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When and why would you use ISO 50?

Brian Lowe

New member
I am curious who uses ISO 50 and why?

And is ISO 50 cleaner than ISO 100?

A lot of you are experienced shooters in this forum and I would like to hear your thoughts on this.



Once again, my inquiring mind wants to know,
Brian
 

nicolas claris

OPF Co-founder/Administrator
Hi Brian
I tried once, some years back with the 1Ds (not MkII), outdoor in Florida, with the big light working its best. I shot the same scene, in "Bayside" marina, from sky to sea including boats. Same framing both at 50 and 100 ISO.
Can't remember all the details of setting, but the results showed that 100 were much cleaner and well exposed than 50 ISO which was burned into the hilights.
Not a scientific test, but I never tried again 50 ISO!
Why don't you try this comparison yourself with different lighting conditions?
We'll surely be happy to read here the results and your comments!
Best
 

Asher Kelman

OPF Owner/Editor-in-Chief
Brian,

Hope you are feeling better!

The 50 ISO is for when you want to open the aperture one stop more for restricting depth of field, but the light is too bright and you havn't go a sufficiently dark neutral density filter.

Asher
 

Tim_Hurst

New member
Hi there,

Thought I could jump in with a useful first post - (been hanging about for a little while)

Tried ISO 50 on a 1Ds2 a couple of times and have learned my lesson and I will not be using it again! Problems occur with clipped channels in the RAW data (red on both occasions for me) not being represented accurately in the preview jpeg histogram. Shooting waves with a long shutter speed produced clipping which wasn't apparent from the histogram at the time of shoot (no blink warning and histogram looked fine). All the highlights came out a lively pink which ruined the files. Secondly, on an interior job, I purposely blew out a window to white and again it turned out a strong pink once processed which needed retouching. (effect was same in RSP, Camera Raw and C1)

Maybe the in-camera jpegs would fair better as canon can compensate but for RAW not worth using in my opinion.

Cheers

Tim

---------------------------------------
www.timhurstphoto.co.uk
 

Alan T. Price

New member
ISO 50, or more generally any slower ISO rating, allows the use of larger aperture for less DOF, or slower shutter speed for more blur effect (waterfalls, panning, etc.).

The penalty in the case of Canon dSLRs is that ISO 50 is slower than the native sensor ISO and is achieved by fudging the data in such a way that dynamic range is also reduced. This may not be obvious when looking at the LCD for the 8-bit histogram and highlight warnings, but may become obvious when looking at 11-bit raw data (one of the original 12 bits is effectively discarded in the fudging) because the hidden headroom has been reduced particularly at the bright end of the scale.

As brightness comprises set ratios of red green and blue it is most likely that blue or red channel data will be clipped at the bright end in the raw data.
 

John Sheehy

New member
Tim_Hurst said:
Tried ISO 50 on a 1Ds2 a couple of times and have learned my lesson and I will not be using it again! Problems occur with clipped channels in the RAW data (red on both occasions for me) not being represented accurately in the preview jpeg histogram. Shooting waves with a long shutter speed produced clipping which wasn't apparent from the histogram at the time of shoot (no blink warning and histogram looked fine). All the highlights came out a lively pink which ruined the files. Secondly, on an interior job, I purposely blew out a window to white and again it turned out a strong pink once processed which needed retouching. (effect was same in RSP, Camera Raw and C1)

Sounds like the converters weren't aware that RAW data clips lower at ISO 50, so when they performed white balance (which generally means multiplying the red channel by about 1.9, and the blue channel by about 1.3 for daylight), the converters didn't clip the highlights where they should have, so the multiplications were preserved.
 

Paul Caldwell

New member
ISO 50 on the 1ds MKII IMO shows a considerable loss of dynamic range. On the 1ds I don't see this as bad. In my work with water, I would prefer to have the base as 50 or even 25. I am looking for longer expsoure times and not having to use a ND filter.

The ISO 50 shots I have taken with the 1dsMKII show to be more washed out and the colors don't seem as true. RAW shots converted with Capture One.

Paul Caldwell
 

Tim_Hurst

New member
John Sheehy said:
Sounds like the converters weren't aware that RAW data clips lower at ISO 50, so when they performed white balance (which generally means multiplying the red channel by about 1.9, and the blue channel by about 1.3 for daylight), the converters didn't clip the highlights where they should have, so the multiplications were preserved.

Aha, so that's what's going on! Thanks for the explanation.

Tim

---------------------------------------
www.timhurstphoto.co.uk
 

Kevin Bjorke

New member
SO if you REALLY want good ISO 50, use a 2X ND filter @ ISO 100. :)

(On the question of "why," not all good photos have short exposures....)
 
Using ISO 50 and why!

Brian Lowe said:
I am curious who uses ISO 50 and why?

And is ISO 50 cleaner than ISO 100?

A lot of you are experienced shooters in this forum and I would like to hear your thoughts on this.

Once again, my inquiring mind wants to know,
Brian

Hi brian.

With Canon ISO 50 is really ISO 100 using Expose to the Right thinking.

This pushes up the exposure about 1 stop giving more transitional tonal detail. Then the raw converters adjust it back automatically. This reduces noise by exposing shadows 1 stop more.(as-well as the rest)

ISO 50 is best used in a controlled environment such as a studio with flat/even lighting, or on an overcast day to prevent overexposure due to the loss of the top stop of dynamic range.

These two photos were shot with My Canon EOS 1DsMKII at ISO 50, You may recognize them from the OPF home page rotation.

WillThompson_VX8Y8407.jpg
WillThompson_VX8Y0509.jpg
 

John Sheehy

New member
Will Thompson said:
Hi brian.

With Canon ISO 50 is really ISO 100 using Expose to the Right thinking.

This pushes up the exposure about 1 stop giving more transitional tonal detail. Then the raw converters adjust it back automatically. This reduces noise by exposing shadows 1 stop more.(as-well as the rest)

If that is truly the case, then that means that Canon isn't using about 30,000 photons of the approximately 80,000 photon limit for each sensor well. That's not to say that it isn't better in some ways than the other option - 1/2 amplification - as that results in less possible DR at 50.

Has anyone actually tested this? The RAW data from a shot of a Kodak grey step card over-exposed by two stops at both ISO 100 and 50 could be used to determine what is, in fact, happening.
 

Brian Lowe

New member
Will Thompson said:
Hi brian.

With Canon ISO 50 is really ISO 100 using Expose to the Right thinking.

This pushes up the exposure about 1 stop giving more transitional tonal detail. Then the raw converters adjust it back automatically. This reduces noise by exposing shadows 1 stop more.(as-well as the rest)

ISO 50 is best used in a controlled environment such as a studio with flat/even lighting, or on an overcast day to prevent overexposure due to the loss of the top stop of dynamic range.

These two photos were shot with My Canon EOS 1DsMKII at ISO 50, You may recognize them from the OPF home page rotation.

WillThompson_VX8Y8407.jpg
WillThompson_VX8Y0509.jpg


Will,

excellent thank you.

What I am trying to understand is, in a controlled environment is ISO 50 cleaner than ISO 100?

Brian
 

Alan T. Price

New member
I've thought of another possible use for ISO 50. That is when you really want a smaller aperture for the available light but you can't afford a slower shutter speed (for subject movement) to go with it and you know that the extra diffraction will degrade sharpness.

Diffraction may be noticeable at f/11 or thereabouts.
 
Brian Lowe said:
What I am trying to understand is, in a controlled environment is ISO 50 cleaner than ISO 100?

The scuttlebut* is that the native ISOs of the Canon CMOS sensors is 100-1600 with the ISO 50 and 3200 being pulled and pushed by a stop respectively. Hence, I would expect ISO 100 to be cleaner. Would you expect a pull processed ISO 100 slide film to be cleaner at ISO 50 than an ISO 50 slide film?

I would seriously suggest grabbing your tripod and shooting some test shots of clear blue skies. Ensure you test over the range of shutter speeds you expect to use and judge for yourself.

enjoy,

Sean

* i.e., I have only heard this, not seen Canon confirm nor deny it.
 

Alan T. Price

New member
Actually the native ISO is most often 100, with 200 for the likes of the 1D CCD sensor. Anything faster than native is a fudge, but one that can be achieved without apparent loss of dynamic range but definitely with an increase of electronic noise.

ISO 50 and 3200 sacrifice some dynamic range. 3200 is still noisier than 1600 but the DR cannot be maintained. ISO 50 may be cleaner than 100 (I have not tested it) but that cleanliness is at the cost of DR.

When a film is processed there is still a certain amount of film grain inevitable from the nature of film. Digital noise is different. A certain amount comes with the sensor capture, but after that digital is as clean as a whistle.

If you were not worried about dynamic range then you could use curves to wipe the darkest parts to pure clean black without affecting the rest. This will take almost all noise with it. Save the image and open it again, and you'll see everything as it was except the very darkest tones will now be pure black.

DR is technically the same as before because you still have black and all of the bright colours, but there is now a small gap in the tonal distribution near black. Is that a problem ? probably not. It would be if it affected skin tones or sky but not for the deepest shadows.

Our problem is that we want it all - high DR and no noise. That's what our cameras try to give us but as yet they are unsuccessful.
 

Asher Kelman

OPF Owner/Editor-in-Chief
Alan T. Price said:
If you were not worried about dynamic range then you could use curves to wipe the darkest parts to pure clean black without affecting the rest. This will take almost all noise with it. Save the image and open it again, and you'll see everything as it was except the very darkest tones will now be pure black.

In fact, Alan, some advocate giving up the first tonalities to black since the printers hardly resolve them anyway. Then one does not have so much to correct in the luminous channel for noise.

Asher
 

Kevin Bjorke

New member
Alan T. Price said:
ISO 50 may be cleaner than 100 (I have not tested it) but that cleanliness is at the cost of DR.
I'm really curious about this, what sort of DR loss are we talking about (or is it just because of the digital pull-processing which means that you're losing exactly one stop from one end?)?
 
Kevin Bjorke said:
I'm really curious about this, what sort of DR loss are we talking about (or is it just because of the digital pull-processing which means that you're losing exactly one stop from one end?)?

My technical analysis sofar (on my 1Ds2) indicates the opposite, namely an increase in overall dynamic range. I think the confusion stems from the fact that at double the exposure time (and less amplifier gain) we can capture 2 stops over medium gray before sensel saturation, compared to 3 stops over medium gray at ISO 100 and higher. However, the double exposure amount also improves shadow exposure, and its Signal to Noise ratio.

To summarize what I've found sofar:
Depending on the color channel, the read noise at ISO 50 is reduced by approx. 10%, caused by lower amplification, and the S/N ratio is improved (by approx. 30%) because of the double exposure level.

Both contribute to lower overall noise levels, assuming one takes care to 'expose to the right', but not clip the highlights. For those using incident light metering (or a gray card), there are not 3 but 2 stops headroom before the highlights clip at ISO 50.

There is 1/3rd of a stop additional headroom in all cases, but that will cause colorshifts in the highlights, due to non-linearity of the sensor response near saturation.

Bart
 
Mike Broomfield said:
Here is a link to a DPreview test that cleatrly shows the lower DR in the highlights using ISO 50

http://www.dpreview.com/reviews/canoneos5d/page22.asp

That particular (over-)exposure example will cause enough lens-flare to reduce (mostly) shadow density (and consequently DR). A less flare sensitive test, e.g. such as described at http://www.clarkvision.com/imagedetail/evaluation-1d2/index.html, gives something closer to sensor response data, instead of system (lens+camera) data which varies by lens used.

I have conducted an even more detailed analysis of my EOS-1Ds Mark II (admittedly a sample of 1 camera unit) which indicates a markedly different (less clipping headroom, but more shadow detail due to lower ADC amplification) response at ISO 50. The net result is a larger DR.

Bart
 

John Sheehy

New member
Bart_van_der_Wolf said:
I have conducted an even more detailed analysis of my EOS-1Ds Mark II (admittedly a sample of 1 camera unit) which indicates a markedly different (less clipping headroom, but more shadow detail due to lower ADC amplification) response at ISO 50. The net result is a larger DR.

What is the ratio of the highest RAW value (minus the blackpoint) at ISO 50 to the read noise (both in ADUs)? What is the blackpoint?
 

Bob Cooper

New member
I am curious who uses ISO 50 and why?

And is ISO 50 cleaner than ISO 100?

A lot of you are experienced shooters in this forum and I would like to hear your thoughts on this.

Once again, my inquiring mind wants to know,
Brian

I'll use ISO 50 when shooting portraits outside in bright sunlight and using fill flash. ISO 50 alows me one more stop before I have to push my flash into High Speed Sync. This gives me faster recycling and more reach with the flash. This one stop also allows me to open the lens for less DOF which I prefer when shooting portraits.
 
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