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7D + 400/5.6 in Tv mode uses 6.3!


tele_tele

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<p>Hi,<br>

with 7D (1.2.2) and a 400mm f/5.6 lens in Tv mode and AutoISO, camera shows and takes pictures at aperture 6.3.<br>

Sometimes it is jumping from 5.6 to 6.3 all the time, mainly settles at 6.3 and then takes pictures at 6.3.<br>

This is happening in AutoISO mode. ISO is greater than 200, so it is not too much light that is problem ( camera is closing down aperture)<br>

Any ideas? :-(</p>

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<p>If you want your lens to shoot at f/5.6 then perhaps you should be using Av and manually selecting f/5.6 yourself. If you want control over both shutter speed and aperture, then choose M.</p>

<p>Maybe I just don't understand your question.</p>

 

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<p>So, either the camera is broken, or it's choosing what it thinks are the best settings.<br>

If the image exposure and focus are on the mark then it's not broken.<br>

If you think you can come up with better settings, then that's what M - manual mode is for.<br>

If you think that the Canon engineers and firmware programmers made a mistake in their calculations, then you should let them know.</p>

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<p>ISO is set on A (Auto ISO) and already high (above 400) as shown in viefinder. <br>

So for example, instead of metering 1/500, f/5.6 and ISO800 camera meters 1/500, f/6.3 and ISO800.<br>

<br />So ISO is already high, but camera wants to close aperture to 6.3.</p>

 

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<p>tele tele, you confused yourself</p>

 

<blockquote>

<p>logically it would increase ISO rather than close down aperture</p>

 

</blockquote>

<p>You should have said: "logically it would <strong>de</strong>crease ISO rather than close down aperture"<br>

or "logically it would <strong>open up </strong>aperture rather than increase ISO"</p>

<p>Also you said "instead of metering 1/500, f/5.6 and ISO800 camera meters 1/500, f/6.3 and ISO800" which should be "instead of metering 1/500, f/5.6 and ISO<strong>640</strong> camera meters 1/500, f/6.3 and ISO800"<br>

If you think the way the camera chose the ISO is not good (bad, wrong, unwise, ... ) then don't use Auto ISO. And ISO 640 and 800 are not much different anyway(maybe a little stop down makes more difference), I believe that happened only for the convenience of the programmer who programs it.<br>

Your conclusion: No more Auto ISO, my conclusion: No more Auto</p>

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<p>Ron:<br />This is how it is on Nikon (which I shoot for years and still do) - in shutter priority mode it always increases just the ISO and leaves aperture at lowest setting (if there is not enough light for base ISO).<br />Only if there too much light for chosen shutter speed it stops the aperture.<br>

I presumed it is the same on Canon or at least documented somewhere if different and how it behaves.<br>

So in situation above Nikon would choose (1/500) f/5.6 ISO 640 (decreasing ISO), Canon choosed 1/500 f/6.3 ISO800 (kept higher ISO, but stopped down 1/3 aperture).<br>

Why would camera close the aperture if there is not enough light in the first place and<br />iso is already high? Instead of ISO800 camera could take picture at<br />ISO640, and picture would be of better quality. <br>

Seems like there is some inside logic that is not documented like ISO-focal length relationship, round ISO number preference or it is just a fault.<br>

Juergen: There is no mention of this in manual - how Tv mode works with AutoISO or AutoISO with focal length. See page 63 and 98-99.</p>

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<p>John:<br />Yes, that was my typing mistake. Thx for correcting.<br>

<br />I really suspect it was programmers choice - keeping round ISO number and prefer to close aperture instead. :-/ <br />From exposure side of things it really doesn't matter (maybe little more noise), but from DOF side (shooting wide open) it does.<br>

BTW Where can one online submit such "errors/request" to Canon?</p>

 

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<p>[[i presumed it is the same on Canon or at least documented somewhere if different and how it behaves.]]</p>

<p>Why not test the reverse? Select Av and check the shutter speeds it picks compared to what you had selected in Tv. (Obviously it would only be a valid test if you did both Av and Tv test in the same light at the same time.)</p>

<p> </p>

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<p>Ron:<br />I think this test wouldn't prove much. Exposure wise 1/500,6.3, ISO800 or 1/500, 5.6, ISO640 is the same. It is camera's choice to close the aperture when light is bad and keep higher ISO, instead of trying to shoot wide open, that is what bothers me and I am also used with Nikon.</p>
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<p>I'll guess, that if you contact Canon, they'll tell you that the noise difference between ISO 640 and 800 is minimal therefore the camera choose 800 and that moving to F6.3 give better image quality than F5.6...... just a guess.</p>

<p>Out of interest, I'd be curious to see a noise comparison test between ISO 640 and 800 on a 7D.</p>

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<p>Brett: I know. Sometimes if background is even and light also I usually shoot in AutoISO off. But if background is very different (dark wood, light grass and bright sky all in same place) and constantly changing light, AV mode with ISO400 for example can be fooled by meter and choose too slow of a shutter speed to freeze action, thus I prefer shutter priority (and on Nikon also Manual mode, since in Nikon Manual mode+AutoISO you can do exposure compensation which you cannot on 7D). But hey, this is another long story... :-)</p>

<p>Marting: I guess you're right. Maybe I wouldn't achieve anything contacting them as they believe that this is right by design (similar to Nikon's preference for wide open aperture in tv). :-( <br />BTW Noise difference isn't any. Picture wise 7D is great. It is the aperture changing that bothered me, good thing is that I changes it always just by 1/3 of stop.</p>

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<p>John: <br /> >Check Custom Function 1-2 maybe you set it to 1 stop<br /> Didn't help. BUT changing Custom Function 1-1 to 1/2 stop (exposure) did the trick! Thanks for helping!<br /><br />It seems as if all this 1/3 exposure up/down twidling causes this effect. <br />Because it always just changed from 5.6 to 6.3 (1/3 stop) no more. Now that this is setup to 1/2 it doesn't have any problems.<br /> I tried it many times, and the "error" is repeatable. BTW I am a SW/HW tester. :-)</p>
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<p>Thanks everyone for quick help, especially John. <br /><br />As this is now understood and cleared, where it is best to contact Canon so they check this option and possibly tweak it?<br /><br />They actually had a similar bug resolved in 1.2.1, which states:<br />"When Auto ISO is used and Program Shift is applied, the ISO setting will be more stable and not change as frequently when the Main Dial is rotated. "<br /><br />This could well be some sort of AutoISO unstable behaviour as it is changing aperture way to quicky. For example I focus on some plant, just moving a centimeter to the right it changes aperture to 6.3 then again few cm away to 5.6 and then it repeats as I move lens a bit. Way too quickly.</p>
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<p>Wow, another bug.</p>

<p>In Tv mode and Auto ISO, setting exposure compensation directly with back dial works (iso decreases), while pushing ISO button and setting exposure compensation with back dial doesn't work (it say so in finder, but metering stays the same).<br /> It seems AutoISO has lots of tweaking to do.<br /> Now seriously, what is the best way to contact Canon international on this issues (in my country there is no service)?</p>

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<p>Maybe it's just a round off display. The correct exposure is possibly F6.0 ISO 800. The camera cannot set F5.6 ISO640 because not enough exposure and it cannot display F6.0 in the finder or in EXIF.</p>

<p>Now you set exposure step to 1/2 stop, the aperture is still F6.0 but it rather displays 5.6 better than 7.2</p>

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<p>All this is fine, but why wouldn't you be shooting in AV to select your aperture and selecting your ISO and letting S/S float within a tolerable range for subject? This way the camera will only be selecting one variable instead of two. Using either TV or AV and then selecting auto-ISO doesn't make sense to me. I use AV and then use +EV or -EV to adjust for the lighting within a scene. I watch the S/S in the viewfinder to make sure it fits my situation and use ISO changes and/or aperture changes to force it up and down.</p>

<p> </p>

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<p>In the various auto ISO threads I see on PN it seems like people are not understanding the purpose of auto ISO. It's not there to help you determine proper ISO because it's somehow hard to do it on your own.</p>

<p>It's there for when low light is an issue and the rapidity of your shooting and decision making does not allow you adequate time to constantly hand-adjust the ISO and test your shutter speed.</p>

<p>It can be very useful in aperture and shutter priority modes. Used this way you're basically in P mode, except that you can give priority to shutter or aperture.</p>

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<p>In exact situation, I guess your Nikon DSLR with Auto ISO would display F5.6 ISO640 Speed 1/500 but since that exposure is not accurate for your situation(remember, I assume ISO 800 F6.0 speed 1/500 is correct). At least one of the three numbers 5.6, 640, or 1/500 are fake you just don't know which ones</p>

<p>Canon just chose the number 5.6-6.3 to be incorrect</p>

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<p>Nicely said. AutoISO saves lives in light challenging conditions. :-)</p>

<p>Using Nikon I see Nikon gets AutoISO better in Tv, M modes and allows more configuration also for A mode and has it available with EC in manual mode (setting both DOF and stopping action while changing ISO) which 7D lacks.<br>

<br />Actually it is the only feature I really miss on 7d and now that I at least know workaround to my original posted problem I can get similar AutoISO behavior on 7D and D300 in tv mode.</p>

<p>Thanks all</p>

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