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Will a high resolution 24x36 camera match an MFDB in image quaility?

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For each generation of higher resolution 24x36 sensor the question "Will this sensor paired with an excellent lens match an MFD in image quaility?"

The answer to that question is a firm and unequivocal "maybe". Why, because we have to many unknown parameters:


  • What is image quality?
  • Which MFD
  • What conditions
  • And some others

Let us look at some basic stuff

Regarding image quality we may discuss 

  • MTF - the Modulation Transfer Function
  • SQF - Subjective Quality factor - an interpretation of MTF
  • SNR - Signal to Noise Ratio - how clean is the image
  • DR   - Dynamic range closely related to noise in the darks

MTF

Well, what is MTF? 

In digital photography we use a lens to render an image on a sensor. That sensor usually has an OLP filter that is used to reduce aliasing and also pixels having physical size.

Any image passed to a system like this will loose some information at each step.  MTF is just a value of how much information is kept. MTF is calculated for detail of different size. A typical MTF curve looks like this:



At left and which corresponds no details, MTF is always 100%. So this says that we are loosing information for small details. This way of presenting lens performance is very useful.

One of the nice properties of MTF is that we can regard different part of the imaging chain to have each's specific MTF and the resulting MTF is just a product of the MTF of the components involved.

There have been a lot of research of MTF and human vision, and it was found that it is the low frequencies that correspond to perceived sharpness, while the medium frequencies define texture. What is a good MTF, by and large a curve that goes high. and drops slowly. The left side is far more important than the right side. As a fact everything above 73 lp/mm will just give artefacts. The optical system above will produce a lot of moiré.

Now, let us assume that we take a picture with my best Hasselblad lens mounted on my P45+ back, let us also take a picture with my best Sony lens on the A7rII and measure MTF for both.


Ouch, here we see that the Sony lens on Sony A7rII has much higher MTF! But it doesn't make it a better combo. Why because we need to enlarge the Sony image more than the the Hasselblad image.

Lets assume that we want to make 100x70 cm print (about 30"x40") the P45+ image is 37 mm in height and the Sony image is 24 mm. So we need to enlarge Sony 100/2.4 -> 42x
while the Hasselblad needs 27 times enlargment. So lets rescale the Sony data with this magnification difference taken into regard.  

Now we get the following curves:

So, now the Hasselblad/Planar/P45+ is on top. On the left side the difference is not that large at say 72 lp/mm the the P45+ has say MTF = 0.3 while the Sony has 0.2 a 50% advantage. 
Now as said before the left part is most important for viewing and here the two systems are close. But when we pixel peep the P45 at 72 lp/mm on the P45+ or 110 lp on the Sony, the P45+ will be at advantage.

Now, the MTF values here are pretty low. In real world we would apply some sharpening.



Here we see that MTF gets much higher and stays around 1.0 up to perhaps 55 lp/mm, but even the parts around 70 lp/mm would be considered sharp. Human vision seems to regard eveything above 35% MTF to be sharp.




A practical example

I shot a subject a few years ago and found at that time that I could see little difference in prints up to A2-size (16"x23") althoug my 39 MP MFD image had better detail than my 24 MP 24x36 mm camera. Now 42 MP has arrived from my 24x36 device maker, Sony. Also I got a new Sony 90/2.8G Macro lens that is said to be pretty sharp. So time for a reshoot. The 90 mm on the Sony is a close match for the Macro Planar 120/4 regarding angle of view. Also the cameras are close in number of pixels, so we can compare the images without rescaling. The Planar P45+ combo was shot at f/11 and the Sony at f/8 which gives roughly equivalence in matters of DoF. The Planar 120 is optimised for near field. Zeiss says that the Planar 120/4 is preferred for subjects small than one square metre, and the Planar 100/3.5 for larger subjects. This subject is a bit borderline. 

The images below show the intended point of focus. My take is that one may be better than the other but there is not an earthslide difference.




P45+Sony A7rII
The image below is a "bokeh" sample, containing parts near optimum focus and parts clearly out of focus.
P45+Sony A7rII

Conclusions

I would not draw a lot of conclusions based on just two images. Obviously there are processing differences, although I made my best to keep them similar. What I see may be that the images are pretty similar. 

Some reflections

  • One of the nice features with the Sony is that it can be useed with a lot of lens solutions due to it's short flange distance. I use it with a Canon 16-35/4 zoom that gives extremely good results but also offers some tilt and shift capability when used with my HCam Master TSI, this is really enbaling technology. I can do things I dreamed of doing for many years. The HCam Master TS also work with all my Hasselblad V-system lenses. 
  • I feel that the Sony A7rII holds up well with low end MFD. I don't think 40-50 MP CCD devices have any advantage over the Sony. The exception may be flash photography outdoors. CMOS devices that is a different thing. I am certain that any 50 MP MFD device has an advantage over Sony's 24x36 mm sensors in most aspects. There is no substitue for square centimetres.
  • For anyone already having a significant film based equipment and wanting to use digital, I would say that a P45+ like MFD is a "no brainer", obviously a good investment.
  • Shooting high end MFD on a technical camera makes still a lot of sense, if money is not a great issue. That technology will be best results, once the stars are right aligned.

Canon, Nikon et al.

  • I happen to shoot Sony, but much of what I see also applies to Canon or Nikon. Canon seems to have made a lot of good stuff on lenses and has great camera with the new 5Ds/5DsR. The are where Canon is behind a bit is DR, which is essentially the same as low noise readout. I don't shoot Canon, but I don't think it is a decisive disadvantage.
  • Regarding Nikon, they are known for very good handling readout, but they are a small step behind Sony in resolution. It may be that they may stay a noticeble step behind Canon in sensor resolution. 







Last Updated on Saturday, 02 January 2016 04:17  

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