medium format reversed lenses

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mtuell
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medium format reversed lenses

Post by mtuell »

I just got a reversal ring 67-52 that allows me to put a Pentax 67 medium format 105 mm prime lens on my Nikkor 55-200 zoom. I've got it on a Nikon D3300 with an APS-C sensor.

I took a stack of a fly tonight, and Lou points out in the post

http://www.photomacrography.net/forum/v ... hp?t=33831

that it doesn't make much sense to use a medium format lens on an APS sensor. First, because you are wasting much of the field, which I agree with, since the FOV of the reversed lens is for a 60 x 70 mm film plane, but APS is only 23.5 mm in the long dimension. And second, that the pixel size of APS sensors is so small, in this case, packing in 6000 pixels in 23.5 mm, or about 3.9 micron pixels, that basically the lens can't keep up with that because the medium format digital cameras use bigger pixels.

Well, this is for a film camera, circa 1990, so it may be better than newer ones that are for digital backs. I went to take pictures of a USAF 1951 target, but realized it wasn't here at the moment. So, I took a picture of a 25 micron pinhole. Since it is a 200 mm zoom with 105 mm reversed lens, that is 1.9:1, with a field of 12.6 mm, with 6000 pixels, that is 2.1 microns per pixel in the field, basically 4 microns per pixel on sensor, as noted. 25 microns / 2.1 is right about 12 pixels across the diameter of the pinhole.

Here is what I found with single exposures with the same setup I was using earlier (iso 100 and 1/200th of a second). Here, no sharpening or other processing is done except what is coming out of the .jpg engine in the camera.

First, a 50% reduced size of the central area of the mount. The pinhole is visible in this image, but tiny.

Image

Next, a 100% crop.

Image

Now, blow that up to 500% to see the individual pixels.

Image

The number of pixels looks right and it doesn't appear that the optics are "pooping out" quite yet... but it may be starting to get close. The flash is certainly saturating the specular reflection areas, so try to ignore that part. :(

So, while it would be nice to have a larger sensor to get more of the field (it would be 19+ mm FOV with a full-frame sensor), it doesn't appear to be a wasted effort to use this particular lens with an APS-C format sensor.

I'll let you know if I get around to actually measuring the resolution with a real target.

Thoughts?

Mike

Lou Jost
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Post by Lou Jost »

That does look pretty good.

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Post by ChrisR »

If that's the centre, you might expect it to show least aberration ;)
Chris R

mtuell
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Post by mtuell »

That's true, Chris... but at 23.5 mm out of 70 mm, you can't be getting too far off center anywhere! :) I'll try it with the pinhole at the edge and corner of the visible field, though. Good suggestion.

Mike

Lou Jost
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Post by Lou Jost »

It would also be nice to see a comparison with a good 35mm lens designed for digital cameras...

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Post by Smokedaddy »

I need to find the time to experiment with that. As you know I have quite a few high MTF 67 and 6x7 Pentax lenses that I use on my D700 with outstanding results. Of course they're all primes except a the SMC 67 55-100.

Very interesting,
-JW:

mtuell
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Post by mtuell »

Thanks for this link, James!

http://www.antiquecameras.net/pentax6x7lenses.html'

According to that, this particular lens is from 1971 era.

Mike

mtuell
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Post by mtuell »

I had said in a different post (http://www.photomacrography.net/forum/viewtopic.php) that this lens was circa 1990, but it was pointed out that this is the older version, first made in 1971. Just to clarify...

I got the USAF 1951 target home and took some pictures. These are 1/30th of a second, ISO 400, no flash. As compared to the fly stack at 1/200th ISO 100 with flash. However, this is with the lens wide open at f/2.4. I will try to get more pictures at at least a couple more f-stops (f/8 and f/22 come to mind as middle and tiny apertures.) No processing was done off-camera except cropping.

While the lighting may be influencing this, it did seem that the corner of the APS-C sensor had somewhat worse resolution. There was noticeably more lateral chromatic aberration at the edge, but there were several confounding factors, so I would have to convince myself it was really an issue (but it does seem likely...)

The center 100% crop shows a cutoff of about "Group 6, Element 3", which is 80.6 linepairs/mm, or 6.2 micron wide bars. (Table of lp/mm posted here : http://www.photomacrography.net/forum/v ... hp?t=32434)

Image

At the very corner of the APS-C sensor, we can say the smallest resolved element is maybe Group 6, Element 1 or 2, but the lighting is a bit weird... anyway, that is 64.0 or 71.8 lp/mm (7.8 or 7.0 microns each bar).

Image

Don't forget that this is without flash and a higher ISO than I was using, and also, importantly, all the way open at f/2.4. The slightly higher ISO and longer exposure may be overriding the wider aperture, since there is a little more noise, as well as vibration is more of a factor. Each of these are single images, but selected as the best from samples of about 10.

I'll try to get back to the pinhole imaged at the corner of the array with flash sometime.

Also, here are a couple of (crappy GoPro) shots showing the setup on my vertical stand on the granite table. Notice the working distance to the resolution target! (And, yes, that is a remote flash softbox behind...)

Image

Image

Mike

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