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Author Topic: Auto iso capability  (Read 4469 times)

BernardLanguillier

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Auto iso capability
« on: September 14, 2006, 09:53:48 am »

Michael,

As a reaction to your valid praise of the auto iso capability of the promising new Pentax DSLr, the Nikon D80 does have a similar feature called "auto ISO".

You can set the minimum shutter speed and max iso, the camera does the rest.

It is however not possible to set a lower limit with the Nikon.

The fearure looks useful at first, but is pretty much unusable with zoom lenses like the 18-200 since you cannot define the limit shutter speed as a relation to the focal lenght. Do you know how Pentax decided to implement their version?

Regards,
Bernard

SteveKoz

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Auto iso capability
« Reply #1 on: September 14, 2006, 10:41:38 am »

I find the Auto-ISO on my Nikon D200 to be useful at times. One technique, while it can't be done from the Mode dial like the K10d, is to turn on Auto-ISO and then select Manual Mode. Lock in shutter speed and apeture.  

There is a race horse training facility that I like to visit occasionally. I get to see some sharp horses and to practice my action shooting. I recently visited and I used this technique to freeze action and lock-in my depth of field.  I found it to give great results.

-Steve
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John Sheehy

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Auto iso capability
« Reply #2 on: September 14, 2006, 11:15:42 am »

Quote
I find the Auto-ISO on my Nikon D200 to be useful at times. One technique, while it can't be done from the Mode dial like the K10d, is to turn on Auto-ISO and then select Manual Mode. Lock in shutter speed and apeture. 
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That's what I would like to see in future Canons, at the least.  Manual absolute exposure with floating ISO.

Limiting the max ISO makes no sense at all in manual exposure mode, though.  Many people have requested that, but it is only going to ruin images, as under-exposure is anywhere from slightly worse to very worse in terms of noise, stop-for-stop.  If you're going to limit ISO, it should only be done if there is a scheme to open the aperture or lengthen exposure (by user preference, preferably).

I am worried that Canon won't do it right, however, if they do implement it.  Canon's 1/3-stop ISO resolution seems to be quality-reducing trickery for the most part, so I certainly wouldn't want to see 30D- or 5D-like 1/3 stop ISOs floating around in manual exposure with auto-ISO.  Rather, only the really optimized ISOs should be used, and there should be user-settable compromises to achieve consistent RAW exposure.  A list of things to do in order should work:

1. open Av 1/3 stop

2. lengthen exposure 1/3 stop

Each of the two settings can be any of "nothing", or the options used in #1 or #2.

It could go further than two, to deal with potential under-exposure at the highest real ISO (usually 1600 for current Canons).  If you really want to stick to the TV and Av settings, this option can be disabled, or "nothing" all through the list.  You could do two stops of "nothing", and then open the aperture when you're at an exposure index of 6400, and then go with Av and TV compromises to maintain the 6400 EI.

An EC control should also be available, to compensate for scene key and contrast like you can do with regular AE modes.
« Last Edit: September 14, 2006, 07:26:58 pm by John Sheehy »
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Ray

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Auto iso capability
« Reply #3 on: September 15, 2006, 09:16:41 pm »

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I am worried that Canon won't do it right, however, if they do implement it.  Canon's 1/3-stop ISO resolution seems to be quality-reducing trickery for the most part, so I certainly wouldn't want to see 30D- or 5D-like 1/3 stop ISOs floating around in manual exposure with auto-ISO. 

John,
That's a good point. I've often wondered why Canon have not already implemented this concept of a floating ISO that adjusts itself to a manual setting of exposure and aperture. The obvious problem, which you've tried to address with points 1 & 2, is the likelihood that for many combinations of exposure and aperture, ISO 1600 is not going to be nearly high enough. The inexperienced user is going to frequently fall into the trap of getting ISO 1600 shots that are not just 1/3 or 2/3 stop underexposed, but 1, 2 or even 3 stops underexposed, and we all know that the consequences of underexposure at maximum (real) ISO are serious noise and image degradation.

Once you start messing around with additional, user settable conditions to the automatic ISO feature, there's not much point in having it. The whole purpose of automatic adjustments is to facilitate the taking of acceptably good shots instantly with the minimum of fuss.  For example, let's suppose I'm in a situation with poor lighting and I want a fast shutter speed and good DoF without compromising image quality. Well, without flash I simply can't get it. I have to sacrifice something. I've basically got 3 choices. (1) Fast shutter speed, good DoF but  underexposure. (2) Fast shutter speed, shallow DoF, accurate exposure. (3) Slow shutter speed, good DoF, accurate exposure but blurred shot.

The solution is bracketing. I can bracket for either 'time value' or 'aperture value' (TV or AV). If my lens doesn't have a wide enough maximum aperture for the autobracketing intervals I've set, I'll simply get a repitition of the same exposure at the same maximum aperture.

What might be useful is autobracketing of ISO in relation to a fixed aperture and time value, but on reflection, I'm not sure just how useful. Lets see what happens in the above example of poor lighting conditions. Since I'm a fairly experienced user, I'll set the ISO to 1600. Generally, a sharp image with shallow DoF is preferrable to a blurred shot with extensive DoF, so I'll set the camera to TV mode and a shutter speed to whatever I think will freeze the action. Depending on the shutter speed, the results might vary from all 3 shots being identically underexposed at the same maximum aperture, to all 3 shots reflecting accurately the EV interval I've set.

If  were able to bracket for ISO, I could expect the same result, ie. the 3 shots would vary from  being all equally underexposed at ISO 1600, to all accurately reflecting the EV interval set. There could be an advantage there if one of the shots was correctly exposed 'to the right' at, say ISO 800 instead of overexposed at ISO 1600 with blown highlights. This option of (sometimes) being able to not use a higher ISO than is necessary, I would consider useful.

Another useful feature which could flow on from a 'floating' ISO is the facility of autobracketing DoF whilst maintaining a fixed 'accurate' exposure (as opposed to a fixed shutter speed or time value). At present, with camera in Tv mode, I can only bracket for DoF concomitant with varying exposure, which means the shot with the ideal DoF might be either under or overexposed. With a floating ISO that changes with changing aperture to maintain a constant 'exposure', I could get a real choice of DoFs easily and quickly. This is something Canon should look into.
« Last Edit: September 15, 2006, 09:34:22 pm by Ray »
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John Sheehy

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Auto iso capability
« Reply #4 on: September 16, 2006, 01:01:34 am »

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Once you start messing around with additional, user settable conditions to the automatic ISO feature, there's not much point in having it.

I don't think so.  A few user-settings can make the camera do the types of things I would do myself if I had the time to pay attention to all the details.  That would make it all the more worthwhile to have.  A system like the one I suggested would allow me to use the real (optimized) ISOs, while also being able to fine-tune the EI with the EC wheel as I do now, for maximum ETTR.  1/3 stop is much more significant to exposure and noise than it is to DOF or action-stopping ability, so fine-tuning EC is always a must.  Unless Canon comes up with a way to maintain its low noise at intermediate ISOs (with consistent highlight headroom), I don't want the intermediate ISOs in auto-mode, and something is needed to reduce granularity of exposure index, hence, the user-settable concessions on Av and Tv values.  All I'd have to do, really, is set my Av and Tv values a little bit on the low-exposure side from my ideal, and the compromises that they may make to perfect exposure will average about my real ideals.


Quote
The whole purpose of automatic adjustments is to facilitate the taking of acceptably good shots instantly with the minimum of fuss.  For example, let's suppose I'm in a situation with poor lighting and I want a fast shutter speed and good DoF without compromising image quality. Well, without flash I simply can't get it. I have to sacrifice something. I've basically got 3 choices. (1) Fast shutter speed, good DoF but  underexposure. (2) Fast shutter speed, shallow DoF, accurate exposure. (3) Slow shutter speed, good DoF, accurate exposure but blurred shot.

The solution is bracketing.
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Bracketing is a good feature, and I think that the cameras should allow bracketing of more things than they currently do, but bracketing is not a practical solution in the type of shooting where I need auto-ISO the most.  Bracketing guarantees that some percentage of your shots are non-optimal, so in the case of timing-critical shooting, you will be reducing the number of keepers.  If you've ever shot small insect-eating birds, you would have windows of opportunity measured literally in milliseconds.  A bird will appear unobscured, on a branch, for less than 1/4 second, and you have that 1/4 second to point (find the bird in the "telescope") focus, and compose.  9 out of 10 times the bird is no longer in position as the shutter opens, and you have to look for it again (if your projection of its motion is wrong).  If you're bracketing, you may be in the poorest bracket when your opportunity arises.
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SteveKoz

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Auto iso capability
« Reply #5 on: September 16, 2006, 09:03:17 am »

Unfortunately Auto-ISO is not the cure for lack of light.    
If Canon wanted to add the feature they might want to beg, buy or borrow a Nikon D200. The implementation is rather straight forward.  Once I set my ISO top end via menu, the EV indicator in the viewfinder will indicate over or under exposure when Auto-ISO can no longer compensate. This is just what you would see if you were working w/ fixed ISO.  Of course in Auto-ISO you have to be paying attention. If metering conditions are challenging, you can adjust EV which changes the “zero” position in Manual Mode.  

Bracketing could also be applied but, like John, when I am using Auto-ISO I am typically in one shot territory.
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John Sheehy

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Auto iso capability
« Reply #6 on: September 16, 2006, 10:43:35 am »

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Unfortunately Auto-ISO is not the cure for lack of light.   
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It won't increase the light, but at least if it goes to the highest (real) ISO, you will get less readout noise, relative to signal, as compared to using the same Av and Tv settings at a lower ISO.  Few things are uglier than an ISO 100 shot exposed with Av and Tv values that should have been shot at 1600.
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Ray

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Auto iso capability
« Reply #7 on: September 16, 2006, 03:31:48 pm »

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  A bird will appear unobscured, on a branch, for less than 1/4 second, and you have that 1/4 second to point (find the bird in the "telescope") focus, and compose.  9 out of 10 times the bird is no longer in position as the shutter opens, and you have to look for it again (if your projection of its motion is wrong).  If you're bracketing, you may be in the poorest bracket when your opportunity arises.
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In this situation, you would probably want a combination of auto aperture and auto ISO in something like a Tv mode, wouldn't you? If you've only got once chance at the shot within a few milliseconds, it would be a pity to underexpose at ISO 1600, which is quite likely considering how variable lighting conditions are in foliage.

Perhaps we could have an option of fixing the time value and aperture in such a way that once the ISO has reached its maximum, instead of underexposing, the camera will open up the aperture to the point where either a correct exposure is acheived or the underexposure will occur at both full aperture and maximum ISO.

But this is hardly better than the currently available option of putting the camera in Tv mode and manually setting the ISO to 1600. What I miss when I look through the viewfinder of my Canon DSLRs is the facility of changing ISO on the fly whilst continuing to look through the viewfinder.
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