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Which of the two lenses "compresses" more the background?

Discussion in 'Adapted Lenses' started by alaios, Aug 25, 2013.

  1. alaios

    alaios TalkEmount All-Pro

    Jan 11, 2013
    Düsseldorf
    Alex
    Hi all,
    I have learned many new things the last two days.
    tomorrow it will be practicing day but still have two three more things to ask.
    1. In the adapted lenses there is typical a small sign at the first value of aperture (image will follow). In the given image this corresponds
    to 1.7 and 2 aperture respectively
    2. Which of the two lenses compresses more the scene? From the "theory" that should be the 50mm to the 46mm. On the given image
    [​IMG]
    If you see the distance meters on the lens you need more aperture to get also 1.5 to 3 meters into focus on the 50mm lens and quite smaller to have those into focus into the 45mm. Does this mean that the 45mm is a better lens to isolate the subject or not? I might also be confused here, very few hours of sleep.

    Regards
    Alex
     
  2. Poki

    Poki TalkEmount Hall of Famer

    Aug 30, 2011
    Austria
    You seem a little bit confused. I try to answer your questions as I understand them.

    1. The longer the focal length, i.e. the smaller the field of view, the more the scene gets compressed. It has nothing to do with the respective lens, only with the focal length.

    2. A 50mm f/1.7 definitely can produce images with shallower depth of field than a 45mm f/2. Don't know exactly what you mean with your question, but longer focal length + faster aperture = shallower depth of field at the same magnification.
     
  3. alaios

    alaios TalkEmount All-Pro

    Jan 11, 2013
    Düsseldorf
    Alex
    Hi,
    I will try to ask again
    1. First question had to do with the small symbol appearing on both aperture rings. 2 and 1.7 respectively. There is an inverten gamma type of letter.
    2. On the left lens, if someone wants to have 1.5 to 3 meters in focus, has to use roughly an aperture of 11. That is what I get from the "dof calculator" of the lens. On the right side the 1.5 to 3 meters need an aperture of 16. That I though is an indicator that the left lens "compresses" more the background compared to the right one lens.

    Did I make it more clear now?

    Regards
    Alex
     
  4. Poki

    Poki TalkEmount Hall of Famer

    Aug 30, 2011
    Austria
    Here we go:

    1. 2 and 1.7 are the maximum apertures of the respective lenses.

    2. No. Depth of field and compression are two completely different things. A 24mm f/1.4 lens will have a much shallower depth of field than a 50mm f/5.6 lens at the same subject magnification, however, the 50mm lens will compress the scene more. Compression has nothing to do with the plane of focus (i.e. how much of the image is 'sharp'), but only with the focal length of the lens.

    Also, your numbers can't be correct. With just such a slight difference in focal length, it can't be that you need to stop down a full stop just do get the same depth of field.
     
  5. Bimjo

    Bimjo Super Moderator

    Oct 28, 2011
    Washington State
    Jim
    Alex, the little mark just shows where that aperture is on the scale. There wasn't room to put the number there without it running into the next aperture number. Otherwise it would look like 22.8 instead of 2[space]2.8 (using the left hand lens in your example pic.

    Based on checking the DOF scales on two different 50mm lenses I have, you're right- if you focus at 2 meters then the lens has to be at f/16 to keep the DOF between 1.5 and three meters (roughly). DOFMaster online calculator says 50mm lens focused @ 2 meters @ f/16 the DOF is between 1.6 and 2.67 meters.

    On the 45mm lens DOFMaster says a 45mm lens focused @ 2 meters @ f/11 has a DOF between 1.64 and 2.56 meters. Changing to f/16 gives a DOF between 1.53 and 2.89 meters. A toss up as to which aperture you'd use based on the subject.

    So you can see that the DOF scale on the lens is a ballpark figure, not something you can rely on totally. It will usually get you close enough.
     
  6. alaios

    alaios TalkEmount All-Pro

    Jan 11, 2013
    Düsseldorf
    Alex
    So If I understand it then right with the 45mm is easier to isolate more a subject as you can smaller DOFs for the same apertures, compared to the 50mm. Is that right?
    A.
     
  7. Poki

    Poki TalkEmount Hall of Famer

    Aug 30, 2011
    Austria
    No. The 50mm lens will give you shallower depth of field, both because it has a longer focal length and a faster maximum aperture. You can't compare the lenses at f/11 and f/16 respectively and tell which lens can give you shallower depth of field.

    By the way, there's much more to 'subject isolation' than just depth of field.
     
  8. alaios

    alaios TalkEmount All-Pro

    Jan 11, 2013
    Düsseldorf
    Alex
    Yes that it is also what I have heard here from the "theory".. but when I was looking on the dof-ring-calculator on the lens, looked like that you need to lower aperture opening much more to keed things in focus for the 50mm lens.
    Regards
     
  9. Poki

    Poki TalkEmount Hall of Famer

    Aug 30, 2011
    Austria
    These lenses are both old and not Leica, Zeiss or similar ones, which means many things. First, if you set the lens at f/11, is it really f/11 or rather f/10 or f/12? Second, the DoF markings are just to give you an idea, these are not very exact at all. Every single 50mm lens will give you exactly the same DoF at a certain aperture, so if you want to know it down to the last mm just use a proper DoF calculator or calculate it yourself.

    There is NO difference between one 50mm f/1.7 and another 50mm f/2.8 lens in terms of depth of field when both are stopped down to the same aperture.

    Again, simply put:
    Longer focal length --> shallower DoF
    Less distance to subject --> shallower DoF
    bigger aperture (i.e. smaller number) --> shallower DoF

    Depth of Field is a quite simple function, the aperture itself is only calculated and doesn't say anything but how big the 'hole' in the lens is at the place the aperture blades are in relation to the focal length of the lens.
     
  10. dbmiller

    dbmiller TalkEmount Top Veteran

    777
    Mar 2, 2012
    New England
    Unfortunately, you didn't put both lenses at the same focus distance, so it's a little hard to tell. But, shorter lenses always have a greater DoF compared to longer lenses focused at the same distance.

    Your picture of the 45mm shows it focused at 7 feet. The DoF calculator at dofmaster.com says at f/16, and focused at 7 feet, a 45mm lens on film would be from 4.68 feet to 13.9 feet for acceptable focus. Which looks pretty close to what the lens shows.

    If the 50 mm lens was also focused at 7 feet, the DoF would be 5 feet to 11.7 feet. Which makes sense. Longer lens, narrower DoF.

    Your picture of the 50mm shows it focused at 10 feet, not 7. So f/16 would give a film DoF of 6.35 feet to 23.6 feet. The marks on the lens looks to agree with that. If the 45mm lens was focused at 10 feet, the resultant DoF is 5.84 feet to 34.7 feet.

    If you want everything from 1.5 meters to 3 meters in range, you should focus the 45mm lens a little closer than 7 feet. You have 1.5 more to the left of the distance mark, than the 3 is to the right of the distance mark. They should be at equal distances from the center to give you the best reading.