Last week in the Daz forum's, Gregorius (aka IuvenisScriptor here at Rendo) wrote that he couldn't figure out why out why the background always seems to come out a bit blurry in all his EnvSphere renders.

I will copy my answer here.

Because you're zoomed in on a tiny fraction of the EnvSphere image.

If you plan to actually see the EnvSphere image as a background, you have to think about how many pixels you're seeing of that image in your render.

Consider - the EnvSphere image, left to right, covers the entire 360 degrees of a circle. Suppose you're using a "50 mm" focal length camera simulating a 35mm film. Horizontally, that covers roughly 40 degrees of that circle. So you are displaying a fraction of the full image that is 40/360 of the entire image, or about 11%.

Now if the environment image is, for example, 5000 pixels across, you are asking it to cover your render using 40/360ths of that 5000 pixels, or about 550 pixels. If your render is 800 pixels across, you're stretching those 550 pixels to 800 - and you get blur. With a 100 mm camera, you're only using 5.7% of the circle, or just around 283 pixels.

So if I'm going to feature my environment image as my background, I usually use a wider camera (lower focal length). For example, a 25 mm lens gives just under 20% of the circle. With a 5K image, that's about 1000 pixels - usually enough to get a good background.

An alternative is to not use the environment image as the background. Keep it for IDL and reflection purposes, but place another photo (a normal one, not an equirectangular one) directly behind the figure on a one-sided square.

I always assumed that the Poser camera focal length was meant to correspond with the field of view produced by that focal length on a 35 mm film camera. Turns out after experimenting, that isn't so. There is a 1.4x crop factor. I can't imagine where they got that. I might have expected the 1.5x crop factor found on Nikon DX DSLR cameras or the 1.6x crop factor found on Canon DSLRs. Where did 1.4x come from?!?

Given a desired field of view, in degrees, and you want to know what focal length to set the Poser camera to, this equation gives pretty good answers:

12.75 / tan(x * pi / 360)

where x is the desired angle.

Did you know that Google accepts math formulas and tells you the answer? It's pretty cool. For example, to find the focal length to produce a 50 degree field of view, type this into Google and hit search:

12.75 / tan(50 * pi / 360)

Google says 27.34 is the answer.

You can also use this formula to calculate pixel ratios. For example, if your environment image is 5000 pixels and you want to see 800 of them across your render, use:

12.75 / tan(800 * pi / 5000)

The answer is 23.19 mm.

Here are a some more (rounded down) based on viewing angle:

10 degrees = 145 mm
20 degrees = 72 mm
30 degrees = 47 mm
40 degrees = 35 mm
45 degrees = 30 mm
50 degrees = 27 mm
55 degrees = 24 mm
60 degrees = 22 mm
65 degrees = 20 mm
70 degrees = 18 mm

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)