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This time I want to cover lens selection.
If you are considering a lens purchase, that's easy. It's going to be the lens with the widest "best aperture" that has the focal range required and fits within your budget.
Or it's going to be the lens that solves a particular problem. My 28-300 is one of those. It covers a large focal range for a FF sensor for a "one lens solution". It's relatively expensive because of that, but it's not a "great" lens; it's best aperture is f/5.6-8. The 50mm 1.4 is also one of those lenses. It's for when you "have to" gather as much light as possible or you have to have the shallowest DOF possible. But it's best aperture is f/5.6 and a razor thin DOF is seldom the goal....
The rest of this is going to be about choosing between the lenses you already own for a particular situation.
First, lets look at perspective.
Perspective is the apparent distance between points/objects within a scene. Perspective is controlled only by working distance.
So starting with a particular FOV/composition and maintaining that composition while only changing the perspective....
Let's say you wanted the scene to look as much as possible like it did to you when you took the picture (not likely in reality, that's usually "boring"). Well, using a crop sensor body the only choice would be a lens of ~35mm used from your current vantage point (working distance). It would be a lens of ~50mm for a FF sensor body.
Lets say you wanted to minimize the distance between points in an image such as to make a nose shorter, or to bring the BG in closer to the "subject". In that case you would use a longer lens from further away.
Lets say you wanted to emphasize the distance between points in an image, such as to lengthen a body, or emphasize the lines of an object. In this case you would use a wider lens from a shorter distance at an angle that causes the lines to move into the scene.
Or you wanted to emphasize the size of something such as the enormity of a tank, or the size of a room. You would use a wider lens from a shorter distance and roughly perpendicular to the subject.
Now, lets look at a situation where I want a large DOF.
Easy, it's going to be the widest lens you have that will frame the composition and set to it's smallest usable aperture (prior to diffraction, usually ~f/11).
The negative is this will minimize the light being gathered resulting in a longer SS. Going wider helps here because a wider lens can be hand held slower. Not enough SS? Time to use the tripod or compromise... Can't use a tripod or the compromises are too much and this is the type of photography you want to do?...NOW it's time to buy new gear.
How about a situation where I want maximum DOF?
Again, easy. It's the same lens with the same aperture setting, but this time focused at hyper focal distance (instead of "on" the subject). Same potential issue with SS as before. Generally speaking, worrying about Hyper focal distance is pointless for lenses much longer than 50mm. With longer lenses you can "focus 1/3 into the scene", that will generally get you about the same results.
***When looking to get large/maximum DOF you may be better off using a wider lens and cropping the image as opposed to using a longer lens which frames the image better initially.
Now for the more difficult part...when you want a narrow DOF.
In the simplest terms, the best lens choice for shallow DOF will be the longest focal length with the widest best aperture that will frame the composition from a given distance. Consider this the "rule of thumb". Done.
If you need less DOF you need to use a wider lens that performs equally as well at a wider aperture from a closer distance (to maintain composition/FOV) or you have to compromise. Can't get there with the lenses you have or the compromises are too much?....NOW it's time to buy a new lens.
As for "Bokeh" (the quality of the out of focus areas)....Get over it. The only time bokeh should be a significant consideration is when the out of focus area of an image has more importance/weight than the subject does (or at least equal).
(you can stop here if you like...it gets a little "windy" from here on.)
Got some time on your hands and you're still interested? Read on.
So, how was that difficult? Lets look at a "common kit" scenario.
For this lets assume you own 3 (somewhat hypothetical) lenses.
- 50mm f/1.4 with a best aperture of f/5.6
- 18-55mm kit lens with a best aperture of f/3.5-5.6
- and a 55-200mm kit lens with a best aperture of f/8
Now, lets say you're going to be taking a portrait and you want a narrow DOF.
Of those lenses my first choice to achieve narrow DOF with a focal range between 18-50mm might be the 18-55 kit lens.
Making this choice has a direct affect on everything else.
It means I'm probably going to have to work closer to my subject than with the 55-200 and possibly even closer than with the 50mm /1.4. It also means the widest aperture available to me is f/3.5.
This choice may surprise some, but it is the lens with the best aperture at ~50mm.
Working closer to the subject will have two effects. It will reduce the DOF and it will change the perspective (as covered earlier).
In simplistic terms, for a given aperture/focal length if you halve the working distance you get half the FOV and one quarter the DOF.
It's not really that simple but it works fairly well when our goal is to use a wide aperture to get narrowest DOF. It can get really complicated because these changes can get you near "hyper focal settings" which has a drastic affect on DOF. But let's leave it at that as the point of these tutorials is to "simplify".
So getting closer reduces DOF. Good, that's the goal and I need to work closer to make up for the DOF gained by using a wider lens. For example the 55-200 @ 100mm (it's usually approximately a 1:1 tradeoff)
And getting closer changes the perspective within an image. That's also good because it helps make an image more unique and interesting.
So lets compare the results of all three lenses capturing the same FOV/composition at their best aperture to get minimum DOF.
50mm f/1.4 @ f/5.6 and 3m- the DOF= .85m
18-55mm @ f/3.5 24mm and 1.5m- the DOF= .68m
18-55mm @ f/3.5 18mm and 1m- the DOF= .4m
55-200mm @ f/8 100mm and 6m- the DOF= 1.2m
And lets compare all of them at ~50mm 3m best aperture.
50mm f/1.4 @ f/5.6 and 3m- the DOF= .85m
18-55 @ 50mm f/5.6 and 3m- the DOF= .85m
55-200 @ 55mm f/8 and 3m- the DOF= 1.2m
Now in the first scenario I can open up the 50mm f/1.4 a stop and get approximately the same DOF as I achieved with the 18-55 @ 24mm, or open up or 2 stops to equal the DOF of the 18-55 @ 1m. But I have given up image quality and I have a more "common" perspective.
In the second scenario the 50mm and 18-55mm are equal on DOF and the 18-55 is maxed out on aperture....If I need less DOF with the 18-55 I can move closer remaining at best aperture and with the 50mm I can open up the aperture giving up image quality.
There is a point at which I will have to switch to the 50mm f/1.4.... when getting closer/wider no longer "works" for the desired composition/perspective or when .4m DOF is just not shallow enough.
The 55-200 was never in the running....
As always with photography, it's a balancing act. There is usually one thing you care most about and you work around that to get the best image possible.
The point of of this tutorial, and the one before, is about how to choose the best settings for your gear in order to get the best possible results. Once you know what the results can be, then you will also know what the tradeoffs are when you can't use those settings.....and then you can make well informed decisions/purchases.
Many never learn what they could get from their equipment and then "upgrade" at large expense and with little actual benefit.
The reality is that cheap Canon 18-55mm kit lens can do as well as the 24-70mm f/2.8 L in 80-90% of situations....and it costs $1300.00 less. My Nikon D7000 can perform just as well as my D3 did in 80% of situations and even better in other situations. The D7000 cost $4,300.00 less.
Gear doesn't make the photographer...
(***note- I am aware of the "inconsistencies" in the examples given. The assumption is all of the "hypothetical" lenses will perform roughly equally when operated at their best aperture. While this is not 100% true it is often "close enough" to not really matter. The point is how to best use your gear/your lenses and not any specific set of lenses)
http://digital-photography-school.com/forum/tutorials/174170-lens-selection-digital-camera-basics.html#post1330381
如果你正在考虑购买一个镜头，说的容易。这将是最广泛的“最佳光圈”，所需的焦距范围，适合在您的预算范围内的镜头。
它的镜头，解决了一个特别的问题。我的28-300是其中之一。它涵盖了一个FF传感器的大焦距范围为“一个镜头的解决方案”。这是由于相对昂贵，但它不是一个“伟大”的镜头，它的最佳光圈是f/5.6-8。这些镜头的50mm 1.4也。这是当你“要”收集尽可能多的光，或你必须有浅水的自由度可能。但它的最佳光圈为f/5.6和一个超薄的自由度是很少的目标... ...
其余这将是你已经拥有的镜头之间选择一个特定的情况。
首先，让我们看的角度。
透视图是明显点/一个场景内的物体之间的距离。透视控制，只能由工作距离。
所以开始与特定的视野/组成，并保持该成分，而只有改变的角度....
比方说，你想现场看尽可能一样，它没有给你当你把图片（不太可能在现实中，这通常是“无聊”）。好了，使用作物传感器本体的唯一选择将是一个镜头从当前的制高点（工作距离）〜35毫米。这将是一个FF传感器体50毫米〜镜头。
比方说，你想减少点之间的距离，如图像，使鼻子变短，或带来更紧密的“主体”的保函。在这种情况下，你会使用一个较长的镜头较远。
比方说你想强调，如图像中的点之间的距离拉长身体，或强调对象的行。在这种情况下，您将使用从一个较短的距离，在更广泛的镜头的角度，使线移动到现场。
或者你想强调的东西坦克的艰巨性，或一个房间的大小等的大小。你可以使用更广泛的镜头，在较短距离大致垂直的主题。
现在，让我们看看情况，我希望有一个大的自由度。
容易，这将是最广泛的镜头，将帧的组成和设置它的最小可用光圈（衍射之前，通常〜f/11镜头）。
是否定的，这会减少光线聚集在一个较长的SS。走向更广泛的帮助，在这里，因为更广泛的镜头可以手持慢。没有足够的SS？使用三脚架或妥协的时间...不能使用三脚架或妥协太多，你想做的事，这是摄影类型？... ...现在的时间购买新设备。
我想最大的自由度的情况下如何？
再次，容易。它具有相同的光圈设置相同的镜头，但这次集中在超焦距（而不是“上”的主题）。像以前那样与SS相同的潜在问题。一般来说，有关超焦距的令人担忧的是毫无意义的更长大于50mm的镜头。你可以使用长焦镜头“聚焦到现场1 / 3”，一般会得到相同的结果约你。
***当大/最大的自由度，您可能会更好使用更广泛的镜头和裁剪图像，而不是使用一个较长的镜头，帧图像更好的最初。
现在比较困难的部分... ...当你想要一个狭窄的自由度。
在最简单的术语，浅景深的最佳镜头的选择将是最长，最宽的最佳光圈焦距，将帧从一个给定的距离的成分。考虑这种“经验法则”。完成。
如果你需要较少的自由度，你需要使用更广泛的镜头，执行同样在一个较大的光圈从距离较近（保持成分/视野），或者你有妥协。无法获得与您已经或镜片的妥协有太多?....现在是时候买一个新的镜头。
至于“背景虚化”（重点领域中的质量)....克服它。只有时间的背景虚化应该是一个重要的考虑因素是，当出重点区域的图像，更重要/重量比主体（或至少等于）
（你可以停在这里，如果你喜欢... ...从这里获取一点点的“刮风”。）
得到了一些时间在你的手中，你仍然感兴趣吗？阅读。
那么，如何是困难的呢？让我们看看在一个“共同的工具包”的情况。
对于这让你自己的（有点假想）镜头。
- 与最佳光圈为f/5.6的50mm f/1.4镜头
- 18 - 55mm镜头套机镜头的最佳光圈F/3.5-5.6
- 和55 - 200mm的套机镜头与最佳光圈的F / 8
现在，让我们说你要采取的肖像，你想有一个狭窄的自由度。
这些镜头，我的第一选择，以实现与焦距范围18 - 50MM之间的狭窄的自由度可能是18-55套机镜头。
这种选择上一切的直接影响。
这意味着我可能将不得不工作接近的55-200比我的题目，甚至可能比50毫米/ 1.4接近。这也意味着提供给我的最大光圈是f/3.5镜头。
这种选择可能会令一些，但它是〜50mm的最佳光圈的镜头。
工作接近主体，将有两方面的影响。它会降低自由度，它会改变的角度（如前面介绍的）。
在简单的条款，对于一个给定的光圈/焦距，如果你的工作距离减少一半，你一半的视野和四分之一的自由度。
这不是这么简单，但它的作品相当不错时，我们的目标是使用较大的光圈以获得最窄的自由度。它可以变得很复杂，因为这些变化可以得到你的“超焦距设置”，其中有一个自由度大幅影响附近。但是让我们离开，它为这些教程的一点是“简化”。
所以越来越密切降低自由度。好，那是我们的目标，我需要工作更紧密的自由度获得通过使用更广泛的镜头。例如55-200 @ 100（它通常约1:1的权衡）
图像内的变化的角度。这也是好事，因为它有助于使图像更加独特而有趣的。
因此，让所有三个镜头捕捉相同的FOV /组成，其最佳光圈得到最低的自由度比较的结果。
@ f/5.6的50mm f/1.4镜头和3M -自由度= 0.85米
的18 - 55mm f/3.5镜头24毫米和1.5M -自由度= 0.68米
的18 - 55mm f/3.5镜头18mm和1M的自由度= .4米
55 - 200毫米，F / 8 100mm和6米的自由度=1.2米
并允许比较〜50mm的3米最佳光圈所有。
@ f/5.6的50mm f/1.4镜头和3M -自由度= 0.85米
18-55 @ 50mm的f/5.6的3M -自由度= 0.85米
55-200 @ 55MM f / 8和3M -自由度=1.2米
在第一种情况下，我现在可以打开了50mm f/1.4镜头停止，并得到大致相同的自由度，因为我与@24毫米18-55取得，或公开或2级18-55 @ 1M等于自由度。不过，我已放弃了图像质量和我有一个“共同”的观点。
50mm和18 - 55mm镜头在第二个方案自由度平等和18-55是最大光圈了....如果我需要与18-55自由度，我可以移动接近余下的最佳光圈和50毫米我可以打开光圈，放弃了图像质量。
在这一点，我会切换到50毫米f/1.4 ....当越来越近/更广泛的不再是“工程”所需的成分/角度或.4米自由度是不足够浅。
55-200是从来没有在运行....
一如既往地与摄影，它的一个平衡。通常有一个最关心的事情你照顾，你周围的工作得到最佳的图像成为可能。
本教程点，前一个是关于如何选择最佳设置为你的装备，为了得到尽可能好的结果。一旦你知道的结果可以，那么你也将知道权衡时，您不能使用这些设置.....然后你就可以做出消息灵通的决定/购买。
许多人从来没有学习他们可以从他们的设备，然后点击“升级”费用大与小的实际利益。
现实情况是，便宜的佳能18 - 55mm镜头套机镜头可以在80-90％的情况，以及24 - 70毫米f/2.8镜头大号....和它的成本$ 1300.00少。我的尼康D7000可以执行的一样好，我的D3在80％的情况下，甚至在其他情况下更好。 D7000的成本$ 4,300.00减。
齿轮不使摄影师...
（***请注意，我知道的“不一致”在给定的例子，假设是“假设”的镜头都将执行大致相等时，其最佳光圈。虽然这不是100％的真实，它往往是“足够接近”不是真正的问题，问题是如何最好地利用你的装备/您的镜片并没有任何特定的镜头） |
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发表于 2011-9-30 08:16:51
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发表于 2011-9-30 08:20:43
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