Focus throw; range, sensitivity, or a conflated measure?

[RokkorDoctor]

I have come across contradicting uses of the term "focus throw" and wondered if anyone can shed some light on the correct definition (if there is one).

1) Some seem to understand it to be the total degrees a focus ring needs to be turned to go from infinity to the minimum focus distance, i.e. a less useful conflated measure of both focus range and sensitivity.
2) Others seem to understand it to be the required degree of turning the focus ring to change focus by a given distance, i.e. a pure measure of sensitivity.

I suspect "focus throw" usually refers to concept 1), which means it isn't really a useful terminology to use for indicating the focus sensitivity of a lens, as it conflates the measures of focus sensitivity and focus range.

Is there in fact any existing terminology for clearly indicating the focus "sensitivity" of a lens? E.g. a useful metric could be the required degree of turn of the focus ring to go from infinity to 100 times the focal length of a lens, so e.g. for a 50mm lens the required degree of turn to go from infinity to 5m focus, for a 300mm lens to go from infinity to 30m, etc.

[D1N0]

1 is correct

2 is probably something video people use but video is not photography

[kymarto]

[D1N0]

[RokkorDoctor]

[visualopsins]

Thanks for topic.

I've understood focus throw to be degrees of turn. How degrees of turn affect distance of focus, i.e. ease of nailing focus, depends on helical fineness, focal throw, and focal length. For example, of two lenses with with equal focal length, equal helical fineness, but unequal focus throw, the ease of focus will be identical, however, one lens will have a closer minimum focus distance. I believe such is the case with common 500mm mirror lenses. I could be wrong.

Ease of focus also depends on contrast and aperture. For example, a high contrast Tamron 500mm lens is easier to focus than a lower contrast 500mm lens if apertures are equal. A faster 500mm will be easier to focus, depending on focus throw and helical fineness.

A relationship between focal length, aperture, focus throw, and helical fineness.

And we thought the four way relationships of RGBW and CMYK color spaces was a difficult concept. LOL

[RokkorDoctor]

[visualopsins]

As long as distance is specified, "rate of change of the diameter of the circle of confusion per degree turn at wide-open aperture".

Reminds me of DOF being wholly aperture and magnification -- focal length and distance determine magnification, of course.

The four factors. Photography seems full of those. ISO, shutter speed, aperture, light intensity.

Indeed, gravity-mass-electro-magnetism; the four DNA chemical bases. The ancients were definitely onto something with air-fire-water-earth. Pairing seems to be a theme as well. Then there is 5-element theory for the addition of metal describing the promotion and inhibiting functions. Anybody up for 6 or more elements? DNA has 6 components. & etc..

[cooltouch]

I prefer not to get caught up in all the complexity. For me, focus throw just means stop to stop. Now, because some are mentioning that the sensitivity extends at closer distances, I just picked up an old Sigma 600mm I happen to have sitting next to me on my desk. This is an early Sigma 600mm, with a focus throw of about 270 degrees. Later Sigmas have a focus throw closer to 360 degrees. After examining the distance scale as it appears along the throw, I pulled out a Tamron 500mm mirror, which has almost a 360-degree focus throw. Examined the distance scale on it along the throw, and guess what I found? They were essentially identical. The scale appears to be logarithmic. At infinity, the distance markings are close together, each one indicating a substantial distance between each other. As I rotate the lens, I see the distance markings grow farther apart while the distances between each marking grow closer together. Finally, at the end of the throw, the differences are in fractions of a meter (or foot) for a fairly substantial amount of remaining throw. I suspect that if I were to chart the values, with distance being the Y axis and % of focus throw being the X axis, where the X axis values would be plotted from Infinity to MFD, I'd find essentially the same charts with both lenses, and the chart would have a strong logarithmic look to them. That is, the curve would be slightly descending with a very slight downward curve but the curve would become sharper and down-turning much quicker as MFD is reached. However, I also suspect that if the X axis were plotted logarithmically, the curve would be linear.

So what this indicates to me is this is just a normal way the lens focuses and that there is no built-in bias toward close-up subjects.

[KEO]

Why not just say "focus throw" for stop to stop, and "focus sensitivity" for measuring by degrees?

They're two different things; they deserve different names.

[RokkorDoctor]

[RokkorDoctor]

[visualopsins]

[stevemark]