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Archive 2013 · Yesterday's medium format digital backs
  
 
jcolwell
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p.8 #1 · p.8 #1 · Yesterday's medium format digital backs


alundeb wrote:
Ok, the steps in USAF1951 are in lp/mm

50.8
57.0
64.0
71.8
80.6

So the accuracy is even worse than I thought. Only two steps to cover the effect of the format difference from 36x24 mm to 44x33 mm.



You should do some more research. USAF 1951 test target resolution is calculated as follows:

R = D (2^G) (2^((P-1)/6)) / f

where, R (lpmm) is the resolution at the sensor/film plane, D (mm) is the distance to the subject (i.e. test target), G is the USAF 1951 "group", P is the "pair" within the group, and f (mm) is focal length of the lens being tested.

There is no dependence on sensor or film size/format; it is resolution at the plane of the sensor/film, regardless of format.



Jul 06, 2013 at 09:37 PM
RustyBug
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p.8 #2 · p.8 #2 · Yesterday's medium format digital backs


mirkoc wrote:
Could it be Mamiya 7 glass?


Mamiya 7 has a registration distance of around 60mm (according to http://www.graphics.cornell.edu/~westin/misc/mounts-by-register.html which is only slightly closer than 645 @ 63mm), which is still longer than most FF, so I don't see why not as long as there is an adapter for it. But ... I'm not versed in whether or not there is some other reason why you couldn't, so I'll defer to others who may know better.

Leaf shutter or rangefinder coupling issues may / may not be of any issue. I've been toying with my Graflex lens (leaf shutter) in front of a dslr ... locked open shutter and hand holding in front of the camera (i.e. no lens mounted). Needs a bellows/mount/adapter/enclosure built for it certainly, but it still projected the image onto the sensor with the leaf shutter locked open, using the dslr shutter instead. I'm curious @ the "look" I'll be able to get with those vintage lenses.

Nothing to get excited about (or make any judgments of) as this is without the lens even being mounted or coupled in any way, nor with any way of shielding extraneous light (open air), nor squaring the lens to the film/sensor plane ... but simply to evidence that locking open the leaf shutter can allow it to be projected/captured on a dslr. If the leaf shutters on the lenses can be locked open, akin to stopped down shooting with other alt glass, why not? Again, I can't confirm the Mamiya 7 glass will work ... just seems worthy to explore if an adapter is available.

One of these days I may play around and make a DIY hack adapter for my Graflex glass.











Jul 06, 2013 at 09:57 PM
alundeb
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p.8 #3 · p.8 #3 · Yesterday's medium format digital backs


jcolwell wrote:
I described how those resolution tests were made, using the USAF 1951 resolution test target, which has discrete step sizes between measurement values.

alundeb wrote:
Ok, the steps in USAF1951 are in lp/mm

50.8
57.0
64.0
71.8
80.6

So the accuracy is even worse than I thought. Only two steps to cover the effect of the format difference from 36x24 mm to 44x33 mm.

jcolwell wrote:
You should do some more research. USAF 1951 test target resolution is calculated as follows:

R = D (2^G) (2^((P-1)/6)) / f

where, R (lpmm) is the resolution at the sensor/film plane, D (mm) is the distance to the subject (i.e. test target), G is the USAF 1951 "group", P is the "pair" within the group, and f (mm) is focal length of the lens being tested.

There is no dependence on sensor or film size/format; it is resolution at the plane of the sensor/film, regardless of format.



It is only the 2^((P-1)/6) factor that matters for the quantization steps. The quantization steps expressed in percentage increase is 12.28%.

The numbers i gave are examples of that, by incidence matching your final calculation.

Look, I asked for objective data suppurting the claim that some MF format lenses are as good as or better than the best of comparable 35 mm lenses. 12% accuracy is VERY coarse when speaking of MTF. It is the difference between a stellar lens and a mediocre lens for center resolution.

I appreciate that you did what you could to find those data, and my reason for knocking them down is only a general frustration about the accuracy of the data, and that there are no other data.

When mentioning the 36x24 and 44x33 mm formats, it had nothing to do with the measurements per se.
What I mean is: Two lenses with different MTF, but when used on their respective formats give identical system resolution, only measure two steps apart in absolute MTF with USAF1951.



Jul 07, 2013 at 06:31 AM
jcolwell
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p.8 #4 · p.8 #4 · Yesterday's medium format digital backs





Jul 07, 2013 at 11:02 AM
theSuede
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p.8 #5 · p.8 #5 · Yesterday's medium format digital backs


The step increment is 1/6 of a log2-step, or 12.5%. Which is quite a lot, when the format difference is 44mm/36mm = 22% - That's what he wrote...

The major shortcoming of the USAF chart isn't the step resolution though - there's at least three major defects to that kind of testing that outweighs that by far:

1) In modern systems you have SHARPENING. At what contrast level do you "disqualify" the resolution result? How do you judge contrast?
2) The target pattern line lengths are just 5 x width. This is not even close to enough to get accurate results, since that means the maximum resolution gives 5 pixels of target line "length". A lens+camera that can accurately image that resolution will not know (from the Bayer pattern) if the lines are vertical or horizontal.
3) If the lens is sharp enough, then the maximum resolution is determined by pure luck. If the target lines are centered on pixel lines in the camera, they are visible. If they are in the middle between two pixel lines, they will not be visible. The lens resolves the pattern PERFECTLY, but the end result is a gray, flat plate.

USAF1951 is an "extinction" test, not a "usable resolution" test. It was supremely suited to large-grain film testing, not lens testing.



Jul 07, 2013 at 11:27 AM
theSuede
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p.8 #6 · p.8 #6 · Yesterday's medium format digital backs


What this means:

If you take three images with the SAME lens and the SAME camera on USAF target, depending on minute changes in target>pixel alignments in the image you will get three totally different results.

Tests that give different results for subsequent measurements are not good for accuracy. They contain a very high amount of pure luck/chance - unless you do ten-twenty measurements and average them. Which is very hard to do subjectively.



Jul 07, 2013 at 11:32 AM
 

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jcolwell
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p.8 #7 · p.8 #7 · Yesterday's medium format digital backs


I agree the USAF 1951 test chart isn't the best system, especially with respect to the subjective decisions required to determine the highest resolving set of line pairs (i.e. contrast). OTOH, differences between individual lenses are consistent and meaningful. Many excellent lenses soon go to the resolution of the camera being used - so what? It's well worth knowing which lenses do and which lenses do not reach this level of performance. N'est ce pas?


Jul 07, 2013 at 12:27 PM
naturephoto1
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p.8 #8 · p.8 #8 · Yesterday's medium format digital backs


Unfortunately there can be much variability in performance of the same lens model. Normally or at least frequently we have insufficient numbers of samples of any one lens to really know the lens norm, though we may know the performance of a specific sample.

Rich



Jul 07, 2013 at 12:31 PM
RustyBug
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p.8 #9 · p.8 #9 · Yesterday's medium format digital backs


theSuede wrote:
1) In modern systems you have SHARPENING. At what contrast level do you "disqualify" the resolution result? How do you judge contrast?


+1

The sharpening algorithm for different glass (Oly vs. Zeiss, etc.) or different camera or mfr (D70s vs.1D MK II) or different lens 24L TS-E II vs. 100/2, or MF vs FF is not a "one size fits all".

Many folks have an expectation that our sharpening should be identical for all glass/camera combinations to produce meaningful or comparable results. When I first switched from consumer grade Sigma on Nikon D70s to Zeiss on Canon 1D MK II, I was totally dismayed by how soft the images coming from the "professional" gear were compared with my former results. Likewise, I didn't care a lot for the change from CCD to CMOS (different subject).

Chuck Westfall of Canon educated me that the "pro" gear would allow for more aggressive sharpening than what I had been accustomed to applying (iirc). While this was of course a Nikon vs. Canon, he also showed me (I just forget which models) the diff needed in the Canon lineup. The salient point here is regarding your final image output ... of which the processing is a significant part of the entire process, it doesn't stop at the lens capture. Kind of like "The Camera", "The Negative", "The Print"

For those expecting to use the same sharpening algorithm that was defined for FF when using MF and judging the comparative results based on that, imo, you are leaving a lot of goodness on the table if you stop formulating your conclusion at that point. There are those will say that using a different sharpening algorithm with your MF glass proves that it isn't as sharp. To this I'll say two primary things:

1) Contrast and resolution are interdependent @ generating perceived sharpness, but not the same thing.
2) Final output rules all

Sharpening algorithms vary between mfr's and models and to a degree is a matter of subjective taste @ the final output that you desire for a given image. So, if you like the "look" of MF, starting with an MF projection is one piece of the final output, sensor capture (strength of AA vs. non-AA, pixel size/density, well depth/noise/etc.) is another and post-capture processing follows. These are (imo) today's version of Camera/Negative/Print as a collective process that takes you toward final output.


In the Alt Forum, we are quite well versed in using different lenses with different drawing styles to get different looks, even within a given format (i.e. FF). We should be able to also recognize that the variability of other process(es) toward final output is "multi-step", even if we need to look back to the significance that others have pointed out at the interdependence of those steps. As such, we should recognize that applying our sharpening (et al) can be different and it isn't anything that is "unfair" or "unequal" or "cheating". It is the final output that is your real objective, not a laboratory comparative of the individual components of the process.

The "look" you get for your final image is going to be predicated by the interdependency of these things. Imo, starting with a projected image from MF glass is the first step in getting more of the MF "look". Starting with MF glass affords me the ability to produce more MF "look" than if I start with FF glass, even if I do use a different sharpening algorithm. This is where you may find that MF really opens things up a bit to the nuance and refinement that you can achieve in post as a product of the original projection transitions (see above @ trig).

Again, my objective here isn't to be comparative to shooting on a larger format. But, if you want to achieve more of the MF look on FF without incurring the $$$ to make the nominal jump to 44x33 (or $$$$$$$$ for larger), starting with an MF projected image and processing accordingly makes sense to me. For others, not so much. I just know that I've taken a liking to using M645 glass to generate what constitutes more of an MF look (smooth transition/tonality/well defined/well corrected) to me ... ymmv.





Jul 07, 2013 at 02:15 PM
alundeb
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p.8 #10 · p.8 #10 · Yesterday's medium format digital backs


Slightly off topic, but since FF and APS-C lenses were mentioned, here is comparison between the Canon EF-S 60 2.8 macro and the Zeiss Makro-Planar 2/50.

On the NEX-7, developed in C1 , unscaled pixel crops from the center of the frame, unsharpened.
The Canon 60 macro is probably one of the sharpest and most contrasty lenses designed for APS-C.









Jul 07, 2013 at 02:46 PM
RustyBug
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p.8 #11 · p.8 #11 · Yesterday's medium format digital backs


Something for the OP @ Yesterday's medium format digital backs

http://www.paulclaesson.com/DigitalInfo/Digtal.MF.Ref.pdf

http://www.ayton.id.au/wiki/doku.php?id=photo:digitalbacks_history



Jul 07, 2013 at 03:16 PM
Sneakyracer
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p.8 #12 · p.8 #12 · Yesterday's medium format digital backs


Johnny B Goode wrote:
Out of curiosity I began researching medium format digital camera/backs lately. I've noticed the $30k cameras of yesterday can be had for a more reasonable (yet still expensive) price. That made me wonder, how many exposures can you be expected to get out of a MFD back before it goes on the fritz? Or is it calculated in the duration of time the sensor is being exposed? Also, is buying an older used medium format digital back like buying a heavily discounted old (but not quite classic) exotic car - it's cool up front but the back end
...Show more

I think most Phase One backs are still serviceable. They are tanks and are made so that most components can be replaced / repaired.

Remember that a medium format digital back is just an image recording device. Where you mount it is of high importance so choose your platform wisely depending on your needs/wants/preferences. There are lots of options.

I own a phase one back and use it on a Hasselblad H1 for people and studio work and on an Arca Swiss tech camera for landscape and architecture. They are two very different beasts but the phase back works great on both. That tells you about the huge versatility of a digital back. It's not a product for everyone but if you know what you want it allows you to build basically a custom digital camera system for yourself since there are MANY back/camera/lens combinations possible! Think of medium format digital as a modular system not just a camera.



Jul 08, 2013 at 05:59 AM
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