Manual Focus Technique

Discussion in 'Nikon' started by geoff_cardillo, Nov 23, 2010.

  1. i've been unhappy with the sharpness of my photos. not all.
    i'm using a nikon d300 with an 18-200mm 3.5-5.6 DX lens.
    i recently took some posed photos with a model using this combo. when i used the on-camera magnification, i zoomed in as far as it would go and could see sharp definition in the model's eyes and skin. so i'm fine with a studio lighting/posed model situation.
    the problem is when i'm shooting in situations that are not studio-lit. outdoors or indoors. i use auto-focus much of the time but seem to always end up with the background content in focus, but not the subject. i've been switching to manual focus - i zoom-in on the subject - to focus - then zoom-out to frame the image. but i don't re-focus.
    the results i've gotten are ... mixed. i viewed the images in the camera - with the magnification all the way in and some are sharp ... others... not as much.
    is my technique valid? i mean... i'm thinking that if i want to frame the shot at 18mm, but i need to zoom-in to 85mm to get the focus correct, should i re-focus? the problem there is that i can't really finely-sharpen the focus inside the viewfinder when my lens is at 18mm.
    i'm sure this raises more questions than can be simply/quickly answered... but if we can start the conversation, that would really help me a lot.
    thank you.
     
  2. Some zoom lenses are parfocal, meaning if you focus and then zoom, you'll still be in focus. Others are not. I don't know if your 18-200mm is. It may also be that your lens does not produce its best results with all combinations of focal length and aperture.
    If you are really concerned with sharpness, though, you probably want to avoid super-zoom lenses.
    Your problem with auto-focus deciding to focus on the background rather than the subject probably has to do with your choice of AF points. I find with most multi-AF-point cameras that if you let the camera decide which point to use, it will quite often choose the wrong one. It's usually best to manually select a single AF point and place it over the subject to maximize the chance of getting focus where you want it.
     
  3. This manual focusing technique of zooming in and then zooming back out with your 18-200 would only work if the lens was a parfocal design, which I believe it is not. Thus, it will require re-focusing after zooming. Over the years, there have been a few threads on parfocal/varifocal Nikon zooms in this forum; I recommend a search if you are interested in more information.
    Agree with Craig on the AF sensor selection issue when the camera decides to focus on the background rather than on the closer subject.
     
  4. thanks craig- i'll check-into parfocal lenses in other photo.net forums.
    the problem that i have will remain with the lens i have, though - i can't see the subject thru the viewfinder to sharpen the focus - do you have any suggestions? is it just... accept it?
    i'm thinking that the 18-200mm lens - while 'versatile' - may not offer the precision i'm seeking. i'll look into the other lenses.
    as for the AF-point choices... i am placing the focal point squarely on my subject's face and still... i'm just not getting what i want.
    to be fair, this often happens in situations where limited light is available and i am shooting with a wide-open aperture... so i don't have much margin for error. i never allow the camera to select the AF-point. i usually use the 9-point AF zone with the intent of limiting the choices the camera would make.
    thanks again for the suggestion to look into the parfocal lenses.
     
  5. Few zoom lenses for still cameras would stay in focus when you zoom. You will have to focus at the focal length you shoot at. Carefully adjusting the diopter correction to match your eye sight is most important.
     
  6. If you are zoomed out, your max aperture may be at f5.6, which is the point where AF systems start having trouble. In a studio, you have plenty of controlled light, and that helps with the focus system. In not so great light, and zoomed out, your AF is working quite hard. It's not that the lens isn't precise, as you have found in the studio. It CAN produce sharp images, it's that the camera has trouble focusing in poor light, at f5.6. ( Assuming you are zoomed out passed 100mm, of course. Below that it's closer to f3.5 ) . by the way, are you using any filters that might reduce the light, out doors ?
     
  7. john - i'm actually NOT zoomed out - quite the opposite. when i'm shooting and having trouble, it's at the low-end of the zoom spectrum (say around 18mm-35mm).
    i am using a UV filter - could this be causing a problem?
     
  8. BeBu - thanks, i'll re-examine the diopter. i do wear glasses and haven't had any trouble focusing on subjects once i'm zoomed-in, but i'll give it a look, nonetheless.
     
  9. The conditions you are describing are not typical of the D300 or the 18-200mm lens. You either have a technical issue or a technique issue. You need to determine which it is and correct it.
    If Craig's suggestion [in his last sentence] does not correct your problem, I suggest you send both your camera and lens to Nikon along with a complete description of your issue and sample photos so they can check out your gear. If your camera is functioning properly, you should get fast, accurate auto focus virtually all the time. And you lens should deliver clear, crisp results at any focal length.
     
  10. I think it is likely your technique. There are a couple of things to consider:
    1) when shooting portraits on location the background is usually close to the subject and not featureless so it's easy for the AF to lock onto the background instead.

    2) slow lenses makes it harder to see what's in focus compared to fast lenses shot at the same small aperture.

    3) the focusing screen in the camera is not optimized for showing focus which makes it hard to visually confirm the focus.

    4) AF sensors are often not the size that is indicated in the viewfinder and they can also use surrounding focus points as "helpers". The AF sensors can also be slightly misaligned. So even using a single focus point might be a problem.

    5) zooming in and out after focusing is not a good solution. Besides the optical design there are mechanical slop as well that might change the focus point.
    6) with studio lighting you are likely shooting with a small aperture and the lens will work well and you depth of field is large. With a larger aperture you lens is not providing maximum resolution and any focusing errors are also magnified as the depth of field is smaller.
     
  11. I may just be missing what the OP is saying, but it sounds to me like the "zooming" that he's talking about is Live Mode zooming for manual focusing. If so, it has nothing to do with whether the lens is parfocal or not.
     
  12. Geoff, I think I might have a solution to your problem. Let the lens focus on your subject, then see what you get. With the magnificent autofocus system of the D300, you should be confident in it. Simple solution...try your old technique, then take another photo allowing the camera to focus and compare.
    Some of us always use the middle focus point to focus on the subject, then recompose. Maybe that will work for you. I've tried the technique you are using and found that I didn't get very good results.
     
  13. Sounds like the focus point is not where you want it to be when you're using AF. Probably that simple...
     
  14. Placing a focus point in the center of a face with a wide zoom may not give enough contrast for the camera to correctly focus. Try focusing on the edge of the head, where contrast with the background may be more optimal for the focus point. I sometimes see htis as helpful.
     
  15. thanks pete - with the studio setup i was shooting at f8 some of the time and f3.5 some of the time. in both cases, i was able to get results that were to my liking.
    as for the focusing screen... it is helpful to learn this - is there any kind of workaround? i do not like the idea of using the LCD as a 'monitor' to focus. i want to look through the viewfinder... what else can be done?
     
  16. Geoff, I shot weddings and faced some of the same problems as you do now albeit with different equipment.
    Weddings are usually faster than portraits, sometimes very low light, people are often not standing still and you don't always get second chances so this may not apply to your situation 100%.
    However I solved my problems over time by (in no particular order):
    1) Converting to using only AF-On for more precise focus control.
    2) Always selecting which focus point to use and never let the camera decide, again for more control.
    3) Always having the cameras in AF-C mode for continuous focusing so the camera will track moving subjects as well as continuously track when I sweep over the subject looking for high contrast edges to focus on. I found this better for low light, moving subjects and as good as AF-S for still life.
    3) Replacing the focusing screens in the cameras to manual focusing screens with split images (for instance katz eye), because they will show what's in focus and what's not over the entire focusing screen.
    4) Only used lenses that was f/2.8 or faster - easier to see what's in focus.
    5) Always used the AF cross sensors in low light as they are faster and more reliable in low light.
    Perhaps a few of these options would be something for you to explore.
     
  17. These guys make good points, but here are two obvious ones to consider first:
    1) When you recompose, how much are you moving the camera? If you have a shallow depth of field (not uncommon close up), then focusing on the subject's eyes and recomposing will usually render their ear in focus. Make sure that when you recompose you're moving the camera laterally, and not turning it.
    2) That lens is awful wide open. In fact, almost any lens with a variable maximum aperture is awful wide open. Try doing the same thing at home on a still life. If it looks good at f/11 but awful at f/5.6, it's a matter of what the lens can and cannot do.
     
  18. When I was in my 20's I could focus very fast with a Nikon type D screen (all ground glass and nothing else, no microprism or split image) that I wonder why they are working on autofocus cameras for. Nowaday my eye sight is quite bad and even with the diopter correction set at max I still need additional glass to focus well and yet not as fast as I could do in the old days.
    Zach talked about recompose after focus, that is the reason why I use a focusing screen with no focusing aid because I can focus any where on the screen. I don't recompose after focusing.
     

Share This Page