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12-08-2008, 04:22 AM   #181
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12-08-2008, 09:22 AM   #182
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QuoteOriginally posted by Wheatfield Quote
I think the lot of you are reading a lot more into this than is called for. What does the viewing distance of the image have to do with whether the focal length of the lens is "normal" or otherwise? The print hasn't changed if you view it from one foot or 10 feet.
I pretty much agree with this about the print not changing. In fact, this is really part of the concept of the normal lens. I'm just explaining the principle behind it.

The idea of a normal lens is really based on the idea that our perception of a print's perspective won't change because of a change in viewing distance. This, however, does not mean that the viewing distance to print size ratio does not matter. Rather, it means that this ratio is considered to be a constant. That is, it is based on the idea that there is a "standard" or "normal" viewing distance that we will (almost) always interpret a photo from (and it scales with the print size). The only question is, where exactly is that distance? The diagonal of the print size has been settled on as at least being close to that distance, but many people seem to feel that in reality it might generally be a bit more than that. This would in turn make normal focal length around the same as the diagonal of the sensor or perhaps a bit more (although some still seem most comfortable with a focal length a bit less than that as well).

It should be clearly understood, though, that the view through the viewfinder matching what you see without it is only as relevant as camera design makes it (which in the case of APS-C cameras is not all that relevant). This is because viewfinder design can be changed to make different focal lengths feel "normal," and has been with different cameras.

Consider medium and large format cameras. Medium format cameras are sometimes SLRs, but I would be highly surprised if a 50mm lens on a medium format SLR made the view through the finder seem anything but wide-angle. Many medium format cameras, and all the large format cameras that I am aware of, have a rangefinder type viewfinder that needs its own lens to show the appropriate angle of view for the camera lens. This type of viewfinder is only accurately tied in to the angle of view of the camera lens because the lens manufacturers go through the trouble of matching that angle with their viewfinder lenses.

There is no focal length that inherently makes the view through every viewfinder normal any more that there is a focal length that inherently creates a normal view for every size sensor. Also, even if there were such a focal length for the purposes of the viewfinder, it would not mean anything for the actual photographic results, and should thus be ignored.

The point is, to know how "normal" your photo will turn out, you need to go by the numbers and previous experience with resulting photographs, not the view through the finder.

Last edited by CFWhitman; 12-08-2008 at 11:04 AM.
12-08-2008, 10:38 AM   #183
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QuoteOriginally posted by Sean Nelson Quote
But "mm of focal length" does NOT describe field of view - it describes the distance the lens is from the focal plane when an object at infinity is in focus. Although the FOV changes, the focal length DOES NOT.

The confusion stems exactly from this - using focal length as a substitute for field of view.

. . .

I don't have a problem with someone saying "this lens has a field of view that's equivalent to a 75mm lens on a full frame 35mm camera". That's an accurate description. But saying that a 50mm lens somehow becomes 75mm (with no other qualification) is very, very misleading.
Exactly. The angle of view is what has changed because the aps-c sensor is smaller than the 35mm film. Therefore, the definition of a "normal" lens now has to take into account the sensor size, film, aps-c, aps-h or FF.

QuoteOriginally posted by audiobomber Quote
I think that's needlessly picky. We all understand that the lens remains the same and the FOV changes. It's a form of shorthand for people who are involved in the hobby. Another example of shorthand is "100% crop". That doesn't make sense to anyone except people who know what it means.
Its not really needlessy picky because this is what confuses a lot of people. It is promulgated in the Mag Rags, company ad campaigns and retailer sales descriptions as a result. How many times have you seen a 300mm lens advertised to be like a 450mm lens on an aps-c body?
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