BetterPhoto Q&A
Category: New Answers

Wide Angle Lenses

As to numbers, I'm confused, because I have the Canon 18-55mm kit lens for the Rebel XT and it's filter size is 58 whereas my new 18-200mm Sigma lens has a filter size of 62. When reading an article, that discussed issues with Circular polarizers and wide angle lenses, it said that issues arose with lenses that were more wide angle than 28mm. So, apparently, one can determine whether a lens is wide angle by the numbers such as 18--55mm and 18--55mm?

Okay, so clearly I dont' know what I'm talking about and am very confused---can anyone staighten me out? Thanks!

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August 30, 2006

Well, a filter size of 62 or 58 I think would mean that it's 62mm threaded or 58mm threaded for that size filter. I don't know if there is a definition for wide-angle, but it means when you are zoomed out. 35mm zoom is the standard, so I would call anything less than that wide-angle.

Now the problems of being wider than 28mm and using a polarizer I guess is caused by the rim of the polarizer showing up in the edges of the pictures?

I hope this helps! Let me know if this isn't clear of if it doesn't answer your question.

Ariel
ScrattyPhotography
ScrattyPhotography Blog

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August 30, 2006

You need to know that I am a long winded old fxxt. So don’t pay me any mind.

Lots of optical principles, in photography, are based on diagonal measure.
Early on, with no enlargement capability, photos were always the actual size of the camera format (e.g. contact print). For this reason, early cameras were big. A camera lenses works because it projects a miniature image of the outside world onto the film’s or chip’s surface. The size of this image is dependent on lens focal length. Short lenses project tiny images while long lenses project big images. Also, all lenses project a circular image however, only the center is unusable. The edges are blurred plus too dim for use (vignette). Thus the film or chip is only allowed to see the central portion. The peripheral image is masked off and baffled to prevent stray light from degrading the exposing image (flair).

Since all lenses vignette, using too short a length on a large format make the vignette noticeable. As a rule of thumb, photo opticians mount a lens with a focal length about equal to the diagonal measure of the cameras format and they call this length normal. Also, a normal lens has a field of view (measured diagonally) of 53 degrees. This angle is widely believed to the normal field of vision of the human eye

Examples of normal (native) focal lengths for various formats:
8x10 inch use 325mm
5x7 inch use 215mm
4x5 inch use 160mm
2 ½ x 3 ½ inches (6cm x 9cm) use 110mm
2 ¼ x 2 ¼ (6cm x 6cm use 75mm
35mm film (24mm x 36mm use 50mm rounded up from the diagonal of 43.3mm
Canon EOS Rebel XT 15.1mm x 22.7 mm use 27.3mm its actual diagonal

Stated another way: If the lens focal length is about equal to the diagonal measure of the film or sensor chip, that’s normal for that media. Lenses significantly shorter are labeled wide angle and lenses a lot longer are labeled telephoto.

Now we have almost 100 years of history for the 35mm film camera and over the years it has become the De facto standard. For this format, 50mm is considered normal (actually it should be 45mm). A lens length of 35mm or below is considered wide angle and 80mm and longer is telephoto. The “nomal” 50mm mounted on your Canon will function as a telephoto. Allow me to explain:
Your Canon EOS has a 27.3mm diagonal. That means the chip is much smaller than a standard 35mm film frame with a diagonal measure of 43.3mm. How much smaller? Divide 43.3 by 27.3 = 1.6. That’s your magic conversion factor (1.6). It helps you figure out how your camera performs with lenses as compared to a 35mm camera. For example; set your zoom to 28mm (about your diagonal measure). That’s a respectable wide angle for a 35mm camera but on your Canon its the normal e.g. 28mm x 1.6 = 45mm which would be about normal if mouned on a 35mm camera. Lets look at a 50mm; 50 x 1.6 = 80. Stated another way; A 50mm normal for a 35mm camera, when mounted on your camera, functions as a telephoto.

That about raps this up until you ask for more. However, the other numbers you are throwing (58 & 62) are the metric thread diameters e.g. the size filters required for the respective lenses you own.

Have fun, don’t let old fxxts like me spoil you good time with boring history and math.

Regards,
Alan Marcus
ammarcus@earthlink.net

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August 31, 2006

Ariel, the reason I thought wide-angle lenses would be wider in diameter, is because any that I've seen always seem to be. Also, it kind of makes sense to simple minds, at least,--the wider the lens is, the more it can take in...

And Alan, all those words were so sweet in intent, that even if I didn't understand any of them (I understood about three), I would still be grateful for the attempt. And they served a purpose---they convinced me that my non-math mind will never understand the concept, and that I should ask a more basic and practical question. And I'll ask two, since no one responded to my question on the other thread, in the hopes that if you two answered one question, maybe you'll answer two!

When looking at a list of lenses for sale, how can I tell, by the various numbers, which are wide-angle and which aren't? I know that if the numbers or at least the second number is big that it's telephoto. I assumed that how small the first number is would indicate how wide-angle it is, but clearly it doesn't because my 18-55 isn't wide-angle and my 18-200 is (it also has a macro function which I like).

Or perhaps it has to "say" wide-angle, like with some macro lenses--you only know they are macro because that's part of the name---like the Sigma 105 macro? But when I look at long lists of lenses, I don't see the words 'wide-angle" as often as I see "macro" and my Sigma 18-200 is not accompanied by the word "wide-angle" on any lists.

Second question, and most important, as I need to purchase some filters for my new lens quickly. I'll make it very basic: I have a choice of buying a normal Hoya circular polarizer for a moderate price for my new 18-200 Sigma, or an ultra thin circular polarizer for about four times that ($135). Is my Sigma 18-200 wide-angle enough to make vignetting an issue or could I just as well go with the cheaper one? And most important, would the more expensive one significantly improve the quality of my pics?

Thanks for any help you can give!

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August 31, 2006

I'm sorry I'm not too familiar with the different kinds of filters. When you say ultra thin, do you mean the band around the filter is thin or the actual glass is thin? If the former, I would expect it to help vignetting to get the thin lens. If the latter, I don't think it makes much of a difference. I have a cheap Opteka circular polarizer filter, and it works okay for me, though I don't go so wide angle. But I think that like with many things in the camera lens accessory word, a more expensive filter would probably produce finer results which might be visible if you look at the picture real up-close.

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August 31, 2006

Sorry my long winded conversation about lenses confused you.

Let me try again:

For your camera, a normal lens would be one with a focal length of 27mm. The Canon 18-55 is a zoom which means it has a variable focal length. When set to 18mm, the lowest value, it functions as a wide angle. As you zoom more and more towards higher magnification, you pass through 27mm. At 27mm, even if unmarked on your lens, the camera is functioning as normal meaning, not wide angle and not telephoto. As you continue zooming, having passed 27mm, you are now in telephoto territory. When you get to 55mm you are zoomed in as close as this lens will go.

Same is true for the 18-200mm Sigma. It starts out the same as the Canon at 18mm. This is the minimum zoom (maximum wide angle position). At this setting both lens are working as a wide angle, and both produce exactly the same view (no difference whatsoever). Now as you zoom the Sigma, again, when you reach 27mm the view is considered normal, not wide and not telephoto. As you continue to zoom the Sigma, and you have lots more zoom to zoom than with the Canon, your view gets closer and closer to the subject. At maximum zoom of 200mm with the Sigma, you can probably see folks smiling in the next county. This is a respectable zoom capability and the lens is definitely working as a telephoto.

Keep in mind that if a lens is set to the same zoom (same focal length), as any other lens, the image size and the angle of view will be the same.

Now for the filter question. It is unlikely that you will see any significant difference between the two filter brands. Normally these differences only show when you are making huge prints. The glass thickness is not a concern. Good filters are flat, that is the most important thing. Both sides must be parallel to each other and the glass free of bubbles or waves or the like. Also the filter must be big enough to prevent getting in the way at all zoom powers. After all the filter adds a little extension to the lens tube. This new length is kind of like a tunnel and it must be wide enough to let the light rays pass without obstruction.

Your right about the fact that wide angle lenses are generally large diameter. This is because they are short tubs that must be positioned very close to the film or digital chip. Most often this closeness gets in the way so the lens maker tries to make them sit more towards the front of the camera, giving more clearance (longer rear focus). This design is much like a telescope except the camera is looking through the tube backwards. To prevent vignette, the front lens must be large.

Please tell me if I helped or confused.

Best of luck,

Alan Marcus
ammarcus@earthlink.net

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August 31, 2006

Some of us love the tech mumbo jumbo, some don't..I'm probably a little of both; and I see you prefer to dispense with the mumbo & the jumbo LOL..so;

A wide angle lens CAN be deemed a wide angle lens by the numbers as long as you know the camera type you are putting it on..35mm, medium format, large format.

In other words, on a 35mm camera with a 80mm lens, there is obviously some magnification going on. On a 2.25"x2.25" square negative film camera, the 80mm is a "normal" lens with no distortion effects..No positive magnification or negative. It pretty much sees the scene as you do.

OK so far? Need a coffee break? LOL

So what is "normal?" A 50mm lens on a 35mm camera is a normal lens..In other words, it pretty much sees a scene as you do, w/o zooming in or backing away.

Our math friends love to talk about FOV (field of view) etc..but knowing that a 50mm is a norm lens on a 35mm is enough for now; right?

Now for the weird stuff: Your Rebel XT is NOT the classical 35mm size. 35mm film, before we had digital measured 35mm x 24mm. The digital sensor in YOUR Rebel is NOT this size..It is SMALLER.
There are what is known as "Full Frame Sensors" which are indeed the size of 35mm x 24mm, but your camera is not one of them. They are usually fairly expensive.

Without getting into all the physics about glass and "how" it redirects light, what you WILL want to know is something called "Crop Factor"

This is nothing more than a conversion used to compare YOUR "focal length" on YOUR camera to the standard of a true 35mm negative (35mm x 24mm)

Here's the math: Let's say you are using a zoom and it indicates you are shooting at 50mm. All you do is multiply 50mm by (1.6) or (1.7) depending on who's camera you use.
Sooo..You're really NOT shooting at 50mm with your digital camera..You are at 80mm if you use the conversion of 1.6
1.6 x 50=80
Again, a lens that will NOT introduce any magnification or reduce the image size is considered normal.

So for you to shoot a "normal" perspective image with YOUR camera, you'd choose a focal length of about 30mm, which when compared to the old standard (35mm x 24mm) comes out to about 48mm, pretty close to 50mm.
30mm X 1.6=48mm Pretty easy huh? ;)

As far as the other sizes you asked about, these are just filter thread sizes.
..and yes, with extreme wide angle lenses, you cam experience vignetting, or seeing the filter ring in the image.

Hope that helps a tad.

For most photogs, they really don't care about conversion factors, FOV etc..They care about getting a great shot.

All the best,

Pete

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August 31, 2006

Another feature of a macro lens is being able to focus on an object that is only inches away ( Some telephoto lenses can only focus as close as several feet away ). The "macro" feature on an so-equipped zoom lens allows the lens to focus closer than it normally could otherwise. And there are other ways to achieve this too, such as the use of close-up filters and such.

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August 31, 2006

On YOUR digital SLR, 18mm and smaller is true wide angle and 18mm to about 20mm is the view offered by most point and shoot cameras that don't have a zoom (a bit wider than normal to give good depth of field with their slow lenses).

28mm is "normal" and you'd still think of it as normal up to about 40mm. After that it moves into the short telephoto range until you get to about 55mm and anything beyond that is "true" telephoto.

None of this has anything to do with the filter mount size. That's just the diameter of the front of the lens. The cheaper filter is fine. The only thing you might consider is to look at Cokin type filters... they come in 3 parts: a ring that screws into the lens filter holder, a filter holder that clips onto the ring, and filters of many types that fit into the holder. The advantages of this system is that you don't have to buy multiple filters: your polariser (and every other filter you buy) fits in the filter holder and snaps on to any lens with an adapter ring, which canm be left on the lens permanently..buy a new lens, get an adapter ring, and all your old filters fit! And rings are a lot cheaper than filters! There are other advantages, too...the filters can be rotated through 360 degrees and positioned higher or lower than the axis of the lens (very useful with graduated and special effects filters).

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August 31, 2006

So, from what I understand, my Canon 18-55 lens is just as "wide-angle" as my Sigma 18-200 meaning that both have the same field of view. And, if I've understood correctly (though I probably haven't!) the reason why my Sigma 18-200 is much wider in diameter is because this is necessary to compensate for the fact that it also has a very large focal distance range---from 18 to 200, rather than just 18--to 55.

And by the same logic, this also means that when set at it 200 focal length, it won't give me any greater field of view than my Sigma 70-300 when it is also set at 200 focal length?

So, what this means is that I haven't really gained anything in buying this lens, except for the convenience of being able to go from 18 focal length to 200 focal length without having to change a lens... (But this one thing is well worth it!)

I will say, though, though I just got the lens and haven't had time to use it, when I borrowed the same lens from someone, it created very beautiful macro and close up images--somehow it does something to the background--throwing it back and creating a lovely blur of colours. That's one of the reasons I bought it.

I've been told that the Canon 18-55 kit lens is "cheap" and even "crappy" for most things, though it doesn't do a bad job with portraits. I must say that it isn't performing as I would like (perhaps do to my ineptitude as a photographer) for the fantastic landscape opportunities I have here in Japan, and I was hoping it would help in that. But not having any wider field of view is dissapointing to me. I want to be able to take in more--give the wonderful panoramic feeling I get when I stand on these lovely mountains and look down over islands shrouded in mist (well, sometimes haze!) and mountain range upon mountain range stretching out into the distance on the shore. I can't stitch at the moment until I get a new computer, my processor is groaning and then just shuts off when I try.

For the future...after I get my computer and Sigma 105 macro, could anyone suggest a lens that would give me startlingly better landscape shots? Let's say for about $600--maybe a little more? Or would I have to save much longer for such a lens? I was looking for a 18-200 Canon L lens, but it seemes they don't exist. I would be prepared to save longer and pay much more for an L lens.

Sorry if this question is branching out....! You've all done such a great job in explaining---I hope I've understood!

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August 31, 2006

Ariel
Nature Photography

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August 31, 2006

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August 31, 2006

You can always get a computer from Dell's site customized to what you think you'll need. HP also makes good ones - laptops and desktops. I personally use an HP Pavilion a1010n.

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September 01, 2006

Adding more lenses in the focal lenth range you already own doesn't seem very useful to me: wider angle leses get pretty pricey, especially if they are zoom lenses, so consider a prime (one focal length, no zoom). These generally have better optical characteristics than zooms, and are faster (simpler to construct, you see).

You Rebel came with Canon's photostitch software; start with tha when you get your computer going. But I'd suggest your regular sortware can do the job better, even if it is a lot more effort. You can get a special adapter for your tripod that keeps the sensor level and parallel with the scene as you move it across the landscape to capture the images you want to capture...a big advantage.

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