Blurry images while using my speed light

Discussion in 'Photography Beginners' Forum' started by vivienne forte, Nov 18, 2020.

  1. Overread

    Overread has a hat around here somewhere Staff Member Supporting Member

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    Ok so your focusing problem sounds like two problems combined

    1) Focusing itself. First understand that when you use focusing the camera doesn't know what you're looking at*; it basically uses all the focusing sensors (points) that you have told it to use and looks for the nearest point of contrast difference it can find to focus upon. This is why focusing can sometimes fail if you're shooting a bright blue sky or a white wall because there's no (or very limited) contrast variation for it to pick up.

    Now if you've got all the AF points active and you are shooting a subject the camera won't know what part of that subject you want in focus; it just finds the nearest point. This is why many photographers will use 1 active AF point. This they can then point at the area they want in focus and the camera can lock into that area. Now for most cameras the centre AF point is the "best", however these days quite a lot of the other points, even in lower end cameras; are very good as well and for a static subject more than good enough. So read the camera manual on how to change your AF mode and points and how to move them around for different situations.

    2) Depth of field. The area of the photo in focus and sharp. Think of it like a flat sheet of paper parallel to the front of the lens. The thicker the sheet of paper the greater the depth of field. This is important to keep in mind because if you recall point 1, many people will use that idea and "focus and recompose". Which means they set the AF to the central point; point it at the subject area they want in focus, focus; then move the camera (recompose) to get the composition they want. The camera doesn't "see" the subject so it just locked the AF position at a set distance. recomposing might move that parallel "sheet of paper" (depth of field) away from the original subject area.


    Now the depth of field itself is affected by several variables (I only outline a few below).
    First off distance, broadly speaking the greater the distance between you and a subject the greater the depth of field. Therefor with a shorter focal length getting you physically closer to the subjects the depth of field, if all else is equal, will be slightly less. This doesn't mean that long lenses are always best, it just means that you need to be aware of the variation.

    Second (and often the most noted) is the aperture. The smaller the aperture (bigger the f number) the greater the depth of field. Your 50mm goes very wide whilst your 18-200 can't go as wide in aperture. Therefore what you're seeing as a difference between the two might well be that when you are taking the shot with the 50mm, esp in lower lighting, the aperture might be very wide (small f number); whilst the 18-200mm you might be doing the same things, but because the lens can't go as wide, you aren't going as wide with it as with the 50mm. Resulting in the difference in depth of field that you are seeing.
    This is likely one of the bigger areas that you're seeing the difference with between your two lenses.



    For a group with multiple points of focus and a general thicker "depth" to the subject you might well want a smaller aperture (larger f number). Don't be afraid of f4 or even up to f8 and such. Creating a thicker "sheet of paper" so that it covers each of your subjects.


    *yes I'm aware some/many modern cameras do have things like facial recognition and such, but that's a set of features unto itself, for now we'll keep it simple


     
  2. wfooshee

    wfooshee No longer a newbie, moving up!

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    The D7100 has the ability to fine tune focus on a per-lens basis. If the lens focuses behind where the camera is telling it to focus, am adjustment factor can be entered into the camera using its menus.

    It requires some test subject setup, but have a look here.
     
  3. petrochemist

    petrochemist TPF junkie!

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    Your title says 'while using speed light' If you're using flash the effective shutter speed is while the flash is lighting the subject typically under 1/1000s. It's unusual to get camera movement in such a short time.
    In such cases I'd minimise the ISO (rather than using auto ISO) to make sure the flash lighting is significant. Outdoors with the wider apertures often used for portraits the ambient light can provide plenty of light so even with the flash 'on' it might be doing very little.

    When flash is not in use a moderately experienced photographer should be able to get images with no significant camera movement when using a 50mm lens at 1/60s. When I first started as a boy I sometimes jabbed at the shutter release (especially if excited about the subject) which caused camera movement - as when handling people a gentle squeeze is much preferable to a jab :)
     
  4. vivienne forte

    vivienne forte TPF Noob!

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    amazing! I did not know this. Thank you
     
  5. Tinstafl

    Tinstafl No longer a newbie, moving up!

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    That is a good setup and you should be able to do it all with the 50. I used to shoot that exact same setup. Don’t turn on auto iso as nikon will posh the iso to the max and introduce noise. For fill take your picture with no flash til the background is how you want it. In manual mode then put on the flash to light the subjects. A fast run and gun way on aperture priority is to put the camera on minus .7 exposure comp and the flash on plus .7. This in aperture priority will work but watch your shutter speeds.
     
  6. wfooshee

    wfooshee No longer a newbie, moving up!

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    ^^^^ Good point about auto-anything, really. Even when you're using flash, the camera's metering system will make every effort to expose with ambient light. If you are in shutter-priority, it will open the aperture as wide as it can go. If you're on aperture priority, you might end up with a 30-second shutter (if it's dark enough.) and auto-ISO, if enabled, will jack the ISO up to what's required, or to your max setting.

    Shooting flash, you want to limit these efforts by the camera's metering and put everything manual. Auto modes with auto-TTL flash will do a decent job of exposure if there is good ambient light, and will then give a fairly good fill flash. Without good ambient light, you don't want any camera settings on automatic. Auto-TTL flash is OK, you'll get a good flash picture.
     
  7. weepete

    weepete TPF Supporters Supporting Member

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    I could be wrong, but it looks like a double exposure to me. If the flash is set so it provides a similar amout f light as the ambient lighting you can end up with two exposures in the same image if the subject moves slightly. It's commonly called dragging the shutter, and while it can make some interesting effects it can produce blur like we see on your shots.

    When using flash, you need to set your exposure so the ambient lighting is underexposed or turn the flash power down and use the flash as fill.
     

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