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Old September 11th, 2008, 06:17 PM
Rhys Sage Rhys Sage is offline
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Default Understanding and selecting lenses

Having just read an elderly book on photography while waiting in the hospital for my wife to have her MRI scan and having spent decades buying, selling and using lenses as well as browbeating with measurebators, I thought I'd wade into the fray. It is therefore my intention to write an article on lenses and how to select them.

I'll probably publish it on my website first as part of a general information section I'm going to put together. Maybe even send it to a photo magazine and earn some cash.
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Old September 11th, 2008, 07:07 PM
Asher Kelman Asher Kelman is online now
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There are many magazines but few people actually get paid. However you may already have an in with an editor with funds. If it gives you pleasure, then it's a great idea!

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Our purpose is getting to an impressive photograph. So we encourage browsing and then feedback. Consider a link to your galleries annotated, C&C welcomed. Images posted within OPF are assumed to be for Comment & Critique, unless otherwise designated.
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Old September 11th, 2008, 07:26 PM
Rhys Sage Rhys Sage is offline
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Few people get paid? Good Heavens, every time I have had an article published they've paid me - even if it wasn't a lot. Last time I wrote a letter designed to stir the readership and they published it as an article and paid me. Sometimes I've put my heart and soul into an article and they haven't published and then I've looked at it a year or so later and realised why.

On the other hand, articles could have followed photos in newspapers with newspapers paying ever less then not at all, which is where agents come in very useful.
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Old September 11th, 2008, 08:20 PM
Ken Tanaka Ken Tanaka is offline
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So, pray tell, how do you select a lens?
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Old September 12th, 2008, 12:27 PM
Rhys Sage Rhys Sage is offline
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Default First draft... Comments please...

Lens Selection.

Many people make lens selection much more complicated than it need be. This article is designed to help the reader to select a lens best suited to the reader's needs.

One of the first resources people turn to when looking for lenses is reviews. These might be on a web site or in a book or magazine. The problem here is that this is not the best way to do so for reviewers review lenses based on the easy things. The easy things are the chromatic aberration and line definition. These are generally the two things people see and thus is all that is usually considered when purchasing a lens. This can provoke some ludicrous debates about the measure of one lens against another which usually gets quite irate with one party supporting one lens at the expense of another when the truth of the matter is that most if not all lenses will perform adequately within their design parameters. The debate raging about the quality of lens X versus Y is usually nothing to do with the lenses but rather the photographers' lack of skill with or choice of that particular lens.

On the market there are many different kinds of lens designed for many different purposes. Generally they can be broken down into the following categories. Of course there will be lenses that fall outside these categories and which cross several categories.

1. Ultra-Wide.
2. Wide.
3. Normal.
4. Telephoto.
5. Super-Telephoto.
6. Fish-eye.
7. Soft-Focus.
8. Macro.
9. Ultra-Wide zoom
10. Wide Zoom.
11. Normal Zoom.
12. Super Zoom.
13. Special-Use lenses such as tilt-shift lenses.

Add to that lens lineup the fact that although most SLR lenses will fit on both 35mm and digital SLRs, some are designed solely for digital SLRs and the fact that many digital SLRs at the time of writing were designed with so-called crop factors. A crop factor is when the image sensor is smaller than the 35mm frame the lens was designed to be used with. This gives an effective multiplier. Thus a 28-80 lens on a 35mm body will seem like a 42-120 lens on a 1.5 crop body or like a 56-160 lens on a 2.0 crop body.

Note that not all lenses are zooms. Lenses of a fixed focal length are called prime lenses. They differ from zooms in several important ways.

1. Zoom lenses are handy all-in one designs that do as many things as possible in one package. They're very popular because of this but suffer from the drawback of being either heavy or having a variable maximum aperture.
2. Prime lenses are usually lighter and have wider maximum apertures.
3. Prime lenses don't suffer from optical distortion at either end of the range unlike many zooms as they have only one focal length and are optimized for that length.

The most common complaints about lenses seem to be:
1. Focusing speed.
2. Focusing accuracy.
3. Flare.
4. Chromatic aberration.
5. Distortion.
6. Low-light focusing inaccuracy.

Of these, some are features of specific lenses but aren't usually the main problem. The main problem is the user and their ability to use a specific lens. Focusing speed is generally quite fast with auto focus cameras. For those that used manual focus cameras in the pre-auto focus era, the complaints about focusing are cause for a certain amount of hilarity. Before auto focus the two main methods of focusing were via a rangefinder or via a viewfinder focusing aid. Both methods needed the photographer to turn the focusing collar on the lens while observing whether the focusing aid indicated correct focus. In low light or in other situations when the focusing aid could not be used, the distance scale on the lens was used. The time taken to focus manually could vary from seconds to minutes dependent upon the circumstances and camera used. It is understandable therefore that the half second or so that most lenses take to focus is a considerable advance. Consequently complaints about slow focusing are generally met with derision from old hands. Most electronic focusing is faster than manual focusing.

Focusing accuracy is an area in which auto focus cameras can appear to let the casual photographer down unless the photographer is prepared to invest time in learning how to achieve correct focus using an automatic aid. Each camera body and each camera lens has its own unique features and the photographer has to learn how to get the best out of each and each combination. Sometimes, this is not possible however. For example, one photographer found that one lens would not focus on one body in certain light levels, throwing an error on the camera body. The solution there was to use the lens/body combination in light levels that didn't throw the error. Of course, that was a somewhat unpalatable limitation and hence that photographer did what most people would, sold the offending lens and bought a different model of lens. The same photographer carried out an experiment, placing a camera on a tripod and aiming the lens at a static subject. Each time that lens was focused, focus would fall at a slightly different point. Taking photographs with that lens wide open was therefore not a good option when the subject was very close. The photographer noted that and chose to use f4 rather than f2.8 when the lens was used for very close subjects. Similarly, many people complained about one specific kit lens that came with a camera body. While the lens was most definitely an economy model, it was perfectly possible to take sharp photos if the inaccuracies were taken into account by using a smaller aperture. No lens that doesn't cost several weeks wages is going to be free of foibles. Often focusing accuracy can be affected by the way focus is achieved. Some cameras need two activations of the focusing button to achieve accuracy. Others of the same model may need only one activation.

One of the major areas of focusing inaccuracy is not a fault of the camera or the lens nor is it a particular foible of a particular lens/body combination. Rather, it is the fault of the photographer. If the subject is not in the center of the frame and the photographer uses a common trick which is to lock focus on the subject then move the frame around the subject to recompose then the photographer falls foul of a little known problem. By moving the frame the center of the frame where focus is accurate has moved also. Focus might be a little in front or behind the subject. It's all to do with the angle of the lens. As it moves up or down, the center and the edge do not occupy precisely the same space. Thus the focus point changes slightly and at wide apertures and close distances this will mean the resultant photograph can be out of focus. Even the center point where focus was achieved will be out of focus as it's now not on the point on which it was focused.

Flare is an issue over which which the photographer has total control. It is caused by light striking the front element or filter obliquely and reflecting inside the lens. It can usually be avoided by adjusting the angle of the lens or by using a lens hood or a combination of both. Even the photographer's hat or hand held up to block the light from striking the lens obliquely can stop flare. Sometimes flare cannot be seen through the viewfinder such as when taking a long time exposure of Christmas lights or the like. There, light might reflect off the front element of the lens, onto the back of the filter and back onto the digital sensor/film. The answer there is to remove filters and to use a lens hood. Sometimes flare still happens in which case often reducing exposure time can help however, it is important to remember that if the photographer can see flare in the viewfinder, that flare will be on the photograph.

Chromatic aberration is normally a problem that simply vanishes when a lens is stopped down by a stop or so. It is caused by the properties of the transmission of light through glass. Glass acts as a prism, splitting up the rays of light into their component colors. Hence blue, red and green (the primary colors of light) will all focus in a slightly different place. On many older manual focus lenses this scale was marked. Often there was a red dot that indicated where to focus if one used infra-red film. The technique was to focus for the visible spectrum then to turn that focus point toward the red dot to achieve correct infra-red focus. Using different types of glass in lens construction and combining the different types, manufacturers have achieved almost perfect alignment of the three primary colors of light. Sometimes this near perfection isn't perfect enough and chromatic aberration occurs. This is commonly seen as a blue tinge around tree branches or as a red or white line above the horizon after sunset. Eliminating this is a simple matter of stopping the lens down. This concentrates the light beams a little more, increasing the depth of focus thus achieving that perfect focus.

Distortion is a property of all zoom lenses. There is no zoom lens that does not distort. At the wide end zoom lenses suffer mostly from barrel distortion and at the long end they suffer from pincushion distortion. Often wide angle prime lenses will suffer from distortion also. Many years ago, a group of thinking photographers got together and came up with a simple formula, having seen how zoom lenses distorted. That formula was 2.8 times magnification. If a zoom lens has more than 2.8 times magnification then it will normally suffer from a certain amount of distortion at each end. There is no escape from this and even super zooms with gigantic lens elements will suffer from this distortion. Some lenses such as fish eye lenses have distortion as a built-in feature that is sometimes sought after. Distortion does not mean, however, that a lens is bad. To those that debate lenses based solely upon reviews and line definition, it is the ultimate condemnation of a lens. This view is not shared by real photographers. If straight lines are needed in a photograph then the correct lens must be used. This will usually be a prime and very often a tilt-shift prime. Zoom lenses are not designed for taking photographs of architecture and straight lines. Take a photograph of a brick wall and the curvature of the lens will be evident. This is why critical photography should be undertaken with the correct equipment. A wedding photographer has no use for a tilt-shift architecture lens and an architecture photographer has no use for a fish eye lens. Similarly, zoom lenses will perform poorly for architecture. Each lens has its own specialized types of use. There is no such thing as a one-size-fits-all lens, no matter what the manufacturers (who are in the business of selling lenses, not truth) tell the public. Most sports photographers use prime lenses because they're lighter and faster than zoom lenses. Certainly fast powerful zooms are available but at a weight penalty making them slower to use. Zoom lenses are jacks-of-all-trades which do all things reasonably well but nothing brilliantly.

Low light focusing accuracy can appear to be a problem. There are many ways around this though. In the old days when manual focus was all that photographers had, if the viewfinder or visual focusing aid in the viewfinder was too dark to use, photographers simply used a flashlight to illuminate the subject or used the focusing indication on the lens body while estimating the distance to the subject. In today's world where the expectation is that technology will do everything, it is not reasonable to expect that technology does not have limits. This is one of the limitation areas hence many manufacturers include a focus illuminator on their cameras or if not on the camera, on the flash. Again, this is an area where many photographers just lack the skills needed. It is not a fault with the equipment. It is a lack of skill by the operator.
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Old September 12th, 2008, 12:28 PM
Rhys Sage Rhys Sage is offline
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Selecting the right lens for the job can be fraught for many photographers. Many will fall foul of the false promises of the zoom lens. It is not reasonable to say that zoom lenses are all bad for all things but it is equally not reasonable to say they will do everything. For the press photographer working for a newspaper, the emphasis is on getting an image and not on getting an image that could hang in an art gallery. The photographer who's going to be running around all day is going to be more concerned with weight and versatility than with high quality. The emphasis there is on news and news is only news while it's new. Speed and portability are of the essence and while the press photographer might well have a wide range of lenses, the normal lens used will be something in the range (in 35mm terms) of 28-135. That lens is wide enough for most groups and long enough for portraits. Occasionally the press photographer may use something longer like a 100-300 for sports. The press photographer does not need high quality images as the print quality in most magazines and newspapers is quite low and the images printed are usually quite small. Together with the casual photographer, the press photographer will find a zoom lens to be almost ideal. The casual photographer frequently has only a small lens budget and thus goes for zoom lenses. The photographer on holiday wanting free hands will probably benefit also from a zoom as it means not having to stop to change lenses as well as not having to carry a camera bag.

Zooms can be a false promise - as mentioned before, they exhibit distortion at both ends. They also weigh more than primes. One photographer used solely primes after finding that the zooms owned were used normally in 3 positions - wide, middle and long and very rarely in-between. Sometimes a zoom can weigh more than the two or three primes it replaces. For the photographer with the usual range of 28mm - 80mm and 70mm - 200mm zooms, these could be replaced by 28mm, 50mm, 90mm, 135mm and 200mm primes. Even then the primes are so closely spaced that the 90mm and 135mm could be replaced by a 100mm. Zooms offer solely one advantage - portability.

Economy lenses can be a good buy for those interested solely in tripod-based work as such photographers are unlikely to need to use wide apertures. Even the humble kit lens complained about by so many will perform just as well as an expensive premium lens given that smaller apertures can be used due to the static photography. Many landscape photographers purchase inexpensive lenses on the basis that landscape photographs are all about maximizing depth of field.

Portrait photographers will be interested mainly in prime lenses also - normally with 35mm a lens of the range 50mm - 135mm is used for portraits. Of special interest also will be the soft focus prime lenses as these can add a hint of romance to many photographs as well as eliminating unattractive skin problems. Filters such as soft focus filters can be used to simulate soft-focus lenses and it can be simulated in software also. The effect is never quite as good as when using a real soft focus lens but it is possible to get close to the effect. Henri Cartier Bresson borrowed a ladies nylon stocking which was stretched over the camera lens to create the soft focus effect for one romantic photograph.

While getting the right lens for the job aids to perfection, it's not essential. It is possible to take quite presentable photographs using humble equipment. One of the most famous photographs of the 20th century - that of President Kennedy being assassinated was taken with a Polaroid instant picture camera. One of the most newsworthy pictures of the 1990s in Britain was taken with a disposable camera.

The one rule to remember about photography is there are no rules. All the rules have been broken in the name of art and have been broken very sucesfully. The trick is in remembering that not every rule can be broken to produce a wonderful photograph. The limitations of the equipment are paramount.
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Old September 12th, 2008, 02:13 PM
Doug Kerr Doug Kerr is offline
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Hi, Rhys,

What an excellent treatise. I really enjoy your writing style here.

I plan to review your oeuvre overall as soon as I can. In the meantime, I just noted one passage on which I feel I need to comment now.

You say:
Quote:
A crop factor is when the image sensor is smaller than the 35mm frame the lens was designed to be used with. This gives an effective multiplier. Thus a 28-80 lens on a 35mm body will seem like a 42-120 lens on a 1.5 crop body or like a 56-160 lens on a 2.0 crop body.
It is a misconception that the matter of "effective focal length multiplication" is the result of the use of a lens on a camera with a smaller format size than the camera for which the lens might have been originally designed (e.g., a full-frame 35-mm camera). A related misconception is that the focal length of a lens is stated in a way that is predicated on the use of the lens on a camera with a specific format size.

In fact, the matter of "effective focal length multiplication" is wholly the result of the format size of the camera of interest being smaller than the format size for the genre of camera that we have (for various reasons, within a certain sphere of photography) adopted as our "reference configuration" for relating field of view to focal length.

For example, a Canon EF-S 60 mm f/2.8 macro lens, used on a Canon EOS-40D (with a so-called "1.6x" size format) will give the same field of view on that camera that a 96 mm lens would give on a "full-frame 35-mm" camera - that is, we say that on the EOS-40D that lens has a "full-frame 35-mm equivalent focal length" of 96 mm. (We may of course use a condensed version of that statement!)

This is true even though that lens was certainly not designed to work on a full-frame 35-mm camera, won't work properly on one, and won't even fit on many of them.

I suspect you know all this, and probably just got "sucked into" repeating a common, but incorrect, description of this matter.

By the way, I never use the term "crop" in discussing this matter. It is wholly inappropriate, and misleading. But that's a story for another time.

Again, my congratulations on what looks to be on the way to being an important paper.

Best regards,

Doug
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Old September 12th, 2008, 05:02 PM
Rhys Sage Rhys Sage is offline
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Thanks for the praise, Doug.

I'm trying to keep my article platform independent so that it will age well. I'm currently reading and referring to a 1965 book by Fellenger. It's good - for its day - and I'm trying to eliminate the elementary errors he makes in his writing.

Eventually I plan to publish this.
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Old September 12th, 2008, 06:07 PM
Doug Kerr Doug Kerr is offline
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Hi Rhys,

Well, now that I've got most of today's other rat-killing done, let me expand on my comment on the word "crop" used in various ways in connection with the implications of a sensor that is smaller than some other sensor.

Sometimes, the term is use in the sense that we can think of the smaller sensor as being a "cut down" (or "cropped") version of the larger sensor. That's a pretty benign metaphor - but not one that really well fits the traditional photographic implications of "cropping".

More often, the metaphor is that the smaller sensor, by virtue of its limited dimensions, "crops" some larger image (in particular, in this "sphere", an image 24 x 36 mm in size) to form a smaller image (22.5 x 15 mm in the case of an EOS 40D) - it crops it to 1/1.6 times its original linear dimensions (and thus the camera is said to have a "crop factor" of 1.6).

But what is that image that is cropped ? Does the lens on the 40D generate a 24 x 36 mm image, of which the 22.5 x 15 mm sensor "crops" out a portion 22.5 x 15 mm in size?

No. If the lens is an EF-S type, it generates a circular image, one large enough to embrace a rectangle 22.5 x 15 mm in size (but not big enough to embrace a rectangle of 36 x 24 mm size). If it is an EF type, it generates a circular image, one large enough to embrace a rectangle 36 x 24 mm in size. The sensor crops its 22.5 x 125 mm image out of one of those circular images.

And on a "full-frame 35-mm camera", said to be "non-crop", its 36 x 24 mm sensor crops a 36 x 24 mm portion out of the large circular image generated by the EF lens!

So, in the metaphor, what is the 36 x 24 mm image that is cropped by the sensor of the 40D to 22.5 x 15 mm? Well, the image captured by the sensor of some full-frame 35-mm camera, some place, some time.

It is as if I said that my 36 x 50 inch coffee is cropped out of a 60 x 76 inch coffee table in Vladimir Putin's office.

So I discourage the use of terms and phrases involving the term "crop" and its derivatives in connection with sensor smaller than 36 x 24 mm.

So, what terms can we use (especially if we are anxious to continue the use of the familiar numerical constants, such as "1.6" for the Canon EOS 40D and its siblings)?

Well, note that, although some decry it, the concept of "full-frame 35-mm equivalent focal length" is perfectly valid. Of course, that is not a focal length of the lens (on any camera). The word "equivalent" warns us of that.

And what is the use we mostly make of the constant 1.6 in connection with, say, an EOS 40D? We, about the only thing we do with it is to multiply the focal length of a lens, one we plan to use on a 40D, by 1.6 to get the "full-frame 35-mm equivalent focal length" of the lens when used on a 40D.

My general, policy is, "when all else fails, call it what it is". Thus, perhaps the best name for the factor 1.6 (which characterizes sensors whose size is about 22.5 x 15 mm) is "full-frame 35-mm equivalent focal length factor. But if we are operating in the sphere where the full-frame 35-mm camera is the "reference" (the sphere inhabited by cameras that are parts of families originally mostly using the full-frame 35-mm format), we can safely condense that to "equivalent focal length factor". Don't shrink from it - it is the name that describes the most prominent use of that constant.

Now, while I have my red pencil out, let me talk about "full frame". A format size we often make refernce to is one properly described as "full-frame 35-mm". What does full frame mean there? It distinguishes this format from the "half-frame 35-mm" format (24 x 18 mm), used on some cameras using 35 mm (type 135) film.

Now, what's a good "shorter name" for the term "full-frame 35-mm". Well, very few 35-mm cameras used the "half frame" format, and it is by definition an "alternative" format (the "half" is the clue). So if we speak of just the "35-mm" format, it is reasonable that we mean 36 x 24 mm. So that would be a good short name.

But unfortunately, many have gotten it the habit of using the short form "full frame" for the 36 x 24 mm format. That's greatly ambiguous. For my 8x10 cameras,. I have backs that operate in both the "full frame" and "half frame" formats (and full frame is 8 x 10 inches!). And for my APS film camera, the "full frame" output is 30.2 x 16.7 mm (also called "APS-H").

Another problem with the term "full frame" is that it sounds like the ne plus ultra of digital formats: if we have that, there is nothing bigger we could have, or even want . If we suddenly ended up with a camera with a 40 x 30 mm format, it is larger than "full frame". Wow, isn't that special.

So I discourage the term "full frame" to mean "36 x 24 mm format". Instead, I would suggest:

"36 x 24 mm" (!)
"Full-frame 35-mm"
"35 mm"

I of course use "full frame" to mean "not cropped " (and I don't mean in the sense of "cropped" I decry above). That is, if I post an image that is the entire image captured by my camera, when giving the technical information for the shot, I will say it is "full frame".

Again, the best practice in nomenclature is "call it what it is".

Best regards,

Doug
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Old September 12th, 2008, 06:25 PM
Rhys Sage Rhys Sage is offline
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I suppose I could clarify but given that most people think of an APS-C sensor as being a cropped 35mm sensor, it might tend to confuse the uninitiated if I suddenly start talking about APS-C.
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Old September 12th, 2008, 06:50 PM
Doug Kerr Doug Kerr is offline
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Hi, Rhys,

Quote:
Originally Posted by Rhys Sage View Post
I suppose I could clarify but given that most people think of an APS-C sensor as being a cropped 35mm sensor, it might tend to confuse the uninitiated if I suddenly start talking about APS-C.
Yes, I discourage all that as well. For one thing, neither APS-C nor APS-H formats (the latter actually being "APS full-frame") are really very close to the digital camera format sizes they are used to "designate".

And of course "APS" by itself must imply full-frame APS (but that's also APS-H), and it is a particularly bad moniker for formats in the general range of 22.5 x 15 mm (but ther are still sometimes called just "APS"-size).

So it's much better to reserve "APS" for when we are talking about APS film, or cameras, or formats, or the Albuquerque Public Schools system.

The actual format sizes in the APS system are:

Taken frame size (full frame): 30.2 mm x 16.7 mm

Delivery sizes

APS-H: 30.2 mm x 16.7 mm (full frame)
APS-C: 23.4 mm x 16.7 mm (a crop)
APS-P 30.2 mm x 9.5 mm (a crop)

Again, when all else fails, call it what it is (and don't call it what it isn't).

Carla has a really nice APS camera with an LCD display screen (for post-shot review). Its sensor is "APS size".

Best regards,

Doug
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Old September 12th, 2008, 06:55 PM
Rhys Sage Rhys Sage is offline
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I don't really think I need to talk a lot about the sub 35mm sensors on dSLRs because I expect my article to outlive the smaller sensors. I imagine the four thirds systems will continue for a while but I expect poorer image quality will lead to them being dropped. Similarly I see a great move away from smaller dSLR sensors toward the 35mm sensors with most dSLR users if not actually changing over, at least thinking they'd like to.
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Old September 13th, 2008, 03:34 AM
Michael Fontana Michael Fontana is offline
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Good morning Rhys

thanks for showing your work; may I add a suggestion?

>Chromatic aberration is normally a problem that simply vanishes when a lens is stopped down by a stop or so<

This isn't my experience since using a DSLR, especially since using wides at FF. As there aren't really good software-based solutions for that problem, I had to replace all lenses having noticable CA.

Now my lenspark includes glass from Zeiss (YC, 5), Leica (1), Nikon (2), Canon (3) Schneider (1) and a bit of zuiko's (5) - all on a Canon FF.


>Many landscape photographers purchase inexpensive lenses on the basis that landscape photographs are all about maximizing depth of field.<

hm, not sure, quite many landscapers look for detail in the trees, grass, rocks, etc; this is demanding on the often used wide lenses lots of contrast, microdetail and resolution power. A inexpensive lens would'nt get that.
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Old September 13th, 2008, 09:47 AM
Rhys Sage Rhys Sage is offline
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It's a work in progress. I'm currently up to 3,100 words and aiming for 10,000 for this particular article.

My plan is to do the same on other topics and eventually to come out with a fairly authoritative tome. Even if I have to publish it myself and to sell it as an e-book, it seems a good idea.
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