2009年6月4日

Reverse Mounting Your Prime Lenses fo...

Reverse Mounting Your Prime Lenses for Affordable Macro Photography

Since purchasing my 50mm prime lens a few years back, I’ve felt like a salesman for it ever since–always telling people how sharp, compact and versatile the little bugger is. Well I have one more reason to recommend it. If you mount this lens in reverse on your camera, i.e. flip it around backwards, you get an affordable and fun way to experiment with macro photography. At a little over $9, I found an off-brand 52mm reverse adapter perfect for my 50mm 1.8 and 1.4 lenses. I took it out for a spin at the National Botanical Gardens. Here are my thoughts and experiences with this fun lens accessory.

Right off the bat, let me say that if you want to get serious about macro (close-up) photography this isn’t the best solution. There are good reasons that lenses are engineered specifically with that type of photography in mind. The downside is of coarse, those specialized lenses come with a price. At only $9 I highly recommend this as a starting point or way of experimenting with macro photography. A surprising secondary benefit I didn’t expect when first using this adapter were the insights it gives you into the relationships of depth of field and aperture. So lets begin.

illustration 1.jpg

illustration 2.jpg

Protecting the Lens

If you haven’t used one before, it will seem strange mounting your lens in reverse. Once mounted, you see some of the guts of your lens that are normally not exposed. All of the pins the lens uses to talk with the camera are visible as well as the internal glass element that moves in and out when focusing.

Unfortunately, leaving the lens on in reverse can potentially let dust inside your lens. I highly recommend you use the lens protector cap that comes with your lens to protect it while not shooting. If you plan on using the lens in reverse often, you can buy a bayonet-to-standard filter thread adapter. This lets you screw on a UV type or other filter to keep nature’s elements away from your lens.

You Lose the Camera’s Brain so Use Your Own

If you haven’t already deduced it, seeing that the pins are now facing out, the camera has no way of communicating with the lens electronically. What does this mean to you? You are going to lose all the cozy automatic features of your camera. No auto-focus since there is no talking going back and forth. In fact, with the lens mounted backwards, your primary means of focusing is going to be moving the camera, which will be just inches from your subject, back and forth until the area you want sharp is in focus. No auto aperture either. I’m guessing that many of you have always adjusted the aperture of your lenses electronically. Say good bye to that and say hello to that manual aperture ring you may have never touched before.

Old manual focus lenses work well here regardless of make. Lenses without an aperture ring such as Nikon G-series and Canon EF lenses may not work. Remember that normally the camera keeps the aperture open so you can look through the viewfinder and see what you are taking a picture of. Just before you take a picture, it stops it down to the appropriate size. Now the camera now has no way to open the aperture for your viewfinder viewing pleasure. This leads us to the next point.

Depth and Field and Lighting

I found it amazing to see the depth of field effects in real time and watch them change as you stopped down the aperture. So when using the reverse adapter, as you look through the eye piece and start stopping down the lens, you will see things get really dark really fast. Unless it’s a very sunny day, twist it all the way to f/16 and you’re staring into a black square. (Many digital cameras have a depth-of-field preview button that forces the aperture to the size designated giving you a similar effect.) Set at 1.4, I could look at a flower and see the razor thin focal plan move up and down the flower as I made tiny movements with the camera.

I lugged in my tripod and a few lighting stands but at the last second decided to shoot everything handheld. Shooting this way can be very difficult for a couple of reasons. First, at large apertures with a nice bright image in the viewfinder, you get a very narrow DOF. The smallest shift in your camera position changes what is in focus. It’s extremely difficult to hold the camera steady enough to get consistent shots.

On the flip side of this, some of my favorite images ended up being the more abstract one with things unintentionally out of focus. If you want a sharper image without that aforementioned razor thin focal plane, you can step down the aperture, but again, you’re faced with guessing what is in focus because you can’t see what your shooting in the dark viewfinder. A tripod would be one solution. You could open the aperture, set your focus, then stop it down to take the shot. I did this a few times without the tripod and it worked out fine.

The three pictures below show the dramatic changes that come with changing the aperture on close up images. These are all the same flower, with changes made to the f-stop. On the first image, only the tiny buds are in focus. On the second image, I moved slightly losing the focus of the buds making them essentially disappear. The last image was stopped down to f-16 to get both the buds and the pink pedals in focus.

Reverse-Mount-1a.jpg

Reverse-Mount-1b.jpg

Reverse-Mount-1c.jpg

This image is of an orchid. You can see how shallow the dof is on the first image and how different it looks in the second which was shot around f/16. All small aperture shots required me to use my flash to compensate for the loss in light. I used it in remote mode and held it in my other hand to get different lighting angles. Leaving the flash on the camera is a bad idea as it usually over shoots your tiny subject and you can’t control the light direction.

Reverse-Mount-2a.jpg

Reverse-Mount-2b.jpg

Finally here are a few other images my brother took with this same adapter in his backyard.

Reverse-Mount-3.jpg

Reverse-Mount-4

Reverse-Mount-5

Reverse-Mount-6

If you want to see the entire collection from botanical gardens, they are on blog.chaselliott.com. Like I said, I found myself liking the soft focus abstract ones and plan on mounting a few of them to frames. If you pick up the adapter, let us see some of your own work.

A few side notes

*This setup would work best in a studio environment. You could use multiple flashes and a tripod or even focusing rails to get pinpoint accurate focusing.

*With this reverse mount technique, you are no longer limited to having a single lens on your camera. A technique called “lens stacking” is popular in some circles where a lens like the 50mm is reverse mounted on the end of a zoom lens. Search around for example shots and explanations.

*You aren’t limited to reverse mounting prime lenses. In fact, you can build your own reverse mount extention tube that will work with any lens. Here are some detailed instructions.

*Reverse mounting a lens is not the only way to get cheap magnifications. If you use the Kenko extension tubes and an AF lens you can maintain metering, autofocus (both AF-s and screwdriver AF), and VR (vibration reduction). A three piece extension tube set will give you up to 6 different magnification ratios plus fine tuning with the lens helix. The reversing adapter will give just one magnification ratio.

*If you already own a nice zoom lens, look into close-up diopter lenses that screw onto the filter threads. The best ones are dual-element made by a few different manufacturers, and cost around $80 each. Some zoom lenses will even work well reversed if you reverse-mount them with the close-up lens attached. In this case, zooming simply changes magnification which is very handy for trimming the composition.

Post from: Digital Photography School - Photography Tips.

Reverse Mounting Your Prime Lenses for Affordable Macro Photography

2009年6月1日

揭秘:一张“哈勃”照片的诞生

揭秘:一张“哈勃”照片的诞生 http://www.sciencenet.cn/blog/user_content.aspx?id=234706

Shea 发表于 2009-05-26 9:04

Jeff Hester 文 Shea 编译

  自1990年发射上天以来,哈勃空间望远镜向世人奉献了无数壮观的宇宙画面。这些照片不仅被刊登在了报纸、杂志以及几乎每本天文学教科书上,它们还出现在了唱片封面、广告、T恤、咖啡杯以及电影场景中。这些照片也激起了公众对天文学的空前兴趣,由此产生的对世俗文化的冲击是近代其他任何一架天文仪器所无法比拟的。

  那么,“哈勃”所拍摄的这些天体真的是如照片上一般色彩缤纷的吗?如果我们飞到这些天体周围,能看到和“哈勃”所拍到的同样景象吗?如果不是,那么是不是如少数人所说的那样,“哈勃”的照片已经被过度地“渲染”了呢?“哈勃”所看到的“真实”宇宙到底是什么样子?

  请跟随我们一起来揭开“哈勃”照片诞生的秘密……

不仅是一张漂亮的照片


图1

  1995年4月1日哈勃空间望远镜上的大视场和行星照相机2(WFPC2)拍摄了鹰状星云的照片。就像普通的数码相机一样,WFPC2也使用电荷耦合器件(CCD)而不是胶卷来记录影像。CCD是一个由光敏器件组成的阵列,其中最小的单元被称为“像素”。而它的作用则是把接收到的光信号转化成电信号。如下面会看到的,在得到最终绚丽图像的过程中最艰巨的工作就是从相机本身产生的干扰信号中分离出那些有用的信号,并且将这些信号转化成对天空中某一点的位置和亮度测量。

  WFPC2的视场大约包含了1600×1600个像素,这使得它大致相当于一台250万像素的数码相机。而且WFPC2所拍摄的图像也不是真彩色的,不过它所能看到的景象比起彩色胶卷来更接近于肉眼。

  在没有开始处理图像之前,让我们先来看一下最终处理完的图像(图1),并且来解释一下照片的右上角为什么会少了一块?

大小不一


图2

  WFPC2事实上是由4架相机组成的——3架大视场照相机(WF)和1架行星照相机(PC1)。如图2所示,除了PC1之外,其余每架相机所拍摄的图像都占据了照片的四分之一。而PC1所拍摄的是局域的放大影像,这使得天文学家可以在右上角看到局部更微小的细节。但是如果最终的图像也采用这样一个不同的比例的话,就会显得非常奇怪了。因此必须先按比例把PC1所拍摄的图像缩小到和其他3架相机相同的程度(见图1)。这就是“哈勃”WFPC2所拍摄的照片总会缺个角的原因。

  现在让我们备份一下图像,并且开始下面的操作。警告:“哈勃”的原始图像和最终的比起来简直“惨不忍睹”。

原始处理


图3

  是的,我已经说过了,“哈勃”拍的原始照片是“惨不忍睹”的。为什么?首先,很显然宇宙线和其他噪音会出现在拍摄的照片中。这个我们下面会想办法把它们去掉。其次,我们现在看到仅仅是来自PC1的图像,一会儿还要处理3架WF相机的图像。再次,我们现在看到是每架相机能拍摄的4种颜色中的一种。就像电视机的色彩是由红、绿、蓝组成的一样,最终的照片也是综合不同颜色滤光片的照片而成的。

宇宙“涂鸦”

图4(左)、图5(右),点击照片可见放大图像

  WFPC2的4架照相机每架都会使用它们的4块滤光片拍摄两张照片,以此来消除照片中的宇宙线。图4和图5中的“雪花”就是宇宙线和CCD相撞的时候所产生的,而宇宙线是以接近光速运动的原子核。每次宇宙线撞上CCD的时候就会留下一道痕迹,干扰我们想要拍摄的影像。幸运的是,宇宙线是随机的,因此它们在两张不同的照片上会留下不同的痕迹。所以只要比较两张不同的照片就可以去除这些恼人的宇宙“涂鸦”。

和宇宙线说再见


图6

  图6就是清除完宇宙线之后的照片。那么具体的是怎么做的呢?首先在两张照片中找出那些只在其中某一张照片中才出现的宇宙线痕迹并且删除它们。然后合并这两张已经没有“雪花”的照片,以此来提高图像信噪比。当然与此同时还有一些诸如“暗流”、“平场”以及“电荷转移效率”等和仪器本身性质有关的改正需要做。不过目前还不需要去关心这其中的细节。尽管我本人也是制造这个相机的团队的成员,但也花了好几年的时间才搞清楚它的特性。

  虽然已经做了这么多处理工作,不过还是会有一些宇宙线和瑕疵需要进一步的处理。

漏网之鱼


图7

  寻找残留的宇宙线和其他瑕疵的一个办法是比较使用4个不同滤光片所拍摄的4幅图像。另一个方式则是把图像和恒星图像的轮廓进行比较。如果某个亮点的轮廓和恒星的不一样,那么就有可能是需要清除的漏网宇宙线或者是其他瑕疵。图7显示的就是用白色标记出的残留亮点,而之所以用红色背景是为了让它们更容易显现出来。

  接下去的任务就是删除这些亮点。

彻底清理


图8

  图8是彻底清理完之后的PC1照片。不过请记住,PC1会使用4个不同的滤光片拍摄4张照片。每张滤光片只能让来自观测目标——鹰状星云——的特定波长的光线通过。

  不同的原子会在特定的波长上发出特有的辐射,因而具有特殊的颜色。这使得天文学家可以只拍摄某种原子所发出光线的照片。对于鹰状星云的照片,WFPC2使用的滤光片使得它可以拍摄来自氢原子、硫离子以及电离氧所发出光线的照片,而第4片滤光片则只能让恒星所发出的光通过。

  我们刚才处理的图像正是PC1使用电离氧滤光片所拍摄的。下一步则是处理另外3张由WF相机使用电离氧滤光片所拍摄的照片。

第2张


图9

  这是处理完的由WF2相机使用电离氧滤光片所拍摄的照片,是最终照片左上角的那一块。请注意,这张以及下面两张WF拍摄的照片已经经过了方向的调整,以便适应我们的视觉习惯。

第3张


图10

  这是处理完的由WF3相机使用电离氧滤光片所拍摄的照片,是最终照片左下角的那一块。

第4张


图11

  这是处理完的由WF4相机使用电离氧滤光片所拍摄的照片,是最终照片右下角的那一块。

  现在就可以把这4张照片拼接到一起了。

拼接照片


图12

  类似于透过放大镜看东西,每张照片都有不同程度的扭曲变形。为了把这4张由不同相机拍摄的照片精确地拼接成一幅图像,就必须要测量并且修正这些扭曲和变形。一旦完成这一步骤,4幅照片就可以完美地合成出一张鹰状星云在电离氧滤光片下的照片了。

  图12是已经修正了像差之后的照片,现在就差最后一步了。

严丝合缝


图13

  好了,现在让我们把图像之间的缝隙去掉。这里需要提醒的是,这张照片拍摄的仅仅是由电离氧所发出的光线。

  对氢原子和硫离子所拍摄的照片也要进行类似地处理。它们之间会有细微的不同。比较不同原子所发出的光线使得我们不仅能得到一张漂亮的照片,还能告诉我们许多其中有用的物理细节。让我们在这里再多花1分钟时间。

照片比较

图14(左)、图15(右),点击照片可见放大图像

  这里是两张鹰状星云的照片。图14拍摄的是氢原子所发出的光线,图15拍摄的是硫离子所发出的辐射。可以看出两者明显的不同。WFPC2上氢和硫滤光片只能分别让波长为6563埃和6725埃的光线通过。而相应的电离氧滤光片的波长则为5007埃。

  现在我们已经有3幅图像,每幅都代表着不同原子所发出的辐射。那么为了体现出这些照片的不同,最好的办法就是不同的照片使用不同的颜色,这样也可以显现出鹰状星云不同区域物理性质的不同。

上色

图16(左)、图17(中)、图18(右),点击照片可见放大图像

  我们分别用蓝色、绿色和红色来表征氧离子(图16)、氢原子(图17)和硫离子(图18)所发出的光。比较不同的照片,你会发现不同的颜色增强了照片中细微结构的差异。接下去就是把这3张处理过的照片合并起来,大功告成!

鹰状星云


图19

  这就是1995年美国宇航局发布的鹰状星云最终处理完的照片。那么这张照片告诉了我们些什么呢?从大体上讲,我们看到的是鹰状星云中一部分浓密的氢分子气体和尘埃云。照片上下的跨度大约是4光年。

  照片中手指状的气体柱中正在形成年轻的恒星。这些恒星胚胎会从周围的星云中汲取物质不断生长,但是(照片上方不可见的)新生恒星所发出的紫外光会“蒸发”星云中的气体,并且造就了这些柱状的结构。

  如果你能回到45亿年前,看看太阳和太阳系的形成过程,那么你也许会看到和今天的鹰状星云非常相似的景象。虽然为了了解这其中气体的物理性质以及恒星形成的过程还有很多工作要做,但是通过“哈勃”拍摄的这张照片至少你立马就有了一个大致的印象。

2009年5月31日

Learning about Exposure - The Exposur...

Learning about Exposure - The Exposure Triangle

200607192012Bryan Peterson has written a book titled Understanding Exposure which is a highly recommended read if you’re wanting to venture out of the Auto mode on your digital camera and experiment with it’s manual settings.

In it Bryan illustrates the three main elements that need to be considered when playing around with exposure by calling them ‘the exposure triangle’.

Each of the three aspects of the triangle relate to light and how it enters and interacts with the camera.

The three elements are:

  1. written a post on ISO - the measure of a digital camera sensor’s sensitivity to light
  2. Aperture - the size of the opening in the lens when a picture is taken
  3. Shutter Speed - the amount of time that the shutter is open

It is at the intersection of these three elements that an image’s exposure is worked out.

Most importantly - a change in one of the elements will impact the others. This means that you can never really isolate just one of the elements alone but always need to have the others in the back of your mind.

3 Metaphors for understanding the digital photography exposure triangle:

Many people describe the relationship between ISO, Aperture and Shutter Speed using different metaphors to help us get our heads around it. Let me share three. A quick word of warning first though - like most metaphors - these are far from perfect and are just for illustrative purposes:

The Window

Imagine your camera is like a window with shutters that open and close.

Aperture is the size of the window. If it’s bigger more light gets through and the room is brighter.

Shutter Speed is the amount of time that the shutters of the window are open. The longer you leave them open the more that comes in.

Now imagine that you’re inside the room and are wearing sunglasses (hopefully this isn’t too much of a stretch). Your eyes become desensitized to the light that comes in (it’s like a low ISO).

There are a number of ways of increasing the amount of light in the room (or at least how much it seems that there is. You could increase the time that the shutters are open (decrease shutter speed), you could increase the size of the window (increase aperture) or you could take off your sunglasses (make the ISO larger).

Ok - it’s not the perfect illustration - but you get the idea.

Sunbaking

Another way that a friend recently shared with me is to think about digital camera exposure as being like getting a sun tan.

Now getting a suntan is something I always wanted growing up - but unfortunately being very fair skinned it was something that I never really achieved. All I did was get burnt when I went out into the sun. In a sense your skin type is like an ISO rating. Some people are more sensitive to the sun than others.

Shutter speed in this metaphor is like the length of time you spend out in the sun. The longer you spend in the sun the increased chances of you getting a tan (of course spending too long in the sun can mean being over exposed).

Aperture is like sunscreen which you apply to your skin. Sunscreen blocks the sun at different rates depending upon it’s strength. Apply a high strength sunscreen and you decrease the amount of sunlight that gets through - and as a result even a person with highly sensitive skin can spend more time in the sun (ie decrease the Aperture and you can slow down shutter speed and/or decrease ISO).

As I’ve said - neither metaphor is perfect but both illustrate the interconnectedness of shutter speed, aperture and ISO on your digital camera.

Update: A third metaphor that I’ve heard used is the Garden Hose (the width of the hose is aperture, the length that the hose is left on is shutter speed and the pressure of the water (the speed it gets through) is ISO.

Bringing It All Together

Mastering the art of exposure is something that takes a lot of practice. In many ways it’s a juggling act and even the most experienced photographers experiment and tweak their settings as they go. Keep in mind that changing each element not only impacts the exposure of the image but each one also has an impact upon other aspects of it (ie changing aperture changes depth of field, changing ISO changes the graininess of a shot and changing shutter speed impacts how motion is captured).

The great thing about digital cameras is that they are the ideal testing bed for learning about exposure. You can take as many shots as you like at no cost and they not only allow you to shoot in Auto mode and Manual mode - but also generally have semi-automatic modes like aperture priority and shutter priority modes which allow you to make decisions about one or two elements of the triangle and let the camera handle the other elements.

A lot more can be said about each of the three elements in the exposure triangle. Check out other relevant posts on the topic at:

  1. ISO
  2. Aperture
  3. Shutter Speed

Post from: Digital Photography School - Photography Tips.

Macro Photography

Macro Photography

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Photo by Daniel Schwen

Macro photography is photography up close, so close you can view all the details in the picture you are viewing. This type of photography is good for wildlife and very small items. The classical definition is that the imageprojected on the "film plane" (i.e., film or a digital sensor) is close to the same size as the subject. On 35 mm film the lens is typically optimized to focus sharply on a small area approaching the size of the film frame. Using the right camera you’re able to get up close look at anything that is you would like to look at.

To achieve Marco photography a digital sensor is close to the same as the object on a

35 MM lens is typically optimized to focus sharply on a small area approaching the size of the film frame. Many 35 MM and micro for insects, flower’s lenses are 1:1, meaning that the image on the film and is the same size as the object being photographed. Lenses that are designed for macro are usually at their sharpest. Recently the term Marco has been used in marketing material to mean being able to focus on a subject close enough so that when a regular 6×4 inch (15×10 move cm) print is made, the image is life sized or larger.

Unlike regular cameras with Marco photography You’re able to get such detail Like the pictures and examples. Making sure you have the right camera and lenses very important. The camera mainly used in Marco photography is Canon MP-E 65 mm.(below)

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Equipment

50–60mm range typically used for product photography and small objects

90–105mm range the standard focal range used s, small objects

150–200mm range gives more working distance - typically used for insects and other small animals like wildlife.

A few zooms provide a macro option, but they generally do not allow a 1:1 magnification. A long barrel for close focusing, called a macro lens. Adjustable bellows, with no optical components; between the camera body and the lens. The lens is from the film or sensor, the closer the focusing distance, the greater the magnification.

Two-element versions being excellent while many inexpensive single element lenses exhibit chromatic aberration and reduced sharpness of the resulting image. This method works with cameras that have fixed lenses, and is commonly used with bridge cameras.

Auxiliary close up lens is an inexpensive screw on or slip on attachments provide to close to focusing it is inexpensive the quality is variable with the different versions and elements. This is very interesting because you really can get into what it is that you want to see. There is all is a picture inside of the picture and the key is to find and capture it. With the right equipment you can get very close with magnification and enhance the full photo itself. focusing in and getting such a close view is great.

Related posts:

  1. Did you say Macro? Here’s a lil’ something about them macro lenses…
  2. Macro Photography 101
  3. Macro Photography 101, Part 2

8 Brilliant Illustrations of the ‘Rul...

8 Brilliant Illustrations of the ‘Rule of Thirds’

I was going through some photos on Flickr, looking for some pictures to illustrate a post that I was writing, and I came across these brilliant photographs that put the ‘Rule of Thirds’ smack in your face. Just looking at them you can make out that they could not have looked better if centered. Here you are with some eye candy then ;)

Why don’t you guys share some of your shots with everyone? We would love to see :)

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