right focal lenght range for wild turkey -- and mathematics to get there

[rob_hanssen]

<p>A couple of my colleagues are turkey hunters and I might be able to join to do some shooting (me with camera only, though). To get a useful estimate what lens to bring, I got into the math of the laws of optics. The results so far:<br>

<br />definitions:<br />d = distance between sensor and object<br />f = focal lenght<br />b = distance between sensor and focal length point<br />v = distance between focal length point and object<br />M = de-magnification as defined by b/v<br>

1) 1/f = 1/b + 1/v<br />2) d = b + v<br />3) M = b/v <=> b = Mv<br />1/f = 1/v + 1/Mv = (M+1)/Mv <=> f = Mv/(M+1)<br />d = (M+1)v <=> v = d/(M+1)<br />=> f = M*d/(M+1)^2<br>

Assuming I would like to fill half the height of the APS-C sensor (landscape orientaion) with turkey (height ~ 80 cm), the de-mag factor M would be 1.6/0.015 ~ 100x. For portrait, this would be 1.6/0.022 ~ 72<br>

Plugging this into the formula for f gives the following table for distance vs. focal length<br />d (m) ; f @ M=100x; f @ M=72x<br />5; 46; 68<br />10; 93; 135<br />20; 185; 270<br />30; 278; 405<br />40; 370; 540<br />50; 463; 676<br />(sorry about the table, can't get it to look good)<br /><br />Assuming I take pictures from about 30 meters, I'd have to use a 300-400mm focal length lens. Does this sound like a reasonable approximation?<br />Thanks,<br />Rob</p>

[rainer_t]

<p>As a rule of thumb ...<br>

D / fl =(roughly)= S1 / S2<br>

where D = distance to subject, fl=focal length, S1=size of subject, S2=size on sensor.</p>

<p>S1=80cm ... S2=0.8cm therefore S1/S2 = 100<br>

D/fl = 100 ... D/100 = fl ... with D=3000cm ... fl=30cm == 300mm</p>

<p>So, yes ... your calculation sounds reasonable.</p>

[stsva]

<p>You might try running your figures through this online calculator to see if it agrees with your calculations:<br>

http://www.giangrandi.ch/optics/lenses/focalcalc.html</p>

[Hector Javkin]

<p>I didn't follow part of your algorithm (Is <em>distance between sensor and focal length point</em> different than focal length?), but I believe you need a longer lens or a bigger turkey. Look at <a href="http://www.bobatkins.com/photography/technical/field_of_view.html">this description</a> , or compare to the results of the program available <a href="http://tangentsoft.net/fcalc/">on this site, </a></p>

 

[jtk]

<p> Wild turkeys are famous for great hearing. Do you know how to call turkeys? That might make up for the distractions your lens and clacking mirror will cause.</p>

[jtk]

<p> Wild turkeys are famous for great hearing. </p>

<p>Do you know how to call turkeys? That might make up for the distractions your lens and clacking mirror will cause. </p>

[orias]

<p>Whatever focal length you calculate, go a little longer, that way you will be more than likely behind the men with the guns rather than in front of them.........</p>

[rob_hanssen]

<p>Hector, some improved definitions (because mine were indeed wrong, thanks)<br>

<img src="http://wwwrestler.chem.tue.nl/~rob/lensmodel.jpg" alt="model for lens" /></p>

[JDMvW]

<p>I have rarely seen a problem more over-intellectualized than this discussion, and I have spent all of my life in Academia.<br /> Let me suggest,</p>

<ol>

<li>ask a hunter what power scope they need on their weapon, OR</li>

<li>get a cardboard box about 80cm high (taking the figure above) and go into the woods with some lenses and see by trial and error what you will need to get a sufficiently large image when you are as close as you are likely to get (ask a hunter again, if you know one).</li>

<li>I find that most people, when they call turkeys, just call them "Tom".</li>

</ol>

<p>These are my favorite two bits of glass for Wild Turkey and wild turkeys.</p><div></div>

[gordonjb]

<p>Just let your friends shoot the turkey first. That way you can use any lens you want.</p>

[jennifer_spencer]

<p>Wild turkeys are some of the most difficult game to find or to view, let alone shoot and take home. My former father-in-law was a good hunter - he grew up hunting his own food - and he had a tremendous respect for the wiles of this bird. He said if a hunter even sees a wild turkey when he was out looking for it, that's impressive. He had stories about the ones that got away, but the best ones were always about the wild turkeys he could find but never, ever catch.</p>

<p>I think that's why this poster is asking so many questions - he has to make sure he'll get the shot the first time. He might not get a second shot.</p>

[JDMvW]

<p>In southern Illinois, there must be a lot of wild turkeys around these days since I commonly see them when I drive around in the Shawnee National Forest. They are hunted at times, but they do not seem especially "nervous" or shy here. Of course, I only see them when I am not packing a camera, so maybe they are pretty wiley, at that.</p>

[bobatkins]

<p>Just take your longest lens. It's that simple.</p>

<p> </p><div></div>

[jpo3136b]

<p>Turkeys are smart animals. 300mm will do. It will also help if you have a, get this, wider angle lens than 300mm. Turkey move so erratically that it's unlikely you will catch them in the perfect pose. I foresee cropping in your future. </p>

<p>I went to a park in middle Tennessee one time; this park was in the middle of a large city. At ten 'til five p.m., the turkey came right on out into the grassy median of the parking lot to gawk and goggle around. Park closes at five. They knew all the people were leaving. It was safe to peck around. </p>

<p>When turkey walk, they look like idiots. I'm convinced that's deception as a survival trait. Good luck with the turkey. J. </p>

[rob_hanssen]

<p>Everybody, thanks for all the input!</p>

<blockquote>

<p>I have rarely seen a problem more over-intellectualized than this discussion, and I have spent all of my life in Academia.</p>

</blockquote>

<p>You haven't seen over-intellectualization until you hang with the engineers :-). Compared to many other discussions I've had, this one didn't even involve writing custom software...</p>

[andrew_gale]

<p>You are thinking way too hard about this one :-)</p>

<p>Use your longest lens that you are happy to carry around (not a 600) 300mm should be fine. 400 if you have one would also be good. Dont forget to take a wider lens too so you can get the hunters.</p>

<p>If your really not sure if it will work, put a turkey sized object on the ground and take some photos from 30 meters or whatever and you can tell if its a good focal length.</p>

[william_livingston]

<p>correct formula for this application:<br>

6.02 x 10 to the 23rd x sq root pi/ mc squared x 1/2 as squared<br>

yeah-thats it</p>

 

[wpahnelas]

<p>it's easier if you find a small one... and even they aren't easy to keep up with once they start moving. i saw one for the first time this year during my walk yesterday afternoon. it'll be interesting to see whether i have any better luck photographing her this year... if you don't see them before they see you, forget about it.</p><div></div>

[JDMvW]

<p>Rob, I hadn't thought about engineeers.... You have a point. ;)</p>

[rob_hanssen]

<p>To get back to Rainer T's remark:</p>

<blockquote>

<p>As a rule of thumb ...<br />D / fl =(roughly)= S1 / S2<br />where D = distance to subject, fl=focal length, S1=size of subject, S2=size on sensor.</p>

</blockquote>

<p>a little reshuffling of my derived formula<br>

f = d*M/(M+1)^2 combined with M>>1 gives you d/f ~ M, so basically we're using the same equation.</p>

[Ed_Ingold]

<p>Turkeys are nearly invisible during hunting season. You might use a shotgun if you have a blind at the edge of a cornfield, but a .270 rifle (if allowed) is better in the open. Your target at rifle range is only about 25 MOA wide, and no camera lens comes close to a FOV that tight. Before your first camera click spooks the birds, remember that your companions are carrying guns :-)</p>

<p>To make a long story short, hunt turkeys with your camera in off-season, by yourself wearing camo or (better) in a blind or in a public park. If they see you, count yourself lucky to get within 300 yards. A car makes a pretty good blind, in off-season anyway.</p>

[dansutton]

<p>by the way, your last implication in your proof there is actually an if and only if. (biconditional implication)</p>

[dansutton]

<p>one other point: bring a telephoto zoom. Physicists in the 70s proved that mathematically, bumble bees can't fly. the point is to beware the limitations of your mathematics. </p>

[Mary Doo]

<p>Just yesterday morning my friend and I set out early to shoot surfers but ended up spotting two groups of wild turkeys strutting their stuff. We both got many good shots. One of mine came out psychedelic shooting through leaves -- and looking romantic. LOL!</p>

<p>I used the 200-400mm lens; my friend used 500mm and 70-200mm. Here are just two examples from yours truly.</p>

<p>Go out to shoot at good light - Good luck,<br />Mary</p>

<p> </p><div></div>

[Mary Doo]

<p>Turkey Romance...</p><div></div>