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Article:
  A Photographer's Review of the Canon EOS 10D Digital SLR
Subject:   Focal Length
Date:   2003-04-02 12:06:48
From:   anonymous2
When talking about the 1.6x multiplier on the focal length. Isn't it a little mis-leading to say that, "Your inexpensive 100 - 300mm tele is now a whopping 160 - 480mm monster lens..." It is my understanding that the field of view is cropped so your lens still has the same magnification and depth of field of the lens you are using. So a 300mm lens still has the magnification of a 300mm lens not a 480mm lens, the image is just cropped like you were using a 480mm lens. You are not actually bringing the object closer. Someone please correct me if I am wrong.


Matt

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Showing messages 1 through 8 of 8.

  • Derrick Story photo Focal Length -- Well...
    2003-04-02 13:20:19  Derrick Story | O'Reilly AuthorO'Reilly Blogger [View]

    If you take your 300mm lens and put it on a 35mm body and look through it, then you take the same lens and put it on the EOS 10D, objects "appear" closer, by 1.6X in fact.

    Maybe a optics expert will jump in here and comment, but in the field, that's what happens.
    • Focal Length -- Well...
      2003-04-02 17:47:34  mattparker [View]

      I still don't fully understand it. Here is a site that helps to explain the multiplication factor: http://www.luminous-landscape.com/tutorials/understanding-series/dslr-mag.shtml

      What is confusing from this article is,
      "‘Crop’ is a fairly good term – the imaging area is physically smaller. Less of the image circle projected by the lens is used, therefore it is a crop. The image remains the same size at the film plane for a given lens and subject distance – it is in no way magnified. It does, however, take up a larger proportion of the (smaller) frame and so it is easy to see why some people call it a magnifying effect. This is also why a tele lens appears so much more powerful – the field or angle of view has been reduced. This is great for nature and sports photographers as the net result is more real pull than before with no trade off of maximum F Stop loss."

      So is the image bigger because we are enlarging the smaller "film size" more? If I use a 300mm on a 35mm camera and then on a DSLR am I actually bringing the subject "closer"? If I am not actually bringing it "closer" does the enlargement end up having the same result of a longer lens? Sorry just thinking out loud.

      It is all very confusing.
      • Focal Length -- Well...Yes and No
        2003-07-06 17:56:00  dafuller [View]

        Yes, the image is larger because you are enlarging a smaller film size more. If you put the same 300mm lens on a DSLR with a 1.6 factor and a 35mm camera, the DSLR will produce the same angle of view as a 480mm lens on the 35mm camera. And it will do it by cropping the image.

        Does the enlargement end up having the same result as a longer lens? No, it does not. There is one important difference: At the same angle of view and f-stop, the DSLR will have greater depth of field.

        This is hard to understand if you have always used only one film format (35mm for example), because within one frame size, depth of field seems to be related only to the f-stop and focal langth of the lens. But there are really three factors at work:
        1. Depth of field increases as the lens is stopped down (the f-number increases).
        2. Depth of field increases as focal length decreases (wider lenses appear to have more depth of field than longer ones telephotos).
        3. Depth of field increases with the subject distance.

        So... If I put the same lens (300mm) on my DSLR, I have to stand farther away from the subject to achieve the same framing, and the depth of field is greater. OR... if I insist on standing in the same place, I have to use a shorter sens on my DSLR. Again, depth of field is greater.

        How important is this? it depends on the kinds of portraits you take. It's one of the reasons large-format film cameras are still preferred by many for portraits: less depth of field. (See, it works in the opposite direction, too.)

        Hope this helps,

        David
      • Focal Length -- Well...
        2003-05-06 07:40:19  anonymous2 [View]

        hi Matt,

        I am no optics expert but I think i can explain the scenarios you are confused about.

        Essentially canon have crammed their CMOS censors into a smaller area than the 35mm back of a standard SLR (1.6 x smaller?). So, only the light rays travelling very close to parallel with each other, directly into the camera, will be represented on the final image.

        A zoom lense does exactly the same thing, except it uses a lense to take a small area of light and expand it onto a 35mm back. This is why they need more light (bigger f-stop) to be used at the same shutter speed. If you pretended that the 35mm back was 70mm and the magnification was 2x then you would have the same pictures....

        You are never bringing a subject closer, only by optics selecting which light rays you want to see. The rest are either out of focus or off the frame...

        Does that make sense?

        cheers
        kris

        • Focal Length -- it's like higher resolution
          2003-10-30 20:43:16  anonymous2 [View]

          if you crop an image, you rae losing image quality, but reducing the number of pixels in the image (assuming you stretch it to its pre-crop size,) if you took a 4MP image and cropped it to a center portion, you would not have as good of quality asif you cropped the same frame in a 6MP image. this is the effect that the magnification factor gives. That is it crops the image a certian ammount while not sacrificing image quality. So you dont get the same light loss or depth of feild changes as you would with a longer lens, but the practical effect is the same
  • Focal Length
    2003-04-02 13:15:20  anonymous2 [View]

    • Focal Length
      2005-05-03 13:11:52  nobody2 [View]

      "So a 300mm lens still has the magnification of a 300mm lens not a 480mm lens, the image is just cropped like you were using a 480mm lens. You are not actually bringing the object closer. Someone please correct me if I am wrong."

      You are correct about Depth of View and Perspective being functions of your physical lens. The crop factor only multiplies the focaln length with regard to Field of View and NOTHING more. There is absolutely zero magnification detail increase done with a "1.5X" crop (or 1.6X crop for entry level Canon DSLR) on the same sensor or film. (i.e. some sensors can resolve more detail or more acuity even if they are smaller/bigger, etc. This is a whole new animal in itself so let us keep the sensor equal in this case for any comparisons)

      Quoted from http://www.naturfotograf.com/D2X_rev06.html#top_page

      """
      The full-frame Canon, given it is placed in the same distance from the subject as the Nikon and both use lenses with similar focal length, will capture a broader field of view. This results directly from image-forming geometry. Such a setup further gives equal magnification of image detail (not equally obvious to people, but nevertheless true). """

      Look for any form of "image magnification" formula on the web or reproduction ratio. You will find out that all of them are functions of physical focal length and distance to the subject. NONE of them talk about the format, because it is 100% irrelevant with regards to magnification.

      As quoted from luminous-landscape.com
      """
      From Ansel Adams in his classic book “The Camera” (New York Graphic Society 1980).

      "All lenses of the same focal length give images of the same size at any given subject distance."
      """

      These are optical laws and facts. Going digital does not change any of these laws.

      But wait, so how come the pictures "look bigger"? Good question and the primary source of why people insist it is some magical multiplier.

      Ever use a scanner before? You can scan a magazine cover to say "4000x2000" image pixels. You can scan a tiny part of the same magazine cover to the same 4000x2000 image pixels. Do you have "more magnification" now?

      Referencing http://www.guides.sk/scantips2/basics3c.html

      """
      It sounds like a problem, but it won't help to use a 600 dpi scanner, because that's not the real issue. The real issue is that scanning a color print simply won't yield more than about 200 dpi of detail. Scanning at 600 dpi will indeed make the image larger, and it may allow your ego to think you're doing better, but realistically, the color print can not yield any more detail over what a 200 dpi scan will give. B&W prints can be a little better, depending on factors like degree of enlargement.
      """

      The answer is no. You just have a bigger image of a smaller part of the region. It is not to say there is a big image compromise, it is still very good of course.

      The magazine cover analogy is somewhat good because the camera can only see what the lens projects on the sensor or film. Just because you are creating more 'pixels' out of a smaller chunk of the same image does not mean you get more image detail (which is the case of magnification).

      But wait, back to the reproduction ratio story, I can print out images far bigger throw the ratio off.

      Well that is because the digital realm gives you the flexibility to change the printer resolution easily. Printer resolution is 100% independent of image resolution. I can print out a 100x100 pixel image as a 8x10 inch photograph if I wanted to but the quality will be bad since the pixels per inch will be pretty low.

      In the end, remember that a camera's "film" or "sensor" sees an image that is projected by the lens. You are not really getting more magnification unless you modify the optics or the sensor itself does some kind of INTERPOLATION. The camera is not really doing any interpolation in this case. It is not adding any more detail, it is just fitting the same visual data across more pixels (think back to the scanner case).

      In this regard it gives you more to work with (more pixels), but it isn't really image detail magnification (it's physically impossible). So it helps your telephoto loosely in that regard.

      So does this mean OH NO we got to wait for Full Frame? No, of course not. 35 mm was not the end all either, look at the older, bigger formats. Nikon is a big promoter of the new 1.5X DX format. Look at Bjorn's d2x comparison where he shows it has more detail only because it's sensor is more details, not because it is a smaller sensor.

      In the end, if you never shot with 35 mm to begin with, you wouldn't even really know about this "cropped field of view". It is only to compare against 35 mm for old film shooters to compare with and something to standardize on rather than renaming everything. The angle of view is still an absolute value you just calculate it a bit differently.

      As this guy mentions
      http://www.naturephotographers.net/articles0703/tg0703-1.html

      In the end, who cares. Look at the viewfinder, and shoot what you see. It is as simple as that.
      • Focal Length
        2005-09-18 21:28:40  dennist1 [View]

        Okay, here is another way to explain it... If you took a 25mm wide piece of film and taped it in the back of a 35mm camera, took a picture, then enlarged it to 8 x10 you would obtain a "closer" image than if you used 35mm film. Due to cropping? Yep. Quality reduced? Yep, because of more enlargment necessary WHEN PRINTING it. So, what this digital question really boils down to is the quality of capture for a given area. If you use a 'super duper' film, with 1/2 the grain for the 25mm piece, you would get the same quality of print with more "magnification" and the same lense. So, applying this to a digital camera, it essentially comes down to the question of what's smaller, film grain or electronic pixel. If the pixel is 1.6x smaller, then you get the same quality (hopefully someday, getting closer). If the pixel is the same size as the film grain, you would get the same quality digitally as you would with the 25mm film.
        Also, someone above stated that anything over 200 dpi is a waste. That is not entirely true .. 200 dpi 4x6 will be very poor if you try to print it at 8 x12 (100 dpi after magnification). However if you start at 600 dpi 4 x 6 and enlarge that to 8 x 12, you still have a respectable 300 dpi. So, higher resolution is very important particularly for large prints. The problem with scanners is that although they create more pixels, they aren't optically detecting any additional detail. Its much the same thing as the 'digital zoom' on consumer level cameras ... Nothing you can't do with the little magnifying glass on your computer screen, yet a great marketing tool to get more money from unwary consumers.