Originally posted by alohadave In your example, you are mixing different focal lengths with different sensor resolutions.
Right. I'm doing it on purpose—to indicate that any magnification effect isn't a result of focal length, but of focal length + sensor resolution together.
Quote: With the 6mp sensor, you have fewer sensels to record the fine detail than you do with the 14.7mp sensor. Of course you are going to see more detail in the K20D file, even with the shorter lens. The K20D sensor has more sensels, so it's going to be able to resolve more detail than the *istDS.
It doesn't change the fact that the 100mm lens is going to fill more of the frame with your subject, at the same distance than the 70mm will. The magnification is based on the focal length of the lens, but the angle of view can be changed by cropping, while the magnification does not change when you crop.
Dave,
You confuse me a little there, when you say that cropping doesn't change magnification. Given the way I see you using the word "magnification," I don't expect you to say that. Actually, when you say that, it seems to me you are looking at things MY way. I thought that by "magnification," you meant "occupying more area within the frame." And when you crop out all those useless trees, etc. in the background, the bird in the center of the shot does occupy more area within the (cropped) frame. Which is what I thought you meant by magnification: occupying more area within the frame.
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I think we are disagreeing here mainly over this word, "magnification." You take the common sense view that it's appropriate to speak of the effect of zooming in (increasing focal length) as a form of magnification. I'm taking the less intuitive position that the word magnification should be left out of the discussion here and reserved for another context where it is more useful.
For me, that other context is the context of the photo's final presentation as a print. The idea of "magnification" really becomes meaningful when I start ordering prints and have to ask myself how large I can print a particular image with satisfactory results. If the same 70mm lens on two different cameras with different resolutions, produces—as it does—image files that don't have the same enlargement potential, well, then enlargement potential (i.e. magnification in terms of the result, which is what matters) isn't a property of focal length alone.
(I could speak also of enlargement potential for screen display, but it's easier to talk about prints, because with screen display, you get into a whole slew of other messy issues.)
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Why is this so difficult to talk about? Because there are actually a number of factors here and it's hard to talk about them all at once, because it's rather difficult to keep 'em in your head at once.
What we're talking about here is a bit like the discussion of depth of field. It's fair to say that a wider aperture (say, f/2.8) generates more depth of field than a narrower one (say, f/8), and in fact, we do talk that way all the time. But when we say this, we're making assumptions about several factors that are as important as aperture to depth of field: we're assuming that the photographer hasn't changed his position relative to the subject, that he hasn't changed focal lengths, and that he hasn't put down his APS-C sensor camera and picked up a full-frame camera instead.
Returning to the effect of focal length: When I shoot, as a practical matter, if I decide that 28mm isn't the right focal length and I switch instead to 40mm or 70mm or 105mm, I don't do it out of a conscious desire to "magnify" anything. I just don't think of it that way. I do it to recompose my shot, that is, to adjust my angle of view. I stand where I stand in order to get the perspective I want on the subject (or because I have no choice), and I use the focal length I use in order to get the "subject coverage" I want (where subject coverage is the effect in the resulting photo of the lens's angle of view).
Virtually the only time that I think of focal length in terms of magnification, is when I do not have an adequately long lens for the task at hand. For me, that usually means that I'm on vacation, trying to photograph wildlife. Then I always wish I had a Bigma handy so I could "get closer."
Will