A Photographer's Review of the Canon EOS 10D Digital SLR
Subject:   Focal Length -- Well...
Date:   2003-04-02 17:47:34
From:   mattparker
Response to: Focal Length -- Well...

I still don't fully understand it. Here is a site that helps to explain the multiplication factor:

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.

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Showing messages 1 through 3 of 3.

  • 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,

  • 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?


    • 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