"P" mode and "Av" mode..whats the difference on a Canon 400d?

Discussion in 'Canon' started by the_niner_nation, May 26, 2007.

  1. As per title, what is the difference between these 2 shooting modes?
    According to the manual, both allow you to define or set your own aperture
    whilst the camera sets a shuuter speed to deliver a correct exposure...if
    thats the case, then why are there 2 modes ro do almost the same thing?

    Thanks!!
     
    the_niner_nation, May 26, 2007
    #1
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  2. the_niner_nation

    Alan Browne Guest

    "P" mode makes an initial selection for you of both aperture and speed
    (based on metered light, ISO setting, max lens aperture and Canon's
    particular P algorithm) and then lets you "program shift" as desired in
    either aperture or speed.

    "Av" locks the exposure speed according to your chosen aperture (and ISO
    setting). You vary the aperture only.

    On top of both of the above is the exposure-compensation that you enter
    to offset for the metering.

    Cheers,
    Alan.
     
    Alan Browne, May 26, 2007
    #2
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  3. the_niner_nation

    HEMI-Powered Guest

    the_niner_nation offered these thoughts for the group's
    consideration of the matter at hand:
    P generally meaning "programmed auto", attempts to reach a best
    compromise between shutter speed and aperture so that you don't
    get camera shake from a too low shutter speed nor too shallow DOF
    due to a too wide aperture. But, it is a compromise.

    Av means "aperture priority" which assumes you know that you know
    the correct aperture for the effect you want, e.g., really broad
    or really shallow DOF, and it adjusts shutter speed to create a
    correct exposure.

    Tv means "shutter priority", which assumes you know what effect
    you want from shutter speed, e.g., some blur to show motion or
    absolute stop-action, and it adjusts aperture accordingly.

    To use Av and Tv effectively, you have to keep a close eye on the
    shutter and aperture shown in the viewfinder to ensure that it
    will be "correct" for what you want to achieve. It REALLY helps
    if you have a good founding in the principles of photography so
    that you can reasonably estimate the shutter speed or aperture to
    any given effect you want to achieve. Could they be combined?
    Yes, but it is simpler to think in terms of what you want to
    control and let the camera do the math on the other setting. A
    true combination does exist - P.

    I find with my Rebel XT that when I try to out-think the camera
    with Av, I usually blow it, so about the only time I use it is
    when I have a specific DOF effect I want to achieve. I don't
    shoot things that move, so I've never tried Tv. All things
    considered, I think the Rebel does a damn fine job of balancing
    shutter and aperture on P.
     
    HEMI-Powered, May 26, 2007
    #3
  4. the_niner_nation

    HEMI-Powered Guest

    Alan Browne offered these thoughts for the group's consideration
    of the matter at hand:
    I added Tv because it is a natural adjunct to the OP's overall
    question. You correctly amplified my remarks by noting that ISO
    modifies how all 3 modes will compute, and the camera's AE can be
    overridden to at least +/- 2 stops. I don't do that much, but isn't
    that EV?
     
    HEMI-Powered, May 26, 2007
    #4
  5. the_niner_nation

    Alan Browne Guest

    eh? I answered first, so you're the one amplifying... ;-)
    I didn't mention Tv as I thought the OP could make that leap of
    connection by himself...

    In the Canon world EV seems to be what everyone else calls "exposure
    compensation". In the real world EV is "Exposure Value" which is
    basically aperture & speed for ISO 100. (eg: Sunny-16 is EV 15
    regardless of actual aperture and speed combo to achieve it).

    Your post correctly pointed out that P sets the shutter speed (or
    should) to reduce camera shake as well as the other factors I mentioned.

    Every time I use a Canon camera I am frustrated by its exposure system
    and it takes me many minutes to adjust. Minolta and Nikon is straight
    nuts and bolts terminology and usage. Pentax too IIRC.

    Cheers,
    Alan.
     
    Alan Browne, May 26, 2007
    #5
  6. the_niner_nation

    Pete D Guest

    Pentax now also has two more modes on the K10D, Sv and TAv.
     
    Pete D, May 26, 2007
    #6
  7. the_niner_nation

    G.T. Guest

    Actually, on a Rebel XT the button is labeled Av+-. And the manual
    describes it as the Av/Exposure Compensation button. In manual mode it
    is used to change aperture, and in other modes it's used to change EC.

    I don't see EV mentioned anywhere.

    Greg
     
    G.T., May 27, 2007
    #7
  8. the_niner_nation

    Alan Browne Guest

    If you have and Av mode then it should mean that and having another
    button labeled "Av+/-" seems like poor UI to me.

    I may have made an error above, but the point remains that Canon have
    one of the strangest exposure nomenclatures. Minolta had it just right:
    A,S,M,P and exp-comp for both available light and flash. What more does
    one need? (Drop the "P" and I'd be just as happy).
    HEMI wrote:
    ""modifies how all 3 modes will compute, and the camera's AE can be
    overridden to at least +/- 2 stops. I don't do that much, but isn't
    that EV?""
     
    Alan Browne, May 27, 2007
    #8
  9. the_niner_nation

    G.T. Guest

    I completely agree.
    I don't know why Canon has to do the Av, Tv thing, and why T instead of
    S anyway?
    Can't speak for HEMI but I was just talking about the manual.

    Greg
     
    G.T., May 27, 2007
    #9
  10. the_niner_nation

    Guest Guest

    aperture value, time value
     
    Guest, May 27, 2007
    #10
  11. the_niner_nation

    ASAAR Guest

    S(hutter) makes more sense, but maybe T(ime) was the intent.
     
    ASAAR, May 27, 2007
    #11
  12. the_niner_nation

    Alan Browne Guest

    Exactly what I was getting at...
    Ah.

    Cheers,
    Alan
     
    Alan Browne, May 27, 2007
    #12
  13. the_niner_nation

    Alan Browne Guest

    Probably. That's what I mean about Canon's offbeat exposure nomenclature...

    Cheers,
    Alan
     
    Alan Browne, May 27, 2007
    #13
  14. the_niner_nation

    HEMI-Powered Guest

    Alan Browne offered these thoughts for the group's consideration
    of the matter at hand:
    I agree it doesn't make sense, but why argue with reality? Just go
    with the flow, pick the setting that does the job for you, and just
    ignore Canon's nomenclature. But, again, Alan, it is when I think
    I'm getting smarter than the camera and switch off P to T or A that
    I tend to get worse results. My Rebel XT does a damn fine job of
    setting a good compromise shutter and aperture with P, so why risk
    failure. The ONLY time I switch to Av is when I KNOW I'll have a
    DOF problem. e.g., suppose I am shooting a car in the foreground
    and there's a building or foliage in the background I also want in
    focus. I MAY switch to Av and go to f/11 or f/16 and even up the
    ISO from 100 to 200. But, those situations are rare for me.
     
    HEMI-Powered, May 28, 2007
    #14
  15. the_niner_nation

    Alan Browne Guest

    In using Av / Tv you really need to understand the meter of the camera.
    What area it's covering and what is the relative reflectance of the
    part being metered. From there using exposure compensation comes in.

    OTOH, P is likewise affected, but it probably weights strongly to the
    overall scene as a starting point so whatever errors get biased out
    fairly well in "average" scenes. In Av/Tv start with scene evaluative
    metering (whatever mode in the camera meters all over the scene) and
    then work towards more spot metered areas using exposure compensation.
    Look up all the meter offset values (ashphalt is 0, grass is -1, skin
    (palm of hand) is +1, yellow is +1, red is 0, etc. .. use a gray card as
    a target and then build your own table of what EC's to use.... Use the
    histogram..

    Cheers,
    Alan
     
    Alan Browne, May 28, 2007
    #15
  16. the_niner_nation

    HEMI-Powered Guest

    Alan Browne offered these thoughts for the group's consideration
    of the matter at hand:
    Yes, e.g., the 3 available modes of metering light. In my case, I
    almost always use Evaluative.
    I would probably agree, Alan, if I had the faintest clue what
    you're talking about! <grin> Well, I have some clue ... But, as
    you've been following along in my comments in this and other
    threads, getting an absolutely, precisely accurate exposure isn't
    really an issue for my car pictures - usually. The bigger problem
    is uneven exposure and dynamic range problems that are really
    difficult to easily fix. e.g. suppose it is daylight, I am in P
    mode on Evaluative, I do an AF lock and allow the camera to
    simultaneously do the AE lock. Rare for me to need to pick a
    different sample point for AE, which is so common is flash
    situations. At car shows, the hoods are almost always up because
    the judges need to see that and because the owners like to show
    off their hard work. When the car is reasonably well exposed and
    there are no serious backlight problems, the engine will be
    almost black. And, the interior will also be quite dark, but not
    as dark as the engine. To fix that. I tweak the entire scene in
    PSP 9 using the usual techniques, then do individual selections
    on area(s) that are either too bright or too dark and adjust them
    until I have a good compromise.

    The main reason I have never taken the time to develop custom WB
    and exposure settings, as you describe, and/or using an image
    already in memory as the base point, is that from car to car to
    car, the problems vary too much to try to diddle around with
    multiple custom settings. And, even with the LCD set all the way
    to bright, in daylight, it is virtually impossible for me to read
    the menus unless I crouch down in a shadow on the car, not to
    mention that there seldom is enough time to be all that careful.

    If that makes me a fool in the eyes of some folks, so be it. I
    think I've clarified my position enough that it isn't what other
    folks think is "right" or "wrong", what the camera books say, or
    what teachers in a photography class say - although I do place
    weight on those things - it only matters what the photographer
    thinks is important and right, for them.
     
    HEMI-Powered, May 28, 2007
    #16
  17. the_niner_nation

    Alan Browne Guest

    It's very simple: 18% grey is about the middle of the exposure range.
    All the relfective parts of the image lie on each side of that. Near
    white is about two stops above (add +2 compensation as the meter thinks
    that white is 18% grey). Dark clothes lie at about -2 to -2.5 so set -2
    to -2.5 (the meter thinks that dark stuff is 18% grey, so it tries to
    open it up to make the dark stuff grey ... you adjust -2.5 to compensate).

    Run the exercise for a bunch of colors v. a grey card reference and
    you'll see what I mean.

    Or with digital, use its built in "white" card. Put a white card in the
    scene and shoot a test shot. Look at the histo. If it shows info all
    the way to the right without overflowing (creating a sharp spike at the
    right), then you're bang on. Underexpose that same scene by two stops
    (the white turns grey) and the histo will be blank from the right to
    about the middle...
    We've gone over this before ... you can't get 10 Lbs of shit into a 5 Lb
    bag. The first thing to do is to control the lighting. If you're
    shooting digital outdoors in mid day with strong shaddows there is NO
    WAY a digital image can fit in all the detail. Use flll flash to
    lighten up the shaddows, but shoot for the daylight.

    Or use negative film and overexpose it a little. You'll get tons of
    detail. (Portra 160 VC would be a good choice exposed at 100).

    Here is where Manual shooting can save your day. Once you figure, for a
    scene, what the exposure should be, set the aperture and speed manaully.
    This will give you shot to shot consistency without the meter making
    errors based on each new scene.

    The point is: the light is constant, so should be the exposure.

    Fill flash will need to vary due to distance.
    That's not the point at all. By using P (ot Tv or Av) you're letting
    the camera evaluate the exposure from shot to shot. But in a given
    scene with available light you can find one exposure value that will
    give you consistency for every frame. In manual mode your camera
    probably has a button you can hold while changing aperture that will
    make the reciprocal change in speed (or v-v).

    For a mid sunny day at ISO 100, if you set f/8 @ 1/500 and the flash at
    -1.5 (for fill light when needed) I would bet you'll get very good
    results and a lot more consistency shot to shot. (adjust the shutter
    speed with something matt white in the scene to fill the histo to the
    right without overflow).

    If shooting the "open shade" side of the car, then you will need more
    exposure (by about 1.5 stops, but sunlit parts will blow out) and you
    may want to set the color temp higher (8,000 - 10,000 K). (If you shoot
    RAW then you can do this on importing the images into the computer).

    Cheers,
    Alan
     
    Alan Browne, May 28, 2007
    #17
  18. the_niner_nation

    HEMI-Powered Guest

    Alan Browne offered these thoughts for the group's consideration
    of the matter at hand:
    You're forcing me to try to remember my Photography 101 from 40
    Please excuse my denseness here, Alan, but what question are you
    answering? I can follow the quotes that you're commenting on how
    to put more science and less seat-of-the-pants into the decision
    which exposure mode to use and how best to use them wrt building
    custom WB and EV settings. Beyond that, I'm not following you.
    Maybe you're replying in general and not to my particular set of
    strange problems ...
    10 pounds of shit into a 5 pound bag is what used to be called a
    "blivet". I understand your point, which is that it is patently
    impossible to get anywhere near 100% correct exposures across the
    entire dynamic range of a pathological case like my example. As
    for me, I am not really complaining, I know how to "fix" the dark
    engines and interiors, I was simply pointing out that it isn't
    always possible to "do it right the first time".
    I think my film days are long gone, and not coming back. But, if
    you've followed my general theses wrt car photography, I am NOT
    all that concerned with 100% correct results on each and every
    photo. I am a documentary photographer interested in the car, and
    less so the technical quality of the image. Other people feel
    differently, and that is their right.
    First, the light is not constant; it varies throughout the day,
    sometimes literally in minutes if clouds are moving. Second, the
    degree to which an engine or interior is in the shade is highly
    dependent on where the car is wrt the sun. e.g., the Walter P.
    Chrysler Museum where the CEMA show will be held on June 9, faces
    exactly E-W, which means that I shoot one series of photos with
    one set of views in the AM and another set in the PM. As the sun
    rises, peaks at around noon, then begins to sink, lighting
    changes dramatically, as does its temperature as well as how much
    the car is actually lit.

    With the number of cars I would like to shoot - can't always do
    what I like, but I try - there simply isn't time to take the
    "correct" approach, and using full manual is problematical for
    many people, including me. If done well, it may save the day as
    you describe. Done not so well and the images will be total junk.
    So, again, P mode is the best all around compromise for me for
    the average car show in daylight.
    Haven't been too successful making fill flash do what it is
    intended to do. I need to do more practicing at home before the
    big shows start.
    I appreciate you many good pieces of advice,Alan. I'm sorry that
    I won't be able to follow you direction however, for the reasons
    I have stated. There is a continuum between two extremes where
    one is lesser quanitity but much higher quality, and the other is
    quantity enough to try to at least get SOME image on a really
    large number of cars, but sacrificing quality to the point where
    it may get pretty dismal at times. Forget me, and just think of
    the general cases I describe. Each photographer has to decide
    where in the continuum they want to be, or can be, and adjust
    their technique(s) accordingly. There are times when I think I
    might be better off giving up my DSLR and going back to a
    "snapshot camera", i.e., a small P & S. It annoys me that I
    cannot take advantage of better methodologies, but that's the way
    the cookie crumbles.

    Have a great holiday!
     
    HEMI-Powered, May 28, 2007
    #18
  19. HEMI-Powered wrote:

    Thanks; I will. Finals of NCAA D-1 lacrosse start in an hour or so.

    Jerry-

    Alan has given a splendid discourse on it, while I've previously
    suggested you use manual, in a very few words. It's not so difficult as
    you seem to make out.

    In any event, it looks like you have had your mind made up some years
    ago on this, and naught's going to change it.

    Happy shooting!
     
    John McWilliams, May 28, 2007
    #19
  20. the_niner_nation

    HEMI-Powered Guest

    John McWilliams offered these thoughts for the group's
    consideration of the matter at hand:
    It isn't that my mind is made up, although it is, it is more that
    I just don't see the benefits of doing it "right" vs. the effort.
    That is not in any way an insult to Alan or anybody, just how I
    feel. I use examples of my personal experiences to illustrate my
    realist and pragmatist approach to everything in my life,
    including photography, cars, PCs, etc. People on this and
    rec.photo.digital apparently confuse my comments as complaints,
    they are not, or requests for help in solving my problems, which
    they also are not. I pretty much gave up asking for specific help
    because my philosophy of doing things is incompatible with
    people's recommendations, which inevitably leads to bad blood. I
    don't want to see that, so I'm just "taking an even strain."

    As to RTFM, I have, but it is like most manuals, poorly written
    and indended to be a reference manual, not a tutorial. If you
    believe manuals are easy to glean information from, I am happy
    for you. I find it extremely difficult and thus extremely
    frustrating.

    In conclusion, again, I do NOT want to insult anyone or diss them
    by appeearing to ignore their advice. Responders to all OPs in
    this NG tend to be more experienced and much more interested in
    superior results than I am. That's fine by me. I wish that people
    would cut me a little slack and allow me to feel the way I do, if
    I am happy and satisfied with my results. Now, if I could improve
    my percentage of first-time good images, I surely would - but it
    has to be with a good eye on the cost-benefit curves. There are
    just so many other things in life more important to me than
    photography of any subject that I have to prioritize my time, and
    that leaves out learning the more esoteric aspects of digital.
    However, I DO keep a folder of what people say, whether to me or
    in general, that I can always print if I want to try my hand at
    improving.

    Let me comment on one aspect of this and rec.photo.digital. OPs
    that are newbies of one sort of another more often than not put a
    couple of paragraphs in that do not have nearly enough detail for
    anyone to respond intelligently. When the thread goes toward the
    highly technical, high quality end, I suspect that the OP doesn't
    want to look stupid, so they simply don't come back in. It has
    been said that beauty is in the eye of the behold, and I would
    submit that so is image quality and print quality.

    Thanks for your comments.
     
    HEMI-Powered, May 28, 2007
    #20
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