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Thread: Please be patient and help my confused brain...

  1. #1
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    Please be patient and help my confused brain...

    I appreciate this is a question that comes up regularly with new DSLR users, so my apologies for bringing it around again, but I'm struggling to get my head around it...

    I've been snapping away for the past year on a 500d with an EF-S 18-55mm lens.
    This week, I purchased a Sigma 24-70mm F2.8 lens (pre-HSM).
    The Sigma, I'm told, is not designed for APS-C sensors.

    With this in mind, I expected to look through the viewfinder (with the Sigma attatched) at 24mm and see the field of view I would normaly get from the 35/36mm area of my EF-S lens.

    However, all through the focal lengths (upto 50mm) they are like-for-like. In fact, 24mm on the sigma actually gives me a slightly wider FOV.

    Is my new lens in fact designed for APS-C or am I one of the many thousand newcomers confused by Crop Factors.

    Thanks in advance.

  2. #2
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    Re: Please be patient and help my confused brain...

    The "crop factor" comes from the fact that the sensor is smaller 22mm compared to th 35mm film. This mens that it will not use the full view of a lens that is design for full frame (35mm). So basicly if you took a picture on a full frame and the croped it down to 22mm you would get more or less the same FOV.
    Your lens should work fine on the APS-C but you will alwais need to applay the crop factor to get the full frame equlent.
    Last edited by Snorri; 03-05-2012 at 21:32.
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  3. #3
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    Re: Please be patient and help my confused brain...

    You are a little confused, I am afraid.

    The 18-55 was, indeed, designed for the APS-C sensor, and gives a view through the lens of approx 29-88 in full frame terms.

    The 24-70, although designed for full frame photography, gives a view through the lens of approx 38-112 in full frame terms on a camera with an APS-C sensor.

    You cannot compare the 24mm end of the 24-70 with the 35mm setting on the 18-55, because you have to apply the crop factor (1.6x) to both lenses, irrespective of what format they were designed for. So, if you are looking through the lens with the 24-70 attached, set to 24mm, you will get a field of view equivalent to 38mm. Likewise, if you are looking through the lens with the 18-55 set to 24mm, you will get the same field of view. I hope this is clear, but if not, just try it - set your 24-70 wide open at 24mm then compare it with your 18-55 set at 24mm - the view should be (approx) the same.
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  4. #4
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    Re: Please be patient and help my confused brain...

    Thank you! It's been confusing me all night and the internet is awash with conflicting answers. To be honest I'm glad, as I do like the view at 24mm, so it's nice to know I haven't lost it!

    Thank you both very much!

  5. #5
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    Re: Please be patient and help my confused brain...

    Quote Originally Posted by Ringo254 View Post
    Thank you! It's been confusing me all night and the internet is awash with conflicting answers. To be honest I'm glad, as I do like the view at 24mm, so it's nice to know I haven't lost it!
    You have lost it - 24mm that is. On your camera you will get a field of view equivalent to 38mm and not 24mm
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  6. #6
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    Re: Please be patient and help my confused brain...

    Quote Originally Posted by Fen View Post
    You have lost it - 24mm that is. On your camera you will get a field of view equivalent to 38mm and not 24mm
    I think this is slightly misleading - 24mm hasn't been 'lost' in the context of the original question...

    Quote Originally Posted by Ringo254 View Post
    I purchased a Sigma 24-70mm F2.8 lens (pre-HSM).
    The Sigma, I'm told, is not designed for APS-C sensors.

    With this in mind, I expected to look through the viewfinder (with the Sigma attatched) at 24mm and see the field of view I would normaly get from the 35/36mm area of my EF-S lens.
    They expected to get ~36mm equivalent field of view, and hence expected to 'lose' the field of view they had at ~24mm on the kit lens... but that's not the case - at 24mm the Sigma will give the same field of view as 24mm on the kit lens, so 24mm hasn't been lost.

    Unless I've got the wrong end of the stick?

  7. #7
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    Re: Please be patient and help my confused brain...

    Yes, a focal length is always the same, the angle of view depends on the format. One advantage, not mentioned, is that a full frame lens used on a crop sensor is using the central sweet spot of the lens and not using the edges which inevitably provide poorer resolution.

  8. #8
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    Re: Please be patient and help my confused brain...

    If you use (as an example) Ringo's 24mm lens on an APS-C and full frame camera do you get the same angle of view? I would have thought this would be so as the crop factor is only applied to the light hitting the sensor.

  9. #9
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    Re: Please be patient and help my confused brain...

    Quote Originally Posted by beatnik69 View Post
    If you use (as an example) Ringo's 24mm lens on an APS-C and full frame camera do you get the same angle of view? I would have thought this would be so as the crop factor is only applied to the light hitting the sensor.
    I'm not sure what you mean by 'the crop factor is only applied to the light hitting the sensor'?

    If you have a 24mm lens on full frame, you will get a wider angle of view compared to using that same 24mm lens on an APS-C camera. Focal length alone doesn't equate to a field of view - you have to take focal length and sensor size into account (or sensor size/film size/ whatever - it's not a 'digital' issue!)

  10. #10
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    Re: Please be patient and help my confused brain...

    Sorry: everyone seems to be confusing the OP. A 24mm lens is a 24mm lens whatever camera it is used with. A "Full Frame" (FF) camera has a physically larger sensor than an APS C camera and so, with the same lens, the latter gives a smaller field of view.

    Thus the 'crop factor' is a function of the camera not the lens. Use the 24mm lens on a Full Frame camera and you can crop the image to APS C size and you will get identical views. However, by cropping the FF image, you sacrifice pixels and, unless the camera has a higher pixel count, you could lose image quality.

    This all arises by our insistence on comparing all digital cameras to a full frame (36 x 24 mm) film camera. This is understandable as so many photographers have an instinctive 'feel' for these parameters on a film camera. However, not only is the sensor area different, but depth of field is also different for the saem lens aperture. Whilst this comparison might be acceptable for an APS C camera, the real differences become very significant when compact cameras with their tiny sensors are involved. This is even more significant when we remember that most compacts have only a very limited range of aperture sizes to control depth of field.

    The rule of thumb is that there will be no confusion if we think of the actual focal length of the lens and not its FF equivalent.

  11. #11
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    Talking Re: Please be patient and help my confused brain...

    Quote Originally Posted by Fen View Post
    You have lost it - 24mm that is. On your camera you will get a field of view equivalent to 38mm and not 24mm
    Sorry, what I mean to say is; When I look through my viewfinder with my EF-S lens set at 24mm I like the view. So, I'm happy that when I look through the Sigma at 24mm, I get the same view (but sharper ).

  12. #12
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    Re: Please be patient and help my confused brain...

    Quote Originally Posted by Ringo254 View Post
    Sorry, what I mean to say is; When I look through my viewfinder with my EF-S lens set at 24mm I like the view. So, I'm happy that when I look through the Sigma at 24mm, I get the same view (but sharper ).
    Yes, you will do! The Sigma is a higher quality lens and you are using the central area which performs better than the edges, also of course the maximum aperture is f2.8 so it will be brighter.

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