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Thread: Prime Lens Question

  1. #1

    Prime Lens Question

    I'm sorry if this sounds like a dumb question and I hope I can explain myself properly.

    What is the difference between manual and auto focus with a prime lens?

    I'm about to get the 50mm prime for my Nikon D40X and as I've mentioned before that model only allows the option of manual focus with that lens. I keep thinking this is a disadvantage.
    But I can't get my head around what this means when I can only compare it to my first film camera, a basic point and shoot. When I went travelling 25 years ago (yikes!) I took with me a little film Pentax Pino - I just aimed at things and when they were printed pretty much everything was in focus. You could aim without looking and get general shots, in focus, or specific shots if you did the walking. But there were no adjustable bits on the camera.
    So how is this different to a prime lens? What/why/how do you use/need autofocus? I 'get' that you focus when you have a zoom lens because there are different focal lengths, but I'm totally confused about this!

    Sorry about the long winded question but I'm finding it hard to explain my question let alone understand it??!!!
    Thanks for your patience and answers
    Donna

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  2. #2
    With manual focus you are literally manually bringing things into focus. So when you look through the lens the distance doesn't change but it will be blurry until you focus it yourself. Auto focus picks a point given the focal length of the lens and that point is in focus. In manual focus you would need to focus the lens whether you were using a prime or zoom.

    Think about it like your eye. If you are focusing on something close up your eye has to adjust to focus on something sitting right beside it even if they are at the same distance. HTH

  3. #3
    I understand the difference between manual and auto focus, but isn't a prime lens a fixed focal length? Why did my fixed point and shoot take pictures in focus without adjustments, yet a prime lens needs to be focused? Sorry if I'm not explaining myself very well.
    Donna

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  4. #4
    The focal length is fixed, but depending on which aperture you choose, your depth of field will be different. The advantage of the 50mm is shooting at a low aperture for a shallow DOF and lower lighting. But, you will have to manually focus on what you want to be in focus. If you shoot at a higher aperture, you might have a better chance of everything being in focus, but you won't be getting the advantages this lens offers - shooting in lower light, nice shallow DOF, blurred bg's, etc.

    I think you'll get used to focusing manually and be fine with it.
    Heidi

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  5. #5
    I think a point and shoot is "auto focus" per se. Becuase it does not allow you to do the focusing manually. Even though they are both fixed length they would be different in that you WILL have to manually focus the 50mm. I have the D40X as well and I am about to get the same lens. As Heidi said, you will get used to it and it wont be an issue. HTH!
    Bailey

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  6. #6
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    If I am not mistaken, I believe Katrina used a 50mm prime for the shots in the below layout. If not, the effect is still there of shallow depth of field.

    As Heidi mentions the advantages of being able to do shallow depth of field if you use the lower apertures, you need to pick what you want in focus and manually focus on that part because the rest is going to be blurry. As you can see in Katrina's layout, she picked different focal points to put emphasis on different elements of the photos.

    So I have found with my learning curve on using the lens, that you may find yourself with pictures at first where the focus isn't on what you intended but they still make for cool photos.

    The prime lenses that auto focus, you have a fixed point of the view finder that it is going to focus itself. So when you point it, it is going to automatically put that portion of the photo in focus. With a manual focus, you are in control of what point in the photo is in focus because you are doing all the focusing work.

    So disadvantage? yes and no. Yes, in that you have to do all the work but no because you are doing all the work and more likely to turn out with what you intended.


  7. #7
    Ahhh Heather! You've shown in a picture exactly what the prime lens can do! (and how funny that you showed with mine )

    And just to add a note...In both photos we were the same distance apart and I used the same aperture. I simply changed which part of him I was focusing on.

  8. #8
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    You've shown in a picture exactly what the prime lens can do! (and how funny that you showed with mine )
    That would be because you do it so beautifully and I am still learning to get what I want in focus and what I want blurred to be blurred and not vice versa.. did learn I had my camera auto focusing in a way that was hurting me not helping me. Manual does sound helpful in many ways.

  9. #9
    it's not a dumb question at all!

    First, you are remembering correctly about the old point and shoot. Many of them are built to work so that any picture you take will be in "pretty good" focus, as long as you are at least 5-6 ft away from the camera. The common disposable point-and-shoots work the same way. They are built to have a relatively small fixed aperture. This means that you'll have very good depth of field (meaning things close and things far both look clear and sharp).

    That's not the BEST way to do it, of course, because when you control focus you can guarantee that your picture will have the sharpness you desire.

    Prime and zoom lenses both need to focus. Even though a prime lens only uses one focal length (for ex a 50mm lens), it still need to FOCUS at that focal length. A zoom lens has many different focal lengths to choose from, as you zoom in and out, and it needs to focus at each one of them that you choose.

    Some lenses have internal focus mechanisms and some have external. On an external focus lens, part of the barrel of the lens actually moves as the focus changes. But this is annoying if you're using a polarizing filter, and many newer lenses have internal focus mechanisms. This means that "stuff" only moves around inside the lens to focus, and the lens won't "grow" as you change focus.

    Keep in mind that focus and depth of field can BOTH change how parts of your picture look sharp or blurry. If you use a very wide aperture (small f/number, like f/2 or f/3), then the thing you focus on, like a face, will be sharp. Things in front of it and behind it will look blurrier. If you use a very small aperture (large f/number like f/22), the thing you focused on will look sharp, and things in front of and behind it will also look pretty sharp. But regardless of which aperture you select, you still need to focus on something or else your entire picture may be blurry!

    I like using autofocus. I have bad eyesight and wear strong glasses, and can't get my eye right up to the viewfinder. Plus, the lenses I own are unforgiving of small changes. That means that as I manually rotate the focus dial, the focus changes DRAMATICALLY and I have to use tiny little movements to get the focus spot on.

    Depending on your camera, though, you can change the focus zone. My camera, a Nikon D200, has many different focus zones. So my camera does not always autofocus on what's in the middle of the screen, which would be a disaster. I can set it up to focus in top left, top right, bottom left, etc. I am ALWAYS changing the focus point when I shoot to account for subject movement. You can also set up some cameras to track the motion of your subject and focus on it continually! For me, changing the autofocus zone is the button I use most often on my camera.

    HOpe that helps. good luck!
    Jennifer

  10. #10
    Thank you all so much for the information. I did pretty much know all of that but it's good to have it reinforced when you haven't actually put it into practice yet! I'll print this out for when I get that lens. (It's time to stop procrastinating!)

    Jennifer, if I understand correctly are you saying the basic point and shoot has a small fixed aperture which is what/why it's always pretty much in focus?

    I'd already decided on the 50mm and understood it's advantages, but I'm just one of those people who has to understand the bigger picture! Just couldn't figure out why my old photos were always in focus when I didn't have to do anything and I could see the camera wasn't doing any auto focusing!
    Donna

    My Gallery

    NIKON D600, 24-70mm 2.8, 50mm 1.4, Macro 105mm 2.8, 28-300mm & FUJIFILM X-E1
    PSE 10, LR 4
    iMac & MBPro / Mavericks


  11. #11
    Not sure about your camera Heather, but mine has a depth of field preview button that I can push and see what is blurred. I don't use it often but it did help to really understand the 50mm

  12. #12
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    Thanks Katrina. I will check into it and see if mine does. I used it today for meeting a friend's newborn in a restaurant for lunch and I got only a couple of good shots but those that turned out, I think turned out fairly well. Would love to get more comfortable with it to feel secure in knowing I will end up with some usable shots in the end.

  13. #13
    yes, many of the point and shoots have a "locked" aperture that you can't change, even if you want to. The manufacturers select an aperture that will allow most of the photo to be in focus as long as you are a few feet away. In addition, some of the disposable point and shoots also have a fixed shutter speed!

    When I was teaching photo classes, I emailed the help desk at Kodak and Fuji just for fun to find out what aperture & shutter speed some of the common disposable point-and-shoot cameras used.

    Here's an example: Kodak Max Outdoor One-time Use Camera has an aperture of f/11, a shutter speed of 1/100, and you must be at least 4 ft away for your pic to be in focus.

  14. #14
    Thank you Jennifer. Somehow comparing my new camera and lens to the old point and shoot helps me understand more about aperture and focus!
    Donna

    My Gallery

    NIKON D600, 24-70mm 2.8, 50mm 1.4, Macro 105mm 2.8, 28-300mm & FUJIFILM X-E1
    PSE 10, LR 4
    iMac & MBPro / Mavericks


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