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Sony Alpha 900 vs. 67 analogue, round 2 - Dynamic range

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Sony Alpha 900 vs. 67 analogue, round 2
Dynamic range
The issues with the red flowers
Good comments from Dominique Ventzke
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What is dynamic range?
Dynamic range is the range of luminance levels that can be handled without excessive noise and acceptable tonal separation. There is a technical definition of dynamic range, that is log(2) of the maximum signal divided by the signal yielding a signal to noise ratio of one. In digital this translates to what is known as "full well capacity" (the number of electrons each pixel can hold) divided by read noise (also in electrons). So DR = log(2) FWC/read noise. There are some issues with this definition of DR for photography, one is that it does not say much about the character of the noise.
It is widely accepted that film has a wide dynamic range. This does absolutely not hold for transparency film, but has a lot of validity for negative film. For film much of the available DR is achieved by compression of shadows and highlights. We can illustrate this by shooting a desnity wedge. The "Stouffer wedge" is one of the more common ones. I have acquired a 41 step wedge for this test, and shot it under identical conditions on three cameras, Pentax 67 with Ektar 100, Sony Alphas 900 and also the Sony Alpha 55 SLT. The film image was scanned on my Minolta Dimage Scan Multi Pro. The images were loaded in the DR module of Imatest, giving the following results.

Th data indicates that both Ektar and Alpha 900 have a similar DR, with the Ektar having about one step wider dynamic. For high quality, the Alpha has wider DR, 6.6 stops instead of 5.3. One very interesting aspect of the curves is that highlights on the Ektar are compressed. Compare the red boxes. This means that highlight contrast is reduced.

 

Note: There are some pitfalls in this measurement, one is being that the scanned data may be affected by stray light in scanning. I plan to do a better test shot and scan both on my CCD-scanner and Dominique's high end drum device.

 

Now, this could also be related to scanner limitations. The pictures below shows a small crop of a 250 MP image. The structure of the wall is here clearly visible on the Sony Alpha image, while the scanned images show little detail. The detail that is visible is much sharper on the "High End" scan than on the other images.

 

Sony Alpha 900 (upsized) Ektar 100, High End Scan at 6096PPI
Ektar 100, Dimage Scan Multi Pro at 3200, upsized

Conclusion: DR on Negative film is similar to a decent full frame CMOS sensor. Film has compressed highlights and may therefore handle highlight smoother at the cost of loosing contrast.

 

Other views on DR

To begin we can look at Kodaks data sheet:

This curve does only show the straing part of the characteristic curve. So it says little about what the shoulder looks like, if we use the pulished data to estimate DR we start at exposure -2.2 end the curces end around 1.2, so that gives a DR of (1.2 + 2.2)  / 0.3 -> 11.3 stops. As said the curves say little about what happens past 20 lux-seconds.

 

Roger Clark has an excellent article comparing discussing film and digital: Clark's article

 

Clark estimates that DR for Velvia to be five steps, Kodacolor Gold seven steps and digital camera to cover 11.7 steps.

My interpretation of Clark's findings is that film gets very noisy at low exposures.

Observations from Dominique Ventzke

What you measured in Imatest was the DR of your particular workflow that includes the signal quality of your scanner. With a different scanner the results may be different.
Tim Gray posted some film DR tests on flickr: http://www.flickr.com/photos/tgray1/collections/72157623656649261/
It’s a pretty good indication of what happens with negative film when over- or underexposed. Note in the 400NC-2 test there is still an image when overexposed 12 stops though the definition and color information is very low at this stage. One of the reasons for the high DR that went into the design of these films was the use in point and shoot film cameras where you have one f-stop and shutter speed, and exposure corrections have to be handled at the scanning/printing stage from the film’s latitude.
You are correct that the increased tonal compression in the shadow and highlight range increases the noise levels within the DR of negative film . I’m not sure how Imatest interprets this as this is actually how we process many images in order to maintain good midtown contrast, and thus the noise gets compressed in the shadow and highlight roll-off. In the highlight part of the curve of color neg film you have reduced levels of detail (resolution) but it still allows the differentiation of extreme brightness levels.
For digital sensors there is no roll-off. You can expose to the right to maximize tonality. But how can you get this soft grading of specular highlights or within light sources like film does? If you want some tonality within those areas you have to set the exposure that nothing clips at all, but that would ruin your shadow detail from the massive underexposure necessary in some situations like high contrast night scenes. Digital technology will probably get there at some point. I found the footage of the Arri Alexa very impressive.
A small remark on the DR of transparency films: These films are designed to look great on a light table or in projection, and thus you get an image with a lot of proccessing already built in which also includes roll-offs in the shadows and highlights. While there is a lot less detail in the highlights and shadows compared to color negs I’ve seen excellent results from slides in high contrast situations. Some reversal films like Fuji Astia 100F or Kodak E100G have a wider DR than others.”

 

 

Resolution and fine detail contrast

Note: I have discovered that the Sony Alpha had some slight camera shake, due to image stabilisation not being disabled. So it would perform a bit better if it was perfectly used. Most findings here are valid. But absolute sharpness cannot be judged. This is of course not much to do about, except to retest, which I will do in due course.

One of the reasons I bought my Pentax 67 was that I wanted to make larger prints with good sharpness and no grain. With digital I started out with APS-C but later upgraded to full frame DSLR. No doubt, I will upgrade when the new 36 MPixel DSLRs arrive, especially after this test. To make large prints we need both resolution and fine detail contrast. The image below demonstrates this quite clearly. The top left image has good contrast (it's blown uop from 24 MP full frame digital), the top right image has good resolution but little contrast, due to highlight compression.

 

 

 

Sony Alpha 900 (upsized) Ektar 100, High End Scan at 6096PPI
Ektar 100, Dimage Scan Multi Pro at 3200, upsized

 

Now, look at the image below, it is another crop from the same set of 250 MPixel images. This part of the image is mid tone. The "High End" scan is definitively the cleanest image.

Sony Alpha 900 (upsized) Ektar 100, High End Scan at 6096PPI
Ektar 100, Dimage Scan Multi Pro at 3200, upsized

Comments from Dominique Wentzke

“I’m not sure if the lack of contrast in the fine wall textures has to do with highlight compression. In my experience this sort of softness or reduced definition of low contrast fine detail is also in the midtones though it is stronger in the compressed highlights. You can also see it in the green leaf crop. The veins of that leaf are more clearly defined in the Alpha image than in the Ektar drum scan.
The tonality in film comes from different sizes of grains and their amount finally building up density. The number of grain sizes in an emulsion layer is limited. Thus it takes a larger area to represent tonal values that only vary very slighty in brightness and hue. Or it takes a threshold contrast to be detected on a small area of film. If the contrast is too low the detail will be lost in the grain structure. Near this threshold film tends to create a textural feel for the detail in a very vague form of rendering. It’s when only a few grains in an area respond to these slight tonal variations. It’s the analog nature that there is something between all or nothing which makes it hard to pinpoint the resolution of a film to a number.
In my experience reversal film is better at the rendition of low contrast fine detail because the tonal range is stored over a larger density range so each recorded f-stop is stretched over much more density than in color negs, and the tonality is not compressed like in color negs. Digital sensors measure the brightness directly and quite accurately wich makes them very efficient at recording very low contrast details as long as it isn’t in the deep shadow end of the signal. I would agree for images where this level of low contrast detail is critical digital is probably better unless you’re using a much larger area of film.”

 



Last Updated on Tuesday, 29 November 2011 00:33  

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