Depth of field and focus distribution II

After a lot of points of good info was answered here

http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00J0u4

there was something about focus distribution that I wasn't able to put in words.

So I translated the DOF scale of my 50mm (for the GL690) into a distance

proportional scale. What came up might be obvious to some, but it was quite

enlightening to me. The biggest thing about it being the focus quality

distribution. In physical dimensions, when we focus a little further away, the

gain in range of high quality focus grows enormously.

Using the DOF scales might be tricky in that even if we get a lot of deep field

focus, we might be getting a lot of very poor deep focus.

I'll post a graphic and go ahead with these ideas, may be you find them interesting.<div></div>

So, if you open the graphic, the X axis is the dof scale in meters, the Y axis is the precise focus point. So, you can see in the same colors as the one in the lens DOF scale, how the focus is distributed.

Now, what I find most interesting is, considering the red range the best in focus quality, and the light blue the worst, how much of each we get when changing the focusing point. So, we can see that while it's temping to go by DOF scales to get focus on the whole range (focusing arround two or three meters and using an according aperture), the impact on overall quality is devastating.

Likewise, we can trade only one meter of good sharpness by a huge amount in the back range.

What's eye opening to me, is that focusing to three meters, and using f22, you might think you are getting great dof, while in truth you are only getting a couple of meters of very good focus past your focusing point, so you'll still have a sharp foreground against a fuzzy background if it's a landscape.

Well, there are several conclusions to be drawn, and as I said, a lot of them might be obvious to some, but looking at this in distance proportions instead of focus range (as we see it in the lens dof scale) gave me a lot of new perspective.

I'm also making the 100mm table, if anyone is interested, I'll post it too.

By the way, I made the graphic on an excel table coloring the cells, so it's not absolutely correct at a decimal level because the work would be a lot harder and I think it works as it is to show the idea. Especially the further limits, which I just approximated, but this could be done properly with a DOF calculator. Also, all the color ranges that reach the right limit of the graphic go past infinity in the lens scale (some barely, some a further).

DOF scales are never accurate as they assume certain parameters that may not apply to you (such as final enlargement size). The only point that is truly in focus and sharp is the point that you are actually focused on, everything else is in varying degrees of unsharpness the further you get away from that focus point. Since you are obviously scientifically inclined, I would suggest a good basic book on photographic optics as it would give you a clearer understanding of what DOF actually is. But you are correct in your observation in that there is more apparent DOF the further away you focus.

Mike, It's not that I love doing this stuff, but it makes the focus mechanic a lot more intuitive once I get it. The big thing I learned is that focus in the back range is not proportionally distributed as in the back. In terms of real distance, in the front range you might be getting 75% of it in very good focus, while in the back range you're getting almost the same in absolute distance, but nearly nothing proportionally. And there IS more DOF in the back when you focus a little further, it's not apparent, you get a lot more meters of sharp depth.

Max:

 

With a wide-angle lens and near/far compositions in which you're stopping down to get both the near and far elements in reasonably sharp focus -- e.g., a plant two feet away and buildings a mile away against the skyline -- my experience has been that relying on hyperfocal distance will make the buildings look softer without making the plant appreciably sharper. Simply focusing to infinity gives you significantly sharper focus at infinity with virtually no cost at the near distance.

 

I started doing this after reading an online essay in which the writer argued infinity focus often beat hyperfocal distance focusing.

 

(Obviously this doesn't apply if you're shooting at f/8.)