Depth of Field Results

Twenty-six degrees Fahrenheit. Solid grey skies. Slight wind increasing the wind chill factor. One hundred and fifty plus photos. That is the physical aspect of my Depth of Field  (DoF) experimenting last weekend.

It was surprisingly difficult to set up the test the way I wanted. In the end, I couldn’t do it and wound up having to move the camera’s aim for two of the lenses: the 400mm prime lens and the 135mm prime lens. The 50mm prime lens was the only one that I didn’t have to re-aim. In every case, the camera was all but on the ground. To ensure that I did not move the camera any more than absolutely necessary for the focusing, I also used a remote shutter release, set the camera to manual focus, set the ISO to 200 instead of my usual 100-200 range, and changed the aperture manually with the lens aperture ring in the first round of photos for each lens.

The first round of photos with each lens was to focus on a point a few feet away and then take a picture at each aperture setting for the lens. The second round of photos for each lens was to focus as close as possible (except for the 50mm lens, more about that in a minute) and compose the frame if at all possible so that there was both blurred foreground and background. From that point I would adjust the focal point outward gradually moving the plane of sharp DoF while being sure to overlap the previous DoF that was similarly clear until I reached infinity. For the 50mm lens, I had to start a little further out than as close as possible because that lens has the capability to focus within inches. Short of putting the camera on the ground, I would have had to aim the camera straight down from the tripod.

So, why didn’t I put the camera on the ground?

Two and a half reasons for that. 🙂  The first is that with the camera on the ground there is no way I could change the focus without moving the camera. I suppose I should have done this anyway and I might go try it just to check. The second is that it is very difficult to see through the viewfinder when the camera is on the ground, a necessity to ensure each photograph was indeed aligned properly. The half reason is that it was 26 degrees F, had been for several days, and the ground was frozen.

It took some searching to find a place that gave me enough distance for the full range of my 400mm lens in a way that would let me see the DoF clearly as I moved the focus outward from the closest point. Part of that is due to the 400mm being 400mm on 35mm film, but on my DSLR it’s the equivalent of a 600mm prime. So I needed someplace that would give me a reasonably flat landscape from a little over 2 feet on out to thousands of feet while also providing easily visible visual markers to use to compare the actual DoF.

Now that I’ve explained the setup, here are my results.

Short answer is that it was very much as I expected from my previous quick and dirty testing that prompted the last blog post. Changing the aperture did indeed change the DoF, but usually only slightly. Checking at two different focal points, which is not the same as the focal length, gave near identical results insofar as the amount of DoF goes. In other words. whether I focused in close or further out, changing the aperture without changing anything else tended to have a slight change in DoF.

There was more variance at the further focal point. I’ll explain how that works in a bit. In addition, changing the focal point but leaving the aperture set changed the DoF. In this case, changing the focal point provided a greater change in the DoF, though still slight per focal point.

Changing the aperture is a change in one of the main variables of the mathematical formula determining the DoF. Don’t worry, I’m not going into the math. As I mentioned in the previous post, I’m one that learns best by hands-on and that helps me remember it better than if I just study it. The tests I did showed me that the DoF changes from focal length to focal length for the same aperture. Also as expected from the formula, the DoF for the 135mm at f/8 is larger than the DoF for the 400mm at f/8. But actually doing the test drove home the point that it’s a percentage of the view as well.

That ties into the next point for me: changing the focal point from near to far also changes the DoF and in fact more than changing the aperture does. However, it appeared to me that the DoF only appears to change in the movement of the focal point. Let me explain….

I’m not using real numbers here, I’m just making them up as I go along. I’m sure there’s a lens out there these numbers will fit, but for this I’m just making them up just to illustrate a point. It’s quite possible I’m wrong about this, but it appears to be valid to me.

At any given aperture, the DoF, that area that’s nice and sharp, is a certain amount of the entire view being photographed, from foreground to background. Let’s say the DoF is 10% at f/5.6. In addition, the total view visible is dependent on the focal point: when you focus in close, say a foot away, you see an area, say, 3 feet deep from foreground to background. 10% of that is 3.6 inches. So your DoF is 3.6 inches in that 3 feet of view in your photograph.

Now without changing the aperture, so that we keep that 10%, let’s take a picture of something further away from the camera. Adjusting our focal point, we now have a picture that is, oh, 20 feet away from you and encompasses a foreground to background distance of 18 feet. 10% of that is 21.6”. Just by changing the focal point (not the focal length) you’ve changed your DoF, the clear, sharp area in the image, from 3.6” to 21.6”.

As your image moves further and further out from the camera, the depth of your photograph’s scene (foreground to background) distance increases, and as that increases, so does your DoF area, but it’s always that same 10%, or whatever the actual percentage is.

As mentioned earlier, that percentage will change with your change of aperture.

Of course, combining the two results in a cumulative effect but if the percentage is very small to begin with, achieving focus can be very tricky to get right manually.

Add to that point that a short focal length (not focal point this time) will tend to have a greater DoF compared to a longer focal length. This was very noticeable in the three prime lenses I used in the test. The 50mm had the greatest DoF, the 135mm was next, and the 400mm had the least for the same aperture.

It gets worse.

These three prime lenses are from my film days. They work, and work rather well, with the DSLR but they were designed for a 35mm imaging area. In both my Pentax DSLR bodies, the actual focal length is actually 1.5x. That means the 50mm works more like a 75mm, the 135mm like a 202.5mm, and the 400mm like a 600mm.

Remember what I said about the DoF being smaller as the focal length increases? That’s definitely the situation here with the 400mm. The jump from 400mm to 600mm is larger than the jump from 50mm to 75mm and I assume, as a result, that the reduction in DoF is similarly larger.

All this explains a lot of things to me and in a way I can understand by having actually played with the lenses, settings, and camera.

I never understood why the 400mm didn’t seem to work as well as it did with film. The DoF is a large part of the reason. With the 35mm film cameras, there was sufficient DoF that I didn’t notice any problems. I also had a better focusing screen in both my film Pentax cameras. With the DSLR, the DoF is much tighter and since I have to focus manually with a much less distinctive view in the viewfinder to indicate when I’m in focus it’s hard to keep a continuous focus working. In my film bodies, I had a split screen to aid in focusing. In the DSLR viewfinder, I have to try and see the sharpness.

So for landscapes, astrophotography, and other non-mobile distant subjects, I can go a reasonably good job getting in focus. For birds or other subjects closer than a thousand feet (304.8 meters) it’s not so easy but at least now I know what I’m dealing with and how to think it through. Even 500 feet (152.4 meters) is difficult. And if the subject is moving erratically closer and further away….

That definitely all makes it harder to use the 400mm the way I’d like. Enough to make me think about a 400mm or even 600mm with autofocus capability. If I can afford one.

On the other hand, when I master these old prime lenses, I’ll really be able to say I know what I’m doing.

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