A Confusion with Lenses and Focal Length

Discussion in 'Photography Beginners' Forum' started by andytakeone, Nov 17, 2015.

  1. andytakeone

    andytakeone TPF Noob!

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    So if a lens's focal length is defined by its distance to the camera's sensor, does that mean all lenses are identical and that the only difference is the length of the cylinder it is built into?

    Of course I know this can't be true and that there are more complications than that. But what are those complications?

    My guess would be that the curvature of each lens has to be adjusted depending on the lens's focal length. But if that's true, than that would mean that a 50mm lens adjusted so that it is 28mm away from the sensor does not make it a 28mm lens--which then diminishes the definition of focal length.

    Can anyone help me with this confusion?
    If you can extend this discussion to prime lenses and zoom lenses that would be the cherry on top.


     
  2. Braineack

    Braineack Been spending a lot of time on here!

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    Wait till you read about telephoto

    using tapatalk.
     
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  3. 480sparky

    480sparky Chief Free Electron Relocator Supporting Member

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    One complication is that if you were to actually measure the distance between your camera flange and the sensor, you'd discover that there appears to be a minimum focal length available. This is due to the mirror movement within the camera, as well as the shutter between the mirror and sensor.

    So optical designers employ a technique called 'retrofocus' to be able to put a 6mm lens that physically cannot be placed closer than 40mm from the sensor.
     
  4. snowbear

    snowbear fuzzy-wuzzy Supporting Member

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    The distance from the front of the lens to the sensor is for a simple lens. I'd wager a paycheck you don't have any simple lenses but compound lenses: those with multiple glass elements so the physical length is really much shorter than in the simple lens of the same focal length.

    Enjoy.
     
  5. Jim Walczak

    Jim Walczak No longer a newbie, moving up!

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    This is just my own opinion, however I think snowbear hit the nail on the head there...when we're talking about focal length in terms of the back of the lens to the camera, we're not really taking about "focal length" in conventional terms. As snowbear said, most camera lenses (even kit lenses) use multiple elements in their design, so we're not talking about focal length in the same terms as we might with say, a telescope (and these days, even scopes usually use multiple elements...even my cheapy Simmons uses 2 elements in the objective). The only real consideration or exception here would be "macro photography", where you might use extension tubes between the lens and the camera sensor in order to get extreme close ups of very small subjects...say the eyes of an insect. That however is a different and rather specialized form of photography and not specifically relevant at this point.

    The way to think about focal length in terms of photography is simply the distance of the camera/lens in regard to the subject. A 55mm lens for example, will be roughly equivalent to the field of view that most people see with the naked eye (always a nice place to start). In other words, using a 55mm lens, if you look thru the view finder at a given subject then pull the camera away from your eye, what you "see"...your subject...should appear as roughly the same distance away as you saw thru the view finder (give or take your own vision of course...some people are near sighted, some are far sighted, which changes that theory a bit). Shorter focal lengths, 28mm, 24mm, 18mm, 12mm, etc., will provide a progressively wider field of view (respectively) and your subject will appear further away from the camera...hence the term "wide angle". Conversely, a longer focal length, 100mm, 200mm, 300mm, etc., will magnify that field of view, making your subject appear closer than it actually is.

    The significance is simply in being able to properly frame your subject with the camera. For example, let's say your interest is "wildlife" and you've spotted a lovely white tail deer across a field...we'll say 100 yards away (for the sake of easy, round numbers). That's the length of a football field, so say you're standing at one end of the field and the deer is at the other end....that deer is pretty far away from you. If you were to use a wide angle lens, let's say 18mm, then your "subject" - the deer, is likely to appear as little more than a tiny little spot in an image that could otherwise be mistaken as a landscape photo. By using a longer focal length...say 500mm, you can magnify the deer in your view finder so that you're taking a picture of the deer. Conversely, if you were to use that same 500mm lens to try and do a portrait of someone standing only 20 feet away from you, you'd likely end up with an extreme close up of the person's nose...you'd likely want to use something closer to a 50 to 100mm range for that (unless of course you happen to like noses, LOL).

    On this issue alone, my advice would be to not worry about the "tech talk" as it pertains to the distance of the lens to the sensor (that is unless your intent is to start designing/building lenses) and just concern yourself with the relevance of focal length in terms of the distance to your subject.

    Now regarding prime vs. zoom lenses, much of this is, for all intensive purposes, the same. Prime lenses also use multiple elements, so in terms of distance regarding the lens to the camera, it's pretty much irrelevant. Likewise, a 400mm prime will magnify your subject where a 12mm prime will make your subject appear further away. Focal length however is different in that a zoom gives you a range of focal lengths to work with. While typically not the greatest lens you can buy, let's consider an 18-200mm "super-zoom" as an example. The advantage of such a lens is that it gives you a very wide focal range to work with...at 18mm, you can stand on a beach and take lovely sunset pictures and with a twist of the lens, you can "zoom in" to 200mm and get a shot of the blue heron that may be down the beach from you...and you don't have to move your position to do this. That's why a lot of folks (myself included) consider these to be a good "walking around lens"...despite their flaws, they are very convenient to use. With primes on the other hand, you'd mount an 18mm lens, take the sunset shots, then take that lens off and mount a 200mm lens to get the shot of the heron...otherwise, you'd simply have to move MUCH closer to the heron to use that 18mm. In other words, in order to effectively cover the whole range of that 18-200mm zoom lens, you'd likely have to carry 4 or 5 additional lenses (or more) with you if all you have are prime.

    The big consideration here is actually about image quality. Some people will argue that a prime lens will always produce a better image than a zoom lens will...and at one time, that was VERY true. With my 40 year old Canon 35mm for example, if you were to compare images shot with a 50mm prime and my old Sigma 28-70mm zoom, there would be no question AT ALL...the prime would produce a better image (regardless of brand). In the old days, zooms were prone to problems with image distortion, chromatic aberration and even a general lack of sharpness. They were good for -some- people, but for many, the advantage of a zoom usually wasn't worth the image degradation (although a great deal of that would depend on the person using the camera and their intent...4 x 6 snapshots with a zoom lens for example were usually fine). That said, the times they have a changed! Thru the use of computer design, modern manufacturing techniques and tremendous quality control efforts, most lens makers today (including 3rd party vendors such as Tamron and Sigma) can produce zoom lenses that are in fact quite comparable to primes. Even the so-called "kit lenses" today can provide a really great value considering their price...that cheapy Nikon 18-55mm I use so much is a FAR superior lens to my old Sigma 28-70 as far as image quality goes. Some people are REALLY stubborn about this, however the truth is that the better modern zooms really do a very good job, even when compared with primes.

    Okies, not sure if that's what you were really looking for, but I hope it helps and gives you a bit of insight.
     
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  6. astroNikon

    astroNikon 'ya all Bananas I tell 'ya Supporting Member

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    Lenses are engineered to provide an in-focus image to the sensor.
    Each camera's sensor is a certain distance from the flange (where you attach the lens). ==> Flange focal distance - Wikipedia, the free encyclopedia

    That provides the engineers a fixed model to develop their lens.
    So multiple manufacturers can make lenses to do anything, at any size, width and tube length BUT it must provide a focused image at the sensor plan from the flange.

    The competing factors are end user price, image quality & manufacturing costs.

    After that, since a lens is made up of multiple elements, everything else can be different: The number of elements, the size of elements, the type of elements, element materials, the total tube height of the lens. New element design techniques are allowing lenses to be smaller and more compact ==> Another Nikon patent for a 300mm f/4 lens with diffractive optical element (DOE) | Nikon Rumors

    As long as the engineered lens provides the in-focus image at the sensor it doesn't matter how long or short a particular lens is (look at pancake lenses as an example ==> Nikon 50mm f/1.8 Series E).

    Here are Nikon's two 50mm lenses.
    a Nikon 50mm/1.8 AF-D lens is approx 1.5 inches tall ==> Nikon 50mm f/1.8 D Review

    whereas a Nikon 50mm/1.8 AF-S G lens is approx 2.1 inches tall ==> Nikon AF-S 50mm f/1.8 G

    Compare those lens elements to a 18mm type lens, which uses much more curved elements to capture a larger width, or field of view. (scroll down a little to see the element design) ==> Nikon 18-35mm f/3.5-4.5 G Review

    Of look at the size difference of the two 24mm lenses on the left of this picture ==> http://www.btobey.com/nikon/images/24mm-size-nikon.jpg

    The lens objective is to provide an in-focus image to the image sensor.
    as the above discussion of the Diffractive Optical lenses, the new 300/4 is shorter and lighter than the old 300/4 while providing a high quality image.

    Thus, in reality the length of a lens and the "tube" really hasn't nothing to do with it's field of view/focal length as the engineering design is for a particular focal length (prime or zoom). The lens elements dictate it's field of view (focused image focal length). The wider angle lenses have much more curved elements in general and the longer ones have much flatter outer elements.

    You "could" make a long 50mm (as long as a 300mm) but it may suffer from image quality and providing a dark image to the sensor - and who would buy such a thing compared to the small ones?

    It's all in the design and whether the engineers use a certain mathematical model which will produce lenses of a certain length, elements/image quality and cost.

    Now if you have a camera that does not have a lens then you don't have to worry so much about the distance from the "hole" to the film, such as pinhole cameras. you just have to worry about the exposure. ==> How does a pinhole camera work?

    And you can use pinhole lenses on dslrs ==> http://www.amazon.com/Holga-299120-Pinhole-Nikon-Black/dp/B007TPXL46
    ==> How to use Holga Pinhole Lens on Nikon D7000 DSLR | Photography Forum


    here's a good read => Camera lens - Wikipedia, the free encyclopedia

    ==> How Camera Lenses are Made - The Beat: A Blog by PremiumBeat
     
    Last edited: Nov 19, 2015
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  7. 480sparky

    480sparky Chief Free Electron Relocator Supporting Member

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    FIFY.

    FOV is not dictated by focal length alone.
     
  8. Jim Walczak

    Jim Walczak No longer a newbie, moving up!

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    You're correct...obviously there's more to it than that. Since the OP seems to be a novice regarding this (indicated by "confusion with lenses and focal length"), I was merely trying to simply things for the benefit of the OP, rather than going on about some otherwise fairly irrelevant techno-babble at this point. Using my deer explanation at 100 yards for example - if you're looking thru an 18mm lens, you ARE going to see A LOT more in your FOV (field of view) than you would with a 500mm...that was the point I was trying to make regarding focal length. This is just my own opinion, but I believe this is far more important for a novice to understand rather than the specific nuances your comment would indicate, let alone specifics like the lens distance to the sensor.

    I would also point out that I did NOT include the comment about "35mm or full frame" deliberately as I didn't wish to confuse the subject even further by introducing the concept of crop factors. In fact, regardless of FF or APS-C, the focal length is the same. The focal length itself does not change based on the crop factor of the camera sensor...a smaller sensor does NOT magnify the image, it just crops off more of the edge. 50mm is still 50mm either way.

    Not trying to be rude - just clarifying my comments.
     
  9. Dave442

    Dave442 Been spending a lot of time on here!

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    One interesting note is that a Telephoto lens is by definition a lens where the physical length of the lens is shorter than the focal length of the lens.
    A prime lens has one focal length when measured at infinity (the focal length can change on some lenses when going to the closer focusing distances - focus breathing). A zoom lens is a variable focal length lens. The zoom lens may include the telephoto optics and if it also covers the wide angle range it may also have a retrofocus set of optics.

    I would suggest some books that cover this in much better detail and more correctly than I can put down here.
    1) "Handbook of Optics" Volume 1 and 2 - all the basics and formulas.
    2) "Applied photographic optics" - geared towards photographic lenses.
    3) "Optical Systems Engineering"
     
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  10. petrochemist

    petrochemist TPF junkie!

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    The lenses focal length is NOT defined by its distance from the sensor.
    It is an intrinsic property of the lens and is not affected by the cameras position.
    (In fact it's actually a series of properties as focal length is dependent on the wavelength of light, but the difference across the visible spectrum is fairly marginal)
    Focal length is actually defined by where parallel rays of light are brought to a point, or the point the rays appear to come from for divergent lenses.

    With a simple lens, the lens is moved further away from the sensor to focus closer, some more complex lenses focus in the same way moving all the elements together. Others move just a few elements which will alter the focal length.
     
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  11. andytakeone

    andytakeone TPF Noob!

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    Wow, thanks for the comprehensive answers everyone.

    Every post helped tremendously.

    Main takeaway was that focal length isn't defined by the distance from the sensor, and as petrochemist said, it is defined by where parallel rays of light are brought to a point.
    I don't completely understand where the measurement (mm) comes into play, but it's a good starting point for me.
     
  12. Jim Walczak

    Jim Walczak No longer a newbie, moving up!

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    I'm sure someone will likely pop in here with the specifics, however personally, while I'm sure I've learned it a few times over the years...the specifics have never been anything worth remembering for me, LOL!!! Part of it I'm sure is just that I've been doing this for so long (over 40 years), that I can usually just glance at a subject/scene and (usually) know which lens I want to use. If I'm shooting critters for example, be it wildlife, animals at a zoo, or even dogs at a local dog park, I know I'm usually going to want a long lens...my 70-300mm works great for many situations, however I know I could certainly put a 400mm or even 500mm to VERY good use....the further away something is, the longer lens you will need. If I'm shooting landscapes (of cityscapes) I know I'm usually gonna want a wide angle (in fact I've been doing a fair bit of interior work here lately, so an ultra wide...something in the 12mm range, has made it onto my wish list). If I'm doing portraits (rare, but it happens), I usually prefer something in the 80 to 100mm range. If I'm working in low light situations, stage photography for example, I've found my 70-200mm f/2.8 to be a good compromise for many situations (although a really fast 50mm also has it's advantages)...you get the idea. I will experiment and play around with lenses sometimes, particularly if I have something new or unusual...I've recently picked up a couple of older manual lenses that I'll likely mess around with a bit to see what their strengths (and weaknesses) are, but again for myself that's more about using the lens and getting to know it, rather than much/any consideration regarding the mathematics of it.

    Not sure if this helps or not, but something I had done (several times in fact) when I first started taking my photography work more seriously, was to take all my lenses to a given scene...say a landscape for example...and take several shots using each lens (at different apertures and in the case of zooms, using different focal lengths), then comparing the results of each lens afterwards (either on my computer or even with some prints) so I could see exactly how each lens would perform.

    I would also add that...for myself at least...there's a common sense rule I follow as well; if you frame your subject in the viewfinder and it's not large enough for the composition, you either need to move closer to the subject or simply use a longer lens! LOL! Conversely, if you look thru the viewfinder and things appear too close/too large, you either need to move back or use a shorter lens. Not really that hard to figure out and doesn't require any terribly proficient math skills :).

    I'm sure there are folks out there who do find the nuances of the math quite useful, but personally I rely on the practical experience of working with my own gear. It's always a good idea to read and learn all you can and books, how to videos and forums such as this are great resources, however at the end of the day I'm a very firm believer in that "we learn by doing". Just take the lenses you have available and go shoot something that interests you...you'll get the idea pretty quick.
     

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