http://www.nikonchat.com/nikkor-6mm/
Will I have a different focal length wide angle 12-24mm Nikkor 18-55mm and 20mm?
What I know is if a wide angle is "more wide "to the same focal length as a normal lens, say a 18-55mm? Since we already have a 18-55mm and bought a 6mm 12-24mm, I'm just winning, in which case a Sigma 10-20mm a better deal than I 8mm profit.
Believe it or not, you will receive a "different" than 20 mm of each lens. Look at this example: http://www.flickr.com/photos/samfeinstein/2214186090/ Will have to see this in the original size and can only move to the 4 th column, where the four objectives are compared. Just pay attention to the field of view and ignore the depth of field. All these are made in 35 mm each respective lens. This was verified by examining the EXIF data of each image. In addition, the D300 is shown that information in the "data" after each shooting mode, so you can adjust each lens until it was Seet to 35 mm. You will notice a slight difference in each case. One might assume that the top row – the first 35mm lens – is a real focal length of 35 mm. If you check closely, you'll see that the center with each lens can be slightly different, but are close enough for comparison. (I was not thinking exactly the comparison you are asking, or I would have been more careful in this respect.) Pick two points in each frame and measure to see what difference there. You obviously can use the edge of the door of the closet in all four box and select a point of reference that is visible to the left of the 4 images, such as the left edge of the center of the daisy yellow top. I have the following Screen measures: 35 mm: 18-70 200mm: 170mm 18-200: 17-55 185mm: 202mm If you can draw a conclusion from this that are better at math than me. The physical length the lens is the point of measuring the focal length at a different point, but the 18-70 lens is the shortest of the bunch. I can not draw a conclusion, although the 17-55 lens costs almost double the 18-200 and 4 times more than the 18-70, so one would expect that it was "more precise." I do not know, either, since no know if the Exif data is from the CPU contacts in the lens or otherwise estimated. Anyway … They are different. More importantly, perhaps, is the overall quality the lens instead of the slight differences noted in focal lengths. If you're looking at the Nikkor 12-24 lens, finished better than Nikon's first goal the same range. (Was 12mm? I forget.) In any case, the 12-24 lens is one of the best lenses Nikon has ever made. Does the Sigma compare with that? 10mm vs 12mm is only a very slight difference. The difference of 20 mm against a 20 mm is even less sure. Lens quality is the factor to take into account:. ADDITIONAL ~~~~~~ was meticulous about keeping the tripod in exactly the same place. It was on the carpet, so it did not move when I changed lenses. I checked carefully to ensure The EXIF data for each frame of 35 mm focal length to read, because I was trying to be as accurate as possible. Data display on the D300 include the focal length and NO is an estimate. Well, it could have been out as much as 0.5 mm, it does not show decimal fractions, but this does not take into account the differences in my comparison. It is possible that the camera moved slightly in the center of the head of the tripod, because I was concerned only on the distance of the main theme of the lens. My photo of the group discussed this at great length and professionals said they had also observed that there is indeed a difference between the critical evaluation of focal and zoom lenses first. It may have something to do with the distance from the front lens element with the theme, but were not able to draw a conculsion. Do you need a extensive knowledge of physics or optics to do that. I stand by all the statements that my test was performed under laboratory standards, but everything to keep distance focal plane of investigation and ensuring that each picture was taken at a focal length of 35 mm. Be your own judge about the results as you look at my tests in their own monitor. I still say that the quality of the lens would be more of a factor in its decision that any trivial difference in effective focal length. ~~~~~ ~~~~~ MORE injanier and P Bill I mentioned the key to this response. Zoom lenses are notoriously inaccurate information on the focal length of the camera. I would say this is so much that you can not count on being a true 20mm 20mm for any purpose, including one sack. I found that there is a difference between 4 Nikon lenses. I'm sure that there is a difference between a Nikon lens and a Sigma lens. GO FOR THE QUALITY YOU WANT. Even if they were very accurate, it will be difficult to tell the practical difference between 10mm and 12mm.
Nikon News