Whats the point of fast lens

[Matt Laur]

<p>The relationships between the distance to the subject, the distance to the background, and the aperture used all interact (in some surprisingly complex ways, sometimes) to create different workable focal plane depths, Benson. The <strong><a href="http://www.dofmaster.com/dofjs.html">DoF calculator</a></strong> mentioned above lets you see how that plays out, instantly, as you change one or more of those variables. You can't really say that the 70-200/2.8 has more or less DoF than the 50/1.8 when each are used wide open ... because they are and can be used in very different ways, and that changes the math.</p>

[Matt Laur]

<p>Ray: I used (as I almost always do), single-point focus with the center focus point in a D300. I usually have the camera in AF-S (single servo mode) rather than AF-C (continuous focus mode) when my subjects aren't running around on me. So in this case, I focused on his eye (the one closest to me), and then slightly recomposed before releasing the shutter.<br /><br />You can tell that his eye, sideburns, and ear are all in the same plane of focus, more or less. The nearest corner of his hat brim is just out of focus on THIS side of the plane, and the far corner of the brim is just out of focus PAST the plane. We're talking in inches, here, but as long as you get the eye(s) in focus, the viewer isn't distracted by that fall-off in focus. Miss the eye, and the image just looks ... <em>wrong</em>. Unless you have an editorial/compositional reason for that, of course.</p>

[john caetano photos]

<p>You guys have all did a awesome job at providing feedback and i am truly appreciated. I need to take all this info and practice. Last night i took the camera and tried shooting at different F #s and also different lengths and had a hard time really seeing the difference but i did. I just need to keep at it and see how the differences will affect it all.</p>

<p>Thanks again for all your time and thoughts</p>

[ned1]

<p>One more point: fast lenses do more accurate autofocus and will autofocus in light where slower lenses will just seek.</p>

[Matt Laur]

<p>John: to make it easier to see, consider practicing along something like a chain link fence or brick wall. Anything that has a repeating pattern. Shoot it at an angle so that it's clear what is and what isn't in focus.<br /><br />For example, I just grabbed these two shots for you, along a wooden fence. I stayed in one place, focused on the same thing, and shot in AP mode, so that as I changed aperture, the camera adjusted shutter speed to keep the exposure more or less the same.<br /><br />This is with a 50mm lens from about the same distance that you'd shoot a 3/4 portrait.</p><div></div>

[Uhooru]

<p>John, I strongly suggest you take a beginning photo class at your local community college. You will then learn some about Depth of Field and how aperture and distance effect it. Your remarks about of fast lens for portraits are simply made out of ignorance. Another example....fast lens-blurred background, not photoshop <img src="http://farm1.static.flickr.com/19/88531751_b30af86bc5.jpg" alt="" /></p>

[Uhooru]

<p>I also wanted to say that everything Matt says is nonsense....no just kidding:) Everything he said is really good. A good book for general understanding is Upton's "Photography" a general textbook that covers a lot of areas including lens length, depth of field, etc. Here's another link that may be useful, it also shows how to calculate DOF for a lens type and length. http://www.cambridgeincolour.com/tutorials/depth-of-field.htm and has a little more explanation of DOF, what it is, what it does and how it works.</p>

<p>Good luck - don't sell those fast lenses yet!</p>

[bensgalguerra]

<blockquote>

<p>The other option would have been to throw on a longer lens, step back further, and keep is body in focus will still keeping the background well out.</p>

</blockquote>

<p>This is what I'm talking about, Matt. But as you said, it is very complex. Dang, if only longer fast lenses we're priced as the 50 1.8.</p>

[ray_l]

<p>Matt, thank you for your response. It was very helpful.</p>

<p> </p>

[john_morris4]

<blockquote>

<p> Dang, if only longer fast lenses we're priced as the 50 1.8.</p>

</blockquote>

<p>The 85 f/1.8 is not too terribly much more expensive. If you can put up with manual focus (and lack of metering with most digital cameras), you can get 105 f/2.5 or f/2.8, and 135 f/2.8 for not too much money, too.</p>

[john_morris4]

<blockquote>

<p> Dang, if only longer fast lenses we're priced as the 50 1.8.</p>

</blockquote>

<p>The 85 f/1.8 is not too terribly much more expensive. If you can put up with manual focus (and lack of metering with most digital cameras), you can get 105 f/2.5 or f/2.8, and 135 f/2.8 for not too much money, too.</p>

[patrick j dempsey]

<p>Benson, the issue there is distance. DOF is a based on a combination of Focal Length, Aperture and Focus Distance. So if you have a 200mm lens set to f/2.8 and you shoot someone's portrait from 20 feet away, you might get a beautifully out of focus background and a perfectly in-focus face. Shooting the same person with a 50mm lens set to f/2.8 and framed the same you will need to be maybe 3 feet away. The 50mm lens will resolve the background with less blur than the 200mm lens because of the focal length. However, the subject will be partially blurry, maybe nose out of focus, because of the much closer distance. So:</p>

<p>Smaller aperture number at the same distance and focal length, means narrower depth of field. </p>

<p>Closer focus distance to the subject at the same aperture and same focal length, means narrower depth of field.</p>

<p>Longer focal length with the same subject distance and same focal length means narrower depth of field.</p>

<p>And of course all of the opposites apply.</p>

[garydem]

<p>john c-please read this reply and the next. there is one point about fast lenses that has nothing to do with shooting in dim light. and that is that lenses begin to reach thier sharpest range about 2 stops closed down from wideopen. the lens may or may not be completely in the best performing range bjt it should be close. with said at wide open or near it a lens lets in a lot of light, BUT that lens is not performing at it best in terms of color and sharpness. lens perform thier best betwenn f5.6 and f11.0, though f4 on some may be very good. however, that range is modified by have to be about the 2 stops from wide open. so a lens that is a f1.4 wideopen with start to perform at or beyond f2.8 but with a lens that has a wideopen of f2.8, the user must shoot at f5.6 or smaller to get past the 2 stop from wide open limit. by the way this does not mean that good lenses shooting at less that 2 stops from the wideopen plus stops make bad images, but they can do much better beyond the 2 stops from wideopen.</p>

<p>it should be noted that because of the crop factor in dslrs, lens focal lengths that once were considered great for portraits now may not be able to be used.</p>

[garydem]

<p >in 35mm terms portraits used lenses from 70-80 to 135mm. the former were the full or 3/4 body shots while the 135mm was for face only. when taking portraits the distances were always in the 10-12 maybe 15ft range in the studio. when one wanted a different type of portrait you simply changed lenses. you did not move the camera to subject distance. if you did and went closer the nose ended up as very pronounced, if you went farther then the face had a very flat one dimensional appearance.</p>

<p >macro lenses are not used for portraits simply because they see too much facial features; no one is going to thank you with every wrinkle or pimple or imperfection shown in all their glory. The image will just not be flattering. if a macro lens is used then you should plan on plenty of pp time to get the bad features back out. it is far simpler to simply use a kit lens size, or a lens similar, that is a 16 to 50 for the 3/4 shots and if desired switch to a 70-200 zoom used at 70 for the face only, which is 105mm, if not tight enough zoom to 90 which is 135mm. but in all this keep the subject distance at the 10-12 to 15ft distance.</p>

<p >for your info- </p>

<p >portraits were done in the studio by pros using, in 35mm terms, about 70mm to 135mm. the distance was fixed you were shooting from 10-12ft. at that distance the 70 gave the 3/4 body shot while the 135 gave a face only. in c sensor the 70mm becomes 47mm while the 135mm becomes 91mm. the distances used were to keep the face and body from distorting from a natural appearance. For digital the f1.4 50mm lenses becomes a very good portrait lens. It can also double a lowlight lens.</p>

<p > </p>

[john caetano photos]

<p>WOW...so much info it's overwhelming.</p>

<p>Anyways I kinda wanted to point out with of course more playing and reading, I do understand about DOF when it comes to Aperture #'s, but I guess what I need to practice more on is how F #s, Distance, and focal lengths (which is kinda like getting closer or farther from the subject) comes into play. I have a slim understanding but can't quite master it on the floor yet when it comes to knowing what will happen before taking the shot.</p>

<p>I guess what I need to do in order to solve my problem if I want to continue to use small F #s is to back up in order to keep multiple faces in sharp focus and then crop in post. If not an option then stop down to get the desired results. (Of course the reason I state to use low apertures is when i need faster shutter speeds and don't feel like bumping up the ISO any higher)</p>

<p>Anyways thanks for all the info guys/gals, going to reread it a few times to get it into my head.</p>