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01/06/2006 05:49:45 PM · #1 |
I just got my D70s, upgrading from an Olympus advanced compact. I had a fair amount of film-based SLR experience in my teens so I'm able to apply most of that (what I remember, anyway) to my new camera. But I've got a couple of questions about lenses and filters related to the crop or magnification factor:
1.) As we know, 50mm is an ideal lens for portraiture among other things. Much of that is due to the lens geometry providing a flat field of view close to what the human eye sees. So, with the D70s's 1.5x magnification factor, do I really want to buy a 35mm lens in order to most closely approximate the naturalism of a 50mm? Or is that going to give me some wider-angle distortion despite the "cropping"? Or should I still use a 50mm and just shoot from further back instead?
2.) The thin filters are recommended for wide-angle lenses, typically anything 24mm or wider. How does the crop/magnification relate to that? Again, the lens geometry plays into whether a thicker filter may cause some vignetting, but will the crop factor negate that? For example, my 18-70mm lens has lens geometry wider than a 24mm, but the effective range is 27-105mm with the crop factor. So do I need thin filters or not?
TIA, Mike |
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01/06/2006 06:05:23 PM · #2 |
Originally posted by Creature:
2.) The thin filters are recommended for wide-angle lenses, typically anything 24mm or wider. How does the crop/magnification relate to that? Again, the lens geometry plays into whether a thicker filter may cause some vignetting, but will the crop factor negate that? For example, my 18-70mm lens has lens geometry wider than a 24mm, but the effective range is 27-105mm with the crop factor. So do I need thin filters or not? |
Don't know about the theory, but here's some empirical data. Using a 10-22 mm wide-angle zoom lens on a Canon 20D (mult. factor 1.6), even at it's widest, I get no vignetting with a filter whose mount is about 0.2 inches thick--hardly thin. |
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01/06/2006 06:14:13 PM · #3 |
| the 35mm pretty much will act like a 50mm lens with 1.5x |
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01/06/2006 06:14:29 PM · #4 |
The crop factor is refering to the size of the sensor inside your camera. The rear element on normal lenses are large to cover 35mm film, but on your camera the image projected onto the sensor will be bigger than the sensor itself, so the image will be "cropped" making it look longer than it actually is. This does not affect image results, just the "view" of the image. With that being said, I would say you would want to go with something more along the lines of an 35mm for your portraits on your 20d.
Message edited by author 2006-01-06 18:14:53. |
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01/06/2006 06:23:25 PM · #5 |
| when you place the 35mm lens on your camera you will see what you woulkd see with a 50mm on a 35mmfilm camera, but the lens is still a 35mm so you don't have any of the aspects that the 50mm would have so depth, distortion are all coming with that 35mm. I say use the 50mm and just get us to shooting a little different. |
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01/06/2006 06:30:19 PM · #6 |
1. That "wide angle distortion" is a myth. The apparent distortion is a function of angular coverage and geometry. Simply put, if you used a film camera and a 50mm lens to shoot a portrait, then replaced the 50mm with a 35mm and shot another exposure, and cropped the 35mm to the same area as the 50mm view, the results would be "geometrically identical" to each other, grain issues aside. That's exactly what's happening with the crop factor. "Distortion" of the sort you're referring to is a function of the relationship of the subject(s)/background to each other and the camera to the subject/background.
For another example, the much ballyhooed "compression" you get with a long telephoto is not an optical phenomenon, it has nothing to do with the lens. Prove it by taking a shot down the middle of a city street or soemthign with the camera on a tripod; do one with your widest lens and one with your longets lens. The crop the wide shot to match the framing of the long shot, blow it up to the same size, and they will be identical as far as apparent "compression" goes.
So-called "wide angler distortion" happens because you move in closer to your subject to get the same relative subject size in the uncropped image. It doesn't exist in the center of the image, it exists around the edges, and it's lens-independent; it's a matter of geometry, the way the light falls on the sensor. To see what I mean, just place a marble on a sheet of white paper and move a flashlight around it; watch the shadow distort. The lower you take the light, the more elongated/distorted the shadow will become. And there's no lens here. When you use a lens with extremely wide angular coverage, the light from around the periphery of the image is reaching the sensor (or film) at a much more oblique angle, and hence shapes are distorted. This is ignoring issues like barrel distortion, incidentally, which ARE optical phenomena associated with certain lens designs. The Canon 10-22mm, for example, is very pure; it has virtually NO optical distortion at the wide end even though it has roughy a 100-degree angle of coverage.
2. Clipping/vignetting with a filter is a function of the percentage of the angular coverage of the lens that you actually use. If you are using a given ultra-wide lens on a full-frame camera, you're using more of the angular coverage the lens provides, and you will vignette with a too-thick filter, but the same lens on a 1.6 sensor, with the same filter, will not vignette because you are cropping out the center of the image. So, no, you probably don't need thin filters on the 18-70 with a cropped-sensor camera.
Robt.
Message edited by author 2006-01-06 18:38:19. |
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01/06/2006 06:42:59 PM · #7 |
Standard Hoya UV HMC on my 18-70, no vignetting.
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01/07/2006 04:56:41 PM · #8 |
| Thanks for the replies -- good info! |
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