Resizing MTF test charts

[Harsh Mishra]

Hi Fred!

I hope you are doing well.
I have just started using MTF Mapper in order to determine the resolution of my imaging system. I use a Cmos camera and a zoom lens objective with different extension tubes foe magnification. If I use a zoom lens set to 110mm and an extension tube length of 10cm how can I try to resize the chart so that most of it can be imaged as now I am only able to see a single edge and it's blurry too... I used lengrid A4 for now... how to resize so that I see most of it and mtf mapper gives better results...
I plan to try different extension tube lengths how do I keep changing the chart any formula which relates it to extension tube?

Thanks.
Regards.

[Frans van den Bergh]

Apolgies for the delayed reply!

I don't think you will be completely satisfied by a standard print (A4) if you are testing at such a large magnification. If I may suggest a rule-of-thumb, you want at least 8 printer dots per image sensor pixel. In other words, if you have a full-frame sensor, say a Nikon D850 with 8256 pixels per row, then you want your field of view to cover at least 8 * 8256 = 66048 printed dots. At a normal laser printer resolution of 1200 dots per inch, that is a print 55 inches wide!

You can compromise a little bit, maybe get away with 5 printed dots per pixel, but the rough edges of the printed black targets will start to show up in the measured MTF.

Given that it sounds like you are measuring in the macro domain, I assume your actual field of view at the focus distance is much smaller, probably below 50 mm (roughly 2 inches, or only 2400 printed dots!), I would recommend you use a razor blade illuminated from behind. This is the only cost-effective way of obtaining an edge that is high enough quality for accurate MTF measurements at such high magnifications. Of course it will be very hard to focus accurately on the razor blade edge, but you can use a focusing rail to step through the optimal focus, meaning capture shots with the focus in front of, roughly on the blade, and behind the blade; the best MTF should correspond to the image closest to optimal focus.

I hope this gives you some ideas, but please feel free to ask more questions!

Regards,
Frans