Looks like you answered your own question...
The DoF calculator on this page also says the total DoF is the same for a 70mm @ 1.4 m and a 140 mm @ 2.8 m (and as an aside; 131 != 2×70 so you might see a tad more DoF in @ 2.8 m)
I can't get my head around it Remco; it looks like if distance is small relative to hyperfocal distance then dof is similar, but the more important question about blur beyond dof looks counter intuitive.
If I wanted to blur the background to these flowers more than this; do I need longer or shorter focal length.
Here is how you can check, use the
DOF Master Depth of Field Calculator
It's a great web site that does the math for you or you can get the formulas, Mmmm formulas, to calculate Depth of Field for yourself.
The formulas are wrong Steaphany. I can work out from my approximation formula that there isn't a lot of difference, but actually there isn't any difference which is what I suspected.
But there is a difference in perspective and enlargement; longer focal length means bigger background and bigger blur.
Hello Steve,
I think your observations of your own experiment are correct. In my view (for non macro photography) there are only three things that influence DOF.
1)Whatever you define as “sharp” or “blurred” - which in the case of your experiment we can take as a constant.
2)The working f stop of the lens – which is a ratio.
3)The reproduction ratio or magnification of the image.
You will notice that the focal length of the lens only matters where it changes the reproduction ratio or magnification – and your results show this to be the case. You may like to try an additional experiment.
1)Mount one of your zoom lenses and set the camera to use a constant aperture
2)Set the zoom to the shortest focal length setting and adjust your position so that a vertical object - e.g. fence post - fills the viewfinder vertical – just. Take a picture.
3)Set the zoom to a longer focal length setting. Move back until the object fills the viewfinder – just. Take another picture.
4)Repeat until you have used the longest focal length setting.
When you compare the pictures you should find that the DOF – in front of and behind the vertical object is the same in all the pictures. That is because all three of our parameters are held constant. The change in the focal length of the lens is negated by moving the shooting position.
The same basic proposition explains why small point and shoot cameras have such apparently great DOF – The ratio between the size of an object and the size of the image of the object becomes very great as the image sensor becomes smaller.
To get more blur in the background of your flower shots you need to increase your magnification, use a big working aperture (if you can), and find a point of view that makes the background that you wish to blur – as far away as possible. The blur behind the point of focus increases with distance – with no limit.
HTH.
Regards,
Nick.
Thanks for that Nick; it is nice to know so that I can plan what I want to be seen better.