Suggestion on use of USB dock accessory for Sigma Art 35/1.4

[Gup]

<p>Well, I've become so used to sharpening in post and assuming I'm to blame if a shot isn't as sharp as desired that I haven't bothered to adjust any of my lenses to date. I'm home alone for a couple of days so I'm going to do some experimenting. I did find some Youtube videos on the subject but nothing too enlightening. It seems there are a few different techniques, one solely utilizing the viewfinder focus dot to achieve calibration... and it looks like a sound technique to me but probably because I'm very familiar with the dot after all these years and various bodies.<br>

Maybe I will find a huge improvement in my old 2.8 zooms. That would be rewarding. I have the 17-35mm and 28-70mm that I loved on previous bodies but hardly use anymore. What focal length would I choose to adjust them at? Or do I take test shots at various points in their range to find out where they are weakest and then try to find a happy medium adjustment? I probably just answered my own question.</p>

[Ian Rance]

<p>This is just a personal feeling, but I found when I started meddling with focus fine tune I started to lose interest in actual photography. Like the phase I went through about sensor dirt, I was shooting more defocussed grey skies at f/22 than was healthy. Same with lens cleanliness. So now, unless I see a continuous and repeating problem with focus I leave it alone. Its when stop down focus shift and shift with zooming get mixed in then I kinow I will never win.</p>

[Gup]

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<p>Critically, unlike the Sigma solution, they should do this while the lens is still mounted to the camera, because the "try a setting, take the lens off, walk back to the dock while covering the mount and rear element to avoid dust while you've got the camera pointing at a distant tree, fiddle with the dock, tweak some settings, return to the camera, iterate" approach is massively frustrating and error-prone. It's nice that the functionality is there at all, but given a dock or a micro-USB port with a rubber plug covering it, I'll take the less faffy option.</p>

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<p>The only thing as complicated and time consuming as this in my world is calibrating this Dell monitor with my Spyder 2. </p>

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<p> My values (not that I've looked them up just now) are wildly different between + and - for both my art lenses, which correlates with the problems I saw when simply trying to use the in-camera AF correction on my 35mm: reliable focus at the 3' range leads to extreme defocus even at 10' if I don't correct. </p>

</blockquote>

<p>This was my finding also. It took me a few trips back and forth to the dock to get something I was happy with. Fortunately for me I did my calibrating indoors (the tail end of the hurricane was shaking my windows all day). <br>

Here are my resulting adjustments for the 35mm art lens at the 4 different focal lengths: +4, +5, +11, +16. I found after careful testing and then retesting I had to then go back to the beginning and bump up the first number from +2. Adjusting the third and fourth measurement affected the first. I used the ruler method as Alan outlined above, measuring with a carpenter's square, a 4' level and a tape measure. I measured from a shooting distance of 1', 1.5', 3' and 9'., measured from the sensor line on the body.<br>

After such an enlightening experience ;) I then calibrated all my other AF lenses. It was rewarding. Initially, I printed a target and mounted it to the wall to utilize the 'dot tune' method I watched online, but found my D800E range finder dot to be too finicky to be accurate the dozens of times necessary with the zooms, so used the ruler method for the other lenses also.</p>

 

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[Gup]

<blockquote>

<p>Like the phase I went through about sensor dirt,</p>

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<p>I was just thinking yesterday how I don't miss all that cleaning. I'm still all tooled up for it, swabs for crop, swabs for FF, lighted magnifying loupes, liquids... The self-cleaning feature is something that seems to work and PS clones out anything that gets by. Careful attention to lens caps, body caps and zipping up the bag seems to be good practice.</p>

 

[Andrew Garrard]

<blockquote>The only thing as complicated and time consuming as this in my world is calibrating this Dell monitor with my Spyder 2.</blockquote>

 

<p>Don't get me started. I spent some time (a while ago, admittedly) talking to Gretag Macbeth about how I wanted a colour profile but not a LUT calibration, because I was using an LCD and the moment you don't have a 1:1 mapping between input colours and output colours, all you've done is throw away some of the colours you could represent. So I wanted non-CMS-capable applications to continue using the full colour range, and CMS-capable applications to use the monitor profile to get the nearest displayable colour to the requested colour, even though the binary values in the display were nonlinear. I never understood why "not throwing away colours" was a useful feature. And then I had an Adobe person give me misinformation about whether Photoshop dithers its 24-bit display. Oh, and I spent a depressing amount of time trying to find a mathematical definition of "perceptual rendering intent" before establishing that there effectively isn't one. I hate colour management. I should just shoot everything in monochrome.<br />

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I'll have to check my calibration values, but your range is not far from mine. I think some values may have been so far off that I had to use lens calibration in the camera in addition to the dock - but I could be imagining that. I, too, used "dot tune" (remember to be in AF mode).</p>

 

<blockquote>Like the phase I went through about sensor dirt,</blockquote>

 

<p>I try very hard not to shoot at small apertures, mostly for this reason (DxO seems to be pretty good at deconvolving diffraction, and the rare times I've used f/22 have come out much sharper than I'd expect). Unfortunately, at 36MP, a combination of the optical limitations of my lenses when shot wide open and the depth of field neeeded to compensate for field curvature (okay, and sometimes to cover the subject) means I'm usually a bit stopped down now - I miss shooting everything wide open on my D700. I hope there's nothing too huge on my sensor (I tend to ask Nikon to clean it before anything important), but I haven't really dared look.</p>

[alan_wilder1]

<p>I've had no expeience with Sigma zooms that accept the USB Dock but in general when using Nikon AF fine tuning on my D750 for zooms, I typically select the middle of the focal range or the long end if that's where I'd expect to use it more often for shooting. Regarding the 35/1.4 ART, my sample had the same distance scale discrepancy prior to adjustment throughout the focusing range, i.e. the index line was <2mm to the left of the correct distance marking on the scale whether focused at 0.3m or infinity. After dialing in +5 at all ranges on the Dock, the scale discrepancy was only off <1mm but focusing was spot on thoughout the range based on the image sharpness with test charts and checking plane of focus aimed on a subject at a 45 degree angle.</p>

[Gup]

<p>Alan, I didn't consult the distance scale on the lens. I didn't even think to check it. I used a tape measure and moved the tripod back as per the distances stated in the dock's software coinciding with the four measurement cells. As I mentioned above, each was out a different amount, all on the + side, and when I tested the lens again throughout the range I discovered that two had to be recalibrated, presumably as a result of the later adjustments. I have yet to test the lens outside since adjusting it.</p>

[alan_wilder1]

<p>Correction, prior to adding +5 to USB Dock, the distance index mark was only off by <1mm and after adding +5 the index mark was off by only <0.5mm.</p>

[alan_wilder1]

<p>Another way to check correct focus is with a Nikon 2x eyepiece magnifier and preferably a second 2x magnifier piggybacked over the 1st one giving a net 4x magnification. The subject I use to focus is a 1951 USAF resolution chart because it's easy to detect even the slightest mis-focus. This assumes the focusing screen focus matches sensor focus. I'm lucky as it does match on my D750. If you have this setup, first manual focus and determine what focus setting gives sharpest focus. Make note of the where the index mark lines up on the distance scale. Switching over to AF, see if it lines up the same way and also confirm in the viewfinder best focus is achieved. If necessary add or subtract points on the USB Dock to get the AF to give settle on the established best manual focus setting.</p>