Timeline for Why would lens be much longer than its focal length?
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| May 11, 2018 at 21:38 | comment | added | Ian Lelsie | The first two paragraphs in this answer are not part of explanation of why the physical lens is longer. Just stick with the remaining paragraphs and you have your answer. In fact @WayneF's comment covers the answer in a shorter package. | |
| May 11, 2018 at 19:21 | comment | added | Jim MacKenzie | @AlanMarcus We veer off topic, but the counterpoint to the sharper-in-the-centre argument is that lenses have resolution in line pairs per mm, and you're recording substantially fewer mm of detail in a crop sensor photo than you are on DX format (or 35mm film). You're getting the best of the lens, but in throwing out a lot of the good-but-not-quite-as-good of the lens, you're losing a lot of resolution. | |
| May 11, 2018 at 17:48 | comment | added | WayneF | In contrast to retro-focus (wide angle) lenses which are longer than their focal length (intentionally moved forward, for example, so that the SLR mirror can be raised behind them), telephoto lens bodies are shorter than their focal length (just a convenience). A long lens (like a telescope) doesn't have to be built shorter, but the word telephoto means "shorter than its focal length", i.e., those that are built that way (most longer camera lenses). | |
| May 11, 2018 at 16:18 | comment | added | Alan Marcus | @ Jim Mac Kenzie -- If you were an insect crawling on the sensor, looking back at the lens from the center (axis) , you would see a circle of light when the shutter opened. If the insect moves to the boundaries, looking back at the lens, an ellipse is seen. The circle has greeter surface area then the ellipse. Thus the center of the image is brighter than at the margins. This is called cosign error. The fact that your lens delivers a satisfactory and uniform image is quite an accompaniment, a tip of the hat to Nikon opticians. | |
| May 11, 2018 at 16:10 | comment | added | Jim MacKenzie | I beg to differ with you. Certainly the centre is sharper, but the corners are more than adequately sharp, especially as you stop down. This lens was designed for full-frame. | |
| May 11, 2018 at 16:04 | comment | added | Alan Marcus | @ Jim MacKenzie -- All lenses project a circular image. The center of this circle is the sharpest and brightest part. The image formed away from this central location dims and blurs towards the edges. Thus only the central portion is photography useful. This is called the circle of good definition. It is difficult to make a super wide-angle that will deliver, without noticeable vignette, an image that covers. The longer the back focus the better the coverage. | |
| May 11, 2018 at 15:09 | comment | added | Jim MacKenzie | Good post, but just to clarify, this lens covers 35mm film and FX full-frame digital. I've owned one since before I had a DSLR. | |
| May 11, 2018 at 14:45 | history | edited | Alan Marcus | CC BY-SA 4.0 |
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| May 11, 2018 at 14:04 | history | answered | Alan Marcus | CC BY-SA 4.0 |