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Exposure guidance, I don't get it
Just got my first DSLR (Canon 40D) and am trying to learn how to use the shutter priority and aperture priority modes. I thought a good way to get a feel of how to set them would be to see what choices the camera made in auto mode and then apply these to the manual modes..

I was shooting indoors in fairly low light with a 24-105 f4 L..

In Auto mode the camera was setting 1/60 f4 ISO400& and the picture came out well.

Similarly in Program mode the camera once again chose 1/60 f4 ISO400 giving the same results.

In Tv mode I then set the shutter to 1/60 to recreate these settings however the aperture value (f4) started flashing which according to the instruction manual means that the camera is trying to tell me that my settings are inappropriate and the image will be under exposed. I took the photo anyway& same results as above..

I then altered the shutter to a higher value, 06 @ f4, to stop the aperture value from flashing which should mean a correct exposure according to the instructions. I took the photo, horribly over exposed..

In Av mode I could not recreated the settings since it sets the shutter automatically at f4 to a value higher that 1/60..

Finally I tried Manual mode set up to 1/60 f4 ISO400 and the exposure level indicator was flashing at -2, again meaning the camera thinks it will be over exposed. It stop flashing at 06 f4 once again..

So my question is, why does the camera calculate exposure correctly in Auto and Program mode but in Tv, Av and Manual modes the exposure guidance always tells me to set it to a different value to what Auto and Program gave me?.

(meetering, white balance, and everying else was set identically in all modes)..

Comments (19)

I do not have this or any Canon DSLR..

I presume you had exactly the same zoom settings and were photographing exactly the same image?.

From the review on this site:.

"In the basic zone the camera will indicate that blur may occur because of slow shutter speeds, it does so by blinking the shutter speed on the LCD panel and viewfinder status bar.".

Http://www.dpreview.com/reviews/canoneos40d/page7.asp.

Commentary below:.

In Auto mode the camera was setting 1/60 f4 ISO400& and the picturecame out well.

Similarly in Program mode the camera once again chose 1/60 f4 ISO400giving the same results.

In Tv mode I then set the shutter to 1/60 to recreate these settingshowever the aperture value (f4) started flashing which according tothe instruction manual means that the camera is trying to tell methat my settings are inappropriate and the image will be underexposed. I took the photo anyway& same results as above..

The maximum aperture on this lens is f/4. My guess is that you had moved the camera slightly and the camera wanted to select f3.5 which was not available. Try to repeat your "test" in better light at say f/5.6 and 1/100th..

I then altered the shutter to a higher value, 06 @ f4, to stop theaperture value from flashing which should mean a correct exposureaccording to the instructions. I took the photo, horribly overexposed..

That makes no sense. In TV mode you set the shutter speed and the camera sets the aperture. So it should have changed to an equiavlent figure. However a change of shutter speed to what looks like 6 seconds would have the aperture closed down to minimum and with nowhere to go. I would have thought the next click stop from 1/60th would have been 1/30th which would probably have got you an aperture of f/5 all else being equal..

In Av mode I could not recreated the settings since it sets theshutter automatically at f4 to a value higher that 1/60..

That tends to indicate that there were slight variations of the scene you were shooting..

Finally I tried Manual mode set up to 1/60 f4 ISO400 and the exposurelevel indicator was flashing at -2, again meaning the camera thinksit will be over exposed. It stop flashing at 06 f4 once again..

I still do not understand "0"6". What shutter speed is that 1/6th? 1/15th? 6 seconds?.

So my question is, why does the camera calculate exposure correctlyin Auto and Program mode but in Tv, Av and Manual modes the exposureguidance always tells me to set it to a different value to what Autoand Program gave me?.

I have a feeling that you have dialed in exposure compensation (EV) at +2. That will have no impact when the camera is fully automatic but will kick in with semi-auto and manual. If "0"6" is 1/15th that is two stops way from 1/60th. That would fit..

Chris Elliott.

*Nikon* D Eighty + Fifty - Other equipment in Profile.

Http://PlacidoD.Zenfolio.com/..

Comment #1

Devnull wrote:.

You did use flash, didn't you?.

How is that going to help explain his results? There appears to be a metering problem. You are just adding another variable by introducing flash..

Chris Elliott.

*Nikon* D Eighty + Fifty - Other equipment in Profile.

Http://PlacidoD.Zenfolio.com/..

Comment #2

DesperadoGary wrote:.

Just got my first DSLR (Canon 40D) and am trying to learn how to usethe shutter priority and aperture priority modes. I thought a goodway to get a feel of how to set them would be to see what choices thecamera made in auto mode and then apply these to the manual modes..

Yes, well, you shouldn't assume that there is any intelligence in the settings chosen by the camera. It can't "think" for you! It can only apply an algorithm from the programing it was built with..

In Auto mode the camera was setting 1/60 f4 ISO400& and the picturecame out well.

Fine. Let's take a look at what the camera has done in a fully progammed mode ....

1) It noted the low level of light and switched to a higher ISO to help ameliorate it..

2) It then selected the slowest shutter speed that might reasonably be expected NOT to cause camera shake at the f-length you had selected.. slowish to help with the low light, but not so slow you got blur. It came up with 1/60th..

3) The camera then assumed it was being pointed at a part of the subject that was about mid-range in tone between the lightest it could record and the darkest it could record in any ONE photograph, and then....

4) It selected the one and only aperture setting that would yield correct exposure at the light level and ISO of (1) above, commensurate with the shutter speed of (2) above and the tonal assumption of (3) above..

Resulting in 1/60th @ f/4 on 400ISO.

That is ALL it did. The camera is not an authority on the finer points of photography. It did NOT choose an *ideal set of settings*. It merely chose settings which would *work pretty well, to give a usable result,most of the time*..

Note: In this case it was particularly stuck on aperture... f/4 was the widest the lens had..

Okay?.

Similarly in Program mode the camera once again chose 1/60 f4 ISO400giving the same results.

That is to be expected!.

In Tv mode I then set the shutter to 1/60 to recreate these settingshowever the aperture value (f4) started flashing which according tothe instruction manual means that the camera is trying to tell methat my settings are inappropriate and the image will be underexposed. I took the photo anyway& same results as above..

Okay. .

I then altered the shutter to a higher value, 06 @ f4, to stop theaperture value from flashing which should mean a correct exposureaccording to the instructions. I took the photo, horribly overexposed..

Let's examine that, bit by bit.......

The reason the 1/60th gave two different responses depending on mode, is because the readout isn't finely enough gradated to show all the tiny fractional parts of a second shutter speeds that are available to the AUTO exposure system. For this reason the DISPLAY ALONE is rounded up or down to the closest value. The actual exposure system is, in all likelihood, giving something slightly different... in this case something 'more', it seems..

I suggest you check the EXIF file. It usually shows a more exact fractional shutter speed for any auto-exposed shots..

Example: 1/134 sec. where LED display read 125..

However, this fine shading of shutter speeds by exceedingly small increments is not available to the M-anual settings of the camera. In Manual the shutter speeds are exactly as displayed, and if that is even slightly out from a theoretically correct exposure, the camera will indicate "incorrect" exposure. In these circumatances, ignoring the display flashing and shooting at an ostensibly incorrect exposure will result in a shot that is (theoretically) incorrectly exposed, of course, but it may be a much smaller error than the eye can see. (1/10th stop is enough to show on a flashmeter, but you would be hard-pressed to see it on any screen!!).

However, in your case we are not talking about tiny errors!!.

Clearly, unless the camera has gone wrong, the severe overexposure is because you wound up the shutter speed to a setting that was much TOO long for the conditions, and here I have an extra difficulty...I'm afraid I do not know exactly what the value - 0"6 - means on your camera; without the manual I'm just guessing......

[six seconds?? one sixth of a second? zero-point six of a second? dunno, I'm floundering].

... nevertheless, I would expect a mere 1/3rd stop increase to be enough to stop the warning flashing in the conditions you described.... a simple 1/3rd increase would have resulted in an indicated exposure of 1/50th [shown thus.. 50] or possibly 1/45th [shown thus... 45].

To close up this point, I do not know why your camera did not warn you of gross overexposure... There may be some other factor involved.. but I can't think of one right now..

[continued in next post.................... ]Regards,Baz..

Comment #3

Chris Elliott wrote:.

I do not have this or any Canon DSLR..

Good start. Let's add some more variables, in the form of my comment on your comment, shall we?.

I presume you had exactly the same zoom settings and werephotographing exactly the same image?.

He probably tried to..

Commentary below:.

In Auto mode the camera was setting 1/60 f4 ISO400& and the picturecame out well.

Similarly in Program mode the camera once again chose 1/60 f4 ISO400giving the same results.

In Tv mode I then set the shutter to 1/60 to recreate these settingshowever the aperture value (f4) started flashing which according tothe instruction manual means that the camera is trying to tell methat my settings are inappropriate and the image will be underexposed. I took the photo anyway& same results as above..

The maximum aperture on this lens is f/4. My guess is that you hadmoved the camera slightly and the camera wanted to select f3.5 whichwas not available. Try to repeat your "test" in better light at sayf/5.6 and 1/100th..

Why don't we have another look at the settings: wide open and 1/60. Both in P and Auto. And that in low light, iso 400. Does not ring a bell? It's very likely because in both A and P mode the camera will select 1/60 sec for flash exposure..

I then altered the shutter to a higher value, 06 @ f4, to stop theaperture value from flashing which should mean a correct exposureaccording to the instructions. I took the photo, horribly overexposed..

That makes no sense. In TV mode you set the shutter speed and thecamera sets the aperture. So it should have changed to an equiavlentfigure. However a change of shutter speed to what looks like 6seconds would have the aperture closed down to minimum and withnowhere to go. I would have thought the next click stop from 1/60thwould have been 1/30th which would probably have got you an apertureof f/5 all else being equal..

This makes quite a lot of sense. The camera metered for the background. Which required 0.6 seconds at f4..

In Av mode I could not recreated the settings since it sets theshutter automatically at f4 to a value higher that 1/60..

That tends to indicate that there were slight variations of the sceneyou were shooting..

I am not sure about what the OP meant, either. I guess the camera wanted longer exposure..

Finally I tried Manual mode set up to 1/60 f4 ISO400 and the exposurelevel indicator was flashing at -2, again meaning the camera thinksit will be over exposed. It stop flashing at 06 f4 once again..

Yup, that's it. Low light..

I still do not understand "0"6". What shutter speed is that 1/6th?1/15th? 6 seconds?.

0,6 seconds..

So my question is, why does the camera calculate exposure correctlyin Auto and Program mode but in Tv, Av and Manual modes the exposureguidance always tells me to set it to a different value to what Autoand Program gave me?.

I have a feeling that you have dialed in exposure compensation (EV)at +2. That will have no impact when the camera is fully automaticbut will kick in with semi-auto and manual. If "0"6" is 1/15th thatis two stops way from 1/60th. That would fit..

Yeah, he must have dialled EC at +5, if you do the math... not..

OP, again, you did use the flash, didn't you?.

It's a bit tricky to explain your results without seeing the images and the EXIF info, but it basically goes like this:.

- in A and P mode, in low light, the camera opens up the iris, sets a 1/60 exposure time and uses the flash to fill in.

- in Av mode, the camera meters for the background and uses the flash to fill in the subject. In low light that will mean LONG exposure times.

- in M mode - which is the normal mode to use the flash with, the camera uses the aperture and shutter speed values you gave AND uses the flash to fill in. You can "control" the amount of flash using flash exposure compensation..

However, you shouldn't have overepxosed images. I would guess two possibilities:.

- having dark/black fooling the flash meteror.

- you have played with the flash settings - but it should be visible in P mode as well..

There is also the posibility you did altered settings between the shots, in which case all bets are off..

/d/n..

Comment #4

Continued from part 1.....

In Av mode I could not recreated the settings since it sets theshutter automatically at f4 to a value higher that 1/60..

By "higher"... do you mean a longer or shorter time value? Remember, lower shutter speeds mean HIGHER volumes of light entering the camera..

Where you point the camera can make big differences to the exposure, for instance, aiming at the wall just one foot closer to the main window can double the amount entering the lens. I suspect that it's something like that happening here. Indeed, no metering comparisons like this are really valid unless the camera is locked down on a tripod and light levels are also stabilised..

Finally I tried Manual mode set up to 1/60 f4 ISO400 and the exposurelevel indicator was flashing at -2, again meaning the camera thinksit will be over exposed. It stop flashing at 06 f4 once again..

Sorry. This I don't understand one little bit. Minus 2 stops is a big difference, and 0"6 (whatever the nomenclature actually means in your camera) is not 2 stops away from 1/60th, whichever way you slice it. Minus 2 stops from 1/60th is 1/250th which would require a very bright interior indeed, even at 400ISO..

On the other hand, if the flashing was indicating the need for an increase exposure of two stops (an unlikely amount, in view of the indications you got in auto modes) then the "correct" setting would be 1/15th second.....

Either way, we know 1/60th is right, or very close to right .... so, unless you are swinging that camera about all over the place... (and none of this is worth a candle)......

...AND bearing in mind that neither 1/125th or 1/15th have got any sixes in them.....

... There's definitely something screwy somewhere!! ... and I will be intrigued to see what other people make of it!! .

So my question is, why does the camera calculate exposure correctlyin Auto and Program mode but in Tv, Av and Manual modes the exposureguidance always tells me to set it to a different value to what Autoand Program gave me?.

I have explained how differences in the displayed values can occur, even when the camera is locked down on a constant subject tone in constant lighting, but why you are getting the actual oddball values you are, is NOT clear..

(meetering, white balance, and everying else was set identically inall modes).

Q1: Does you standardisation include fixing where the camera was pointed, and the levels of light falling on the subject?.

Q2: Are you able to verify that you are reading this 0"6 figure correctly, and ascertain what fraction or multiple of a second it represents?.

(for instance, how many clicks away from 1/60th was it, and what were the intermediate values that appeared in your readout? That would help me... )Regards,Baz..

Comment #5

It would have been helpful if you had mentioned that..

I will leave you to sort it out with others rather than waste more of my time..

Chris Elliott.

*Nikon* D Eighty + Fifty - Other equipment in Profile.

Http://PlacidoD.Zenfolio.com/..

Comment #6

I'll try it out this weekend in day light to see if I get anything more sensible out of it. I've only had time to play after work so far, when it's dark..

Admittedly my tests haven't been under strick scientific conditions, sat on the sofa aiming at the same object on the floor..

So basically, in auto & program modes it gives me a shutter speed of 0.016 seconds at f4 and in Tv and Maunual modes it suggests a shutter speed of 0.6 seconds would be more appropriate at f4 i.e. 36 times longer resulting in overexposure..

The camera is probably assuming slightly more intelegience from the user in the Manual modes...

Comment #7

DesperadoGary wrote:.

Flash was used.

(sigh) Thanks for letting us know. A bit late, perhaps, but there you go..

Myself, I do not have sufficient knowledge of just how the camera's flash capabilities are woven into it's different modal exposure algorithms to help you further....

... so I will leave you in the hands of the other more Canon-savvy responders..

(At least I found out was screwy.. (shrugs)).Regards,Baz..

Comment #8

Devnull wrote:.

You did use flash, didn't you?.

You were right. I will leave you to sort out why he got the results he did. No doubt there is some other variable the OP just happens to have forgotten to mention..

Chris Elliott.

*Nikon* D Eighty + Fifty - Other equipment in Profile.

Http://PlacidoD.Zenfolio.com/..

Comment #9

As I stated, the same settings were used in all scenarios..

Therefore since auto mode will automatically apply the flash in low light with no way to override (unless you turn the dial to no-flash mode, which isn't auto-mode), it can be read that I used flash in the program modes, otherwise the test would really not be valid...

Comment #10

The exposure modes work differently when a flash is mounted (or the pop up flash is up) from when there is no flash. (it will be in the manual on the flash pages)..

With the flash mounted you have two sources of light, the ambient light and the flash light. The flash duration is very brief (always less than 1/500 s) so in normal situations the shutter speed ahs no effect on the flash exposure providing it is not faster than shutter sync speed of 1/200 s..

In the auto modes the flash is used as the main source of light. A pre-flash is fired micro seconds before the exposure flash, and the light reflected back form your subject is used to set the aperture for the correct exposure for the real flash burst..

On Canon EOS systems, AV and Tv are fill flash modes. The camera meters for the ambient light, sets the shutter speed and aperture for the ambient light, and the flash is fired a little below the ambient reading to fill in the shadows. The exposure given by the meter in the viewfinder is always for the ambient light. It shows the settings you would need for a correct exposure without the flash..

In Av and Tv you should see the same meter readings whether you have the flash up or not..

With a flash mounted in ETTL mode (which the pop up always is), M mode on the camera is a semi-automatic mode. The flash exposure is automatic. The exposure given by the meter in the viewfinder is for the ambient light, as it always is. You can vary how much ambient light you want in the exposure by controlling the aperture and shutter speed..

M mode on the camera is generally seen as the flash node on Canon cameras. You can set the shutter speed to anything you can handhold, set the aperture of DoF, and ignore the meter and let the flash expose the shot. Or you can use the meter to vary the ratio of flash to ambient..

Why is it overexposing in Av and Tv?.

We would have to see the shots to evaluate that. But it is using settings based on the metering, so something is throwing the metering off. Perhaps you are shooting something dark and should be using negative exposure compensation (EC)?.

Brian A...

Comment #11

Chris Elliott wrote:.

I will leave you to sort it out with others rather than waste more ofmy time..

If you didn't have time to waste you wouldn't be in the Beginner's forum in the first place.........Regards,Hank.

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Comment #12

Hank3152 wrote:.

Chris Elliott wrote:.

I will leave you to sort it out with others rather than waste more ofmy time..

If you didn't have time to waste you wouldn't be in the Beginner'sforum in the first place..........

Thank you for your incisive contribution to solving the OP's problem. And thank you for encouraging me to help others..

I spent 20 minutes plus puzzling over the OPs problem. Of course I am cross at his lazyness of thought! Why shouldn't I be? Cynical comments like yours are becoming typical of the attitude on this site. Whilst there are lots of decent newbies who post here there are getting to be far too many people who expect others to do the work for them and cannot be bothered:.

A) to read the manualb) to engage their brain and describe their problem accurately..

And more experienced people like you who snipe at the side lines at others who are trying to help..

P.S. Comparing our respective message count it is obvious who is the time waster..

Chris Elliott.

*Nikon* D Eighty + Fifty - Other equipment in Profile.

Http://PlacidoD.Zenfolio.com/..

Comment #13

Gary,.

Please ignore my outburst immediately above. It is unfair to single you out. You seem to have a peculiar problem and it is obvious from your brief posting history that you have asked for advice and acted on it with the purchase of your 40D..

I am sure you agree it was an oversight not to mention your use of flash! Hence signs of exasperation from two of us here..

Chris Elliott.

*Nikon* D Eighty + Fifty - Other equipment in Profile.

Http://PlacidoD.Zenfolio.com/..

Comment #14

Chris Elliott wrote:.

I spent 20 minutes plus puzzling over the OPs problem. Of course I amcross at his lazyness of thought! Why shouldn't I be? Cynicalcomments like yours are becoming typical of the attitude on thissite..

Whos's being cynical........the OP had a legit question and you were flat rude by your remark..

P.S. Comparing our respective message count it is obvious who is the time waster..

If you read any of my posts you'd find that most are all helpful contributions/suggestions and not derogatory to anyone.......

Chris,.

I'm impressed that you do have a shred of decency by your last apology to the OP so I'll afford you the courtesy and respect you deserve.Regards,Hank.

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Comment #15

DesperadoGary wrote:.

Admittedly my tests haven't been under strick scientific conditions,sat on the sofa aiming at the same object on the floor..

I dont think this is the main issue here; its not slight variations in the exposure due to different aiming of the camera or whatever that is causing the confusion..

So basically, in auto & program modes it gives me a shutter speed of0.016 seconds at f4 and in Tv and Maunual modes it suggests a shutterspeed of 0.6 seconds would be more appropriate at f4 i.e. 36 timeslonger resulting in overexposure..

No, I dont have your model of camera (so some of the detail in what Im going to say may not apply, but I think most of it will), and I dont have all the details in my head, but heres how it works, in very general terms:.

In Auto mode, the flash is always enabled. The camera selects the slowest shutter speed that is reasonable for hand held work (1/60) and a wide aperture. The camera knows this will be an underexposure, but thats OK because its going to use the flash..

In Program mode, the flash may or may not be enabled - this is up to you. If the flash is enabled, the camera behaves pretty much as it would in Auto mode. If the flash is not enabled, you need to note that:.

In Program mode you do NOT get the full range of exposure options. The camera will not select extreme (e.g. 0.6 second) shutter speeds, something like 1/60 is as low as it will go. In Program mode, you are offered a selection of reasonable exposures (shutter/aperture combinations) from a sort of table in the cameras memory. So the result is that you will still get an exposure of something like 1/60 at f/4 - even though that is (without the flash) an underexposure.Program mode is really just a slightly more flexible version of Auto mode..

In the other modes (Aperture priority, Manual etc) you are in total control of the exposure. The camera will now warn you that the shutter/aperture combination is inadequate. Now you are seeing the real exposure - 1/60 is nowhere near enough, you must go down to 0.6 second to get a good exposure using the available light. If you have the flash enabled, then you can stay on 1/60..

But the key thing is - the reason the camera is warning you of underexposure in Manual (etc) mode, is because in that mode it is attempting to show you the correct exposure for the available light, regardless of the flash..

Moral of the story: If you want to learn about the exposure modes, and exposure in general, do it outside in good light with the flash disabled. (You probably cant disable the flash in Auto mode but thats OK, Auto mode is not the place to learn about exposure!) Take the flash out of the equation - minimise the variables, learn one thing at a time..

The camera is probably assuming slightly more intelegience from theuser in the Manual modes..

Yes, yes it does .

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Comment #16

Thanks Chris, Appreciated..

PS just used your Zenfolio Referral code, cheers ..

Comment #17

Hank, maybe you like to waste your time on really dumb questions asked badly by people too lazy to even think..

You're special..

But for people like Chris who work hard to provide good informtion, getting sent down the wrong trail is a complete waste of time,a nd drives good helpers away..

Can you fill the gap?.

BAK..

Comment #18

I skipped over all the arguing..

Anyway..

I'm wondering if the differences in exposure may be due to auto ISO in the fully auto and 'P' modes, where in 'AV' and 'TV' the ISO may have been set differently!.

Also, it was said in one of the posts that the 40D won't choose anything slower than 1/60th of a second in 'P' mode..

This is wrong..

'P' mode stands for 'Programmed Exposure', which means that the camera has a set of predetermined exposure values (ie.- EV - shutter speed and aperture settings) for all the different EV's within the cameras range of shutter speeds, ISO and the lens that is mounted on the camera..

There is no minimum shutter speed that the camera will allow in any exposure setting..

It is up to the camera operator to know and understand all of these things..

Perhaps the OP needs to learn how to use the in-camera exposure meter, then he will understand the settings that the auto modes are selecting..

J. D.Colorful Colorado.

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Remember.always keep the box and everything that came in it!..

Comment #19

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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