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Thread: Setting aperture with flash

  1. #1

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    Setting aperture with flash

    When I set my camera to P it allows me to change the aperture, but when I attach a flash it no longer allows me to change it. Assuming that the preflash sets the exposure by shortening the duration of the flash, why does it not allow me to change the aperture, but uses a default of f4, even when attaching a lens with a wider aperture? AV on the other hand does, but I have to make sure that the shutter speed does not exceed the sync speed. Is this only a Canon quirk?
    Last edited by Ken MT; 27th October 2012 at 03:09 AM.

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    Re: Setting aperture with flash

    Quote Originally Posted by Ken MT View Post
    AV on the other hand does, but I have to make sure that the shutter speed does not exceed the sync speed. Is this only a Canon quirk?
    Hi Ken,

    I never use P mode, so can't help with the first part of your question, but with regards to Av mode, the camera won't allow you to go beyond max sync speed UNLESS you have HSS enabled (at which point the flash behaves like a constant light source).

    Hope this helps.

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    Re: Setting aperture with flash

    I cannot explain the apparent anomaly of not being able to adjust the f/stop in P mode while the flash is mounted and turned on except to say that it seems to be the nature of the beast. There is nothing wrong with your camera or flash...

    The P mode will usually give decent exposure both with and without flash. The difference is that without flash, you have some control over the shutter speed/aperture combination while with flash mounted and turned on, the camera/flash acts more like the full automatic setting. It selects the shutter speed and f/stop and you cannot adjust the combination.

    If you want control over your shutter speed and f/stop when using flash, I recommend either using Av or Manual camera exposure mode and ETTL for the flash. Either of these modes will allow you to adjust your f/stop and shutter speed.

    As Colin mentioned, the camera will not allow shutter speeds faster than 1/250 second, unless High Speed Sync (HSS) is chosen which shoots a series of flashes rather than a single flash.

    HSS is an important facet for me since I often shoot fill flash and will want exposures faster than 1/250 second with the accompanying wider f/stop.

    The HSS mode will reduce the power of your flash but, that is not usually a problem since when using a faster shutter speed, you will be shooting with a wider aperture. I believe that the loss of light (when not considering the increased aperture size) is actually around one stop. Since I most often shoot my flash fill (which is where I would use the higher shutter speed) at -1 EV; the loss of light is somewhat of a moot point.

    One nice thing about HSS is that when you have that mode selected (at least on my 430EX and 550EX flashes) if your shutter speed is at 1/250 second or slower, the camera/flash automatically reverts from HSS to standard sync. IMO, that is a win-win situation.

    Please correct me if I am in error but, I see no reason for having my flash units in other than the HSS mode. That mode will allow the maximum flexibility in selecting a shutter speed and, therefore, the f/stop combination. Standard sync at 1/250 and below and high speed sync when my shutter speed is faster than 1/250 second. I don't know if other non-OEM flashes which are equipped with HSS have the automatic reversion capability...
    Last edited by rpcrowe; 27th October 2012 at 04:48 PM.

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    Re: Setting aperture with flash

    Another angle to this is that light falls off in the inverse square of the distance [ ie if 10ft needs say f/5.6 then 5ft is f/11 of the same subject] so the aperture is detirmined by the flash to subject distance when the flash is in manual mode.
    So if you want to adjust the aperture you would need to adjust the output of the flash as well if both are in manual modes.
    Sorry if this is basic stuff you already are aware of Ken.
    I never use HSS becuase I don't have a flash capable of working this way

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    William W's Avatar
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    Re: Setting aperture with flash

    Quote Originally Posted by Ken MT View Post
    When I set my camera to P it allows me to change the aperture, but when I attach a flash it no longer allows me to change it.
    It will - it should.

    In P Mode (most) EOS Cameras allow ‘Program Shift’. However this function is NOT available when a dedicated Flash is ‘active’ and attached to the Camera’s Hotshoe.

    However, when a Flash is attached, one can still adjust the Aperture when the camera is in P Mode and this is done by using Exposure Compensation – in fact using Exposure Compensation with an active Flash Attached, will ONLY allow the Photographer to adjust the Aperture and not allow any adjustment to the Shutter Speed.

    Conversely, if a Flash is NOT attached and the camera is in P Mode - Exposure Compensation will change BOTH the Shutter Speed and the Aperture and the change will be in accord with the software program for P Mode and also be dependent upon the Lens which is attached.

    If there is a Flash attached and active and one chooses to adjust the Aperture using Exposure Compensation, then it is (usually) sensible also to use Flash Exposure Compensation – in fact the two compensations go hand in glove for adjusting Flash as Fill when using P Mode.

    It is also necessary (or worthy) to note that P Mode and a Dedicated Flash will work is TWO distinct ways depending upon the Ambient Light Level
    – for low level Ambient Light (about EV10 and below) the ETTL Flash will assume the Flash is to be Key for the Foreground Subject.
    – For high level Ambient Light (about EV13 and above) the ETTL will assume the Flash is to act as Fill for the Foreground Subject.

    It is when the Ambient Light is between EV 9~14 that EC and FEC is the most useful (to me) to attain a range of Fill/Key light balance from the on camera Flash, when the Camera is being used in P Mode.

    If one uses P Mode and Flash (with or without EC and or FEC) it is very useful to plan the ISO to be used such that there is maximum flexibility within the general EV (Light Levels) to be expected.

    However – do not misinterpret these comments as suggesting the necessary use of P Mode for Flash Fill shooting scenarios – I am merely answering the question – not making suggestions regarding what Camera Mode to use.

    However it is worthwhile mentioning that it is my experience that P Mode is the MOST MIS-understood and LEAST used automatic Mode – and P MODE is the only Automatic Mode about which there is much confusion and much (rampant) misinformation, posted on the web (but not here at CiC).

    ***

    Quote Originally Posted by Ken MT View Post
    Assuming that the preflash sets the exposure by shortening the duration of the flash, why does it not allow me to change the aperture, but uses a default of f4, even when attaching a lens with a wider aperture?
    Speaking about NO FLASH ATTACHED -
    P Mode does not use a default of F/4.
    P Mode is a sophisticated program which (in simple terms) evaluates the LENS which is attached and the LIGHTING CONDITIONS (using the TTL Meter) and then sets both the Aperture and the Shutter Speed.

    Speaking about FLASH ATTACHED and ACTIVE -

    Again P Mode does NOT use a default of F/4 – HOWEVER – it is very likely that the combination of the lens and the lighting conditions always rendered a choice of F/4 even though you were using a faster lens - this is very common for many lighting conditions when a Flash is attached and active.

    But F/4 is most certainly NOT a default aperture for P Mode – either with or without Flash. And with a faster than F/4 lens attached to the camera and an active Flash on the hotshoe – there are certain lighting conditions / shooting scenarios which will make P Mode select an Aperture larger than F/4.

    You might like to try a something like a tight shot of reasonable dark object in a moderately lit room (EV about 6~7) using an F/2 or faster Medium Telephoto Lens, maybe F/2.8 lens . . . P Mode with a flash attached and active will select something around F/3.2 or maybe even F/2.8 - - - see the samples below.



    ***

    Quote Originally Posted by Ken MT View Post
    AV on the other hand does, but I have to make sure that the shutter speed does not exceed the sync speed.
    As already mentioned, there is a safety feature on EOS cameras such that you cannot exceed the Maximum Flash Sync unless you are using HSS (High Speed Sync).

    But that ‘safety feature’, does has a work around.

    However, the only reason I can think of wanting to use the work around is to make pictures like this, to show why the safety feature is really sensible and exists in the first place:

    Setting aperture with flash
    EOS 20D and 580 EX attached to hotshoe – the top image is at Max Flash Sync @ 1/250s,
    Then - 1/500s; 1/1000s; 1/2000s and finally 1/4000s


    Quote Originally Posted by Ken MT View Post
    Is this only a Canon quirk
    I don’t think so.

    As I recall Nikon’s P Mode tends to also hover around F/4 when a dedicated Flash is attached and active – and also I recall that most Nikon Cameras have safety features such that the Photographer cannot exceed the Flash Sync when the camera is in Aperture Priority Mode and Flash is attached – (and not using Nikon’s equivalent of High Speed Sync).


    ***


    Samples -

    Test equipment for these samples is an EOS 5D and EF135/2 and a 580EX – direct Flash and using no EC or FEC
    Partial Metering Mode and ‘P’ Camera Mode.
    The JPEGS are SOOC:

    SAMPLE 1:
    The first scene is with the bright Desk Lamp in shot at the side and the TTL Meter Reading in P Mode was:F/2.5 @ 1/200s @ ISO400

    The Flash is turned on shot is made and ETTL and P Mode computed: F/4 @ 1/60s @ ISO400.

    Setting aperture with flash


    ***


    SAMPLE 2:

    The second scene is below the desk and focussing on the chair leg with dark carpet behind and a black jacket occupying much of the foreground shot, TTL Meter Reading was: F/2 @ 1/8s @ ISO400

    The Flash is turned on shot is made and ETTL and P Mode computed: F/2.8 @ 1/60s @ ISO400
    Setting aperture with flash



    I just whipped these two images up a few minutes ago - that screen is the draft of this reply.

    I made these two quick shots just to show on this thread - but the point is I knew the general Shooting Scenario which would trigger the results I wanted, so it was easy for me to do - (relevance of that comment to reinforce the fact that P Mode being most misunderstood by many folk).


    I've several examples of EC and FEC in P Mode for Flash as Fill (outdoors) and Flash as Key (indoors) - but it is more fun for interested parties to make their own experiments.


    ***


    Other bit:


    Quote Originally Posted by rpcrowe View Post
    Please correct me if I am in error but, I see no reason for having my flash units in other than the HSS mode. That mode will allow the maximum flexibility in selecting a shutter speed and, therefore, the f/stop combination. Standard sync at 1/250 and below and high speed sync when my shutter speed is faster than 1/250 second.
    Hi Richard,

    There might not be any reason for you not to deviate from HSS – but that would be (I expect) because your experience tells you that your general shooting scenarios will allow you to always have enough Flash Power.

    On the other hand – one most critical effect of using HSS is the deterioration of the MAXIMUM FLASH WORKING DISTANCE.

    (for one example)
    If one is shooting on camera Flash as Fill on a sunny day, using HSS can cut the effective MFWD from about 15ft MAX down to 8ft or even less.

    That can mean a group portrait could be ‘impossible’.

    (Based upon a 580EX Flash – outdoors in Sunlight - Sun as Backlight – On Camera Flash as Fill - Ambient EV = 15)

    So I would suggest that you’ve got your bases well covered and you just haven’t reached those limits of HSS - where the shot becomes ‘impossible’.


    WW
    Last edited by William W; 30th October 2012 at 02:35 AM. Reason: cotrrect spellung mistuck

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    Re: Setting aperture with flash

    Bill,

    I think that you might have misinterpreted my remark about leaving the flash in HSS mode, or else I didn't express myself clearly. I don't always shoot in HSS mode, even outdoors. However, since the flash automatically reverts to standard sync when the shutter speed is at 1/250 or slower (for my xxD and 7D - I suppose it would be 1/200 for the xxxD series cameras), I see no reason to manually switch it back to standard sync - even when I am shooting at 1/250 second or slower.

    If I want standard sync, I shift my shutter speed to 1/250 second or slower.

    I am always open to ideas... Can you think of a reason to switch manually to standard sync when the flash switches automatically?

    I am not sure I have explained myself explicidly. Here is a better explanation of what I wanted to say...
    http://www.rpphoto.com/howto/view.asp?articleID=1026
    Last edited by rpcrowe; 30th October 2012 at 02:53 AM.

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    William W's Avatar
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    Re: Setting aperture with flash

    Quote Originally Posted by rpcrowe View Post
    Bill, I think that you might have misinterpreted . . .
    Yes I completely misunderstood your meaning.
    Thanks for clarifying.

    ***

    Quote Originally Posted by rpcrowe View Post
    I am always open to ideas... Can you think of a reason to switch manually to standard sync when the flash switches automatically?
    No I cannot. Except for the ‘special’ situations explained in the link – but for general working day to day with On Camera Flash as Fill I also work that exact same functionality.

    Yes, I have read that article before – it is likely you’ve posted it previously - I read a quite lot of what you post.

    Yes I note the para:

    “Note that for Canon and Nikon, you can leave high speed sync "on" all the time, and it will only "kick in" when it is needed. At shutter speeds at or below the sync speed, the flash operates in the "standard" mode, and there's no power loss. I recommend leaving high speed sync enabled all the time, unless you're doing something advanced with your flash where you know you don't want it (like hummingbird photography, or using your hotshoe flash to trigger studio strobes).”

    REF: Ralph Paonessa; http://www.rpphoto.com/howto/view.asp?articleID=1026 op. cit.

    WW

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    Re: Setting aperture with flash

    Hi Bill you said that
    "However, when a Flash is attached, one can still adjust the Aperture when the camera is in P Mode and this is done by using Exposure Compensation – in fact using Exposure Compensation with an active Flash Attached, will ONLY allow the Photographer to adjust the Aperture and not allow any adjustment to the Shutter Speed"
    With exposure compensation do you mean FEC or EC. With FEC I suspect that you are changing the flash duration and not the aperture.

  9. #9
    William W's Avatar
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    Re: Setting aperture with flash

    Quote Originally Posted by Ken MT View Post
    . . .Bill you said that
    "However, when a Flash is attached, one can still adjust the Aperture when the camera is in P Mode and this is done by using Exposure Compensation – in fact using Exposure Compensation with an active Flash Attached, will ONLY allow the Photographer to adjust the Aperture and not allow any adjustment to the Shutter Speed"

    With exposure compensation do you mean FEC or EC.
    Thanks for the question to clarify -


    I meant it, exactly as I wrote it: Exposure Compensation, i.e. - “EC”.

    ***

    Quote Originally Posted by Ken MT View Post
    With FEC I suspect that you are changing the flash duration and not the aperture.
    Yes, that is correct.

    And I purposefully mentioned Flash Exposure Compensation (“FEC”) here:

    “If there is a Flash attached and active and one chooses to adjust the Aperture using Exposure Compensation, then it is (usually) sensible also to use Flash Exposure Compensation – in fact the two compensations go hand in glove for adjusting Flash as Fill when using P Mode.”


    ***


    What camera do you have?

    WW

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    Re: Setting aperture with flash

    William, tiny tiny footnote. Yes, FEC is separate from EC with Canon gear, but on the Nikon side of the fence, iirc, EC is tied to FEC. To quote from Neil van Niekerk's article on flash exposure compensation:

    ... With Nikon, the overall exposure compensation and flash exposure compensation is cumulative .. to an extent. For example, if you were to dial in +1.0 exp comp and -1.0 flash comp, it would cancel each other – but only for this scenario where the ambient light is low, and your flash is your main source of light.

    Where the ambient light levels dominate, and flash is used as fill-flash only, then different algorithms come into play, and you have other factors such as max sync speed and available apertures affecting the scenario as well .. and hence the flash and exposure compensation might affect ambient light exposure differently then.

    But with Canon, flash exposure compensation and general exposure compensation aren’t linked, as they are with Nikon. So with Canon, in manual exposure mode, you can only set flash exposure compensation and not overall exposure compensation.

    (It is no use asking me how it handles this in any of the auto exposure modes, since I use my cameras nearly exclusively in manual exposure mode. You’re on your own there.)

  11. #11
    William W's Avatar
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    Re: Setting aperture with flash

    Hi Kathy,
    Just another area where Canon has superiority!


    On a serious note – thanks for that footnote – I didn’t know that: but more importantly it is useful for all those Nikonians who might be reading this thread.

    Certainly all my comments on this thread have been Canon specific, I trust that’s been quite clear.

    Regards,

    WW

  12. #12

    Re: Setting aperture with flash

    Quote Originally Posted by William W View Post
    but more importantly it is useful for all those Nikonians who might be reading this thread.
    Can they read? (ducks the thrown shoes).

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    Re: Setting aperture with flash

    Quote Originally Posted by inkista View Post
    William, tiny tiny footnote. Yes, FEC is separate from EC with Canon gear, but on the Nikon side of the fence, iirc, EC is tied to FEC. To quote from Neil van Niekerk's article on flash exposure compensation:
    As an aside, I was surprised to read that Nikon have changed this behaviour on the D4 so that in essence it works the same as Canon now.

  14. #14

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    Re: Setting aperture with flash

    Bill (William W) asked What camera do you have?

    5D Mk 2 and 580 ex, except for macro when I use a 30D

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    William W's Avatar
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    Re: Setting aperture with flash

    Quote Originally Posted by Ken MT View Post
    Bill (William W) asked What camera do you have?

    5D Mk 2 and 580 ex, except for macro when I use a 30D
    OK
    Thanks, I wanted to know in the case that you asked any more detailed questions, so I could reference a camera specifically, rather than answering generally about Canon gear.


    WW

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    Re: Setting aperture with flash

    Except when using HSS and equivalent modes in other camera [?] that have it, the limit on shutter speed is a characteristic of the focal plane shutter which achieves faster shutter speed by a slit traveling across the sensor so [ I guess ] with a sync speed of 1/250 you get 1/1000 by only exposing a quarter of the sensor at any one split second of time. The camera has two shutter blinds and at sync speed the second one only starts to cover up the sensor when the first one has reached the 'far side' ... above Sync speed the second shutter starts to cover before the first one has fully uncovered the sensor.

    The leaf, compur, electronic shutter opens the sensor to all the light at the same time and before HSS was invented with flash units that strobe their light, was the only way to sync above the SLR's 'sync speed' which used to be between 1/30 and 1/50 in the old days when blinds moved across the 36mm of the 35mm film frame instead of vertically as they do today.

    So if I wanted to do sync sunlight work I used my Rollieflex TLR and not my Pentax SLR
    Before that when I first saw Syncro-sunlight I dumped my Leica [1/30 sync] for a Japanese fixed lens camera with a leaf shutter, Topcon 35S.

  17. #17
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    Re: Setting aperture with flash

    Here is a web page from Canon explaining the use of high speed sync with Canon cameras and Canon HSS capable flash units.

    http://web.canon.jp/imaging/flashwor...igh/index.html

    Note the last sentence in the third paragraph, just above the illustration of the way HSS works,

    "The camera automatically reverts to normal flash firing when the shutter speed is set slower than the flash X-sync speed."

    This is not a secret but, may not be well known. In fact a Canon guy presenting a seminar at Calumet Camera in Escondido, CA on Saturday, Nov 10th did not know this. In fact he challenged me that is was not true. HE WAS WRONG!

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    Re: Setting aperture with flash

    Quote Originally Posted by rpcrowe View Post
    Here is a web page from Canon explaining the use of high speed sync with Canon cameras and Canon HSS capable flash units.
    The thing I like to point out to folks when it comes to HSS (in fact, a couple of things) ...

    1. That in HSS mode you're dealing with a CONSTANT LIGHT SOURCE so (a) it may not freeze the action as well (eg at 1/320 in HSS mode where the stopping power is 1/320th -v- 1/200th in non-HSS mode where the stopping power may be 1/10,000th) (depending on the ambient portion of the light though), and (b) the old saw about "shutter controlling ambient and aperture controlling flash" doesn't apply (I might add that even below HSS sync speeds the aperture still has an effect on both ambient AND flash, so I really think that saying creates more confusion than anything else).

    2. In HSS mode the lighting that appears in your image is simply a "pure drag race" between the flash and the ambient light.

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