Flat field macros for scanning.

Started 7 months ago | Discussions thread
OP bharathmsk Regular Member • Posts: 162
Re: Flat field macros for scanning.


I can confirm that at 1:2 magnification the Oly's field curvature is less noticeable. With the match native Oly 25mm extension tube, I suffer both the field curvature (the problem discussed here), and glare (which I minimize by masking stray light as best as I can).

To level components, I check to make sure the light box, and camera are leveled. My light box also has high contrast black cm/mm markings; so, I also make sure the whole range of "cm/mm" markings go out of focus (or in focus) in unison, as the focus ring is moved. Focus peaking helps substantially to visualize this.

After all this, I am sure of the field curvature and my leveling too to a good degree, because the level of blur is quite similar on both ends of the frame. Both can be brought into focus at similar levels (as the center goes out of sharp focus). I also checked to make sure this is not an issue with the film not being held flat, by checking on a sheet of printed paper held flat and taught on the lightbox.


guyfawkes wrote:

bharathmsk wrote:

Hey Everyone,

I got into film recently, started fiddling with extension tubes on my kit lens to do the DSLR scanning. I could never get the center and corners sharp. The corners were so horrendous that I thought my 35mm film equipment/lens was bad. Trying the scans with various lenses, over and over, I found out that I have been experiencing the effects of field curvature all along. What a hard way to learn about field curvature!

I then went for a vintage 1:2 Olympus f/3.5 macro that with an appropriate extension tube goes to 1:1. The lens is gorgeous, but it still has some field curvature that renders it not so great for flat copy work I am after.

Further search along these lines leads me to the Micro-nikkor 55mm f/2.8 or the 55mm f/3 .5 AI-s. Any other alternatives? I also came across a hefty Vivitar f/2.8, but I am unable to quite get a sense of its flat field through my search.

Any help is greatly appreciated! Thanks

Two things come to mind here. Firstly, the Olympus lens should be fine at 1:2 but if it was not envisaged as a true 1:1 macro despite the extension tube, then it is possible that it will not reproduce a flat field across the whole frame. I am unable to confirm this, it's just a thought.

More likely is the set up you are using. It is not easy to ensure that all the components are accurately aligned with respect to the optical axis and with the magnification you are working with any slight deviation will readily lead to out of focus images at the frame edges. At 1:1, just being 1/2 degree out of true will be noticeable.

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