Red fringing on slide camera scans

Hello

I’m doing some experimentation before scanning a big pile of slides and negs, to get the best out of the process. My setup is a DSLR (R5), a macro zoom (RF100) a stand, viltrox light and EFH.

As I expected, the slides I’m testing are slightly sharper with 2.8 aperture, as this lens has some well known focus shift issue unfortunately at higher ones (and I don’t want to manual focus thousands of slides). I would love to scan at 2.8, but I also noticed some red fringing in transitions, which is very prevalent at 2.8 and very much less if any at higher ones.

I put a small example below

Do you know where it can come from and how to fix this ??

Purple fringing can exist on high contrast edges like described on this page (scroll down to find the text and image)

It could also be trace of longitudinal chromatic aberration, but it’s hard to tell from the tightly cropped image.

The effect should go away with higher aperture values or change of focusing or object distance.

Just a quick question here. Why would you like to scan at 2.8? You should check your lens’ focus at every aperture and only use the sharpest aperture for your scans. F2.8 often isn’t a lens’ best focal length for sharpness. My Sigma 70mm f2.8 macro lens is sharpest at f13. So, all of my scans are done at that aperture. You should do the same with your camera and lens combination.

Also, shooting at f2.8 gives you the smallest depth of field available for your lens. If you are out of focus a little, it can throw the whole scan out of focus. A higher aperture gives you a wider depth of field and more latitude for focus. To me, using a higher aperture number gives me a better focus and a sharper image.

In the case of this particular lens, 2.8 seems to provide the best results (despite the short DOF) because of the focus shift issue. I would prefer a higher aperture, but I’m losing details unless I manage to manually focus each slide which would ba a painful longer process.

According to this article, focus shift and LoCA must be the cause of your trouble. Focus shift in combination with a lack of ability to focus with closed aperture makes this lens difficult to use in our context.

Maybe you can still use the lens with AF calibration, but I suppose that it cannot be set on a mirrorless camera.

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You may be seeing fringing in your film originals. I know I have captured fringing on slide originals shot with older, film era lenses. But if your duping lens is itself fringing, the only solution is to stop down to the point it becomes minimal or is gone, typically around f/5.6 or certainly by f/8. You will not get any sharper results shooting past f/8 effective aperture, as diffraction is a physical fact. There is no useful working depth of field at 1:1 obtainable by shooting at small apertures. The best duping lenses (and scanner lenses) are optimized for maximum aperture, often f/2. For those of us using a major camera manufacturer macro lens, your likely going to optimize all lens aberrations at around f/4 to f/5.6. But remember, at 1:1, a lens marked f/2.8 is at effective (actual) aperture f/5.6, and marked at f/4 it is actually f/8! Resolution of any lens will be diffraction limited at f/8 and smaller apertures. It might be “acceptable” but not “optimal.”

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You would definitely need to stop down to about f/5.6 to get best results. Not sure why you need to manual focus every slide. Once you get the first one in focus the next ones should all be in focus already without touching the lens.

You are right, this specific lens has this focus shift issue I mentioned and I can’t calibrate … seems like there is no real solution

Different lens…check out Lens Tests — Close-up Photography

Sadly, zoom lenses are almost universally less sharp than fixed focal length lenses and suffer from a number of other problems as you have noted. You would be much better of with a good second-hand macro lens such as the Nikkor 55mm F2.8 micro. It’s outstandingly sharp and surprisingly cheap on ebay.

Thanks but I’m not gonna buy another zoom or even switch to Nikon for that purpose :slight_smile:

Sorry, I was taking what you said in your original question at face value - that you wanted to get the best out of the process. With respect, that goal is not achievable using (a) a zoom lens (b) a lens with a host of problems.

Well I wasn’t clear and using the right wording as in France it’s common to call a “zoom” any type of lens (for a non pro photographer). My bad. My RF100 macro is not a zoom lens, and I don’t want to use a zoom lens. The problem I have is that this lens should be sharp enough for the goal, without the “focus shift” problem this series of lens is crippled with.

The last thing I dis not investigate is playing with the light source. Gonna continue searching

Why should it be? I don’t mean to be snide. The lens design is for non-macro, typical work. Any “macro” notation is for a casual shot where flatness of field, etc., is typically not noticed with 3D subjects. Buy a lens that is designed for the situation you want to use it in, otherwise accept the limitations.

Whoops excuse me and you have my apology! I misread what you wrote and thought the lens was a zoom. If you still get fringing with a lens designed for this work, and that fringing is not in the original slide, then your lens is not performing well. For focus shift, try focusing in live view at the taking aperture.

This lens RF100macro, putting aside the focus shift issue, is supposed to be one of the best macro on the market … I suspect this fringing issue might also come from the lightning system. Gonna do more tests. Regarding the focus shift, live view does not really solve this

There is no phenomenon of normal or typical lighting that I know of that can cause fringing. Fringing wide open should diminish rapidly by stopping down one or two stops maximum. I’m not clear on your full use of this lens: at f/2.8 you have fringing? At smaller f/stops you have focus shift? Despite its reputation, these are not hallmarks of a best in class lens. The only workaround is focusing at taking aperture where fringing is dimished or gone. I suggest trying f/4 or f/5.6 (marked aperture, effectively f/8 or f/11), which should still allow accurate live view focusing. Fringing in your film originals will not go away.

Can you close the aperture with the DoF button and focus manually?
(not ideal, but might do the trick)

Yes of course, but I just didn’t want to buy a new lens for this

I know all this, and as mentioned earlier, stopping down indeed solves this fringing phenomenon, but then the focus shift effect that this lens has pops up, that’s why I’m trying to find a way to scan full open at 2.8 where I get the sharpest shots by far