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07-14-2010, 04:19 AM   #16
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QuoteOriginally posted by wildman Quote
Is he talking about apparent magnification?
I think so--basically the same issue you addressed here.

07-14-2010, 09:48 AM   #17
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I do like the fist approach for visualizing this while you get comfortable with just "seeing" the fields of view unaided.

Another interesting way of looking at this, related to the question as asked and as others are discussing here but it putting it in slightly different spin, is to think about a specific subject and the distance you'd need to shoot that subject from a given distance. As has been suggested, if the subject is birds, basically, the answer is, you want the longest focal length you can afford or can stand carrying around or accept the image quality from. and similarly for most forms of wildlife. So that's not a very productive way to think about focal lengths. But for *people*, that's not such a bad model. Think about taking a head and shoulders portrait - so the field of view is about 2 feet or so (somewhat under a meter) at the main subject distance. You could indeed work out what lens to use from a given distance based on that.

But given that you're usually just guessing distance to subject, you're still probably better off just learning to visualize field of view directly.
07-14-2010, 11:01 AM   #18
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QuoteOriginally posted by jaieger Quote
Simply put, how does the Xmm translate into Xmeters/feet ?
OK I think I've got it.

I would phrase the question differently however:

How close must I get to a typical sized woodland bird, for a given FL, so that the image scale is large enough to reveal fine plumage detail?

Try this (and I have actually done this) -

Print approximately a 3" diameter bold circle on a piece of paper and tack it to a post.

Select any FL you wish. Move back and forth until the diameter of the circle is about 1/3 to 1/4 the distance of the horizontal axis of the image in landscape mode. Note the distance between you and circle.

Do this with a variety FLs noting the distance between you and the circle.

You will quickly get an intuitive sense of how close you must get to a bird for a decent image scale at any given FL.

Try it with your 105mm and see if you think you could get that close to a normal healthy bird in the wild.

I think you will be surprised at how much FL is needed to get a good image scale on wildlife at a realistic distance.

FL means nothing if you are a completely silent invisible photographer. In that case you could get great bird shots with a PS.

Try this little empirical test it's not as complicated as it sounds here.

Last edited by wildman; 07-14-2010 at 11:23 AM.
07-14-2010, 11:43 AM   #19
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QuoteOriginally posted by jaieger Quote
Is there a certain calculation or formula or ratio or something that lets me know how far a given focal length (let's say 105mm) will zoom in?
See Marc Sabatella's comment re your use of the word "zoom." I would suggest that you might have wanted the word "reach" here. It's not a technical term but seems to convey what I think you are thinking about.

IE. let's say I find myself constantly x feet too far to get a good shot of, let's say, easily startled birds. How do I know if 105mm will be able to get me close enough, or 125, or 75, etc. ? Simply put, how does the Xmm translate into Xmeters/feet ?
Honestly, I think it's an odd question. Unless you are doing some sort of scientific experiments, I don't see why you ever need to think this way.

I can tell you that a 105mm lens (on a Pentax body) is generally NOT long enough to get good photos of "easily startled birds." You can figure this out for yourself pretty easily—and perhaps you have already. Just go out and try shooting birds with a 105mm lens. You'll fairly quickly realize that the birds are all pretty small in the photos you take with that lens.


You want to shoot birds? You start somewhere around 200mm (again, I'm thinking of 200mm on a Pentax DSLR body) and you go from there, to 300mm and beyond, if you can afford it.

As a practical matter, it boils down to two things. First, you learn how far (and how wide) different lenses will reach by using them. It's easier to learn than it sounds. And second, you ALWAYS use what you've got. In either case, there's no need to calculate.

If you are in the country and see a beautiful bird, you might decide not to bother shooting at all, if you knew your lens simply wasn't up to the task at all. You see an eagle a quarter of a mile away by a river, and you've got a 21mm lens on the camera, well, you're not going to get much of a photo. OR you pick up the camera and shoot and hope for the best. If you have a zoom lens, you zoom until the bird is the right size in your finder—or until you realize that you can't go any further. If you have a 300mm f/4 prime on the camera, then you either take the shot, or you think about "zooming with your feet," that is, moving closer or farther away from the subject, as needed, and if possible.

Again, if you want to shoot birds, get a zoom that goes at least to 300mm. The Tamron 70-300 Di LD macro lens is a decent lens for amateur bird photography and sells right now for less than $200 from Amazon—a very solid deal. Add a 1.4x teleconverter if you need one.


Finally though, keep in mind that, when you're shooting wildlife, focal length is very far from the only thing that matters. Other factors to consider are:
  1. max aperture of the lens at the focal lengths you expect to use
  2. lens acuity (sharpness or resolution)
  3. how noisy your camera's sensor is at different ISO settings.

Let me explain why these things matter, too.

The longer the focal length, the greater the need to have a fast shutter speed to counteract camera shake. And even if you put the camera on a tripod, you may still need to shoot fast because wildlife doesn't always freeze for you for long exposures.

It's taken me a long time to figure this out, but I realize now that I would rather have a 200mm f/2.8 lens that was optically really, really good (in other words, an expensive, high-quality lens) than a 300 f/5.6 lens whose optical qualities were okay but not great. A bird shot with the shorter lens will be "smaller" in the picture, but I will probably be able to crop that 200mm photo and still come up with a better (sharper, clearer) photo than the one I took at 300mm. Even apart from its optical superiority, the 200mm f/2.8 would permit me to shoot at a lower ISO to keep noise down and get a sharper photo.


07-14-2010, 11:50 PM   #20
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QuoteOriginally posted by WMBP Quote
Finally though, keep in mind that, when you're shooting wildlife, focal length is very far from the only thing that matters. Other factors to consider are:
  1. max aperture of the lens at the focal lengths you expect to use
  2. lens acuity (sharpness or resolution)
  3. how noisy your camera's sensor is at different ISO settings.
Not that I chase birds, but I'd add these to this list
  1. Your Patience
  2. Your ability to sit still
  3. Abillity to learn the birds habits
07-18-2010, 09:47 AM   #21
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The easy way to figure this out: Borrow a Bigma (Sigma 50-500) zoom. Tack bird-size targets to a pole. Stand at various distances and see what focal length(s) work for you. Don't worry about buying a Bigma, it's too heavy and slow and expensive, but it's a good learning tool.

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