Wide angles, flashes and the Himalayas with D7000

[adam_nash2]

<p>Hi Andrew,</p>

<p>Thanks for taking the time to write such a detailed post. I have read about this kind of thing in a previous post and after a few reads I am starting to understand the mechanics behind it a bit better. No doubt due to your clear explanation.</p>

<p>I am aware that an f-stop is a ratio but are you saying it is a direct ratio between the aperture of a lens and its focal length? If that is the case then an aperture of f5.6 would vary in size between lenses. Or have I misunderstood you and f5.6 is always a fixed dimension no matter the focal length of the lens? I actually cant quite believe I am asking this as I type this. It seems like it such basic knowledge but clearly something I still don't quite know.</p>

<p>I also presume using the long end of the 70-300 on a mountain some way off with stars in the background would produce the same flattened perspective as discussed earlier and possibly make the stars look even closer, enveloping. </p>

<p>I will keep you posted on how the Benro does. I was tempted to get the Gitzo model but prices were really beginning to creep up and I had to keep myself in check. </p>

<p>Thanks,<br>

Adam</p>

[Andrew Garrard]

<p>Hi Adam. I'm glad I'm helping. :-)<br />

<br />

Yes, an aperture of f/5.6 is a different size for different focal lengths - "f" stands for focal length, so f/5.6 is "focal length divided by 5.6". This is the <i>effective</i> aperture, so the actual size of the bits of glass tends to vary.<br />

<br />

For a long lens, the front element is typically roughly the size of the aperture. In order for the lens to focus at infinity and still have this speed, the front element has to be at least this big - the cone of light leaving a distant object and entering the lens is effectively a parallel-sided cylinder with the aperture's diameter. (There are some macro lenses which don't focus to infinity that have a smaller front element than their aperture, because the cone of light is still expanding inside the lens.) The front element of a 200mm f/2 is about 200mm/2 = 10cm across.<br />

<br />

This is not always true, especially for wide lenses (a 14-24 has an effective aperture of 14/2.8 = 5mm at the wide end but the front element is huge; even more so for, say, the 6mm f/2.8). For these, not all the light which hits the lens gets as far as the sensor - for example, if you look at some <a href="http://weblog.froglab.it/cose-tanto-belle-e-costose-quanto-inutili/">images of a 6mm f/2.8</a> from different angles, you can see that the view of the sensor through the lens "follows you around". Typically, the farther you get from a normal lens, the bigger the front element has to be - at least when the wider lens is retrofocal. And the size of the front element is a good indicator of price.<br />

<br />

Telephoto zoom lenses are usually limited by the aperture of the long end of the zoom. The 70-300 f/4.5-5.6 has a 300/5.6=54mm(ish) aperture, and the front element is slightly larger than this, at 67mm. That's actually surprisingly large: the Canon 70-300 f/4-5.6 IS takes 58mm filters. A 70mm f/4.5 prime could have a 16mm front element, which is comparatively tiny in spite of the larger aperture (and a 70-300 will show more stars at the long end of its zoom than at the short end for this reason). For smaller lenses, the minimum filter size tends to be inconveniently small so there's a lot of spacing (like the 50mm f/1.8 AF-D and AF-S); when the filter gets big on a longer lens, like on a 120-300mm f/2.8 Sigma, the filter is barely bigger than the aperture. In fact, 105mm is <i>smaller</i> than 300/2.8, but most focal lengths and apertures are a bit approximate.<br />

<br />

So the effective entrance aperture (the size of the "light gathering hole" as seen from the front of the lens) does change with focal length, although because you're looking at that "hole" through some optics the actual size of the glass of the lens might be a bit different, especially with wide angles. Note that if you look at a 200mm f/2 lens the size of the entrance aperture as it appears to you would be the same as the size of the entrance aperture of a 100mm f/2 lens that was twice as close; this is why if you keep the size of the subject in the frame the same size, depth of field mostly depends on aperture and is almost unaffected by focal length. (It <i>is</i> slightly affected, but only because the amount of magnification in front of and behind the lens varies.)<br />

<br />

Don't worry, while it's "basic" knowledge, it's something that you don't need to worry about 99% of the time, and there's no reason you should want to know if you're not as inclined as I am to poke into the mathematics. But the other 1% of the time is the reason that I encourage people to say f-over-something rather than f-something.<br />

<br />

Perspective is a function of relative distance rather than focal length. If you're taking a photograph of a mountain from one mile away at 100mm or three miles away at 300mm you'll get the same amount of mountain in the shot (or you would, if it were a cardboard cut-out of a mountain all at exactly the same distance from you), but you'll get more of the sky in the 100mm shot. Similarly if you want to make the moon look bigger than the foreground in a shot, you need a <a href="http://petapixel.com/2013/04/27/silhouettes-in-a-giant-moonrise-captured-using-a-1200mm-lens/">really long lens</a> and to shoot from a long distance away. You do need to make sure there's something interesting in the sky in the background, though!<br />

<br />

I hope all that helps, or failing that you can print it out for reading on your flight. :-)<br />

<br />

Good luck with the Benro (and the rest of the trip!) Here endeth the lesson. :-)</p>

[adam_nash2]

<p>I am completely interested in the mechanics behind it. And slowly but surely it is all sinking in. Again a need a few reads to make the knowledge innate but I'm getting.</p>

<p>I wont print these posts off but they are now safe and sound on my Kindle and iPad for future reference. Say whilst in a freezing cold tea-house somewhere in the mountains of Nepal (current bird flu crisis permitting).</p>

<p>Once again thank you for taking the time, your posts have been most insightful.</p>

<p>Much appreciated,<br>

Adam</p>

[Andrew Garrard]

<p>Glad to help Adam (and I hope I've mostly got my information right - please double-check anything that sounds dubious!) At least my thoughts might be going on a cool trip somewhere pretty, even if I don't get to go myself. :-)</p>

[sunray1]

<p>As for bags: the F-Stop Loka would be a great choice if you use porters on longer treks. It sits very solid to your back which is great for a more active style of outdoor activity (I loved it in the Dolomite via ferrata's!). Highly recommended!<br />If you want something slightly bigger, more comfortable, trekking style, the F-Stop Tilopa would be nice too!</p>

[mightypir]

<p>Adam:</p>

<p>If you are going to be on foot most of the time, strongly recommend that you carry as little as possible. I don't know how fit you are, but with the thin air any form of exertion gets pretty intense (though if you are trekking you are probably in good shape). I have been to that part of the world several years ago and I can tell you that it was pretty tough especially with a loaded backpack (not with camera equipment!). Accessing equipment from a backpack while on a trail etc is also a pain. I ended up keeping my FM2 and 28mm around my neck and only used my other lenses when I was a rest house etc. </p>

<p>I would also be wary of keeping expensive equipment not on my person. </p>

<p>Best of luck,</p>

<p>Farooq</p>

[adam_nash2]

<p>Thanks Ray, They are my top choices</p>

<p>Farooq, Yes I'm in pretty good shape so I'm not that worried about the physical exertion but two days in I may be eating my words. I will be doing some training before hand too up a few mountains in Oman as a trial run. I hadn't considered the safety of my gear actually but probably should. That said it will be on my back at all times. We will have two porters. One will go off ahead with our clothes and sleeping stuff and one will be with us for...... Well I actually don't know what but my girlfriend seems to think it's a good idea. </p>