i'm going to start out by apologizing to all the old heads here who've probably answered something like this before.

    But everytime i think i get my head around it, something else crops up or someone else says something, i start to doubt what i know.

    Now i understand that both EF and EF-S can both be used on crop cameras. whereas only EF can be used on full frame.

    my confusion comes in when you get something like a 18-55mm EF-S lens and you put it on a crop body, do you still need to multiply it by 1.6x so in fact the focal length is about 29-88. Or because it is an EF-S lens on a crop body, it remains a "true" 18-55?

    if someone can put me out of my misery and confirm this, it would be greatly appreciated!

    Trent

    Canon 60d | Canon 450d | Tamron 17-50 f2.8 | Canon EF-S 55-250 | Canon EF 50 1.8 II | Canon 430exII speedlite

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    The lens is always the focal length that is advertised. so an 18-55 is an 18-55 lens.

    the multiplication factor doesn't come into the equation until it gets put on a camera. For canon crop cameras this is usually 1.6 like you mentioned. Or 1.3 for the 1D for example (if the lens would fit). Or even 1.5 for Nikon (again if it would fit).

    Hope that clears it up for you.

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    Administrator ricktas's Avatar
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    The focal length of your lens doesn't change. A 50mm lens is a 50mm lens, no mattter what camera body it is mounted on. it doesn't magically suddenly become a 75mm lens just cause you put it on a camera with a 1.5 cropped sensor. What changes is your 'field of view'.

    Have a look at this, it explains it very well : http://www.ausphotography.net.au/for...ctor-w-example
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    Ausphotography Regular gcflora's Avatar
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    Rick, just because YOUR 50mm lens does not magically change to a 75mm lens doesn't mean that mine don't. I've often used this to "upgrade" my lenses. I have a full-frame camera, so what I do whenever I get a new lens is to first put it onto a cropped body which increases my lens's focal length. Using your example it transforms my 50mm lens into a 76mm lens (Canon crop) so now when I put it on the full-frame it's still 76mm because the lens has already been upgraded to the larger focal length.
    Craig

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    Ausphotography Regular gcflora's Avatar
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    I just had a cunning thought! Now that my 50mm lens has been upgraded to a 76mm lens I wonder if I can upgrade it further by putting it onto a cropped camera _again_. Then it might make my 76mm lens a 121.6mm lens. I wonder if I could do this indefinitely...

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    Don't go and confuse the poor guy even more.

    Trent, to put it in simple terms:
    an 18-55 on full frame = 18-55,
    an 18-55 on a crop sensor (x1.6) = 29 - 88.

    Doesn't matter if it is an EF or EF-S. Technically as Rick pointed out, it is actually your field of view that changes, not the focal length.
    Last edited by wmphoto; 22-10-2010 at 11:18am.

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    thanks for the quick replies guys.

    i guess what i was trying to wrap my head around was whether the manufacturers, in my case canon, had actually made an 18-55mm lens a 11-34mm lens but branded it 18-55mm because it can only be used on crop sensor cameras. i hope that makes sense.

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    Ausphotography Regular gcflora's Avatar
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    I apologise if I confused you more tmd.. ignore my above comments they are tongue-in-cheek

    To answer your last question, no Canon (e.g.) do not make a 11-34EF-S Lens and brand it as an 18-55... it's a true 18-55 that just won't fit onto a full-frame camera. The main reason for making an EF-S 18-55mm lens is that because of the position of the back element of the lens they can make an 18-55mm lens that fits the cropped camera with less glass (elements) (the back element of the lens can be closer to the sensor) and therefore it's lighter and more compact than a comparable EF version... does that make sense?

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    haha no confusion gcflora, all good i understand the basic elements which go into the difference between the EF and EF-S lenses, it was just moreso curious as to whether the manufacturer was playing games with us.

    appreciate all your responses everyone! all set now!

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    In Training MarkChap's Avatar
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    Quote Originally Posted by 2BAD4U View Post
    Don't go and confuse the poor guy even more.

    Trent, an 18-55 on full frame = 18-55,
    an 18-55 on a crop sensor (x1.6) = 29 - 88.

    Doesn't matter if it is an EF or EF-S.
    Wrong - you talk about not confusing the poor guy and you post that.

    An 18-55 Lens is an 18-55 lens, a 24-70 lens is a 24-70 lens, always was, always will be, end of story.
    The smaller sensor crops the outer portions of the image circle to give you a narrower angle of view when compared to a 35mm sized sensor, plain and simple. Focal length and there fore lens perspective does not change.

    And ALL cameras have a "Full Frame" sensor, some may be APS-c, some may be APS-h, some may even be equivelant to 35mm film, but each sensor is a full frame.(thanks Shelton)
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    In Training MarkChap's Avatar
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    OK, so I just re-read my last post

    I will climb down of my soap box now

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    Quote Originally Posted by MarkChap View Post
    Wrong - you talk about not confusing the poor guy and you post that.

    An 18-55 Lens is an 18-55 lens, a 24-70 lens is a 24-70 lens, always was, always will be, end of story.
    The smaller sensor crops the outer portions of the image circle to give you a narrower angle of view when compared to a 35mm sized sensor, plain and simple. Focal length and there fore lens perspective does not change.

    And ALL cameras have a "Full Frame" sensor, some may be APS-c, some may be APS-h, some may even be equivelant to 35mm film, but each sensor is a full frame.(thanks Shelton)
    You obviously replied before I had added the last little bit to my post about "field of view".

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    Ausphotography Regular gcflora's Avatar
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    Incidentally, and related to my post above, it's possible to mount an EF-S lens on a "full-frame" camera if you do some surgery to the lens mount. The problem that occurs though is because the back element of the ef-s lens is further back than on an EF lens the mirror can hit the back of the lens when it flips up when you mount the modified ef-s lens on the full-frame. You therefore risk doing some major damage. People have modified the ef-s 10-22 and stuck it on a 5d2 (for example) though for a super-ultra wide angle and/or for experimentation... it's not something I'd do though

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    In Training MarkChap's Avatar
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    Quote Originally Posted by 2BAD4U View Post
    You obviously replied before I had added the last little bit to my post about "field of view".
    Yeah the replies were coming thick and fast, there like 5 more between when I started typing and when I posted

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    So when I look through the view finder, Using my 24-105 EF on my 30D APS-C Senser, I see a Field of View of Around 39-160 , But It's still at 24-105mm It's enough to put you on the
    Last edited by William; 22-10-2010 at 11:48am.
    Canon : 30D, and sometimes the 5D mkIII , Sigma 10-20, 50mm 1.8, Canon 24-105 f4 L , On loan Sigma 120-400 DG and Canon 17 - 40 f4 L , Cokin Filters




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    In Training MarkChap's Avatar
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    Yep, you got it.

    And that is the reason that the theory of "Zoom Factor" has perpetuated the digital world. Because it is easy to explain

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    Just to complicate things further, focal length can actually change. The designated focal length is determined with focus is at infinity. When focusing on a close object the effective focal length can be less than what's stamped on the barrel. This is fairly pronounced on the Nikkor 18-200 for example, but I have yet to find it a problem in practice.


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    Some Nikon stuff. I shoot Mirrorless and Mirrorlessless.


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