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Portrait DOF & exposure problem.

Discussion in 'Photography Forums' started by RimBlock, Oct 24, 2005.

  1. RimBlock

    RimBlock
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    Studio portrait.

    As I understand it you need a large aperture and therefore a fast shutter speed to get the nice blurred background.

    My issue last night was that when opening the aperture wide I ended up with over exposure (160th/s). I sped up the shutter speed with no real difference until it got to a point where there was only the top half of the frame captured (500th/s or there abouts). The rest of the frame was black.

    I am using 4 studio lights and the two main were on 1/4 power using a softbox and bounce umbrella. The two other lights (one backlighting, the other side lighting) had diffusers on them.

    Only with the stock lens could I go (and needed to go) below f8 for a reasonable exposure.

    Needless to say that although a number of the pics have come out very well, there is no bluring of the background.

    The camera is a EOS 350D and the lenses were mainly fast (1.8 or 1.4) primes.

    Any ideas short of shooting in darkness :).

    Cheers
    RB
     
  2. sorbiegunner

    sorbiegunner
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    Were you on the lowest possible iso setting?
     
  3. RimBlock

    RimBlock
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    :D Not to start with.

    Was on iso 200 and dropped to iso 100 when I noticed it.

    Not a massive amount of difference. Allowed me to drop from f11 to f9 ro f8. Only lens that could go lower was the stock lens without overexposure.

    More than a little worred about the partial shot at 500th/s though.

    Cheers
    RB
     
  4. tomson

    tomson
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    have you thought of using an ND filter?
     
  5. nobackwheel

    nobackwheel
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  6. nobackwheel

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  7. RimBlock

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    So the sync speed is causing a problem.

    Now a little worried with the problem happening at 500th/s and the article stating that upto 2000th/s is normal.

    Now a sync lead is being used to the master flash head which sets off all the flashes. I cannot find any specs on the portaflash 366VM heads sync speed though.

    Anyone have any links to the specs for these ?.

    Any suggestions on getting over the this problem. Only thing I can think of now is using less lights and more reflectors :( .

    Cheers
    RB
     
  8. RimBlock

    RimBlock
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    The sync is a rating of the camera and not fo the flash then.

    The 350D is rated with an X-sync of 1/200 so the fastest shutter speed usable with flashes is 200th/s. Is this correct ?

    The 20D is only rated at 250 :( .

    Cheers
    RB
     
  9. Peakoverload

    Peakoverload
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    At 1/500th you are definitely going to exceed the max flash sync speed which is why you are getting the half black frame. The black you are seeing is actually probably the rear curtain shutting. Basically flash has to synchronise with the front and rear curtains opening and shutting, too fast and you get the half and half effect you are seeing.

    The 350D has an x-sync speed of 1/200th meaning that you should not use a shutter speed higher than that. However depending on what Flash gun you have (dont know if such an equivalent is possible with studio lights) you can set the flash gun to FP (Focal Plane) mode which does allow for much faster shutter speeds. The way it gets around the rear curtain problem is by, instead of firing just one burst of light, it fires a pulsing light. The only downside to this is that the throw of the flash is greatly reduced (the higher the shutter speed the more the throw is reduced).

    In your studio setup you should set an aperture suitable for your subject and focal length. For example a portrait you could shoot at 90mm f/4 for example to get a nice dof or you could shoot at 200mm f/8 and get the same (or near enough) albeit it with a differenct perspective. Once you have got the dof you want you then need to worry about your shutter speed. If you are only getting 1/60th then you either need to use a tripod or wind up your ISO. If you are getting 1/60th at ISO 100 then upping it to ISO 200 will give you a shutter speed of 1/125th which is much more usuable.

    Don't forget that you also control your flash via your aperture NOT your shutter speed so you may find that f/4 (depending on how close your subject is to your lights) is too strong so you may need to drop down to f/8 but will also need to then lower your shutter speed. The reason for this is that a burst of flash is incredibly fast, MUCH faster than your shutter speed, so altering the shutter speed will have no affect on the flash as no matter how fast you set it your burst of flash light is faster. Changing your aperture though will control how much of that light passes through the lens. So if f/4 is giving you blown highlights, drop down to f/5.6, f/8 etc (but watch you shutter speeds and if neccassary, bump up your ISO. Remember that the difference between any two ISO's, apertures and shutter speeds is always 1 stop. So if you go from f/4 @ 1/125th ISO 100 to f/5.6 but still want a shutter speed of 1/125th, bump your ISO to 200.

    HTH
     
  10. RimBlock

    RimBlock
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    GUys,

    Thanks for the input :thumbsup: .

    Peak, yes that has cleared up a few points so thanks very much for that but the problem I am having is over exposure and not being able to reduce the aperture to get the blured background.

    For example....
    [​IMG]

    Shot at;
    135mm
    F7.1
    1/160 sec
    iso 100

    Still had to drop the exposure compensation by 0.5 to get this and due to the aperture there is no bluring of the background.

    So it is the bluring I am after so I need to open the aperture but I cannot do so as I cannot get a fast enough shutter speed to allow less light into the sensor.

    Regards
    RB
     
  11. Peakoverload

    Peakoverload
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    Yes but you are only at ISO 100. Just upping your ISO to 200 will allow you to take exactly the same shot at f/5.6, going to ISO 400 will allow you to shoot at f/4. Looking at your example I do wonder why you feel the need to knock the background out anyway? It's plain black and whether its in focus or not will still appear black (incidently a black background could also be the reason for achieving low shutter speeds as your light meter may be reading partially off of that). If you still feel the need to knock the bg out of focus then just up your ISO.
     
  12. RimBlock

    RimBlock
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    Peak,

    As I understand it the iso value is the sensitivity to light with 100 being the lowest and 800 being the highest (my camera can manange).

    The f-stop is the size of the aperture with the lower number being the larger 'hole'.

    Therefore to let less light in when making the aperture bigger to give a blured background you need a lower iso and if you are at the lowest sensitivity (i.e. iso 100) then you need to speed up the shutter.

    The reason for the bluring is to pull the subject more into the foreground. Not all of the backdrops are black, I have a patterned cream and a black velvet backdrop which I was using.

    To give anotehr example with the white backdrop;

    50mm, f4.5, 1/200sec, iso 200 - almost totally washed out.
    50mm, f8.0, 1/125sec, iso 200 - some more details visible but still pretty much washed out.
    50mm, f13.0, 1/160sec, iso 200 - Sharp picture.

    Black background;
    85mm, f2, 1/160sec, iso 100 - All skin features washed out, most detail gone from clothes.
    85mm, f7.1, 1/160sec, iso 100 - nicely exposed.

    These are taken from the exif infor on the raw pics shot on the day.

    If I am missing something then please let me know.

    Cheers
    RB
     
  13. RimBlock

    RimBlock
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    Oh sorry, forgot.....

    The camera is in manual mode so all the settings are set by me and not by the cameras' logic.

    Cheers
    RB
     
  14. Peakoverload

    Peakoverload
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    Rim,

    No you are getting mixed up.

    ISO 100 is LOW which means it is LESS sensitive to light and will therefore result in LONGER shutter or LARGER apertures e.g. f/2.8 at any one aperture.

    ISO 800 is HIGH which means it is MORE sensitive to light and will therfore result in FAST shutter speeds or SMALLER apertures e.g f/22 at any one aperture.

    Now ISO in itself bares no relation at all to depth of field, all it means is that given the avaliable light a max aperture is possible at that particular ISO. Increase the avaliable light or increase the ISO and you will be able to knock the background further out of focus.

    Basically you have 4 elements that go to make your photo.

    Avaliable light - either natural or artificial
    ISO sensitivity - how sensitive your sensor is to the light. The more sensitive the smaller aperture and faster shutter speed
    Shutter speed - the slower the shutter speed the more light hits the sensor
    Aperture - the larger the aperture the more light that hits the sensor.

    Now you are in a situation where you know the aperture you want to use and you know that there is a minimum shutter speed you can use before you get motion blur so you can only do one of two things. Increase the amount of light or increase your camera's senstivity to the light that's avaliable.

    Now to take the examples you listed. What you've done is gone the wrong way around. You should have been using ISO 100 on the white background as there will be more light but ISO 200 on the black background as there will be less light.

    HTH
     
  15. tomson

    tomson
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    I think there's some confusion as to what rimblock is needing here.

    He needs to reduce the amount of light entering his lens. His options are:
    Smaller aperture - but he wants to retain a large aperture to throw the background out of focus so he can't alter this too much.
    Faster shutterspeed - but sync problems are arising.
    ISO - already using the lowest possible on his camera.
    Available light - studio lights are already set at 1/4 power which i presume is the lowest setting.

    Hence my suggestion of an ND filter - we used to use them in cheap studio setups where the non-adjustable lights were causing a similar problem to what rimblock is experiencing.

    Have a read: http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/filter/filter-ND.html
     
  16. RimBlock

    RimBlock
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    Tomson,

    Yep you are right, that is exactly what I am after in terms of results. I will have a look at ND filters and see if they help.

    Cheers
    RB
     
  17. sorbiegunner

    sorbiegunner
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    Is this the opposite side of the coin re the larger sensors on dSLR cameras? The larger size gives more sensitivity which is great in low light conditions but causes headaches at the other end.
    My Oly 770 can go as low as 1/1000th f8 iso 64 and this still caused me problems on holiday recently.
    Presumably SLR lenses can give you bigger f numbers though (when shallow DoF is not a priority)
     
  18. RimBlock

    RimBlock
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    Well it looks like ND filters are the way to go although wanting to drop 4 stops on the aperture means two filters (Hoya 8x and a 2x). Hopefully the Jessops on the way home will have them in 58mm.

    The only other option I can see is blocking some of the output from the flashes using multiple diffusers (i.e. sheets of white paper) but as I am at least trying to look like I know a little bit of what I am doing in front of the model I think I will leave that as a last ditch option :D

    I do have a poloriser at 72mm for the 28mm-135mm which I completely forgot about and to be honest some of the best shots IMHO were taken at the 135mm end of that lens.

    Thanks for all the advice.
    RB
     
  19. thebigkung

    thebigkung
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    Hi Rim,

    When shooting with Flash changing the shutter speed has no affect on the exposure of the flash. You cannot go above the sync speed of your camera, as you have found out, otherwise your film plane will not be exposed to the light from the flash.
    Low shutter speeds will allow too much ambient light to enter resulting a 'ghosting' type effect.

    You have a lot of lights! Try removing one of the front lights.
    If you want to reduce the amount of light falling on the subject, move those lights further away, space permitting. Also if you can use a more telephoto lens, these have narrower DOF which will help to blur the background. Also, again space permitting, move the subject further away from the background.
    And as others have mentioned, you could ND filters but this will make it harder to look through the viewfinder if you don't have good modeling lights.

    Do you use a flash meter? you should invest in one if you don't.

    Have fun. :D
     
  20. RimBlock

    RimBlock
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    Hi TBKung,

    Ok the four lights may be a bit much considering the size of the room which while not small is not very big. Large bedroom but not huge.

    Maybe reducing to 3 lights and doubling the flash used for the backligh as the hairlight as well by placing a large reflector above the model and the backlight shooting upwards may help. Having the two front lights I though was quite important with the key light providing the strongest light source to illuminate the subject and the second used to the other side to soften the shadows created by the first.

    Space is an issue so moving the lights further away is difficult as is moving the model further away from the background. I can use the 28mm-135mm IS lens although this will then restrict me to head portraits again.

    I finally managed to get a 4x ND filter and it was the only one I could find without buying on-line (had a shoot scheduled for Sunday just gone but that is a whole different story). Jessops had none including their flagship New Oxford Street store and Jacobs Digital just had this one in 58mm. Will have to try with the two stops it will give me.

    Thanks
    RB

    Oh for an unused garage :)
     
  21. thebigkung

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    Hi Rim,


    :thumbsup: You do need a Key light and a Fill light but they don't have to be 2 flash heads. You could use the soft box (behind and slightly to one side of the camera), and a white reflector. If the reflector is near the subject and angled correctly it will have quite a high intensity, and as you move it further away it will have less. This way you can vary the lighting ratio on the subject. As you use only one light the fill light will never be as bright as the main light and flood the subject (as the light off the reflector travels further to the subject) but will fill the shadows. Give it a go if you've no luck with the ND filter. :D

    Have fun.
     
  22. RimBlock

    RimBlock
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    TBKung,

    Yeah I can see the logic behind using the reflector. A am not sure the best way to go about it though as the majority of portrait shots I am aim for are either full length (not so easy given the space) and half length.

    What I think I need is something to attach a reflector to a flash stand. I can take off the flash head and use that to position and angle the reflector. Any idea of any products out there which will grip a reflector and enable it to be angled as desired ?.

    One other thing.... Back lights. As of now I am using a slave flash head (diffused) attached to the models chair pointing up. This seems to work reasonable well. Any other suggestions ?. Maybe a reflector at 45 deg behind the chair and a flash firing along the ground so teh light goes under the chait and then bounces up. Only problem I can see with that is the chair legs obviously blocking some of the light. Trying not to get reflectors in the shot can also sometimes be an issue.

    Thanks for the advice.
    RB
     
  23. thebigkung

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    Hi Rim,

    Ah, Half and full length, hmmm, coud be tricky. Not one I considered from seeing the head shoot post of one of your portraits. :eek:

    ad for the reflector. I'm sure you could get some kind of bracket, but I'd just use lighting stand and some gaffer tape. :D

    The Flash slave on the back of the chair is a goodn :thumbsup:
    Not sure I understand the reflcetor and flash on the ground thing.
     

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