Focussing a 55-100mm zoom on a P67ii body


rickbehl

Link Posted 06/06/2011 - 04:08
Hi,

I recently was fortunate enough to be able to pick up a 55-100mm Zoom lens for my Pentax 67 II camera. This is my first zoom lens for the camera and I'm very much looking forward to using it after hearing very positive things about its quality.

Most of my photography with the Pentax 67ii is Landscape work and my main lenses were the 45mm, 55mm, 75mm and 105mm. I expect the 55-100mm zoom to replace 3 of these lenses in my bag. However I have a question regarding how best to focus the zoom lens. As most of my work is in landscape I am almost always looking to maximize depth of field without losing too much sharpness. I found pretty much all the primes mentioned to be great quality even stopped down to f22. I couldn't really see any softness introduced by diffraction, etc.... When focussing the primes I would use the lens barrel to put the infinity mark just between 22 and 16 on the left side in order to maximize depth of field (assuming I had a shot which included a far horizon/mountain, etc).

Now my question is what should my technique be for obtaining maximum depth of field with the 55-100mm zoom? I don't believe I can now use the lens barrel to see what the limits of the focus are going to be. Should I now rely on adding hyperfocal distance 'markers' to the lens at various focal lengths?

Any opinions on what approaches are available would be much appreciated !

Cheers, Rick

johnriley

Link Posted 06/06/2011 - 06:52
If you are maximising DOF the easy "rule of thumb" is simply to focus one third of the way into the scene. The scene is defined as the area containing the nearest and furthest objects that you want to be in focus.

The DOF will be the same at any given focal length for both the prime lenses and the zoom. You might therefore just note what your focus distance has been and just set the zoom to the same. No need to focus at all through the viewfinder, just compose.

Finally, set the focal length on the zoom before focusing if you do use the viewfinder for focus. Not all zooms hold focus accurately when they are zoomed.
Best regards, John

rickbehl

Link Posted 06/07/2011 - 16:41
Thanks John!

Banjo

Link Posted 22/07/2011 - 09:54
Hi Rick,

I have the same 55-100 zoom and the same issue with it as you have.

It seems all lenses of the same focal length may have the same hyperfocal distance at the same aperture, so you can look up the relevant formula and calculate the distances in question, or consult appropriate tables: I did the latter and made up a little abridged table, which I keep handy in the gadget bag along with the lens.

For what its worth, here it is:


Hyperfocal distance (in meters)

Focal Length: 50mm 80mm 100mm
Aperture

f/8 9 27 38

f/11 7 20 28

f/16 5 14 19

f/22 4 10 14

f/32 2 7 10



As you can plainly see, focusing one third into the scene doesn't really cut it.

My little table is very skeletal, and you will need to make some approximations for intermediate values as needed.

Anvh

Link Posted 22/07/2011 - 19:23
With this you can make a hyperfocal distance chart.
http://www.dofmaster.com/charts.html

Banjo wrote:
As you can plainly see, focusing one third into the scene doesn't really cut it.

It's almost never true this rule so i'm curious where it comes from?
With Macro photography for example you actually have a DOF spread of 1:1.

Here is a whole read about it.
http://toothwalker.org/optics/dof.html
Stefan


K10D, K5
DA* 16-50, DA* 50-135, D-FA 100 Macro, DA 40 Ltd, DA 18-55
AF-540FGZ
Last Edited by Anvh on 22/07/2011 - 19:24

johnriley

Link Posted 22/07/2011 - 19:43
The one third in works perfectly well in the circumstances it refers to - that is using small apertures at moderate distances, such as in landscape photography.

In the same way the "sunny 16" rule also works extremely well, but the photographer needs to know how to apply it.
Best regards, John

Anvh

Link Posted 22/07/2011 - 19:49
John what is the full offical rule because you state two different things in your comments.

-simply focus one third of the way into the scene-
vs
-using small apertures at moderate distances-

I'm getting a bit lost


ps. a more recent article about why the third rule doesn't work.
http://www.cameratechnica.com/2011/06/12/alternatives-to-the-one-third-rule-for-...
Stefan


K10D, K5
DA* 16-50, DA* 50-135, D-FA 100 Macro, DA 40 Ltd, DA 18-55
AF-540FGZ
Last Edited by Anvh on 22/07/2011 - 19:58

johnriley

Link Posted 22/07/2011 - 20:07
I'll start again from the beginning. This is a simple "rule of thumb" (approximation if you like) for getting as much of a scene in focus as possible.

Set a small aperture (maybe f16, maybe f22)
Focus on a point one third of the way in, between the point closest that you want in focus and the point furthest (often infinity).

So if the closest point you wanted in focus was 30 feet and the furthest was 120 feet then you focus at about 60 feet.

Obviously citically a lens always is focused on one distance only, but the circle of confusion allows some leeway that we call depth of field.

This also assumes something close to a normal lens. It's not going to be much use with a 1000mm f8.
Best regards, John

Anvh

Link Posted 22/07/2011 - 20:32
Thanks John for explaining

johnriley wrote:
This is a simple "rule of thumb" (approximation if you like) for getting as much of a scene in focus as possible.

It's not even an approximation, just do the math with what you just said.
You will need to try very hard to get an example where the rule will work.

If you'll use a 55mm lens at f/16 and you focus on 60 feet to get 30 to 120 feet in focus you're really way off... you can use f/2 if you focus at 50 feet...
With f/16 and 55mm you can get 5,4 feet to infinity in focus if you focus at 10,5 feet.
If you focus at 10 feet the distribution of the dof will be 3/97 so not even close to 1/2.
Stefan


K10D, K5
DA* 16-50, DA* 50-135, D-FA 100 Macro, DA 40 Ltd, DA 18-55
AF-540FGZ
Last Edited by Anvh on 22/07/2011 - 20:33

johnriley

Link Posted 22/07/2011 - 21:03
Focused on 10m (30 feet) a 55mm lens has a DOF from 3.71m to Infinity.

Not too shabby as an approximation. Comes down to 3.01m on f22.

The figures I quoted were only to give an idea of the scale of the measurements, not to suggest they were actual accurate figures.

The point is made that everything would be within the DOF. If it didn't work, then generations of photographers would have been wrong.
Best regards, John

Anvh

Link Posted 22/07/2011 - 21:33
johnriley wrote:
The point is made that everything would be within the DOF.

But how it is done is simply to crude, it's like trapping a mouse with an elephant trap...

johnriley wrote:
If it didn't work, then generations of photographers would have been wrong.

It simply doesn't work, not even in general.

The one third rule is based on the false fact that 1/3th of the focus is infront fo the focus point and that 2/3th of the focus is behind it but that's never the case.

here is an quote that explains it.

The rear DOF is twice the front DOF.
This is just a very silly statement. In the macro regime the DOF distribution (front:rear) is 1:1, in the portrait regime it is still close to 1:1, and a landscape captured at hyperfocus will have a distribution of 1:infinity. Somewhere in between there will be a scenario that gives 1:2, but you will have to look for it. Perhaps the misconception is due to the observation that a 1:2 distribution represents some kind of average over many photographic situations. So much is clear, every scenario should be considered individually.
Stefan


K10D, K5
DA* 16-50, DA* 50-135, D-FA 100 Macro, DA 40 Ltd, DA 18-55
AF-540FGZ
Last Edited by Anvh on 22/07/2011 - 21:35

Blythman

Link Posted 22/07/2011 - 21:37
Stefan, be grateful if you'd share a rule of thumb that II could quickly use. I have to say I am useless with estimating distances, and distance markings on lenses are almost useless.
Alan


PPG
Flickr

Anvh

Link Posted 22/07/2011 - 21:54
But it isn't a rule of thumb, because it doesn't even come close to what is true or general correct.
Stefan


K10D, K5
DA* 16-50, DA* 50-135, D-FA 100 Macro, DA 40 Ltd, DA 18-55
AF-540FGZ

Blythman

Link Posted 22/07/2011 - 21:56
Anvh wrote:
But it isn't a rule of thumb, because it doesn't even come close to what is true or general correct.

So you've demonstrated. Which is why I'm asking you if you can suggest an alternative to help me out, now you've convinced me to scrap using it.
Alan


PPG
Flickr

Anvh

Link Posted 22/07/2011 - 22:10
Ah sorry, didn't read it well

There is no rule of thumbs for focusing because there are to many variables.
There is no way you can get focal length, aperture, focus distance, circle of confusion in an easy package.

For most subjects you simply focus on the subject so there is no need for a thumbs of rule.
For landscapes you could use a hyperfocal chart like this one.
(this chart is for APS-C)



It's quite simple.
- pick the focal length at the bottom
- move up till you hit the correct f-number line
- move to the right to read the hyper focal value
- as a rule of thumb, the focus begins roughly at half the hyper focal distance.

I wished the camera had a hyper focal switch that simply put your lens on it.
Stefan


K10D, K5
DA* 16-50, DA* 50-135, D-FA 100 Macro, DA 40 Ltd, DA 18-55
AF-540FGZ
Last Edited by Anvh on 22/07/2011 - 22:19
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