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 Post subject: How to test macro-lenses
PostPosted: Sun Sep 18, 2011 6:36 pm 
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Or: Where's my 5000dpi test-target?

In preparation for some macro-testing I'm still at a loss where I can get a test-target with enough resolution for say 1:2 testing. We're talking about a target with 100-200 lines per mm (simple calculation: The sensor of my D300 has approx 200 scan-lines per mm): this is 2500-5000 dpi! :shock:
Unfortunately non of my printers can resolve anywhere close as we're talking here about clean black/white transitions, not some fake 2400 dpi dithering with multiple ink-blots :roll:
I was thinking about using a optically minified (as opposed to magnified) version of the Siemens-stars I usually shoot. Imagine holding a 50mm lens at 1m distance aiming at a standard target. Behind the lens there is an image of the test-target that is 20 times smaller than the original, with a resolution that should reach and exceed the sensor resolution. If you now place the camera with the mounted macro-lens pointing at the image you should be able to capture an image of the projected test-target and see how the macro-lens resolves it.
Well, trying this setup handholding shows me that it could really work, but only if you align everything up perfectly and keep the 50mm's aperture open at something around f/5.6.

But what I'm really wondering is: Do you have any ideas of a test-target for macro-shots?

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PostPosted: Sun Sep 18, 2011 6:56 pm 
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What about someting as simple as a dead fly?

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PostPosted: Sun Sep 18, 2011 9:27 pm 
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Something 3-dimensional is very challenging to use as a test-target: The minimal dof at magnifications around 1:1 or 1:2 mean that so little is in focus at larger apertures that you cannot really a good (and reproducible) shot. Perhaps a wing of a fly could be, like in that shot...

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PostPosted: Sun Sep 18, 2011 11:45 pm 
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Hmmm, tricky one.

I think OldCarlos might be onto something - an ideal natural subject would have sufficient detail at very close range, but ideally it needs to be flat.

To help people visualise it Thomas, how big would the subject be that you need to shoot? Is it twice the size of an APS-C sensor?

You know, it could still be interesting to see how printed material looks at that close range, as you'd see some paper texture details...

Hang on, I think I just worked it out - you need to use camera film! Imagine you're building a slide scanner with the macro lens. Obviously medium format would be better than 35mm, as a frame would cover your entire target area.


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PostPosted: Mon Sep 19, 2011 5:43 pm 
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Hmm, yes. I thought about film already. But need to produce some quality slides or high-res b&w negatives from a siemens-star test-target.
Let's see: I have to check whether my old Oly OM-2N still works, get a roll of b&w (any suggestions for a maker and type, e.g. Ilford PAN F plus) and shoot a large test-target at a distance that outresolves the the lens+film combo and find someone who develops the shots.

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PostPosted: Sat Oct 29, 2011 3:00 pm 
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Finally the Ilford Pan F b&w negative film arrived from the Lab and I just did some test-shots with my trusty old micro Nikkor AF-S VR 105/2.8G. See one of the images:

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Macro Test Target 32541 by Thomas, on Flickr

This one was shot from the film-negative at around 1:2 magnification with linear inverted curve.
The original film was shot with my Olympus OM-2N with F-Zuiko 50/1.8 at f5.6. So this is sort of a "scan" from the negative film.
Both the image quality of the Olympus lens on the analog film and the Nikon macro lens on my D300 influence the outcome. But the trick is to have the analog reco´rding on the Ilford film in such a high quality that you can detect IQ-differences in macro-lenses when shooting the film-negative at say 1:2 magnification.
The initial outcome looks promising but I'm still searching for the very best negative in the series for use as a macro test target.
---
You can pixel-peep the large original by clicking through the image and choosing "all sizes".

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PostPosted: Sun Oct 30, 2011 7:11 am 
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Nice result Thomas. Did you photograph the original target onto film at all apertures? One may have been sharper than f5.6...


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PostPosted: Sun Oct 30, 2011 10:58 am 
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Yep, I went up to f11 plus shot at +0, +2/3, +4/3 EV.
So I have some digging to do for the sharpest shot...

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PostPosted: Sun Oct 30, 2011 8:07 pm 
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From my film days I remember, but I may be wrong, that a lens could resolve in the range of 50 -100 lines/mm. Film resolution is, and may be a lot more than that. I’ve used photographic plates in holography with resolution in the range of 2500 lines/mm, but the sensitivity was about 3 DIN, like photographic paper. Of course, in that setup there was no lens between the subject and the plate.
So, If you choose the film with the lowest sensitivity you can get, you’ll have to wary only about your lens’ resolution limit. It would be nice to have access to a large format camera and record your chart on a glass plate. Apart from very good resolution, you would get a perfectly plane chart, and if you record it’s emulsion side, as you should, it would be reflection free too.

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