Depth of Field Experimenting

Last week, I posted about being reminded about using the Depth of Field (DoF) indicators on my older lenses. Today, I’m posting the first result of that reminder.

By trade, I’m a test engineer. I develop tests for the products put out by the company I work for. It was natural, then, for me to develop a test suite that would let me play with the DoF and actually learn, hands-on, exactly what the DoF for each setting is and how to best leverage it for the other camera settings I might need, or for a particular photograph. I’m one of those that can read a book and understand it, but to be honest about it, I won’t grok it until I actually DO it. Without the hands-on, I learn more where to look up the information if I need it than how to use it.

So, last week I figured out what my problem most likely was, what I needed to do, and re-visited in the depths of my mind using the DoF markers like I did when I was shooting film with those lenses. I understand DoF, and I understand how it changes with aperture. BUT, and for me this is a big but (no smart ass comments, please ;-), knowing it from logic and study is quite different from knowing it from actually using it. I need to revisit that experience-based knowledge after ignoring it for so many years.

One very interesting surprise I discovered is that my more modern 80-200mm (I have two, an old manual one and a newer one that has autofocus) appears to also have a DoF indicator. I had noticed that little window on the lens (see image below) but hadn’t really paid any attention to it before now.

This is the depth of field indicator on my newer 80mm to 200mm Pentax zoom lens.

This is the depth of field indicator on my newer 80mm to 200mm Pentax zoom lens.

I thought it was DoF related, but without the other markers I was used to I just didn’t see how to utilize it. Looking closely, I see three markers: one labelled 80, and one close to the center marker. It definitely doesn’t seem as accurate as my other lenses’ indicators with their different lines for different apertures. However, I suspect that the red line close to the center indicates the 200mm focal length field of view. This seems more a DoF indicator for the focal length of the 80mm-200mm zoom lens. This is important because it ties in closely with the next two paragraphs.

While I ran some tests last weekend, I’m not happy with them and will likely re-run these tests later. The problem, I discovered, is that the field of view on some lenses is so small (the 400mm especially comes to mind) that I need to change the view in order to be able to utilize the full focus range of the lens. The aperture, of course, I can change without having to modify the view. The reason for this concern becomes evident in the next paragraph.

One important lesson that I took away from all this initial testing is that it’s not just the aperture that controls DoF. Indeed, the focal distance seems to have a much more broad impact. I am not talking about the focal length of a zoom lens (e.g. 80mm, 90mm…200mm) but rather how far in front of the camera you focus. Again, my 400mm is an excellent example: the focal length remains the same 400mm regardless, but I can adjust the focus from a few feet in front of the camera out to thousands of feet in front of the camera.

Changes in aperture did result in a change in DoF, but for each lens it was apparently very slight, given the little bit of testing I did with a couple of lenses. On the other hand, changing the focus point appeared to result in a more noticable change in DoF.


That wasn’t something I was expecting, but thinking it over and comparing that idea to the images in last week’s blog post, it makes sense. The nature of the telephoto, especially in the longer lenses, tends to subtly hide that effect, at least it did for me. The very premise of the telephoto, initially, is long distance photography. So, as you focus further and further out, the field of view moves out as well. That tends to “hide” the DoF effect due to moving the focus if, like me, you forget that it’s a moving range that likewise moves out with the focus. While the DoF changes as you focus further away, the ratio of the DoF to the distance doesn’t change all that much, so unless you’re looking for it you might not really notice the DoF is actually changing. Especially since the field of view can hide a foreground that would otherwise appear blurry.

So far, everything seems to support the premise that three things impact the DoF. In apparent order from least impact to most impact, right now it appears to be (1) focal length, (2) aperture, and (3) focal distance. The focal length is only important on zooms, of course, being a fixed value on fixed, or prime, lenses.

At any rate, while my tests last weekend resulted in some data, that data is incomplete. I need to re-do the test out in a field where I have plenty of visual markers to show what is in focus and what isn’t. I tried with the street I live on, but the curve of the road and the slight hill didn’t let me get continuous line of visible focus points without having to re-aim the camera. I want to go out to a location where I have an uninterrupted continuous view off into the far, far distance and where I can set up the tripod really low. That way, the camera will be able to see both near and far objects without having to re-aim the camera. There are plenty of places nearby where this will be easy enough to do, so if the weather co-operates, I hope to be able to do this next round of testing next weekend.


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