depth of field... calculator

Hello
I am using this website: http://www.dofmaster.com/dofjs.html

Recently bought a Tamron 17-50 f2.8 - I believe the sweet spot is f4 (haven't tested yet but I found a website the other day, can't find it anymore, where has a test of sharpness for every single lens at every single aperture, so you can even compare your lens twice with different aperture) so thought I'd see what I can achieve for landscape at f4...

Using the model with the following settings:

Example 1:

Pentax Kx
17mm
f4
4 meters focus

Results
Near limit 1.9 m
Far limit Infinity
Total Infinite

This means: if I shoot a landscape at 17mm f4, focus on something only 4m from me, I'll have everything in focus from 1.9m to infinity? that should be the sharpest shot ever right?

Example 2:

Pentax Kx
17mm
f8
2 meters focus

Results
Near limit 0.95 m
Far limit Infinity
Total Infinite


If I do this it means I can include a proper foreground (just a meter away from my camera) and still get something pretty sharp (I think f8 seems fair)

Am I reading this correctly?

Just checked your results on the application I've stored on my windows smartphone, and they correlate.

Here's what I have http://home.comcast.net/~jeffrey.kohn/DepthCalc/DepthCalc.htm

Just to clarify:

The DoF calculator will give you the hyperfocal distance. That shows you what will be acceptably in focus.

The MTF (Modulation Transfer Function) of the lens is another thing again
That's the point at which the lens exhibits the best resolving power - effectively the point at which the lens 'shines', or its 'sweet spot'. The MTF will vary according to focal length, aperture, and lens.

Not sure if the Kx has it, but there is an custom option to choose the best aperture according to the MTF (it's called the 'Program Line') - then the camera will use info stored in the lens to try and pick the sweet spot automatically.

Matt

Thanks for the info. Not sure how to check that though!

But basically i have the feeling that using the camera on "auto" might actually give the best results!?

Just checked the Kx manual - doesn't look as though you can change the Program Line.

Not a big problem. Personally, I use Av mode almost all the time and have a 'feel' for the MTF of each lens I use. Often the best aperture on a modern lens is around one stop down from wide open (e.g. f/4 for an f/2.8 lens etc), but depending on the lens YMMV.

Just experiment and don't worry too much
Matt

I know that's my whole point, instead of shooting a lanscape at f16 I might have a much better result at f4 as will have almost everything in focus (except the very close foreground)

FWIW, I have tested my own Tamron 17-50 and on my one the sweet spot for peak sharpness all across the frame is at f5.6. From memory the centre is about the same at 4 and 5.6, but the edges are sharper at 5.6.

good to know
cant wait to test it, just have nothing to shoot..!?

Matt, does your comment mean that some Pentax DSLRs do have this feature? If so, which one(s) please?

Basically the top of the range ones (this feature was even on the later top of the range film SLRs)
Andy

Thanks Andy. Found it in my K20D manual. Very interesting.

Glad to be of help
Andy

I first used it on the film Z1.
The *ist-D had it, the K10D and K20D, and of course the K7.
It can be handy, but I tend to work with just Av more and more (having ISO on the other control wheel).
Matt

Ditto
Andy
Having typed that, had ISO on K10D wheel - but have epxosure compensation set on K7 on front wheel