ISO Expansion - ISO 50, Good or Bad?

[Waddizzle]

I am fairly new to DSLRs, with just over 2 years under my belt, but not SLR photography.  I am still learning all of the nuances that digital photography brings to the table, compared to my film world from 30 years ago.  Let's say I took a sabbatical from the hobby, so I could raise a couple of sons.  The drug store disposables were more economical than using an SLR.

Enough of that.  I have recently rediscovered that my 6D has ISO expansion at the top end, like most Canon DSLRs.  But, it also has an expansion in the low, "L", direction, too.  The higher ISOs tend to introduce noise, and freely admit that I have never used them. 

However, I recall going through the menus early on, and making a decision not to make ISO 50 available for use.  I have run into several occasions where a wide open aperture [f/1.4], ISO 100, and a 1/4000 shutter speed, added up to an overexposure.  I found that I either had to stop down the aperture, which upset the bokeh I wanted, or resort to using an ND filter, which frequently introduce their own set of WB and CA problems.

I do not recall the rationale behind my decision to make ISO 50 unavailable for use.  Does the ISO 50 setting put extrain strain on the sensor or electronics?  At the moment, I cannot think of how it could be harmful.  Obviously, there is some drawback associated with it.  Otherwise, why would it be made an option, like high ISO expansion, instead of simply making it available at all times?

[RobertTheFat]

I may have read somewhere a while back that sensor performance isn't as good at very low ISO settings, especially when the ambient temperature is unusually low. That's the only reason I can think of for avoiding it. If I were in your shoes, I guess I'd give it a shot and see how it works out. If it were liable to damage the camera, I think the instruction manual would warn you.

[Waddizzle]

---

@RobertTheFat wrote:

I may have read somewhere a while back that sensor performance isn't as good at very low ISO settings, especially when the ambient temperature is unusually low. That's the only reason I can think of for avoiding it. If I were in your shoes, I guess I'd give it a shot and see how it works out. If it were liable to damage the camera, I think the instruction manual would warn you.

---

Nah, I'm not worried about damage, no more than using ISO expansion at the top end is a risk.  I guess I'm thinking more about image quality, than actual physical harm.

So, far my test shots at ISO 50 don't look any different from ISO 100.  But, I'm waiting for a brighter sunny day to test it out, instead of the intermittent rain and cloudiness I've had for the past 10 days.   

[kvbarkley]

Generally, the lowest ISO setting is the one that takes the signal directly from the photosensors. All other higher ISO's have amplifiers to boost the signal.

I would guess that an ISO of 50 actually *attenuates* the signal from the photodiodes, so it won't harm the camera but it won't work any better than ISO 100. Think of it as a neutral density filter.

[Waddizzle]

---

@kvbarkley wrote:

Generally, the lowest ISO setting is the one that takes the signal directly from the photosensors. All other higher ISO's have amplifiers to boost the signal.

I would guess that an ISO of 50 actually *attenuates* the signal from the photodiodes, so it won't harm the camera but it won't work any better than ISO 100. Think of it as a neutral density filter.

---

I was thinking the same thing, except that the amplifiers are still in the signal path at ISO 50, but are operating at such a reduced gain level that extra noise is introduced?

[diverhank]

Waddizzle, I think you pretty much answered your own questions on this topic 🙂

ISO 50 is an artificial setting taken from ISO 100 and reduced 1 f/stop by the camera.  The advantage is, as you stated, it allows you to open the aperture or decrease the speed 1 more stop.  The cost of ISO 50 is reduced Dynamic Range.  I do not believe that there's a discernible difference in the noise level between 50 and 100 so ISO 50 should be only used when you need to use the extra stop.

[Waddizzle]

---

@diverhank wrote:

Waddizzle, I think you pretty much answered your own questions on this topic 🙂

ISO 50 is an artificial setting taken from ISO 100 and reduced 1 f/stop by the camera.  The advantage is, as you stated, it allows you to open the aperture or decrease the speed 1 more stop.  The cost of ISO 50 is reduced Dynamic Range.  I do not believe that there's a discernible difference in the noise level between 50 and 100 so ISO 50 should be only used when you need to use the extra stop.

---

Why would Dynamic Range be reduced?  I would not be surprised if would be.  Obviously, there is some down side to it.  Maybe, the sensor is being operated at such a low power, or amplification, that noise creeps in.  Sort of like how driving at a very slow speed dramatically reduces your gas mileage?  That's the best GUESS that I can come up with.

[diverhank · Accepted solution]

---

@Waddizzle wrote:

---

Why would Dynamic Range be reduced?  I would not be surprised if would be.  Obviously, there is some down side to it.  Maybe, the sensor is being operated at such a low power, or amplification, that noise creeps in.  Sort of like how driving at a very slow speed dramatically reduces your gas mileage?  That's the best GUESS that I can come up with.

---

I guess because the image at ISO 100 is darkened to make it ISO 50, you end up losing some highlight end...Dpreview did a review on the 5DMark II and mentioned this... I'm cutting an excerpt from it below:

http://www.dpreview.com/reviews/canoneos5dmarkii/25

[Waddizzle]

Nice info, thanks.

Hmm, that seems a lot like a frequency response thing.  Audio amplifiers can suffer from a similar fault.  They're designed to amplify a range of audio [light] frequencies equally and uniformly at all gain levels. 

However, frequency response of the gain at either the low or high end of the frequency spectrum can drop off, which means the bandwidth becomes narrowed at low gain levels.  

In the case of ISO 50, and also at the high ISOs, the drop off in gain must be fairly uniform across most visible light frequencies.

[ebiggs1]

All sensors can work outside of the ISO settings available in the menus.  A sensor dosen't care what ISO setting is used.  It works the same at whatever ISO setting. The sensor sends the data to the circuitry and it does the rest. The main reason that more ISOs are not offered is they are too far out of reasonable acceptability by Canon.    It does not damage the sensor. 

The early Nikons started at ISO 200.  ISO 100 was considered low.  The settings outside of the ones considered normal are for, you just have to get the shot no matter what situation.  But it may not be up to expected quality.