Let’s Talk Photography Ep.71 – Aspect Ratio & DOF Listener Q

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In this solo show Bart explains the term ‘Aspect Ratio’ and answers a listener question on Depth of Field.

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Reminder – you can submit questions for future Q & A shows at http://lets-talk.ie/photoq

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Show Notes:

Aspect Ratios
  • Lenses actually cast circular images, so the most efficient shape for photographs would be circular, but somehow, we just don’t seem to like that as humans. If you need to have orthogonal straight edges, then the square is the most efficient shape.
  • Humans seem to prefer rectangles to squares though, so, for much of the camera’s history we’ve used various flavours of rectangle for our plates, film frames, and digital sensors.
  • We’ve also taken to cropping our images after we shoot them to change their shape.
  • Our finished photos are almost always some flavour of rectangle.
  • We describe these different flavours of rectangle with Aspect Ratios.
  • The aspect ratio of a photograph is simply the ratio between the width and the height. A square has an aspect ratio of 1:1 (appropriately pronounced ‘one to one’ in mathematics, but often ‘1 by 1’ by photographers!)
  • Any aspect ratio other than 1:1 is rectangular, and must fall into one of two categories:
    1. If the photo is wider than it is tall then its aspect ratio will have the largest of the two numbers first, and it will be in so-called landscape orientation.
    2. If the photo is taller than it is wide then its aspect ratio will have the smallest number first, and it will be in so-called portrait orientation.
  • Mathematically, you always reduce ratios to their smallest whole-number versions — i.e. a mathematician would always describe a square as 1:1, never 2:2, or 3:3, or …
  • Photographers are not always great mathematicians 😉 Because aspect ratios were originally manifest as print sizes in inches, and 6“x4” is very different than 18“x12” or 3“x2” a lot of once common print sizes remain in use in our digital world as aspect ratios within editing apps. Mathematically, no crop tool should give 6:4 or 4:6 as preset choices, it should be 3:2 or 2:3, but because 6“x4” was one of the most common print sizes for decades, many do!
  • Every camera has a native aspect ratio which is determined by the shape of the photosensitive material at its heart (film frame, slide negative, digital sensor etc.).
  • Every editor, even the most simplistic, lets us crop images to any arbitrary flavour of rectangle, but most give us the option to constrain to one of a number of popular aspect ratios, including:
    • 1:1 (square)
    • 3:2 AKA 6×4 (35mm & most DSLRs)
    • 4:3 (most point & shoot cameras, phone cameras & micro 4/3 cameras)
    • 5:4 AKA 8×10 (many medium and large-format cameras)
    • 16:9 (HD video)
    • 3:1 (panoramic cameras)
  • When photographs had to be printed to be seen photographers had to choose from one of the common aspect ratios unless they planned to create a custom mat.
  • In the digital age there is generally no constraint on aspect ratio, but there are exceptions like Instagram’s practice of forcing every image into a square.
  • Even though we don’t generally have to confine ourselves to commonly used aspect ratios anymore, many photographers still choose to. The old ratios were chosen because they feel right to humans, and since humans haven’t changed, these aspect ratios still feel somehow right today.
  • Aspect ratio is now mostly an artistic choice
    • Different subjects lend themselves to different aspect ratios
      • Round things like flowers work very well centred within square images.
      • Landscapes work very well in wide aspect ratios that close to the aspect ratio of human vision like 16:9.
    • There is a strong relationship between aspect ratios and compositions. A composition that looks harmonious within a square is very unlikely to work well as 16:9 HD, let alone 3:1 panoramic!
Listener Question on DOF

Reminder — you can submit your own questions at lets-talk.ie/photoq

Listener Cass asks:

Does a change in sensor size change the “Depth of Field” or does the change of Focal Length change the “Depth of Field?” The reason I ask is I have taken-up 4×5 photography. I have a 127mm lens on my field camera and I also have a 127mm lens on a 6×7 camera and the resulting depth of field seems the same when shot at the same aperture. Both use the same film, Kodak Tri-X. Am I just imaging this or is it real. I am confused as this goes against what I have read and heard on the internet. Do larger sensors make thinner “Depth of Field?”

Depth of Field, or DOF, is not determined by one factor, but by at least four (think it’s actually four, but I might be forgetting one 😉).

  1. Aperture — this is the most obvious factor because we use the aperture setting to assert creative control over the DOF as we shoot. Keeping everything else equal, dialling the aperture down (opening up the lens more) will decrease the DOF, and dialling it up (stopping down) will increase the DOF.
  2. Focal Length — longer focal lengths result in shallower depths of field. Wide-angle lenses will have deep DOFs, while telephotos have shallow DOFs. (You can have fun with this by getting in close to flowers or butterflies with a wide-angle lens and shooting a macro that has a near-in-focus background to give context, e.g. www.flickr.com/…).
  3. Focus Distance — everything else the same, the closer you focus in, the shallower your DOF gets. This is why the same lens on the same camera with the same aperture setting will give you a deep DOF when you focus far away to shoot a landscape, but a very shallow DOF when you get up close to a flower. This is also why macro photographers have to work so hard to get their entire subject into focus, even having to resort to complex processing techniques like focus stacking!
  4. Sensor Size — the smaller a sensor is, the deeper the DOF, and the larger, the shallower the DOF. This is why phone cameras have to resort to software trickery like portrait mode to emulate the pleasing shallow DOF a full-frame camera will naturally give you when shooting portraits.

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