Minimum Aperture vs Focal Length Range

Discussion in 'Photography Beginners' Forum' started by zrheath, Apr 5, 2016.

  1. zrheath

    zrheath TPF Noob!

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    Hey all, I joined this forum quite some time ago when I bought my first DSLR and I am finally exploring the real potential that it has. I have been using Aperture Priority mode a lot lately to really get that down, but one thing has me stumped. I own two lenses, an 18-55mm 3.5-5.6 and a 55-200mm 4-5.6 that I have been experimenting with. I have found that although the 18-55 has a lower minimum aperture, I cannot get nearly as good portrait shots as when I am using the 55-200 and standing further away and zooming into my target. Can anyone explain why this would be?


     
  2. table1349

    table1349 Been spending a lot of time on here!

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    Post examples with each lens of the same subject. Include the exif data for the photos. Hard to tell with out seeing examples.
     
  3. john.margetts

    john.margetts No longer a newbie, moving up!

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    What is a 'lower minimum aperture'? If you mean a small value for the f/ stop, that just means a wider aperture when you have the lens zoomed to 18mm - it has no direct bearing on image quality. Shooting at f/3.5 will give you a smaller depth of field but otherwise a poorer image quality. Most lenses have a 'sweet spot' at f/5.6 to f/8. Practise will tell you the best aperture for each of your lenses, but it will not be at the 'lowest minimum aperture'.
     
  4. Dave442

    Dave442 Been spending a lot of time on here!

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    To get that larger aperture in the 18-55 you need to be close to 18mm and at that focal length the depth of field is large (try out an online depth of field calculator to see the numbers). At 200mm and f/5.6 and close enough for a portrait you have a small depth of field.
     
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  5. robbins.photo

    robbins.photo Yup, It's The Zoo Guy Supporting Member

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    It has to do with your depth of field. Using the zoom lens you stand further back from your subject, which affects your depth of field, as does the focal length of the lens.

    So lets take your 18-55mm as an example. Lets say your taking a portrait where your standing 5 feet away from your subject and shooting at say 18 mm at f/3.5 using a camera with an APS-C sensor. Your depth of field is 3.91 ft.

    Take that same shot standing say 10 feet away using the 200 mm zoomed out to 200 mm at F/5.6, and your DOF drops to 0.16 feet. So even though your shooting at a higher aperture number, both the focal length of the lens and the distance from the subject cause the DOF to actually become more narrow, which is what gives you that more out of focus background.
     
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  6. table1349

    table1349 Been spending a lot of time on here!

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  7. fmw

    fmw No longer a newbie, moving up!

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    You would need to define what better shots means but the difference is possibly perspective. As you increase the distance between camera and subject you flatten perspective. We call it foreshortening. I often use long lenses, for instance, with fashion photography because of this phenomenon.
     
  8. zrheath

    zrheath TPF Noob!

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    Thanks to everyone for the replies. I have only self-taught to this point so I am definitely new to the correct terms when it comes to some of this but I will try to explain the best I can.

    In this instance I am specifically talking about getting the background to blur, so I guess I worded my question very badly. From what I have read, a wider aperture makes more a background blur. When taking close up photo, I can never seem to get the background to blur as much on the 18-55mm as I can on the 55-200mm, even though the 18-55 has a minimum of 3.5 compared to the 55-200 having a minimum of 4. What I somehow did now realize is that as you zoom in, the width of the aperture decreases. So this obviously explained a lot when I loaded the photos in and looked at the EXIF data, however I am still seeing a difference in the photos when taken at the same f-stop. The two photos below are the best I have to describe what I am talking about. They are both taken at f/5.6, but the first is taken at 55mm and the second is taken at 200mm with the zoom lens. The second has more of a blur in my opinion. I have realized, however, that it is just too difficult to get close enough with the 18-55 and still have a wide aperture, making zoom lenses much easier to work with in this case.

    [​IMG]DSC_0019 by zrheath@sbcglobal.net, on Flickr

    [​IMG]DSC_0018 by zrheath@sbcglobal.net, on Flickr
     
  9. robbins.photo

    robbins.photo Yup, It's The Zoo Guy Supporting Member

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    The aperture of the lens, the focal length your shooting at and your distance from the subject will all affect your depth of field, which will in turn affect the amount of blur in the background.

    The further away your subject is from the background and the more narrow your DOF, the more blur you'll see in the background.

    Here's a nice link that will let you play around with it a bit.. give you a better idea as to how background blur relates to your distance from the subject, your lens aperture and the focal length your shooting at:

    Bokeh simulator & depth of field calculator
     
  10. zrheath

    zrheath TPF Noob!

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    This pretty much explained everything I was asking, so thank you for taking the time to help. It's clear that getting the out of focus background deals much more with the DOF and much less with the actual f-stop. I clearly have a lot more research to do in order to completely understand this.
     
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  11. Overread

    Overread has a hat around here somewhere Staff Member Supporting Member

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    This is because background blurring is affected by your distance as well. For the shot with the longer focal length lens you are further away when taking the shot and thus the increased distance causes an increase in the degree of background blurring that takes place.
     
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  12. robbins.photo

    robbins.photo Yup, It's The Zoo Guy Supporting Member

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    No worries, aperture does affect DOF - but it's only one factor of several. I included a link in a previous response that has kind of a neat little simulator that will give you a better idea how the various factors affect DOF and there effect on background blur.
     

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