{"id":276,"date":"2006-11-08T22:54:38","date_gmt":"2006-11-08T13:54:38","guid":{"rendered":"https:\/\/regex.info\/blog\/2006-11-08\/276"},"modified":"2006-11-08T22:54:38","modified_gmt":"2006-11-08T13:54:38","slug":"focus-charts-and-retesting-my-nikon-70-200-f28-vr","status":"publish","type":"post","link":"https:\/\/regex.info\/blog\/2006-11-08\/276","title":{"rendered":"Focus Charts and Retesting my Nikon 70-200 f\/2.8 VR"},"content":{"rendered":"\n\n
\n\n
UPDATE:<\/b><\/td>A much<\/span> improved\nversion of the chart used in this post is available as Jeffrey's Autofocus\nTest Chart<\/a>.<\/td><\/tr><\/table>\n\n<\/div>\n\n

\n\nIn my previous post, I worried<\/span> that there were focus problems<\/a> with my new\nNikon 70<\/span>-200mm f\/2.8 lens. I'd based<\/span> this on a few<\/span> test pictures taken\nwhile visiting the park with Anthony, and concluded with the realization\nthat I need<\/span> to do more rigorous tests to discount the possibility that my\nown errors were causing the problems.\n\n<\/p>

\n\nI later posted a short summary of this to a forum<\/span> at Phil Askey's the most\nexcellent Digital Photography\nReview<\/a> (see\nthread here<\/a>), and got some comments reiterating that it might be my\nfault. Modern SLR camera auto-focus works on finding contrast, and while I\nthink<\/span> that the subjects in my test shots stand out with high contrast from\ntheir background, some of them also had highish-contrast aspects within the\nbackground, so it's possible that the autofocus latched on to them instead\nof my intended target.\n\n<\/p>

\n\nI've never run into this problem with my 18-200mm f\/3.5-5.6, either because\nit doesn't have focus problems, or even if it were to have one, because its\ndepth-of-field is much deeper. With the zoom at 200m, a subject<\/span> 5 meters\n(16 feet) away has a depth<\/span> of field of about 5.3 inches at the lens's\nfastest aperture of f\/5.6, more than twice the depth as the new lens at its\nfastest aperture of f\/2.8 (a depth<\/span> of field of only about 2.6 inches).\nTesting the new lens wide open (at f\/2.8) places the most demand on all\naspects of focus (camera, lens, and me).<\/p>\n\n

<\/div>\n\n

So, today I did more tests.<\/p>\n\n

Focus Test Charts<\/b><\/big><\/p>\n

\n\nThere are many focus test charts available — I've used this<\/a> and this<\/a> previously.\nI've learned a lot<\/span> with them, but felt I could<\/span> do better, so I finally<\/span> made\nmy own. A very<\/i> shrunk version is shown at right.\n\n<\/p>

\n\nThe black bar in the center has very high contrast from the background,\nsurrounded on all sides with very low contrast. The auto-focus can't latch\non to the low-contrast surroundings, thereby guaranteeing that if the\ncamera focus spot is pointed anywhere near the center bar, it will focus on\nit and only it.\n\n<\/p>

\n\nThe chart is tipped back away from the camera, so that everything below the\nblack focus bar is closer to the camera than the focus bar, and\neverything above the focus bar is further away.\n\n<\/p>

\n\nA properly working camera and lens (and operator ) focusing on the black\nbar should result in the black bar being in focus, as well as everything to\nits left and right. That includes the far edges of the test chart, which\nhave various repeating high-detail patterns and text. Things above and\nbelow the main focus line, because they are different distances from the\nlens, should get progressively out of focus.\n\n<\/p>

\n\nThere are numerous little features about my chart that make it more useful\nthan the others I've seen, but I'll save that discussion for a later<\/span>\nwriteup. One thing<\/span> I'll mention now is that the focus bar is surrounded not\nby blank white, but by text — low-contrast text that is invisible to\nthe auto-focus, but not in the final picture. It helps<\/span> a lot.<\/span>\n\n<\/p>

\n\nHere's a close-up of the focus bar and the low-contrast text that surrounds\nit (which doesn't look so low-contrast in this image, but does so when\nprinted, at least on my printer):\n\n<\/p>\n

<\/div>\n\n

When printed at 600 dpi, the black bar is 1.8mm tall (a bit<\/span> over 1\/16th of an inch).<\/p>\n\n

Here's a close up of the detail seen to the left of the focus bar:<\/p>\n\n

<\/div>\n\n

That repeats up and down both sides of the chart.<\/p>\n\n

Retesting my Nikon 70<\/span>-200mm f\/2.8 VR<\/big><\/b><\/p>\n\n

With the new chart in hand, I set it<\/span> up in the sun by a window.<\/span> Here's what one shot looked like:<\/p>\n\n

<\/a>\nNikon D200 + Nikkor 70<\/span>-200\/2.8 VR   —   200mm f\/2.8, 1\/5,000th<\/sup><\/sub> sec, ISO 100Focus point is about 3.4 meters (11 feet) away\n\nAcceptable focus extends about 0.6 inches in front and behind the focus point\n<\/span><\/div>\n\n

Here's a full-resolution crop with exaggerated contrast to highlight the\nlow-contrast area, showing that the focus bar is in front of the in-focus\narea. This matches exactly the results I saw<\/span> before<\/a>.<\/p>\n\n

<\/div><\/div>\n\n

Here's a test at the other end of the zoom range: 70mm:<\/p>\n\n

<\/a>\nNikon D200 + Nikkor 70<\/span>-200\/2.8 VR   —   70mm f\/2.8, 1\/4,000th<\/sup><\/sub> sec, ISO 100Focus point is about 1.8 meters (5.9 feet) away.\nMinimum AF for this lens is 4.9 feet.\n\nAcceptable focus extends from about 1.3 inches in front of, and 1.4 inches behind the focus point\n<\/span><\/div>\n\n

Here's the full-resolution crop.<\/p>\n\n

<\/div><\/div>\n\n

Compared to the test above, more of the chart is in focus, and the focus\nchanges more gradually. This is because the chart is not at such a steep<\/span>\nangle to the lens, and also because at this distance and focal length, the\ndepth of field is more than double. So, it's harder to pinpoint just where\nthe center of focus is, but it seems to me that its at about the 7<\/b>\nto 9<\/b> range (black numbers on the left; you can scroll the image),\nand not the 0<\/b> that it should be:<\/p>\n\n

Conclusions<\/big><\/b><\/p>\n\n

It's still a work in progress, but I'm now more confident of my\ntechnique and of the results: I have<\/span> a bit<\/span> of back focus with my camera\nand\/or lens.<\/p>\n\n\n\n\n\n\n","protected":false},"excerpt":{"rendered":"

In my previous post, I worried that there were focus problems with my new Nikon 70-200mm f\/2.8 lens. I'd based this on a few test pictures taken while visiting the park with Anthony, and concluded with the realization that I need to do more rigorous tests to discount the possibility that my own errors were causing the problems. <\/p>

I later posted a short summary of this to a forum at Phil Askey's the most excellent Digital Photography Review (see thread here), and got some comments reiterating that it might be my fault. Modern SLR camera auto-focus works on [...]","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[],"_links":{"self":[{"href":"https:\/\/regex.info\/blog\/wp-json\/wp\/v2\/posts\/276"}],"collection":[{"href":"https:\/\/regex.info\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/regex.info\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/regex.info\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/regex.info\/blog\/wp-json\/wp\/v2\/comments?post=276"}],"version-history":[{"count":0,"href":"https:\/\/regex.info\/blog\/wp-json\/wp\/v2\/posts\/276\/revisions"}],"wp:attachment":[{"href":"https:\/\/regex.info\/blog\/wp-json\/wp\/v2\/media?parent=276"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/regex.info\/blog\/wp-json\/wp\/v2\/categories?post=276"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/regex.info\/blog\/wp-json\/wp\/v2\/tags?post=276"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}