Bokeh question

[astroNikon]

So I was out trying to take pictures of hawks the other day. They were "no shows"
So I wandered through a park trying to take pictures of the woodpeckers, but they weren't cooperating much with posing for me.

I was using my Nikon D7000 and a 75-300mm AF f/4.5-5.6 lens (made in 1989-1999)

I assumed this lens really didn't have much bokeh. But there were a few bees about 10 feet away so I took a few shots because I had my camera in my hands and the woodpeckers weren't cooperating.

But the bokeh on these lens is pretty nice, I am surprised.
At 300mm it has some nice bokeh at f/6.3,
but at 220mm it's not much at f/6.3.

The 220mm f/6.3 you see back ground defocusing but you can still make out everything
The two 300mm f/6.3 the background is pretty well totally out of focus, pretty neat.

My question is basically why the difference in bokeh ?

BELOW: 220mm


BELOW: 300mm

 

[Braineack]

it's just kinda the nature of a telephoto lens, if you shot at 85mm and f/6.3 it wouldn't be nearly as smooth/blurred.

 

[astroNikon]

FYI Here's one "okay" picture I got of one of the woodpeckers. 300mm f/11

 

[Designer]

The out-of-focus areas (the background in this case) varies with the focal length.

The QUALITY of the out of focus is called "bokeh", and is inherent in the lens.

Telephoto lenses will exhibit a more pronounced out-of-focus area when the subject-to-lens distance is less (closer).

 

[KmH]

You're asking about depth-of-field, not bokeh. Bokeh and DoF are not the same thing, though they can be related if DoF is shallow enough that the bokeh a lens produces can be seen.
Bokeh is only adjustable by changing to a different make/model of lens.

One of the major factors that affects DoF is point of focus distance relative to focal length.

A more useful test would be to keep the subject size in the image frame, and keep the same lens aperture the same for each shot.
That way you are changing the point of focus distance for each focal lengt.

Understanding Depth of Field in Photography

Plug the appropriate values from your images into this calculator and compare the total DoF values - Online Depth of Field Calculator

http://i307.photobucket.com/albums/nn290/CougarWilbur/200mm56Feet20.jpg

If subject scale is kept constant in the image frame, the total DoF of say f/4 at 300mm, is the same as the total DoF of f/4 at 50 mm.
Because the point of focus distance changes to keep subject scale in the image frame constant.

But the difference is, a lens at 300 mm magnifies the blurred background elements making them larger.

 

[KmH]

it's just kinda the nature of a telephoto lens, if you shot at 85mm and f/6.3 it wouldn't be nearly as smooth/blurred.

It would be if subject scale in the image frame was maintained.

In fact it would be pretty much exactly the same.

 

[Braineack]

it's just kinda the nature of a telephoto lens, if you shot at 85mm and f/6.3 it wouldn't be nearly as smooth/blurred.

It would be if subject scale in the image frame was maintained.

In fact it would be pretty much exactly the same.

If you had to move closer to frame the subject the same, the background bokeh would also be closer and the effect would be dampened, would it not?

If you stood in the same spot, and same f-stop, then yes, I'd assume it to be exactly the same.

 

[astroNikon]

Thanks for the info guys. Slowly but surely everything is starting to make more sense, rather than just looking at the exposure meter and making all is set when in Manual. I've played alot with my 50/1.8 to understand DOF but i never correlated it to anything more than a "static" understanding from my fixed 50mm versus a zoom which varies .. if that makes any sense.

 

[KmH]

If you had to move closer to frame the subject the same, the background bokeh would also be closer and the effect would be dampened, would it not?

First, the only change in the bokeh would be if a different model or make lens was used, not because a different focal length lens was used


But no, because the shorter focal length doesn't magnify the background as much as the telephoto lens does just by getting the shorter focal length lens closer to the subject to maintain image scale.