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Understanding Lenses: Part I, and is the third in a series of lessons about camera lenses. Links to the others are at the bottom of the article.
No lens is perfect. No matter how good the quality, there will be lens aberrations of one kind or another. It’s just that on a good quality lens there are less of them.
One of the benefits of digital photography is that it is easy to correct some types of lens aberration using software. Before digital cameras came along there was little you could do except accept what your lens gave you, and splash out for more expensive gear if you weren’t happy with its performance.
Now, as long as you shoot Raw, you can use software such as Lightroom and the latest versions of Photoshop CS to eliminate or minimise the most obvious lens aberrations*. While this doesn’t mean that your lenses will suddenly take on the qualities of their most expensive counterparts, it does help you get the very best out of the equipment you already have.
*If you shoot Jpeg, some cameras have a facility for correcting vignetting and chromatic aberrations in-camera. You can also get rid of the worse effects of chromatic aberration, vignetting and distortion with Photoshop, but it takes longer. Using Raw makes it much easier.
There are three types of lens aberration that you can correct using software. The photo above, taken with a discontinued Canon kit lens, has them all:
This is a characteristic of wide-angle lenses and is where the lens bends straight lines outwards. You are most likely to see barrel distortion at the widest end of inexpensive zooms and super-zoom lenses. You can see it in the above photo and in the opening photo (a really extreme example).
Pincushion distortion is the opposite of barrel distortion and is a characteristic of telephoto lenses. Straight lines are bent inwards. The effect is usually slight and you would normally only notice it if you are photographing something with straight lines, like a building.
Chromatic aberrations are the coloured fringes along the highlight edges within your image. They are caused by the inability of your lens to focus all wavelengths of light at the same point. You may also see purple fringing caused by charge leakage from your camera’s sensor.
The crop above shows chromatic aberrations along the edges of the trees.
Vignetting is where the edges of the frame are darker than the centre. No lens is exempt from vignetting. It is strongest at the widest aperture of the lens and gradually disappears as you stop down.
There are times when vignetting is a benefit, and you won’t want to correct it. A good example is a portrait – if the edges of the frame are darker it helps direct the viewer’s eye towards the sitter.
But there are times when vignetting is detrimental, and it’s useful to know how to correct for it.
Broadly speaking there are two types of vignetting. One is caused by the optical characteristics of your lens. This is the sort you can correct in post-processing.
The other is caused by stacking filters or using the wrong lens hood. You can’t do anything about this type of vignetting in post-processing except crop.
I use Lightroom for most of my Raw processing so that is the software I will demonstrate in this article. The same functions are available in most Raw processors, including the latest versions of Adobe Camera Raw in Photoshop CS and the free software that comes with your camera. Check the documentation or the help files to see that the capabilities of your software are.
I’m a Canon user and I’m familiar with Canon’s Digital Photo Professional. While it’s not as quick and convenient to use as Lightroom, and doesn’t support discontinued Canon lenses or lenses from other manufacturers, it still does an excellent job. You can learn more about DPP in my ebook Understanding DPP.
The first thing to do is go to the Lens Corrections panel, click on Profile and tick the Enable Profile Corrections box. Then go to Make and select the brand of the lens you used to take the photo from the drop down menu. Lightroom should detect the lens automatically from there, if not you can select it yourself. If there is more than one profile listed, stick with the Adobe profile. Depending on how pronounced the lens aberrations in your photo are, you should see a difference right away:
There are three sliders at the bottom of Lens Corrections panel: Distortion, Chromatic Aberration and Vignetting. The default setting for each is 100, and you may adjust the sliders to vary the strength of the corrections.
For instance, with this particular photo, I felt that Lightroom had overcompensated for the barrel distortion, so I dragged the Distortion slider left until I felt it was right. A grid appears when you do this to help you see whether the lines in the image are straight. I also had to increase the Chromatic Aberration slider to 200 to eliminate the chromatic aberrations. The comparisons below show what a difference it makes:
You may also choose to reduce the vignetting correction, as mentioned previously, for creative reasons.
Now what happens if your lens isn’t included in Lightroom’s profiles? This may happen, for example, if you have a new lens like Canon’s EF 40mm f2.8 pancake lens, a recent purchase of mine. In that case you have to make corrections manually.
Click on Manual in the Lens Correction panel and you’ll see a different set of sliders (above):
These sliders are for correcting barrel and pincushion distortion. You may also rotate the image to straighten the horizon. If you use the Distortion slider, tick the Constrain Crop box to tell Lightroom to trim the image accordingly.
This is for making the edges of the image lighter or darker. The Midpoint slider determines what area of the image is altered by the Amount slider: move it right to lighten or darken the very edges of the photo, or left to include the central areas.
Removing chromatic aberration is a little more involved. Set Defringe to All Edges then use the Red/Cyan and Blue/Yellow sliders to try and eliminate chromatic aberrations. You’ll need to zoom into the image to check the results. You should scan the entire image, because sometimes the slider settings required to eliminate colour fringing in one part of the image introduces it in another.
Sometimes it is a matter of compromise, and you have to settle for minimising chromatic aberrations rather than getting rid of them completely. You may also get better results by setting Defringe to Highlight edges, depending on the content of your photo.
My previous article sparked some debate in the comments regarding the effect of subject distance on perspective. Here are three photos taken from the same position with a 17-40mm lens (on a full-frame camera) set at different focal lengths. You can see that using the longest focal length has the effect of the bringing the island in the distance closer, and the widest focal length pushes it away. This is the effect that the focal length of the lens has on perspective.
These are the previous articles in the series. My next article will be about flare; exploring the causes, how to prevent flare and also how to use it creatively.
If you liked this article then take a look at my latest eBook, Understanding Lenses: Part I – A guide to Canon wide-angle and kit lenses.
September 21, 2012 06:11 pm
These articles are written for a general audience of photographers, most of whom are amateurs trying to better themselves as hobbyist photographers. For you and other self-impressed Ansel Adams Mini-Me types, perhaps other sites would be more suitable.
September 10, 2012 12:19 am
Interesting analysis. Ultimately, the photographer is much more important than the equipment.... but I still like to have good glass!
September 5, 2012 05:56 pm
“Why Lens Quality Doesn’t Matter Quite as Much as You Think it Does”
Exactly the kind of statement one would expect from a site that is writing guides on how to take "creative control of your iphone" ...
On the subjects of focal length and perspective... Get your facts together :-(
The more articles I read on this site, the more my BS meter goes into the red zone.
September 4, 2012 03:29 am
The problem with PERSPECTIVE arises because of the numerous definitions, this is one of the best in my opinion:
The technique of representing three-dimensional objects on a two-dimensional surface so as to give the right impression of their height, width, depth, and position in relation to each other.
Then, if you change lenses and the height, width, depth and position in relation to each other of the elements is the same in both pictures, you did not change perspective.
I took a picture of some friends whith the 10mms end of my Canon 10-22mm and found the feet enlarged in comparison with a picture with my Canon 28-135mms with the indicator on the 50mm mark, so, according whith the previous definition, in this case, I did get different perspective.
September 2, 2012 11:21 pm
Erm - closing date to order your ebook? There is no "Friday September 31st". There is never a Sept 31. Ever. (Don't worry - I'm just being pedantic:-)
September 2, 2012 05:06 pm
Certainly, if you are using your camera to make a living, I would agree with you that you should buy the best quality lens you can afford. Not only is the lens going to be sharper but it is going to be built more ruggedly and will take much more abuse and be less prone to fail. However, for the rest of us that have jobs outside of photography and view this more of an avocation rather than a vocation, a strong case can be made for supplementing an inexpensive lens with post processing software. I understand that your work demands efficiency and that you can't spend all of your time behind a monitor. I don't know whether you have tried DxO Optics or not. If you have not I suggest that you may want to go to their website and observe first-hand what this software is capable of doing. And to relieve your mind, virtually all of the corrections are performed automatically. It can do this because they have measured aberrations of camera/lens combinations from all of the popular manufacturers. Once your camera/lens combination has been automatically identified through your EXIF file, the proper software correction module is selected and the aberrations are automatically corrected right before your eyes. This makes a kit lens very good and an expensive lens great. I hope that you realize that all I would like to do is to make your life a little easier. I think that proper selection of post processing software could do just that. I wish you well.
September 2, 2012 10:24 am
"Why Lens Quality Doesn’t Matter Quite as Much as You Think it Does"
Uh, yeah, it does if you plan on making any kind of money on your photography. A high quality lens will out perform a low quality lens every time, regardless on which post processing software you use. If I had a choice, I'd take a lower end camera with a high end lens over a high end camera with a low quality lens. Another reason lens quality matters is that there is less post processing with a high quality lens. I can't be efficient if I have to spend all day behind a monitor correcting a poor quality lens. Time is money for me.
September 1, 2012 04:20 pm
"So DPS is now saying that it doesn’t matter how poorly you capture an image, as long as you can fix it in post."
I'm not trying to be argumentative, but I didn't get the feeling that either the article or any reader's comments suggested that one should not strive to capture the best image possible in the camera. My opinion is that any of the post processing software discussed in the article or in the threads should be viewed as additional tools available to the photographer to enhance what has been captured by the camera. Even Ansel Adams recognized the need for post processing in the darkroom. To quote," The negative is the score, the print is the performance." I realize the I am talking film versus digital, but in the final analysis, the medium makes no difference, it's the final image that counts. And the route that one takes to get to the final image is inconsequential.
September 1, 2012 02:49 pm
So DPS is now saying that it doesn't matter how poorly you capture an image, as long as you can fix it in post. Whatever happened to capturing the highest quality possible in-camera?
August 31, 2012 05:34 pm
Totally agree with what Mathew has said. Focal length has nothing to do with perspective. It is the POSITION of the lens in relation to the subject that causes perspective distortion as we know it.
August 31, 2012 03:16 pm
Interesting and instructive article. But it would be so much better if the 'before' and 'after' images could somehow be presented side by side so that comparisons are immediate . Scrolling up and down one loses the mentally held first image by the time the next one is up .
August 31, 2012 10:34 am
Since this article, to a great extent, discusses lens aberrations, I would have thought that the author would have delved into the advantages of using DxO Optics. The originators of DxO Optics have become the standard in defining and measuring camera/lens combinations. They have taken their findings and are applying reverse algorithms to correct aberrations. When applied to a "kit" lens, the results can approach that of an expensive piece of glass. When applied to a top-of-the-line lens, the resulting image can be stunning. My personal opinion is that this piece of software, which costs about $100 US, can go a long way to making kit lenses perform up to the standards we desire and at a cost far less than purchasing high end glass.
August 30, 2012 10:43 am
Great article. I am new to Lightroom. Thank you for sharing on how to use lens correction in Lightroom.
August 30, 2012 10:41 am
Lens matters a lot. For resolution I can definitely tell when a cheaper (or sometimes older) lens has been used and when it's not down to a focus screw up. I have a 50mm f/1.4 that isn't all that great, it vaguely gives a soft touch blurriness to everything rather than really sharp detail no matter how close I am to the subject. Autofocus or manual, aperture, shutter speed... none of the settings matter. That's just what the lens does and, even though I've learned to work around that, I'm sorry I bought it used because I thought it would be ok because it was from a reputable store. I feel like I got ripped off a little. My other two lenses give me the kind of sharp detail I want. If sharpness matters to you in your style of photos then lens quality matters immensely.
August 30, 2012 10:32 am
I think everything matters. We are artists not just photographers. I click a button and use that image to extra-create in an image enhancing program.
August 30, 2012 10:13 am
Most of the correction methods mentioned in the article employ interpolation, remove information at some part of the photo, and introduce calculated at others.
A good lens will bring in more real/correct information. Software can not create new data that is not already in the photo, it can only transform or loose them.
August 30, 2012 08:37 am
I understand the style used in the title of this post but it really messes with the quality of the article. A good quality lens will give you excellent resolution both center and at corners, nice bokeh, contrast, micro-contrast and an easier workflow through your digital work.
This post should be titled to be used as a guide to know your lens and get the best of it and the cheap title used to lure readers is not worthy. Sure, you will probably get more readers but at the expense of some of them ending up feeling like they are not capable of know what a good lens provides or feeling the article uses a misleading title and that reflects badly on the reputation of the website.
August 30, 2012 06:47 am
This post is conflating perspective and focal length. As was discussed in the comments of the previous post that you linked, if you keep the same distance to your subject and change focal length then the perspective (relative position, distortion, movement of subjects within the frame) does not change. If you stand in the same spot and put on a longer lens you just get more magnification (i.e. you crop content out of the frame). For example if you take your example 17mm shot and crop it in photoshop to frame the same contents as your 35mm shot you will see that the two images look identical. The perspective is unchanged, it's just that the 17mm shot has more stuff in it.
I confirmed this myself using your two images as best I could with the small web samples. Since you have the originals it would be a nice post to demonstrate this fact later if you are interested by showing those crops.
And to be clear, if you switch to a different focal length and move closer to your subject to keep the same framing THEN your perspective changes, but it's because you moved the camera, not because you changed lenses.
August 30, 2012 04:15 am
Great article and I tend to agree that under most circumstances, if you shoot RAW, one can correct for many lens abberations. However, there is nothing like having a great, well constructed, responsive lens in your arsenal if you can afford it.
One cool thing about post correction is that you can change focal lengths, like what I did with this shot of Marilyn Monroe in Chicago...cool, eh?
August 30, 2012 02:55 am
There is one statement that I cannot agree with at the end of the article. Let me quote:
"This is the effect that the focal length of the lens has on perspective."
In short: focal length, and I cannot state that more clear, has nothing to do with perspective. Focal length is responsible for field of view, and perspective depends only, and only, on the position of a camera. The three sample photos were taken from the same place, hence the perspective is the same, its just that field of view gets more narrow. As a proof, you can see that objects in front (wall) and in the background (island) are in the same relationship to each other in every picture, its just that every next one is basicaly a cropped version of a previous one. Exchanging perspective is a common mistake between amateurs, and I am deeply surprised that it had appeared here. The mistake is taken from the fact, that when widening the lens, you generally get closer to the subject to fill the frame, so the perspective changes. Still, they are two different variables of a photograph. It's partially also a reason when people, mostly amateurs, tend to just stand in one place with their 30++X zoom compact camera, and don't even think about moving themselves.
August 30, 2012 02:19 am
I agree with both the article and Edmond. Lens is not relevant at all to take good pictures, but it could be very important for good image quality pictures, i.e., charpness, contrast, low light performance. Thanks for posting.
August 30, 2012 01:53 am
too bad quality lens will keep better detail and sharper than cheaper lens, which, cannot be recover in lightroom
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