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Thread: Lens Focal Length and frame size - beginner a bit confused

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    Lens Focal Length and frame size - beginner a bit confused

    Hi everyone, this forum has been such a help already.

    I'm a beginner with maybe a silly question... but here goes. I understand that if you have a full frame camera with lets say a 70mm lens gives me the same field of view as a 70mm lens with a 35mm camera.

    If I put the same lens on a small frame camera, it gives a "magnified" field of view equivalent to say roughly a 83mm (depending on the sensor size) lens on a 35mm camera? Is this correct?

    And if I have an canon EF-S 70mm lens which is designed for a small frame camera on an actual small frame camera, am I still getting the equivalent 83mm field of view that I would with 35mm?

    Thanks for your help. I'm still trying to grasp some of these concepts.

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    Re: Lens Focal Length and frame size - beginner a bit confused

    Incorrect, although I can understand your confusion because some do explain it that way. With a full frame lens on either format camera nothing is magnified more than on the other. It's still the same lens so the lens projects the same image no matter what camera it's on. The difference is with how much of the image is registered on the sensor. It's not "magnified" it's restricted. Look up "Digital Camera Sensor Sizes" in the CIC tutorials. The smaller the sensor the less it can "see" of what the lens passes through. When you blow up the photo to the same size it appears larger but what you really have is just less of the scene and therefore a tighter field of view. Try not to think of it as magnified but rather, cropped. I will try to find and post a really good diagram I had that explains it quite simply.

    Lenses built for smaller size sensors would to some extent not provide a full circle of view on a larger sensor. Some amount of vignetting would occur.
    Last edited by Andrew1; 13th January 2012 at 01:29 AM.

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    Re: Lens Focal Length and frame size - beginner a bit confused

    Andrew is right.

    The light rays that would have landed on a FF sensor went through the lens in the same manner, but the smaller sensor didn't pick up the light rays around the edges. Oddly enough, it's analogous to why a hockey goal-tender wears such a big catching glove - it catches more pucks than a small one (albeit not all at the same time).

    Glenn

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    Re: Lens Focal Length and frame size - beginner a bit confused

    Thanks, I think I got it. So, given the same lens, a SF camera takes the same image that a FF sensor would capture and crops it to fit the smaller sensor. Therefore you get a smaller fov. When people say that a 70mm lens on SF camera is the equivalent of an 83mm lens full frame, what they are really saying is that the fov of the resulting image would be the same as if using an 83mm lens on full frame. In other words, using the SF sensor is really reducing your fov to a higher focal length but not really increasing your magnification.

    If I'm still confused, someone please smack me over the head.

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    Re: Lens Focal Length and frame size - beginner a bit confused

    Previous replies have clarified what's really going on with crop-sensor cameras... the "equivalent focal length" for a crop-sensor camera is just a way of stating that what focal length would give the same field of view on a full-frame camera at the same distance.

    Now to address your second question:

    Quote Originally Posted by woty87 View Post
    ...if I have an canon EF-S 70mm lens which is designed for a small frame camera on an actual small frame camera, am I still getting the equivalent 83mm field of view that I would with 35mm?
    To which the answer is yes... the EF-S 70mm lens is still a 70mm lens, and so produces the same field of view on the sensor as an EF 70mm lens. In other words, the EF-S 70mm lens would produce the same field of view as an 83mm lens would yield on a full-frame body in your hypothetical example.

    Hope this clarifies things a bit

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    Re: Lens Focal Length and frame size - beginner a bit confused

    Let me add that I too am confused, I have been looking at a sigma 150 to 500mm for my Nikon D7000.
    reviews seem to use it as an advantage that the lens gives the equivalent to a 225 to 750 odd on a FX body, but the way I see it now, it is actually a disadvantage that you are not getting any closer to the subject, in fact all you are doing is losing some of the picture that the lens is made to capture.
    So why do they use it as a selling point that the lens is good for a ASC sensor?
    I would definately do well to stay away from FX lenses unless I get a FX body
    Peter

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    Re: Lens Focal Length and frame size - beginner a bit confused

    Quote Originally Posted by Poider View Post
    So why do they use it as a selling point that the lens is good for a ASC sensor?
    I would definately do well to stay away from FX lenses unless I get a FX body
    Peter
    Actually, there are some advantages. The poorest image quality a lens produces is almost always near the edge of the image, so when a lens designed for a full-frame body is mounted on a crop-sensor camera, the region of poorest quality is cropped by the sensor. Use that same lens on a full-frame body, and the sensor will include the poorer quality region in the image. I had a 70-200mm Canon lens with a serious vignette problem (too large to correct in PP), but I never even knew the problem existed until I bought a full-frame 5DMkII... the crop-sensor 7D I had been using cropped the vignette out completely, so I couldn't tell the defect existed.
    Last edited by Snarkbyte; 13th January 2012 at 09:55 PM.

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    Re: Lens Focal Length and frame size - beginner a bit confused

    Quote Originally Posted by Poider View Post
    ...
    I would definately do well to stay away from FX lenses unless I get a FX body
    Peter
    Only if you don't plan on buying a full-frame body in the (near) future. Lenses that are designed specifically for a crop-sensor won't fill the full-size sensor (cf. the vignetting mentioned by Al, above). So, if ever you change to a FX body, you'd have to buy again the whole range of lenses, this time in FX version. One argument in favour of lenses designed for cropped sensors is price: afaik, they are cheaper than FX equivalents. Then again, the FX bodies are the top of the range, so one might expect that the lenses designed for them are also top of the range (optically and mechanically).

    And, in descriptions of SLR lenses, they always mention the real focal lengths in the designation (be it FX or DX lenses), so in that respect there is NO difference.

    Remco

    (one advantage of DX bodies is that, for a given FoV, the lens is a lot lighter, and cheaper, for equivalent image quality )

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    Re: Lens Focal Length and frame size - beginner a bit confused

    Hi Poider,

    A crop camera has a much denser sensor than a full frame camera. A 21MP Canon 5D has the equivalent of 8.2MP in the centre area of a Canon crop camera. The latest Canon crops have 18MP.

    When using a zoom lens such as the 150-500mm the subject will be very far away and likely in the centre of the image. In this case it works well on the crop camera which has far more pixels in the part of the projected image that you want to capture.

    A second point is that you still need a lot of lens and glass to make a 500mm focal length for a crop camera. The advantage of the smaller sensor needing less light is lost because the lens still needs to be very long. Erefore it is not really economically viable to make long lenses for crop cameras. They would cost nearly as much as those for full frame cameras. With the doubling of the number of lenses made by the camera companies this may put prices up even more for each one.

    Alex

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    Re: Lens Focal Length and frame size - beginner a bit confused

    Thanks for that Al. and Remco and Alex

    Peter
    Last edited by Poider; 13th January 2012 at 07:53 AM.

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    Re: Lens Focal Length and frame size - beginner a bit confused

    I feel the need to correct a few errors and clarify some confusions in this thread. I'll start from the top.

    Quote Originally Posted by woty87 View Post
    Hi everyone, this forum has been such a help already.

    I'm a beginner with maybe a silly question... but here goes. I understand that if you have a full frame camera with lets say a 70mm lens gives me the same field of view as a 70mm lens with a 35mm camera.

    If I put the same lens on a small frame camera, it gives a "magnified" field of view ~
    Hi woty87,

    It isn't a silly question, and to my mind 'magnified' is quite a good way to describe the effect.
    Yes, an FX digital is exactly the same field of view as an old 35mm film camera.

    As others have touched upon;
    A traditional interchangeable lens for a DSLR has a published focal length (or lengths if a zoom), that is 'fixed' and a matter of the physics and mechanics of its construction.
    It doesn't matter whether it is an FX lens, an APS-C lens, a DX lens or a 4/3 lens.
    However, I specifically exclude non interchangeable lens bridge cameras from this discussion, because they confuse the issue by often quoting other figures.

    Back to DSLR lenses.
    A lens may be built for use on any of those camera.
    If it is built for an FX body (or 35mm film camera) it will project a certain size of circle of light onto a piece of paper held where the sensor should be, this will be a bit more than the diagonal measurement of a 35mm frame (36mm x 24mm), which is 43.3mm.
    If you shine this onto a smaller APS-C sensor, you only get the central part of the image (minus many of the peripheral defects as has been said), so yes, the view apperas magnified and may be thought of as a longer focal length on an FX camera or FF film camera.
    If you buy an EF-S lens, the focal length 'magnification' is still the same, although the projected image circle now only just fits the APS-C sensor. In theory, you wouldn't normally use such a lens on a FF/FX camera, because it will leave the edges of the larger sensor dark and you'll only get a much smaller number of MP of image from it.

    Quote Originally Posted by woty87 View Post
    If I put the same lens on a small frame camera, it gives a "magnified" field of view equivalent to say roughly a 83mm (depending on the sensor size) lens on a 35mm camera? Is this correct?
    While the principle is correct, NO (sorry to 'shout' but several people have not corrected the erroneous maths).

    You do have to be very specific about the sensor size if giving numeric examples and no format I know of will turn a 70mm lens into an 83mm equivalent. Here's the maths;

    (real) focal length x 'crop factor' of sensor = a figure which may be thought of as an equivalent focal length.
    As I can see you do appreciate, it is the field of view seen by the sensor that actually changes.

    Since you have mentioned Canon and EF-S, their most common crop factor is 1.6 (for APS-C), so 70 x 1.6 = 112mm (not 83mm)

    Quote Originally Posted by woty87 View Post
    And if I have an canon EF-S 70mm lens, which is designed for a small frame camera on an actual small frame camera, am I still getting the equivalent 83mm field of view that I would with 35mm?
    Yes, you get the equivalent magnification effect on field of view, but no; it won't be 83mm.

    Later ...

    Quote Originally Posted by woty87
    Thanks, I think I got it. So, given the same lens, a SF camera takes the same image that a FF sensor would capture and crops it to fit the smaller sensor. Therefore you get a smaller fov. When people say that a 70mm lens on SF camera is the equivalent of an 83mm lens full frame, what they are really saying is that the fov of the resulting image would be the same as if using an 83mm lens on full frame. In other words, using the SF sensor is really reducing your fov to a higher focal length but not really increasing your magnification.
    In principle, most things you say here are valid, except the bad example figures.

    I would say the FF image 'overflows' the smaller sensor, but yes, you could think of it as cropping I guess.

    Later still ...

    Quote Originally Posted by Poider
    Let me add that I too am confused, I have been looking at a sigma 150 to 500mm for my Nikon D7000.
    Reviews seem to use it as an advantage that the lens gives the equivalent to a 225 to 750 odd on a FX body, but the way I see it now, it is actually a disadvantage that you are not getting any closer to the subject, in fact all you are doing is losing some of the picture that the lens is made to capture.
    So why do they use it as a selling point that the lens is good for a ASC sensor?
    I would definately do well to stay away from FX lenses unless I get a FX body
    Peter,

    On a Nikon DX body (like the D7000), the FX field of view equivalent is 225 - 750mm, because the crop factor is 1.5 for Nikon DX - the sensor is a tad bigger than Canon's APS-C. This is very definitely a good thing for you with that body, the smaller field of view IS getting you closer!

    However, if you splash out and get the new Nikon D4 you'll be much further away, as the lens will only give you the field of view of a 500mm lens (because it is on a FF body).

    As others have said, using the middle bit of the image circle on a DX body avoids problems like CA, vignetting and softness. If you own APS-C or DX snsor cameras, be sure to read lens reviews in the context of a similar camera body, it should show that FX lenses will return better results than cheaper DX lenses of same focal length.

    Quote Originally Posted by Poider
    I would definately do well to stay away from FX lenses unless I get a FX body
    I would say No to this, for the reasons above, plus; if any of us do ever go FF, at least any FF lenses we have will work, although we may be less satisfied with their performance on the new body. However, if you only have EF-S or DX lenses, you have nothing really usable at all, huge extra expense, probably prohibitively so


    I recommend people should not think in terms of "a smaller sensor needing less light" - it just confuses things for the purposes of this discussion, we're discussing angles of view, not f-stops and exposures.


    I am not very happy leaving incorect maths here for others to read and become confused by in future.
    Do people want to edit their posts to correct any bad figures? (replace "83mm" with "112mm")
    In researching this, I saw other factual errors on the interweb, so no wonder people get confused.

    Hope that helps,
    Last edited by Dave Humphries; 13th January 2012 at 09:20 AM.

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    Re: Lens Focal Length and frame size - beginner a bit confused

    Thanks Dave, Maybe the better way to demonstrate the difference would be to find somebody with a crop sensor Camera and a full frame sensor and show pictures of the same subjects taken from same place with ech camera using the same lens.
    Peter

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    Re: Lens Focal Length and frame size - beginner a bit confused

    Quote Originally Posted by Poider View Post
    Thanks Dave, Maybe the better way to demonstrate the difference would be to find somebody with a crop sensor Camera and a full frame sensor and show pictures of the same subjects taken from same place with ech camera using the same lens.
    Peter
    Maybe, (volunteers?) but I think we all know what it will show; a more 'zoomed in' picture with the crop camera body for a given focal length (e.g. using say, a 300mm prime lens).

    It's sorting it out in our heads that is the difficult bit.

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    Re: Lens Focal Length and frame size - beginner a bit confused

    Quote Originally Posted by Poider View Post
    Thanks Dave, Maybe the better way to demonstrate the difference would be to find somebody with a crop sensor Camera and a full frame sensor and show pictures of the same subjects taken from same place with ech camera using the same lens.
    Peter
    Maybe this will help...
    Lens Focal Length and frame size - beginner a bit confused

    The circle on the left is the image projected by an EF lens designed for, but not always used on a full frame camera. The red crop marks indicate the framing of a full frame camera, the green crop marks indicate the framing for a 1.3x crop camera and the blue crop marks are what a 1.6x camera captures. You will see that the mountain is the same size in the full frame, 1.3x and 1.6x captures. You just see more or less of it due to the sensor size.

    The circle on the right indicates what is projected by an EFS lens (designed for 1.6x crop cameras) of the same focal length as the lens on the left projects. You will see that only the image from the 1.6x sensor is fully contained within the circle projected. The result would be that the 1.3x and full frame images would vignette (black edges) if you could use an EFS lens on one of those cameras. Fortunately, you cannot physically fit an EFS lens on either a full frame or 1.3x camera.

    You will also see that the mountains are the same size in both the circles because a EF and an EFS lens of the same focal length will project the same size subjects. It is only the portion of the image captured that is different.

    To confuse the issue, there are some Tokina lenses designed for 1.6x cameras which can physically fit the full frame or 1.3x cameras and which will vignette if used on these cameras at the wider focal lengths. However, that is another subject.

    Hope this helps...
    Last edited by rpcrowe; 13th January 2012 at 02:47 PM.

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    Re: Lens Focal Length and frame size - beginner a bit confused

    Focal length is a simple concept. I think we're over explaining it!. woty87: Doesn't http://www.cambridgeincolour.com/tut...era-lenses.htm answer your inquires?.

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    Re: Lens Focal Length and frame size - beginner a bit confused

    I posted the original question and am so grateful for all the info that's been passed along so far. This forum is awesome. I hope I know enough one day to be one of the knowledge providers on this site. Thanks everyone!

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    Re: Lens Focal Length and frame size - beginner a bit confused

    Hi, I took the liberty of taking rpcrowe's images, 1.6x & 1.3x sensors, and resizing both to a 3:2 aspect ratio..eg 6x4 photo. You can see how the smaller señor image on the right crops and narrows the field of view and makes the photo more "magnified".

    forum.jpg

    To me, this begs the following question. If both of these cameras are say 16 megapixels, aren't they packed in more densely into the smaller sensor? So if I resize each photo to 6x4, don't I get the same resolution and aren't I, in fact, truly getting better magnification at 1.6x? I mean on the 6x4 piece of photo paper, I'm getting a larger image and the same resolution with the smaller sensor. Am I not? Somehow, I think the answer is NO, but I can't figure out why that would be.

    Thanks!
    Attached Images Attached Images

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    Re: Lens Focal Length and frame size - beginner a bit confused

    Quote Originally Posted by Poider View Post
    Thanks Dave, Maybe the better way to demonstrate the difference would be to find somebody with a crop sensor Camera and a full frame sensor and show pictures of the same subjects taken from same place with ech camera using the same lens.
    Peter
    Peter:

    In a sense, I demonstrate this to myself quite often - I have both the Canon 30D (crop) and a 5DII (FF) bodies, and I have only one macro lens, a 100 mm f/2.8.

    For four years I had only the 30D, and when I added the 5DII I thought I had passed into nirvana.

    Basics: The 30D sensor has 8.1 megapixels (MP), and the sensor dimensions are 5/8ths of the 5DII sensor which has 21 MP.

    I immediately noticed that with the 5DII I had to get quite a bit closer to frame the same flower closeup. The trouble was by getting much closer, I was losing depth of field (DOF).

    If I positioned the 5DII camera + lens at the same location that I would have for the 30D + lens, I'd have to crop quite a bit from the edges. The amount I'd have to reduce the 5DII image is by the ratio 5/8ths (note that 5/8 is the inverse of 1.6). But this is where things fall apart - one must apply this reduction factor to both the height and the width of the image.

    Doing the math: 5/8 x 5/8 x 21 MP of 5DII sensor = 8.2 MP. The number of pixels remaining from the cropped FF 5DII sensor image is only slightly more than the 8.1 MP of the 30D crop sensor image. I really haven't gained anything at all in terms of resolution by using the 5DII.

    I often see someone on the Canon forum say proudly, "I've upgraded to a FF body". From my point of view, this is not necessarily a true statement. It can be great for landscape photography. But for for closeup, macro, and wildlife work, the reverse is true.

    In order to utilize the resolution of my 5DII FF body, I must acquire another longer/expensive macro lens - and they are not cheap. Looking back, I should have bought the 7D not the 5DII.

    The crop sensor has major advantages for closeup flower photography and for wildlife photography (most wildlife photogs use crop bodies).

    So, all you people with the "inferior" crop bodies, be careful what you wish for.

    Glenn
    Last edited by Glenn NK; 13th January 2012 at 08:28 PM.

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    Re: Lens Focal Length and frame size - beginner a bit confused

    Quote Originally Posted by woty87 View Post
    Hi, I took the liberty of taking rpcrowe's images, 1.6x & 1.3x sensors, and resizing both to a 3:2 aspect ratio..eg 6x4 photo. You can see how the smaller señor image on the right crops and narrows the field of view and makes the photo more "magnified".

    forum.jpg

    To me, this begs the following question. If both of these cameras are say 16 megapixels, aren't they packed in more densely into the smaller sensor? So if I resize each photo to 6x4, don't I get the same resolution and aren't I, in fact, truly getting better magnification at 1.6x? I mean on the 6x4 piece of photo paper, I'm getting a larger image and the same resolution with the smaller sensor. Am I not? Somehow, I think the answer is NO, but I can't figure out why that would be.

    Thanks!
    Hi "Woty87" (what's your first name?)

    You're catching on

    Folks often refer to crop-factor as being a psudo focal length multiplier (which it is) - BUT - (and as you can see, it's a BIG but), folks then go on to assume that this equates to a (say 1.6x) advantage of a 1.6x crop-factor camera over a full frame camera (in terms of resolving power), and that just ISN'T the case because pixel counts also enter into it.

    Case in point - I was shooting at a sports event with a FF Canon 1Ds3; and an associate was shooting with a 1.3x Crop-factor Canon 1D3. My associate "claimed the advantage" because of the 1.3x focal length multiplier, but in reality, if I cropped a frame from my 21MP 1Ds3 to the same field of view at the 1D3, I end up with a shot that has 13MP of information to his 10MP -- so I out-resolve him by a significant 30%.

    In reality though, a lot of crop factor sensors still have a lot of pixels crammed into their smaller dimensions, which does give them an advantage in this situation (but puts them at a disadvantage in other situations eg wide-angle & noise characteristics etc).

    Hope this helps.

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    Re: Lens Focal Length and frame size - beginner a bit confused

    Quote Originally Posted by Colin Southern View Post
    Case in point - I was shooting at a sports event with a FF Canon 1Ds3; and an associate was shooting with a 1.3x Crop-factor Canon 1D3. My associate "claimed the advantage" because of the 1.3x focal length multiplier, but in reality, if I cropped a frame from my 21MP 1Ds3 to the same field of view at the 1D3, I end up with a shot that has 13MP of information to his 10MP -- so I out-resolve him by a significant 30%.
    From 13M points to 10M and same aspect ratio there is only a 14.0% increase in sampling frequency for each direction. Resolution (Properly speaking) is a different but related parameter diffraction, anti aliasing filtering, color filtering, failure to properly focus the intended object to be resolved, etc... Fortunately your experience and skill as a photographer is much more important than the spatial sampling frequency of your camera's sensor!.

    P.S: What Ken Rockwell has to say about the subject: http://www.kenrockwell.com/tech/resolution-myth.htm.
    P^2.S: http://bythom.com/ccds.htm
    Last edited by Photon Hacker; 14th January 2012 at 05:10 PM.

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