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Thread: how do decide which lens to use for a given shot

  1. #81

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    Re: how do decide which lens to use for a given shot

    Nothing confrontational at all, Joe,
    it was just a humours note - when I started to express the same thought, I was immediately accused as being confrontational. That accusation was then deleted by its overzealous author.
    Live well and don't be afraid to express what is correct - life is defended in the same way. Cheers.

  2. #82

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    Re: how do decide which lens to use for a given shot

    Quote Originally Posted by Willie View Post
    If you own only one lens the choice is a bit easier. More than on good photographer has gone that route.
    So true!
    "I would feel more optimistic about a bright future for man if he spent less time proving that he can outwit Nature and more time tasting her sweetness and respecting her seniority"---EB White

  3. #83
    stradibarrius stradibarrius's Avatar
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    Re: how do decide which lens to use for a given shot

    Joe, Thank you for going to the effort to provide such a good answer! I am going to do just what you have explained. I am certain it will improve my results.
    Quote Originally Posted by Joe Smigiel View Post
    'The "view" that you are trying to achieve' is the best perspective. Semantics... Perspective is viewpoint (aka view, POV, etc.). You are really asking a question about perspective and referring to it as "framing." Moving forward or back (or left,right,up,down) changes the perspective and that changes framing as well as image shape, size, apparent distortion and so on. You can only obtain the same framing by changing both the format and the focal length while keeping the perspective the same. Any other combination (subject distance + focal length, lens angle-of-view + image size, etc.,) will result in a different "view."

    That being said, the resultant changes may not be important to you (e.g., more or less background) but they will still occur and change the appearance of the image/object/view.

    The only way to use two (or more) different lenses of different focal length and get the same framing is to stay in the same spot and use for example, a 150mm lens on a 4x5 camera or a 300mm lens on an 8x10 camera or a 600mm lens on a 16x20 camera, etc., (assuming the same lens design formula and angle of view) and then enlarge the smaller format film(s) to match the image size of the larger formats. The perspectives ("framing") will be the same then (although depth-of-field, graininess, etc., will change, other things being equal). If you change only two variables such as focal length and subject distance, you will not have the same framing. (You will need to change the format as well.)

    Here's a pic with a musical instrument that shows 3 different crops from the same position in space, i.e., the same perspective. If you consider the the central crop from the smaller sensor, it has an identical perspective and relative size and shape relationships from the machine heads to the bridge regardless of the sensor size. However, to match the absolute image size of the instrument and person between crops, you would have to enlarge the smaller sensor's image to match the full frame and the result would be less resolution (and less background).



    I think I fully understand your question and what you are curious about but as in the portrait example, to maintain the shape and size of any object in the frame (or any part of an object such as a violin), you have to stay at the same spot. If we use a violin as an example, if you photograph so the tuners (forgive my lack of knowledge in terminology here) are a certain size and shape, the rear of the violin will take on a certain size and shape based on the perspective and constraints in angle of view. Changing the focal length and subject distance (referenced here to the tuners) will result in the rest of the instrument assuming a different size and shape relationship. It will either elongate or compress visually with a change in distance. The amount of included background will also change as is clearly illustrated in the portrait example.

    The two pictures of the violins a few posts previous illustrate the effects. In the first example taken with a short focal length lens, the head is exaggerated in size and distorted from a normal view of its shape. It is a striking effect but rest of the violin looks diminutive compared to the head. It analogous to the exaggeration of the nose in the portrait examples. In contrast, the second view of the violin appears more normal, less dramatic, and the relative size and shapes of the head and body of the instrument are more normal in proportion. It also helps that the viewing angle is more planimetric. The latter picture was probably taken with a longer lens from a greater distance.

    The portrait example earlier changed subject distance and focal length to maintain the head size (object size on film). The violin examples show a change in perspective and focal length which altered the relative size and shapes of the instrument. The digital sensor crop shows identical perspective but a change in coverage and format and therefore magnification and image size affecting resolution. A change in focal length holding perspective and lens angle-of-view the same but allowing format to change affects things like depth-of-field and object size on the film. But, with the latter you can blow up the smaller format pic to identically match the view of the longer lens image taken from the same spot. Doing so changes the magnification and graininess of the image, but not the perspective or "view." You can see that effect as well as the background/foreground size and shape distortion caused by maintaining the object size and changing subject distance in this additional example :



    The key to all this really is the cutout viewfinder card (or having some familiarity in advance with a very limited number of focal lengths). Don't dismiss the card without giving it a try. It will only take a few minutes to make. Take a piece of matboard and cut an opening equivalent to your film format in it. Then place your violin on the surface of choice with the background you would like to use. Approach it using the viewfinder and moving it around forward and back, up and down, at different distances from your eye, etc., until you find the framing you want. That's it. You will now have the correct perspective, the preferred relative size and shapes of the instrument will be what you see through the opening and also have probably envisioned, and the distance of the card from your eye will tell you the focal length to use. It is much simpler to do than explain or debate. Give it a try and I think you will be amazed at how simple it makes the focal length decision and shows you the best POV.
    Generalizations are made because they are Generally true...

  4. #84
    chassis's Avatar
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    Re: how do decide which lens to use for a given shot

    It's a matter of perspective, since the stated assumption is the lenses are "equal" in performance. If you have access to a CAD program or 3D viewer software, a test can be made to simulate the OP's question, by setting the focal length of the viewing software at different values, and setting the camera location at different locations in order to achieve roughly the same image framing. Another, more practical, way to do this is to study images on this site and others to understand lens selection for different situations.

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