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07-12-2010, 05:16 PM   #1
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How Can You Easily Tell How far a Given Focal Length will Zoom?

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? 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 ?

07-12-2010, 05:49 PM   #2
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There are various lens calculators on the Web, but I don't know if any will tell you exactly how many feet viewing distance is provided by a given focal length. I can give you a small rule of thumb calculation. It goes like this...

A 50mm lens has about the same native magnification through the viewfinder as the human eye


I am not talking about field of view. That can be just about anything depending on lens coverage and media size. I am just talking about focal length and how big things look. So...

A 100mm lens is about 2x, a 200mm about 4x, a 300mm about 6x, and so on. Now here is the bad news...you need a pretty long lens for bird photography. 300mm (6x) is considered to be the entry point for birds in the field and that is not that great (for comparison, the average binoculars are 8-10x)! Better results are to be had in the 400mm-500mm range.

That being said, I have gotten some pretty good shots of larger wading birds and some very indulgent hawks with a 200mm lens mounted to my K10D. Those were lucky and I would go crazy trying to shoot birds with that short of a lens unless I was set up at a feeder with remote shutter release.


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07-12-2010, 06:07 PM   #3
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Honestly, just bolt a few lenses on and you'll have a 6th sense about it trust me.
07-12-2010, 06:17 PM   #4
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QuoteOriginally posted by Alfisti Quote
Honestly, just bolt a few lenses on and you'll have a 6th sense about it trust me.
This link will give an idea if you dont have all the lenses to bolt on yet.
pick "digital" ie APS-C crop, or "35mm" full frame and then use the slider 11mm up to 500mm to see the differences. Or pick one of the lenses from the list on the right. (half of which arent available on K-mount unfortunately)

Focal length comparison tool, Tamron USA

07-12-2010, 06:38 PM   #5
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It's really all down to a little math

Image size = subject size x focal length / subject distance

Just remember an ASP-C sensor is 0.024 x 0.016 meters
07-12-2010, 08:50 PM   #6
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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? 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 ?
Here's an easy mental calculation method:

Look through the viewfinder at a scene. If you want to adjust the lens so the scene is 1/3 as wide, choose a focal length 3 times longer. If you want to adjust the lens so the scene is 1/2 as wide, choose a focal length 2 times longer. etc....

Another way to put it is if you want a bird to be three times larger in the viewfinder, get a lens three times longer. If you want the bird to be 4 times larger, choose a focal length 4 times longer. etc...

Here's a method for making an initial estimate I posted earlier today:
QuoteQuote:
Hold your fist in front of your face with your elbow bent at a right angle; use your knuckles to select the lens you want for the scene behind your hand:

4knuckles:50mm, 2:100, 1:200.

The trick is figuring out where to hold your fist so the above rule holds. Try it a few times.

Take a photo of a scene with a 50mm lens (or look through the camera's viewfinder with a 50mm lens in place.)
Put the camera down & hold your fist in front of your face such that the 4 knuckles just fill that scene height.
Remember where to hold your fist next time you want to select a lens.
Dave in Iowa

Last edited by newarts; 07-12-2010 at 08:59 PM.
07-13-2010, 07:06 AM   #7
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One easy form of the math is:

Focal_length_mm= 24*Distance/Width

Say the scene you want to photograph is 4 meters wide and 10 meters away.

F=24*10/4=60mm

Dave in Iowa
07-13-2010, 09:37 AM - 1 Like   #8
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BTW, on that that will help in reading about this is to realize you are using the term "zoom" incorrectly. "Zoom" refers to the *changing* of focal length. So no single focal length ever zooms at all. A 20mm lens, a 50mm lens, and a 500mm lens all have exactly the same "zoom": none at all. The word you are looking for doens't really exist, because photographers don't tend to put things in quite the way you are, but the closest thing would be to use the word
"telephoto" to mean what you mean by "zoom". Telephoto isn't a verb, though. There is no commonly used verb to mean what you are using "zoom" to mean. Maybe "magnify", but that has a somewhat different connotation in photogaphy, because it's usually concerned more with closeup photography than photography over distances (eg, microscopes, not telescopes).

I say this because if you go around thinkng zoom = telephoto, chances are high you'll end up misinterpreting a lot of what you read, and you will sometimes get inappropriate answers to questions.

Beyond that, while mathematical formulas can be used, I agree with those saying tht with experience (and it doesn't even take that much) you'll develop a feel for it and won't need formulas. It's kind of like if I asked how one knows how heavy different weights are. Someone could explain to me that a 25-lb weight is acted upon by gravity according to the Newtonian formula blah blah blah - or they could just set me down in t a room with bunch of different weights and let me experiment for myself to see how heavy 25 lbs is.

07-13-2010, 11:43 AM   #9
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In re-reading the OP, I think we have all missed the point.

the OP is asking "IE. let's say I find myself constantly x feet too far to get a good shot of, let's say, easily startled birds. " and "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 ? " in the case of wild life and easily startled or small birds, it does not matter what he has, it will be too short

but actually, aside from that realism, regardless of how people approach the issue it all comes down to the math I put a few posts ago.

I keep coming back to this because sometimes it is useful to know if you know how far away you will be, and what the subject is, you can calculate what you need and cur what you carry.
07-13-2010, 01:03 PM   #10
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QuoteOriginally posted by Lowell Goudge Quote
it all comes down to the math I put a few posts ago..
True but, since counting knuckles is pretty much the limit of my math skills, I'll go with newarts method.

Seriously though, I can do the math--it's the estimating I find most challenging.
07-13-2010, 06:21 PM   #11
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QuoteOriginally posted by Lowell Goudge Quote
In re-reading the OP, I think we have all missed the point.

the OP is asking "IE. let's say I find myself constantly x feet too far to get a good shot of, let's say, easily startled birds. " and "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 ? " in the case of wild life and easily startled or small birds, it does not matter what he has, it will be too short...
Lowell,
I think that is what I told him, though in way more words. It is a tragic truth in photography that bird photography always requires longer glass than what you own and that when you finally get the long stuff, you are too old, weak, and shaky to effectively use it


Steve
07-13-2010, 09:08 PM   #12
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my brains is a bit soft today, but once upon a time when toteing my LF camera around, I used to carry a card with 4"x5" cutout window in it. When holding it up to your eye about 150mm away, that would be the view of a 150mm lens, so you could do the reverse, hold it so it framed your view how you wanted to record it, then estimate what lens you were going to need by measuring or guessing the distance from card to eye. When you only have 2-3 focal lengths at your disposal this is feasable. Not sure if, with a bit of basic maths (to accomodate for the sensor size and a different sized window), you could use a similar approach .
07-13-2010, 11:24 PM   #13
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There are some online and Smartphone calculators that are easy to use that can give you the FOV for a given lens length and range to subject. The one I use is on my Droid.

Here is an example of how I might use it. Letís say I wanted to do some sports photography. I am going to stand at about mid field so my max range to subject is 150-175 feet. I want to get full height shots with some grass and a little margin. So that is about 7-8 feet FOV height of frame with the camera in landscape. The calculator gives with a 300mm 9.1 feet high at 175 feet and 7.8 feet high at 150 feet.
Is this the kind of thing the OP is looking for?

DAZ
07-14-2010, 12:09 AM   #14
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Is he talking about apparent magnification?
07-14-2010, 12:30 AM   #15
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QuoteOriginally posted by wildman Quote
Is he talking about apparent magnification?
Hard to say. I think that Lowell got it correct in that he is frustrated by not having long enough glass to approach skittish birds and wants to know how to tell ahead of time what lens to mount.


Steve
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