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Old Jun 22, 2005, 4:20 AM   #1
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Well, I'm puzzled ...

50mm on aSLR camera with format 24x36mm (french system, I guess it is 50mm in proper english ...? well, let's say 350D, 20D, etc) is supposed to be the natural focal length, i.e. the focal length which gives a picture that corresponds to what a (set of) human eye(s) would see.

So, that's why it is often advices to have a 50mm lens with a SLR ..."it makes you a better photographer!"

BUT, with the 1.6x crop factor (on a DSLR), you end up with 80mm equivalent focal length so a smaller angle than human eyes, right?

Thus, we should all get a 35mm lensfor a SLR with 1.6x crop factor to get back ~50mm (56mm actual)!

Am I

1. right (and why)?

2. wrong (and why)?

3. It does not matter?

JM
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Old Jun 22, 2005, 8:57 AM   #2
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Well you're right about 50mm being effectively 80mm on a smaller sensor camera. I don't know about the "natural" perspective making you a better photographer though. I guess it's more to do with the fixed focal length making you think a bit more about what you are getting in your shots rather than just zooming to fit everything in. Also primes, and especially 50mm ones, are much easier to make than zooms with the result that they tend to be cheaper, better optically and much faster (wider maximum aperture) than zoom lenses. Using these might give you a bit more control and encourage you to learn a bit more about photography. I've heard that one of the reasons 50mm lenses are easy to make has to do with the focal length being roughly the same as the frame size (diagonal of 24mm x 35) so maybe we'll see some very cheap 35mm lenses for smaller sensors soon. In the meantime, 80mm is a bit long for taking most photos (for me anyway) but is fantastic for portraits.
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Old Jun 22, 2005, 9:42 AM   #3
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Normal (whatever that is :blah human vision is actually closer to a 45mm lens :-)
A 28mm on a 1.6 crop camera gets you to 44.8mm .

Lot of 28mm's available, and zooms that either start at 28mm or include it in their range.

Using a fixed focal length lens helps you think with that view in mind, and use your other zoom feature (your feet ) to hunt out and locate better viewpoints.

Zooms let you quickly frame and isolate your image, but IMHO make for lazy photographers who never really explore the subject.

Peter.
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Old Jun 22, 2005, 10:27 AM   #4
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PeterP wrote:
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... Using a fixed focal length lens helps you think with that view in mind, and use your other zoom feature (your feet ) to hunt out and locate better viewpoints.

Zooms let you quickly frame and isolate your image, but IMHO make for lazy photographers who never really explore the subject.
I agree. Deciding where to stand is IMHO the most important factor in photography. Just for simple things like avoiding a telephone pole growing out of Aunt Emma's head.

All sorts of arguments can be made for the "optimum" focal length, and the better of those arguments will be based on what you are shooting. In general, wide for landscapes, slight telephoto for portraits, long telephoto for sports, about 50mm (equiv) for general purpose. Since lenses that sell the most tend to be cheaper for the same quality, I'd suggest a 50mm since it is the biggest seller. As you noted, that works out to be a slight telephoto with a 1.6 crop factor.
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Old Jun 22, 2005, 10:40 PM   #5
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polarwasp wrote
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BUT, with the 1.6x crop factor (on a DSLR), you end up with 80mm equivalent focal length so a smaller angle than human eyes, right
Rather than use the 1.6 crop factor term you can use sensor width. That may help make sense of things. Sensor widths for dSLRs can vary, but are around 23 or 24 mm.

This page has some examples.

http://homepages.ihug.com.au/~parsog/photo/sensors1.html
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Old Jun 23, 2005, 6:56 AM   #6
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Quote:
Rather than use the 1.6 crop factor term you can use sensor width. That may help make sense of things. Sensor widths for dSLRs can vary, but are around 23 or 24 mm.
Agree, and it is a very decent web site .... BUT (from the link above)
Quote:
For instance, the Nikon D100 uses a 23.7 x 15.6 mm sensor and has a factor of 1.5, if you use a 28mm lens on the D100 then it will behave as a 42mm lens (28x1.5=42).
So what matters to my point is the factor 1.6 and whether using "human eye angle", i.e. 45 to 50mm "equivalent" focal length is important.

If it is, you are better off with a 28 to 35mm lens. But it seems what matters is learn to move around your objects, so what you really want is a fixed lens!
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Old Jun 23, 2005, 8:57 AM   #7
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I agree that is a good page for reference.
It is missing a few of the current popularish cameras crops.

Most notable are:
Sigma is a 1.7 crop
Canon 20D,RebelXT are 1.6 like their predecessors
Canon 1d-mkII is a 1.3 crop
Canon 1ds-mkII is a 1:1 crop (36mm*24mm sensor)

And I also agree, knowing the crop factor helps you quickly work out the effective focal length and from that the field of view of the lens you want to use.

Peter.
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Old Jun 23, 2005, 8:42 PM   #8
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polarwasp wrote:
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... So what matters to my point is the factor 1.6 and whether using "human eye angle", i.e. 45 to 50mm "equivalent" focal length is important. ...
I'm not sure that there is a meaninfull focal length equivalent to the human eye. The eye has a very small area of high acuity so in some sense, we see with a "long lens". We also have pretty good peripheral vision that detects motion very well, so in some sense we see with a very "short lens".

Also, our vision is much more panoramic than the standard photo and we see in 3-D. The eye and a camera are at least as much different as a sports car and a dump truck. Knowing something about one will help understand some of the other, but you better not push the details to far.
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Old Jun 23, 2005, 11:13 PM   #9
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To me photos taken with a 50mm lens tend to look flat or boring. Wide angle photos tend to make the viewer's eye move around to comprehend the image. Telephoto photos have some excitement because the relationship between the background and foreground is enhanced. I am oversimplifying here but these are my impressions and may help answer your question. Another way of putting this is that with a 50mm lens you have to work harder to engage the viewer of the finished photograph.
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Old Jun 24, 2005, 2:19 AM   #10
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So, if I may conclude this.
  • The fixed focal length lens will make you walk around and look for the best point to shoot your photo.[/*]
  • 50mm (equivalent) focal length is "boring" :blah:.[/*]
  • A 50mm lens (80mm equivalent) is great for portraits[/*]
  • The human eye story is not so meaningful!
[/*]
If you have the discipline to keep you zoom lens on the same position and walk around (ANDtake your time...) You have the learning while your bank account refills for thefixed lens you dream of!

This of course does not consider the other advantages of a prime lens.

Thanks guys !
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