Focal length, focus distance, and depth of field

Discussion in 'Photography Beginners' Forum' started by robertwsimpson, Feb 10, 2010.

  1. robertwsimpson

    robertwsimpson No longer a newbie, moving up!

    Joined:
    Aug 3, 2009
    Messages:
    2,471
    Likes Received:
    30
    Location:
    West Palm Beach, Fl
    Can others edit my Photos:
    Photos NOT OK to edit
    There seems to be a lot of confusion about how focal length (the mm part of what your lens is called) and focus distance (how far away from the front lens element you are focusing) affect a photo's depth of field. People have been arguing that a photo taken with a longer photo length (ie the long side of a zoom lens) will have a smaller DoF, given the same f-stop setting. As I learn best by doing, I have some exhibits that should help show what is actually happening. Hope this helps. Keep in mind that the basis of this is "similarly framed photographs."

    Exhibit 1:
    [​IMG]
    This photo was taken with my 18-55mm f/3.5-5.6 kit lens with it set at 18mm and f/3.5. The first thing that everyone should take notice of is that a blurred background is very achievable with one of the worst and cheapest lenses that canon makes.

    Exhibit 2:
    [​IMG]
    This photo was taken with my 18-55mm f/3.5-5.6 kit lens with it set at 55mm and f/5.7. This is the widest aperture setting available at this zoom.

    Exhibit 3:
    [​IMG]
    This photo was taken with my 18-55mm f/3.5-5.6 kit lens with it set at 18mm and f/5.7. This is the same aperture value as the maximum available at 55mm.

    First, lets dispel the myth that on a variable aperture lens, the DoF becomes smaller at the long end of the lens. This is clearly not the case. The easiest way to tell this is by looking at the coffee cup behind the stuffed animal. At 55mm and 5/5.7, the black line at the top of the cup is distinct. At 18mm and f/3.5, it is barely distinguishable at all. Another way to tell is by looking at the dark spot above the hind leg of the stuffed animal. In the 18mm shot, the spot is blurry. In the 55mm shot, some of the fibers are distinguishable. In this instance, logic prevails. A shot at 18mm and f/3.5 does in fact have a smaller DoF than a similarly framed shot at 55mm and f/5.7.

    Next, lets compare the two shots with the same f-stop values. Comparing the coffee cups reveals similar levels of detail. The black ring is distinguishable, although blurred. The spot on the stuffed animal's leg is blurred, but some of the fibers are visible. I would confidently assert that these two photos have very similar depths of field.

    Thus ends the data dissection of this experiment. I do have some other observations. First, Depth of Field has far more to do with focus distance than focal length. Let me explain. Everyone is familiar with the focus graph. If not, it looks something like this:
    [​IMG]
    So, the closer the object is to the minimum focus distance of the lens, the smaller the DoF becomes. We see this with macro lenses. That is why giant f-stops (tiny apertures) are used in macro photography. The farther away an object is from a lens' minimum focus distance, the larger the DoF becomes. That is why the maximum focus distance on a lens is infinity, rather than some arbitrary distance. The farther away one focuses, the DoF becomes so gigantic that for intents and purposes, everything far away is in focus.

    One reason that I think people assume longer focal lengths give a shallower depth of field is that when compared to shorter focal lengths, longer ones act to compress the apparent distance between an object and its background. This can be seen in the size of the coffee cup in the 18mm shots vs the 55mm shot above. I did not move the cup, but it appears closer to the camera in the 55mm shot. Since it appears closer to the camera, and it is still blurred by the same amount as the 18mm shot, it appears that the 55mm shot has a shallower DoF than the 18mm shot.

    I hope that this helps everyone. Also, I hope that when people make assertions on the internet, no one just believes them without reading up or doing his own experiments. Not only is it silly to just accept what other people say, but you can learn a lot by checking things out for yourself.
     
  2. robertwsimpson

    robertwsimpson No longer a newbie, moving up!

    Joined:
    Aug 3, 2009
    Messages:
    2,471
    Likes Received:
    30
    Location:
    West Palm Beach, Fl
    Can others edit my Photos:
    Photos NOT OK to edit
    By the way, comments and questions are very welcome. We're all here to learn. If you think I've gotten something wrong here, please let me know. Just be respectful.
     
  3. robertwsimpson

    robertwsimpson No longer a newbie, moving up!

    Joined:
    Aug 3, 2009
    Messages:
    2,471
    Likes Received:
    30
    Location:
    West Palm Beach, Fl
    Can others edit my Photos:
    Photos NOT OK to edit
    also, please excuse my tax documents and our business plan, which are sitting there for the world to see.
     
  4. Dao

    Dao No longer a newbie, moving up!

    Joined:
    Mar 24, 2008
    Messages:
    6,252
    Likes Received:
    418
    Location:
    St. Louis
    Longer focal length do have a shadower DoF

    i.e.
    Camera with crop factor of 1.6

    Focal Length = 50mm
    Aperture = F/5.6
    Subject Distance = 10ft
    DoF = 2.62 ft

    Focal Length = 200mm
    Aperture = F/5.6
    Subject Distance = 10ft
    DoF = 0.15 ft


    Now try another experiment.
    Use a 50mm Lens and take a portrait shot with background far enough to create a blur. Set the Aperture at F/5.6

    Then use a lens with 200mm focal length and take a portrait shot with the same background. But this time, the camera need to move back further away from subject so that the framing is about the same as 50mm.

    You will see the result. :)
     
    Last edited: Feb 10, 2010
  5. robertwsimpson

    robertwsimpson No longer a newbie, moving up!

    Joined:
    Aug 3, 2009
    Messages:
    2,471
    Likes Received:
    30
    Location:
    West Palm Beach, Fl
    Can others edit my Photos:
    Photos NOT OK to edit
    I just did that experiment. The results are posted above?

    I'm not arguing that a 50mm lens and a 200mm lens both have the same DoF at a focus distance of 10 feet.
     
  6. Rosshole

    Rosshole TPF Noob!

    Joined:
    Mar 23, 2009
    Messages:
    536
    Likes Received:
    14
    Location:
    WI
    Can others edit my Photos:
    Photos OK to edit
  7. robertwsimpson

    robertwsimpson No longer a newbie, moving up!

    Joined:
    Aug 3, 2009
    Messages:
    2,471
    Likes Received:
    30
    Location:
    West Palm Beach, Fl
    Can others edit my Photos:
    Photos NOT OK to edit
    How are you guys not getting that I just did all of this in real actual life and posted the photos above so that you can see them? No need for theoretical calculations. Just look at the pictures.
     
  8. Rosshole

    Rosshole TPF Noob!

    Joined:
    Mar 23, 2009
    Messages:
    536
    Likes Received:
    14
    Location:
    WI
    Can others edit my Photos:
    Photos OK to edit
    Ok, I understand your testing, but you didn't post how far you wer from the focus point, which is an important part of the equation.

    Your 18 mm shot seems VERY close to the subject. The shorter the distance to the subject, the shallower the depth of field. These principals are easier to see when comparing a 70 mm shot to a 200 mm shot.

    EDIT: because there are three variables in this equation, a 3d graph would be in order.... the 2d graph doesn't express all of the proper values. Also, you are changing two of the variables in each photo (distance to focus point, focal length), try again and change only one at a time. The results should be apparent then
     
  9. robertwsimpson

    robertwsimpson No longer a newbie, moving up!

    Joined:
    Aug 3, 2009
    Messages:
    2,471
    Likes Received:
    30
    Location:
    West Palm Beach, Fl
    Can others edit my Photos:
    Photos NOT OK to edit
    Well, clearly, to frame a photo the same, one must be farther away from it at 55mm than at 18mm. I even talked about this in my original post. Did you read it all?
     
  10. robertwsimpson

    robertwsimpson No longer a newbie, moving up!

    Joined:
    Aug 3, 2009
    Messages:
    2,471
    Likes Received:
    30
    Location:
    West Palm Beach, Fl
    Can others edit my Photos:
    Photos NOT OK to edit
    3d graph is not needed, as the aperture value remained constant for the main part of the test.
     
  11. Rosshole

    Rosshole TPF Noob!

    Joined:
    Mar 23, 2009
    Messages:
    536
    Likes Received:
    14
    Location:
    WI
    Can others edit my Photos:
    Photos OK to edit
    Your DoF will remain essentially the same if you increase the length to focal point while increasing the focal length.
     
  12. robertwsimpson

    robertwsimpson No longer a newbie, moving up!

    Joined:
    Aug 3, 2009
    Messages:
    2,471
    Likes Received:
    30
    Location:
    West Palm Beach, Fl
    Can others edit my Photos:
    Photos NOT OK to edit
    Exactly. That is why I said that the DoF is essentially the same for similarly framed photos with equal aperture values at different focal lengths. It's all about focus distance.
     

Share This Page

Search tags for this page

focal distance vs focal length

,

focal length and focus

,

focal length focal distance

,
focal length vs actual distance
,

focal length vs focal distance

,
focus distance
,
focus distance focal length
,
focus distance vs focal length
,
focus length
,
focus vs focal length