Getting the Correction Factor right...

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dkperez
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Joined: 22.03.2014 02:30

Getting the Correction Factor right...

Post by dkperez » 24.04.2015 17:35

I've been playing. This is what happens when you're a geek and don't have a life.

All my examination of the TIF output of the stacks was in Bridge CC with no sharpening, at full screen, at 100% on my calibrated 30" monitor. Images are 7360 x 4912 px.

I took my Nikon D810 with the Tamron SP 90mm macro. I initially tried a grid of lines and placed it parallel to the sensor, and shot as the manual said at f2.8. At CF1 I got no out-of-focus (OOF) bands. At 5.6 none. At 8 none... I finally decided I couldn't have the target parallel to the sensor and set up so the camera was 45 degrees from the target.

Started shooting the 10-shot stacks with CF1 and went through 8. At CF5 I could JUST start seeing a slight banding, but my total focus movement was so small it was difficult to tell. So, I changed to f5.6 instead of f2.8 and changed my target to one of the back-focus targets with markings from +2 inches to -2 inches and added vertical lines every 1/4 inch so it would be easier to see the OOF bands. I don't shoot stacks at f2.8, so 5.6 was more realistic for me - I generally use 5.6, 8, or on occasion f11.

Eventually, after looking at a lot of stacks, I decided that at 90mm a CF of 4 was the highest I could go without seeing any OOF bands. This gave an interval of 10.

THEN I switched to my 28-300 and did the same thing, starting at 90mm again. Got the same results, so I decided any lens on my body at 90mm would provide a CF of 4. Then I went to 180mm and shot with CF1, 2, 3, 4, 5, 6, 8, 10, 12, 14,16, and 18. And got twice the CF before I started getting bands. With CF between 8 and 10, things looked good, with 10 just starting to show OOF bands. At CF12 they're quite visible. On my camera, a CF8 also equals an interval of 10.

Switched to 270mm, which if I understood what was happening would move my CF another 50% (presuming it went something like 90mm CF4, 180mm CF8, 360mm CF12)... Tested again using CF 8, 10, 12, 15, 16, 18 and found that indeed a CF of 10 appeared to work. CF11 might also work but at CF12 I just started to see very slight banding.

I expect that if I tested at 45mm I'd get a CF of 2, but since I've never shot stacks at a focal length that short, I didn't test it.

BUT, I didn't get FEWER images when doing a "real" stack because the interval at different focal lengths changed. For example, to cover +2" to -2" at 90mm and f5.6 might give me a A - B interval of 50. Doing the same thing at 180mm gave me a higher value for interval between A and B (maybe 60 or 70). So, my CF went from 4 to 8, but the "DOF in interval values" stayed the same (10), and the number of shots increased because the overall interval increased and each focus adjustment was relatively smaller as the focal length increased. At least that's what I looked like to me.

In the end, the short answer is that for my D810, with a fixed 90mm I use a CF of 4. With a zoom, I have a linear graph that lets me pick a CF that's right for the focal length at which I'm shooting with CF4 at 90, CF8 at 180, and CF10 at 270mm...

THEN I tried it with a friend's Canon 5D Mk III with a Canon 100mm macro. TOTALLY, COMPLETELY DIFFERENT. With the Canon "Size of focusing steps" set to Big, we got OOF bands at CF1. Switched to "Small" and CF1 was ok, but we'd get OOF bands at CF2. So, for that system, leaving the CF at 1 and just using the interval provided by the DOF function appears to work better.

We tried the SAME thing with a Tamron 28-300 on the 5D MkIII. Got the same results. Values were different, but the bottom line was that for that camera, it appeared that leaving it at CF1 worked best. At any higher value the intervals got large enough to see visible OOF bands.

Needless to say, your results may vary DRASTICALLY from mine - I'm certainly no testing expert and was simply trying to get some empirical data to help me be more systematic when I'm sitting in a bog.

My next effort will be testing my Nikon 70-200 with the extension tubes. I frequently use this combination for close-up/macro work because it's a lot more flexible than the fixed 90mm macro. It'll be interesting to see what effect the extension tubes case with respect to the focal length....

Now if the stinkin' weather will just improve enough for a spring flower to stick it's head out of the ground, I can try it for real!

Stas - just curious. Does what I've been doing make sense in terms of getting correction factors for zoom lenses? I didn't change the Nikon "Size of focusing steps" in the preferences from 15 other than to try one test at 150 and got GIGANTIC steps between shots. So, I put it back to 15 and left it.

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mike_mccue
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Joined: 28.03.2015 18:03

Re: Getting the Correction Factor right...

Post by mike_mccue » 24.04.2015 18:31

Thank you for posting this. I just started using Remote with my Canon 5dmkII and Canon 180mm Macro lens. I've been searching the forum and re-reading the Remote help files and I appreciate the attempt to illicit more info or discussion about these features.

I am using the Canon preferences "Big Step" choice and trying to learn to use the DOF calculator. It seems as if using a Correction Factor of 1 works reasonably well. I can't imagine trying to use a larger Correction Factor to reduce the number of frames I am shooting. The results I am getting look a bit softer than the work I was doing with Canon EOS Utility or a focus rail that I have which allows me to easily work in 0.0105 inch increments.

I realize that I had been overshooting but now I feel as if I am under shooting.

I'd like to experiment with smaller increments using correction factors below 1 but it seems to confuse Remote when I set the factor below 1. I can't seem to shoot in less than a increment of "1", and if I follow the instructions carefully to try to use the DOF calculator I usually end up with "?" marks in my steps and increments windows and so Remote takes a single shot, then racks the focus and stops. I get the feeling I am trying to use DOF calculator to take more shots within a given range when it was actually designed to help you take less.

Today I plan to make tests with the Preferences>Canon Small Step choice to see if my impression of softness is confirmed by comparing to stacks made with relatively more images/range.

FWIW, I've been using a metric ruler as my target. I have it set as nearly perpendicular to the Camera sensor as practical. I can see each millimeter mark (which because of the slightly angled positioning of the ruler are spaced slightly closer than 1mm)

I have tried correlating the DOF charts available on the Zerene Systems website with the length of an "increment" in Helicon Remote with a Canon "Big Step". I get the impression that the charts found at Zerene's website predict that the movements I am making with Helicon Remote are not small enough to achieve the potential for finest detail.

I am hoping to adopt a best practice use of the Canon Small Steps setting to get a sharper over all result. I anticipate that this will include changing the <<<, <<, >>, >>> settings to something suitable for small steps.

I wish the help files and tutorials explained the best application of the adjustments, or at the very least described the ideas behind the adjustments in a more detailed and contextual manner.

Having said all that, it does seem like increments of "1" at a Correction Factor of "1" seems to work resonably well at every distance to subject that I have tested with my Canon 5dmkII, Canon 180mm Macro lens, and "Big Step" preferences. setting.

dkperez
Posts: 48
Joined: 22.03.2014 02:30

Re: Getting the Correction Factor right...

Post by dkperez » 27.04.2015 20:48

From my experiments, I BELIEVE (for what that's worth) the "Big" step is 4X the "Small" step on a Canon. When experimenting, I found that the SMALL step gave smoother results - less banding and smaller increments.

For me, the number of frames I can shoot generally depends on the environment. If I'm outdoors photographing wild flowers, even a MINUTE breeze is a problem so even when using a wind tent, capturing the depth wanted with the fewest exposures improves the chances of getting a usable stack.

When I'm INDOORS - other than a conservatory with the fans on - I have the flexibility to let the camera use smaller intervals.

I've noticed on my Nikon D810, using a smaller correction factor, and thus smaller intervals just results in more overlap, which won't hurt anything, but having 25 images when 12 will cover the range doesn't SEEM to improve the quality of my stacks. BUT, given the choice I"ll overshoot rather than undershoot 'cause things get REAL UGLY real FAST when you don't adequately cover the range or get bands that are out of focus.

When I was fiddling with the Canon, I tried to use a correction factor of less than 1 - .8 or .5 for example, and I couldn't get it to work. What I COULD do was leave the correction factor at 1 and change the DOF interval value to a smaller one. In fact, I recommended to the friend with the 5D that she use an interval about HALF what the correction factor provided.

I wish the help files had more geeky information too, but I think they're just trying to give people basic information to get them started... For those of us with no life and/or an insatiable appetite to experiment, we just have to rummage around and try to figure out how to optimize the results for out equipment.

In the end, if it works best for you to shoot in really small steps, go for it. I can't imagine that it'll hurt anything, and as long as your computer can process the larger number of images with no problem, and your stacks look good, you're happy and that's what matters most.

One of these days when I'm traveling I'm going to find a spot with an interesting flower in front and great mountains way off in the background and set up with the foreground 2 inches away from the camera and see if I can go from the front of the foreground item all the way to infinity for the mountains and get something interesting! The "A" to "B" should be QUITE a number!

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Stas Yatsenko
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Re: Getting the Correction Factor right...

Post by Stas Yatsenko » 30.04.2015 09:49

Thanks for sharing the results of your research. First, it is a mistake on our part that you have to adjust correction factor when changing the focal length of a zoom lens. The factor is supposed to work for the specific lens and not just for the lens at specific focal length. So we've improved the formula, which means the correction factor values you're using right now will become obsolete in the new version of the program.

There is no problem with correction factor less than 1. There is, however, a problem with interval less than 1 focus step. 1 is the minimum increment. So, let's say the calculator suggests the interval of 10 focus steps. But it's too large a step, and you find that CF of 0.5 and the actual interval of 10*0.5 = 5 steps works well. That's perfectly fine. But imagine that you get DOF of 1 step when, in fact, you need even less. So you would like yo set CF to 0.5, but you can't do half a step. This clearly indicates that your DOF is too small for the current focus step size. You have to switch from Big to Small if it's a Canon camera, or lower the step size value for Nikon (default is 15, minimum is 1).

dkperez
Posts: 48
Joined: 22.03.2014 02:30

Re: Getting the Correction Factor right...

Post by dkperez » 02.05.2015 23:23

Thanks Stas... I look forward to fiddling with the new version.

The updated version for the Nexus 9 works great - it cut out all the jittering and big screen lag.

To make the whole correction factor EXTRA interesting, I frequently use a Nikon 70-200 with extension tubes. And the size of the tube 12mm, 20mm, or 36mm, depends on how close I get and/or how large I need the subject. I haven't tested but I suspect the extension tubes will completely throw off the correction factor and any relationship between focal length and interval...

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Stas Yatsenko
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Re: Getting the Correction Factor right...

Post by Stas Yatsenko » 05.05.2015 15:19

dkperez wrote:I suspect the extension tubes will completely throw off the correction factor and any relationship between focal length and interval...
Right, you need to find the correction factor for each of the tubes independently and specify it in the program when using that tube (or ring, or a close-up filter).

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