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Author Topic: Another Stupid Camera Test: IQ 260 / A7r / 5d2 / Epic  (Read 21765 times)
Hans van Driest
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« Reply #160 on: January 13, 2014, 01:51:55 AM »
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Hi,

The short version:

Sensors have gotten better. Making pixels smaller reduces the DR per pixel but the increased number of pixels helps a lot.

The long version, see below:

DR is a bit tricky. The technical definition is FWC/readout noise, with WFC is being Full Well Capacity, the number of electrons a pixel can hold. Readout noise is the noise in reading out the electrons.

For medium size pixels like (6 my) the FWC may be around 60000 electrons FWC is by and large proportional to pixel surface, even if I think some progress have been made. The major factor in DR has probably been the reduction in readout noise. Modern CMOS may have say 3 electron charges in readout noise while CCD is more like 12-15 electron charges.

Let's assume we have a P45+ with 6.8my pixels, and let it compare to a 3.4 my pixel sensor using modern CMOS-technology. Let's assume 65000 electron charges (EC) per pixel for the P45+ and 16 EC in readout noise. So we have 65000/16=4062, converting to EV we get 11.99 EV. DxO-Mark measures 11.75 EV at 50 ISO (in 'screen, mode).

Now, let's take a CMOS sensor with half of that diameter, and assume FWC  65000/4 = 16250, if we assume 4EC in readout noise we would get a DR of 4062 that is 11.98, nearly the same as the "fat pixel CCD".

Now the small pixel CMOS has four pixels instead of one. Would be print both at the same size and assuming a sensor of the same size, the combined FWC would be 65000, read noise adds in quadrature, so we would have sqrt (4 * 4 * 4) = 8

So DR would be 12.98 EV, the smaller CMOS pixels would have a 1 EV advantage over the large pixel CCD. Would we do the same math with a CCD instead, FWC would still be at 65000 but readout noise would be 32. So DR would reduce to 10.98 EV.

Newer generations of CCD sensors have less readout noise than the older ones, but they are still in the low tens. Regarding CMOS, some sensors have external analog-digital converters (ADC), the off chip ADCs tend to be noisy, so these cameras have CCD like readout noise. Many modern CMOS sensors have thousands of on chip converters that work in parallell, these are much less noisy.

What complicates the issues is that DR I describe is a technical definition, and represents a level of noise where the signal is barely perceptible. In photographs we want a better Signal Noise ratio (SNR), on the other hand lower end of the DR scale is normally what I would call deep shadows where we would have little detail anyway.

Midtones and highlights are more affected by "shot noise", the natural variation of incident photons, which is independent of the sensor technology and depends only on exposure, sensor surface and quantum efficiency QE). QE is the percentage of the incoming photons that are detected.

Hope this helps! Here is a recommended paper, describing it more deeply: http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/index.html

Best regards
Erik





When doubling the number of pixels, the dynamic range on a pixel level is indeed reduced. but when looking at the whole 'picture', the dynamic range stays exactly the same (theoretical), since the noise is averaged over more pixels. averaging over double the number of pixels reduces the noise (assuming it really is noise), by 3dB, just like halving the size of the pixel (surface) resulted in an increase of 3dB.
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dayfornight
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« Reply #161 on: March 03, 2014, 08:44:45 AM »
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The thing that surprised me the most is that the 645/35mmSchneider combo doesn't outperform the 35mm/24mmCanon lens one, in terms of distortion.
If anything the walls look more bowed in the former.
I'm very close to spending an awful lot of money on a Silvestri/Rodenstock/phase set up in an effort to make interiors look more natural.
I'm having second thoughts.

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torger
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« Reply #162 on: March 03, 2014, 09:07:03 AM »
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The thing that surprised me the most is that the 645/35mmSchneider combo doesn't outperform the 35mm/24mmCanon lens one, in terms of distortion.
If anything the walls look more bowed in the former.
I'm very close to spending an awful lot of money on a Silvestri/Rodenstock/phase set up in an effort to make interiors look more natural.
I'm having second thoughts.

The 35XL has less distortion than 24mm Canon (the Canon is not too bad though). I'd guess that the 35XL shot is done without distortion correction, and the 24mm Canon is done with. From measurements/data sheets: the 35XL has about 0.5% barrel distortion in its 90mm image circle (I'd say only 75mm of it is high quality though in terms of sharpness), and the TS-E24 has about 0.9% (which is not bad, not sure if it's entirely comparable though as the TS-E24 number is only for the 44mm image, I'd guess the true number is more like 1.5%). Due to the simple symmetric design of the 35XL it's not surprising that it's lower than for the heavily retrofocus TS-E24. In fact I would guess that the 35XL is the lens currently on the market with the least distortion of comparable focal length and image circle.

If you're worried about distortion note that the retrofocus Rodenstock lenses can have some, take a look at the data sheets to get an idea, the 23mm reaches 2%. Retrofocus design has many advantages but one of the disadvantages is that it introduces distortion.

(I own both the 35XL and the TS-E 24 lenses so I could make a test for you if you want to, I'd suspect that the TS-E24 distortion would detoriate more in shifted positions than the 35XL. I'm very pleased by the low of distortion in my 35XL, but I generally shoot landscapes so I'm not looking at straight lines as often as an interior photographer... attached a few examples of the 35XL with straight lines I happened to find in a quick browse, some with substantial shift, none of them is distortion corrected. The bridge might look a bit distorted anyway, because the bridge itself is distorted Smiley )
« Last Edit: March 03, 2014, 09:26:42 AM by torger » Logged
tho_mas
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« Reply #163 on: March 03, 2014, 09:30:07 AM »
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http://www.alpa.ch/en/products/tools/alpa-tools/alpa-lens-corrector-product.html
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torger
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« Reply #164 on: March 04, 2014, 01:51:27 AM »
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An unfortunate thing with lens correctors is that shift numbers are not stored in the exif data. This is actually also the case of DSLR shift lenses. So you need to write them down manually. One thing I like with my symmetrical lenses is that they have so low distortion I can ignore to correct it, so I don't need to keep track of which movements I had. But for extremely critical architecture work maybe even the SK35XL's 0.5% is too much.
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tho_mas
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« Reply #165 on: March 04, 2014, 02:12:39 AM »
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An unfortunate thing with lens correctors is that shift numbers are not stored in the exif data.
With Leaf backs you can add the shift values in the EXIF. Not sure about the IQ backs ...
But you can also just shoot a reference image unshifted to recalculate the shifts later on.
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torger
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« Reply #166 on: March 04, 2014, 02:27:18 AM »
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With Leaf backs you can add the shift values in the EXIF. Not sure about the IQ backs ...
But you can also just shoot a reference image unshifted to recalculate the shifts later on.

I have a Leaf Aptus myself, yes you can enter stuff but I find this to be more cumbersome than to shoot the LCC shot, I'm not too good at using the touch screen.

If back makers really cared they could make a number of things in the GUI to make life easier for tech cam users. Say if you could pre-configure your back with what lenses you have and movement range you have, and had android/iphone style scrollboxes for selecting focal length (which you'd only do when changing lens) and aperture, shift X shift Y tilt and swing, instead of having to type all those details on a on-screen keyboard.
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Gigi
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« Reply #167 on: March 04, 2014, 03:10:44 AM »
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Without any distortion correction, the 35XL holds up pretty well.

Mies at IIT Crown Hall.
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Geoff
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« Reply #168 on: March 04, 2014, 07:01:14 AM »
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I have a Leaf Aptus myself, yes you can enter stuff but I find this to be more cumbersome than to shoot the LCC shot, I'm not too good at using the touch screen.

If back makers really cared they could make a number of things in the GUI to make life easier for tech cam users. Say if you could pre-configure your back with what lenses you have and movement range you have, and had android/iphone style scrollboxes for selecting focal length (which you'd only do when changing lens) and aperture, shift X shift Y tilt and swing, instead of having to type all those details on a on-screen keyboard.
I don't use a Leaf back... but I can imagine that editing the EXIFs is somewhat cumbersome (but at least they offer a solution... which is essentially positiv).
For some reason I can always remember the shift values applied for a certain capture (but I don't shoot high volumes ... and only very few captures of a respective motif). But, again, shooting a reference image unshifted to recalculate the shifts later in post is not a big deal.
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