Can someone explain about the focusing

Discussion in 'Digital Photography' started by G.T., Jan 10, 2010.

  1. G.T. macrumors 6502a

    Joined:
    Jul 12, 2008
    #1
    OK here we go, in this example I will use this lens: Lumix 20mm f1.7 Lens.

    It is a prime lens so fixed focus, so let us only talk of primes only.

    In terms of auto focusing how does it work if I manually keep the aperture at f1.7 so shallow depth of field, more light etc. Now it is fixed focus so if I have the aperture fully open can I only have a subject in focus up close, or is focusing different to focal length etc.

    If I put it this was, I have the camera on end of table and a flower in front of lens with aperture at f1.7 it has nice bokeh etc. Then I choose to move flower further back but aperture at f1.7 will it be able to focus on the flower or not? If I was using this lens would it be shutter speed that I control and leave the camera to change aperture to keep subject in focus at different distances.

    Please help I'm confusing myself lol, and without camera/lens to test I can't work it out through experimenting

    Do people shoot shutter priority over aperture or do they set it to manual and control both
     
  2. Westside guy macrumors 603

    Westside guy

    Joined:
    Oct 15, 2003
    Location:
    The soggy side of the Pacific NW
    #2
    I think you're getting "fixed focus" and "fixed focal length" mixed up. A prime lens is a fixed focal length lens, not fixed focus. A pinhole camera is a fixed focus lens.
     
  3. Designer Dale macrumors 68040

    Designer Dale

    Joined:
    Mar 25, 2009
    Location:
    Folding space
    #3
    ^^^ Yes ^^^

    As a concept, focal plane is movable. The focus ring moves the lenses and causes the focal plane to travel from a set minimum to infinity. Think of it as a plate parallel to your lens that moves back and forth in space while you turn the focus ring. f/1.4 makes that plate very thin while f/11 makes it quite a bit thicker.

    Dale
     
  4. G.T. thread starter macrumors 6502a

    Joined:
    Jul 12, 2008
    #4
    OK that make sense, I was't sure the relationship :eek:.

    In relation to fixing aperture size so I keep it wide open at f1.7 like in the example will the camera be able to focus. Or does it use changing aperture to allow it to focus on objects of different distance. If someone can take a couple examples with a prime in relation to my scenario so I can see the out come would greatly help. So keep aperture fixed an widest like f1.7 and move object on table to focus on it.

    More specifically I'm looking at the gf1 with said lens but I don't want to buy camera till 1. it is a bit cheaper and 2. I know I understand what I'm buying.

    I have a film canon eos that was given to me, I love how the pictures are produced but I kept on auto. We have a bridge camera at home so I've been tacking it off auto and learning settings, with digital it is "cheap" to make mistakes but with film I wouldn't know how they'll turn out or if I got the shot I would want.

    I suppose it comes down to me wondering the limits of this prime but it has had such good review and f1.7 would be good to allow for fast shutter speeds considering it doesn't have built in image stabiliser. It would be cheaper and give more flexability with the kit 14-45 lens which has mega OIS.

    Could I get an adapter for the micro 4/3rds for my 28-85mm lens that I have with the cannon.

    Hope it all makes sense.

    So at f/1.4 say the focus can move to infinite, but if a subject is closer it has the ability to give better bokeh. But if it was at f/11 then when subject is close the bokeh is less (more of image is in focus, even if focus plane was at same point as when aperture was set at f/1.4).

    Would the inifinite focus be sharper for f/11 then f/1.4 (Where does diffraction come into this or is it different altogether.)
     
  5. Designer Dale macrumors 68040

    Designer Dale

    Joined:
    Mar 25, 2009
    Location:
    Folding space
    #5
    Depth of field and aperture have an odd relationship. At f/1.7 you will have a very shallow depth of field an a close focus (think macro) but it will be deeper if the focus is on something farther away like a landscape. I don't really understand why. It's just something I know.

    Dale
     
  6. Phrasikleia macrumors 601

    Phrasikleia

    Joined:
    Feb 24, 2008
    Location:
    Over there------->
    #6
    You're confusing some terms here. "Bokeh" is the quality of the out-of-focus areas of an image (pleasing bokeh is smooth and buttery, whereas undesirable bokeh is jittery and distracting). What you're actually referring to is "depth of field." You really only ever have one point of focus, but the area in front of or behind that point that is also acceptably sharp to the human eye is variable. This total area of acceptable focus (from front to back) is your depth of field.

    Diffraction is another phenomenon entirely. It is a property of physics that governs when you are no longer resolving an optimal amount of detail, and this happens as you reduce the size of your aperture (that is, as you stop down the lens, say from f/4 to f/11). Some physicists may be able to improve upon the vocabulary I've used, but that's the basic idea.
     
  7. G.T. thread starter macrumors 6502a

    Joined:
    Jul 12, 2008
    #7
    Like using a zoom lens you mean. Or if I had the prime and it focused on something further away? Can you find example image online to help me picture it thanks
     
  8. Designer Dale macrumors 68040

    Designer Dale

    Joined:
    Mar 25, 2009
    Location:
    Folding space
    #8
    Yes. See my note on the relationship between aperture and depth of field.

    I don't think infinity would be all that sharper from f/1.4 to f/11 on the same lens. You're usually shooting something big and distant like a landscape.

    Photography really is an odd science. Kind of like visual alchemy.

    Dale
     
  9. G.T. thread starter macrumors 6502a

    Joined:
    Jul 12, 2008
    #9
    OK thanks I do mean DoF, I'm wrongly using bokeh as a term for it :eek:. OK let us ignore the diffraction thing then.
     
  10. iBookG4user macrumors 604

    iBookG4user

    Joined:
    Jun 27, 2006
    Location:
    Seattle, WA
  11. G.T. thread starter macrumors 6502a

    Joined:
    Jul 12, 2008
    #11
    Ignoring the benefit of large aperture for fast shots low noise ect, if I used solely for the purpose of amount of DoF desired, basically on the prime I can shoot at any f stop and the camera can focus on either a close subject to landscape. However, if I desired the close subject to be isolated from the background more then I would use a larger aperture (and obviously change other settings accordingly for exposure)
     
  12. Phrasikleia macrumors 601

    Phrasikleia

    Joined:
    Feb 24, 2008
    Location:
    Over there------->
    #12
    Yes, if it's subject separation you're after, get as close as you can to that subject and use a wide aperture. The closer you are and the wider the aperture, the greater the amount of separation.
     
  13. G.T. thread starter macrumors 6502a

    Joined:
    Jul 12, 2008
    #13
    So like with this link http://www.luminous-landscape.com/tutorials/dof2.shtml looking at the 28mm and 17mm cause the are closer to the 20mm lens, the depth of field is the same but because more is seen and things shift they appear shallower. What if I was to go closer the DoF would appear shallower because I am closer??
     
  14. Designer Dale macrumors 68040

    Designer Dale

    Joined:
    Mar 25, 2009
    Location:
    Folding space
    #14
    With a prime, say an f/1.4 50mm, When you look through the viewfinder at a subject and turn the focus ring the subject will go in and out of focus but not change size. With a zoom, turning the focus ring has the same effect, but turning the zoom ring on a lens like my 28mm-300mm Tamron change the focal length of the lens up or down within that range. The subject gets larger or smaller but the focus stays the same.

    For the purpose of this discussion, avoid zooms. The components inside the lens move in complex ways that change things like aperture that are consistent in prime lenses.

    Here are some link from DP Review (see my sig) and one from Wikipedia. Optics are a complex and important aspect of photography that needs more attention like this. Those of us who have been around know this, but the megapixel thing is distracting sometimes.

    http://www.dpreview.com/learn/?/Glossary/Optical/Depth_of_Field_01.htm

    http://www.dpreview.com/learn/?/Glossary/Optical/Focal_Length_01.htm

    http://en.wikipedia.org/wiki/Bokeh

    Dale

    Ignore what I just posted, then. Each science or art has it's own vocabulary and it's important that we all speak the same language. That's not always easy...

    Dale
     
  15. Phrasikleia macrumors 601

    Phrasikleia

    Joined:
    Feb 24, 2008
    Location:
    Over there------->
    #15
    OK, now you're talking about perspective, which is independent of aperture/depth of field. The further you are from your subject, the larger background objects appear. What the author of that link doesn't make explicit is that the camera had to move with each focal length to keep the framing of the gremlin the same.

    So although getting closer to your subject will decrease your DoF and therefore give you greater separation, getting further away and then using a really long focal length will enlarge the background a great deal, making details in it less distracting.
     
  16. G.T. thread starter macrumors 6502a

    Joined:
    Jul 12, 2008
    #16
    Oh I know, I might not be a serious photographer (yet) but I'm very technological minded and know a lot about what is important and what is over sold like the megapixel myth. Obviously a decent amount of pixels is required depending on the size you want to print but I know that optics is what can be very important. That is kind of why I'm making such a fuss on the lens and learning how the aperture etc can be used to my advantage. I want to make the right choice with the lens because I know the body will be acceptable. I like the micro 4/3rds portability and it is a real contender but right now i'm not too fussed about the body or even I might just get a dSLR but first I want to understand the lens ability properly first.

    I love this forum its so helpful and I know this isn't a photography site but the people on here (and specifically most who visit this section) are photographers and know what they are saying.

    OK thanks that makes sense. I noticed it said he had to move camera closer to keep it similar to others. So what I said in relation to a prime I would have to go closer to keep framing the same but DoF won't change.

    If this was a problem I would want to use a longer focal length or widen the aperture if available?
     
  17. Designer Dale macrumors 68040

    Designer Dale

    Joined:
    Mar 25, 2009
    Location:
    Folding space
    #17
    Very good example. If you drop a 2.8/50 on a tripod and focus on a subject and shoot, then change the lens for an 2.8/85 and shoot the subject size will be the same. The separation between the subject and background will be greater with the short lens than the longer one.

    This ability of a lens to stretch or collapse the separation in the image is very important in Portraiture. A wide angle lens elongates the face and a telephoto will make it look shallower. Portrait photographers shoot with 85-105mm prime lenses. There is a mathematical formula that we used in film to find this. It was something like adding the width and height of the film plane. On 35mm film, that came out to 105mm.

    Dale

    Yes, I just noticed that. Pays to read... But the focal length still has the ability to generate or amplify that separation.

    Dale

    Depth of field is complex. It defends on several variables, change one and the other changes too. Aperture is one component. Wide aperture produces shallow DOF and tight ones give deep DOF. Distance from the lens is another. The farther you get from a subject the deeper the DOF, even at the same aperture. Confused, Yet?

    Dale
     
  18. G.T. thread starter macrumors 6502a

    Joined:
    Jul 12, 2008
    #18
    Actually that makes sense, think i'm getting this guys :)
     
  19. toxic macrumors 68000

    Joined:
    Nov 9, 2008
    #19
    focal length does not inherently lead to exaggerated or flattened perspective. perspective depends only on distance. 80mm on 645 is not the same as 80mm on 135, which is not the same as 80mm on APS-C.

    Perspective Control in Images - Focal Length or Distance? (a tutorial)
     
  20. G.T. thread starter macrumors 6502a

    Joined:
    Jul 12, 2008
    #20
    OK well that link says "focal length has absolutely nothing, in itself, to do with perspective in images". Therefore, what would you recommend, I understand that the 20mm prime has good aperture range and would help with night shots but is the 14-45mm that is in kit with GF1 maybe better all suited lens it also makes package cheaper. Can you really create a nice portrait image with 20mm lens?? Maybe if cropped and not to close? I would more likely take landscape shots but even on flickr the 20mm shows promise with close up shots.
    Anyone have any opinions/experiences?
     
  21. Phrasikleia macrumors 601

    Phrasikleia

    Joined:
    Feb 24, 2008
    Location:
    Over there------->
    #21
    I wouldn't recommend a 20mm lens for portraits. You'll get some nasty perspective effects on faces that way (big noses, ugly distortion). A more "flattened" look is preferable, and you get that with a longer lens. I don't know about the 14-45mm lens you've mentioned, but I'm going to guess that if it's a kit lens, it's on the "slow" side, meaning it doesn't have a very wide maximum aperture. So it will make a good basic lens for landscapes and portraits, but it won't excel at the latter.

    A perspective example

    Here is an example of why a wide focal length is less desirable for portraits. The shot on the left was taken with a 24mm prime lens, and the one at the right with a zoom set at 55mm. Both were on APS-C sensor cameras. You can see there is a huge difference in how the face appears. On the left, Homer has a bulbous look with bulging eyes and a receding hairline, whereas the shot on the right has a more natural appearance.

    (click to enlarge)

    [​IMG]
     
  22. G.T. thread starter macrumors 6502a

    Joined:
    Jul 12, 2008
    #22
    One other thing to get best of both worlds and save money, if I got a Canon EF lens mount for the micro four thirds then I can attach my canon 28-85mm lens which would give me more freedom when I want to get closer to a subject and when I need portability I would only need the 20mm.

    Or I could only buy the body because I have a lens and get mount to save even more. I don't think it would work for taking videos though? And some people had issues with AF on some lenses, I wouldn't want to take the risk, if it worked great if not then I only have one lens.

    Have a look at this review it shows good promise for only using the 20mm http://craigmod.com/journal/gf1-fieldtest/

    Yes the 14-45mm is kit lens and is slow thats why I like the appeal of the 20mm while it can be bundled as a kit it is of high quality. If u see my post before this I have mentioned a lens I have the would let me take better portrait shots, which like I say I would take less than landscape, as I would want to take camera around then the 20mm lens is more portable and will let me capture more of the surroundings.
     
  23. Phrasikleia macrumors 601

    Phrasikleia

    Joined:
    Feb 24, 2008
    Location:
    Over there------->
    #23
    If portability is what you really want, then go ahead and get the 20mm pancake lens to start out. It will be a fun little prime lens to experiment with. Use that for a while and then expand your kit when you have a better idea of what you need most to accomplish your goals.
     
  24. G.T. thread starter macrumors 6502a

    Joined:
    Jul 12, 2008
    #24
    I think thats what i'll do and get a canon EF mount for GF1 so I can use my other lens cause i'm a student and so must spend money wisely ;).

    If prices don't drop I can get body and mount and will be set then get the prime but I fear I would take the camera out with me less.
     
  25. Designer Dale macrumors 68040

    Designer Dale

    Joined:
    Mar 25, 2009
    Location:
    Folding space
    #25
    So then is a 105 on a film 35 is good for portraiture because it places the camera at a distance which yields best perspective for a shoulder-up framing? 135 moves it farther away and a 50 moves it closer, right?

    I understand the relationship between sensor size and focal length. 28mm on my XSi has the same view as 42mm on a 35mm film camera.

    Dale
     

Share This Page