brenz

I understand the larger opening of the aperture and the smaller F-stop number the more you can blur the background.
My question is how does the focal affect the it.

SharpShooter Loc: NorCal

Brenz, I just did a post on composition and dof last week.
Look for it and see if it will give you some insights! ;-)
SS

brenz

Thanks

Uuglypher Loc: South Dakota (East River)

hi, Brenz,
Are you asking "how does the focal length of the lens" affect the blurring effect? or "how does the distance of focus" affect the blurring effect?. they are two different questions...and two good questions to ask to help clarify the matter.

There were two really long, extensive recent threads on "depth-of-field" . That is the zone in front of the camera within which, at a given aperture, things will appear to be in reasonable focus...appear to be acceptably "sharp".

Between the lens and the close limit of that zone...the DOF...Thing will be blurred and out-of-focus.

Beyond the far limit of the DOF things will also be blurred and out-of-focus.

That depth-of-focus will be very thin/"shallow" with a large aperture(lower number aperture)
and
the depth-of-field will be comparatively much deeper/thicker with a small aperture (higher number aperture).

I hope that helps get you started.

Dave in SD

brenz

Are you asking "how does the focal length of the lens"

Yes
Thanks
Brenda

SharpShooter Loc: NorCal

Brenz, let me simply answer your questions directly.
DoF is governed by aperture opening.
The larger the aperture, the shallower the dof.
Since DoF is governed by aperture, two lenses of the same aperture have the same exact dof regardless of focal length. In other words, a 500mm f4 lens has the exact same DoF as a 24-105 f4 zoom.
BUT, the minimum DoF (max blur) occurs at any lenses minimum focus distance. That min. focus distance is fairly constant relatve to the ratios of their focul lengths. Let me explain that, if that sounded of the wall! :lol:
A 50mm lens with a minimum focus distance of 1 foot, is relatve to a 200mm that will have a min. focus distance of 4 feet(4 times focul lenghth, 50mm x 4 = 200mm, hence 1 foot x 4 = 4 feet, or their relative min. focus distance ratio).
So a longer lenses does NOT produce a shallower DoF at the same f-stop.
The advantage of that longer lens is that if a 50mm lens at f4 has a shallow dof of say 1" at 2 feet away, a 500mm f4 lens will give the same DoF at 10 times that distance, or at 20 feet away, giving you a lot more working room.
Just the same way that with a 50mm macro lens you need to be right on top of a bug to get 1:1, a longer macro lens will give that same 1:1 with a much greater working distance.
Hope that helped a little?
Now I'll let others tell you if the longer lens will produce more or less visual compression or better or worse OOF, or Bokeh!! :lol:
SS

Uuglypher Loc: South Dakota (East River)

Sorry, Sharp Shooter, your first response sounded like you wanted Brenda to go back and wade through that last thread on her own. That's why I started her out at the start...couldn't have been more basic.

Since you have now "...simply answered her questions directly" you go ahead now and take it from here where you've left it. Sorry to have stepped in in front of you. I obviously misunderstood your intent. No hard feelings, I hope!

Best regards.
Dave

brenz

Sharp Shooter, Thank you so much. That helps me so much! I get it

brenz

Thanks Dave. I do now understand. My instructor really confused me yesterday. I appreciate all the help here.

Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)

Distance - the shorter the distance (higher magnification) the narrower the depth of field, and the blurrier the background.

Longer focal lengths will provide higher magnifications/shallower depth of field at a given distance than a shorter focal length.

[edit] Keep in mind that any depth of field calculator or table will provide "approximate" information, and generally assumes average print sizes and viewing distances - your mileage may differ. For really sharp images I will use half the depth of field indicated in DoF tables or phone apps, and for aperture if I want less DoF I will use a larger lens opening - when possible.

CO

Canbridge in Colour has a flexible depth of field calculator online that is very good. You can enter sensor size, aperture, lens focal length, and the focus distance. Check it out here:
http://www.cambridgeincolour.com/tutorials/dof-calculator.htm

Revet Loc: Fairview Park, Ohio

This is how a photo enthusiast optometrist explained it to me. (Optometrist's get a pretty hefty education in optics)

The only absolute way to change DOF is with Aperture. DOF can also be relatively changed by varying focal distance and lens focal length but this is a function of magnification (you blur the background by magnifying it). Saying this, all three ways are effective at blurring the background which is what is really important here.

dsmeltz Loc: Philadelphia

zigipha Loc: north nj

SS, that is a bit misleading. DoF affected by F stop, distance to subject and focal length. If you keep the angle of view the same by moving feet back and forth, then the DoF does not change much wrt focal length. But if you focus 100M away, and start changing focal lengths, then DoF will change drastically.

Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)

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