Dark Night Focus

[Stagecoach]

Canon certainly has, at the least, acknowledged that there is a difference.

Different expansion and contraction rates of individual elements, within the lens, has been cited as one reason for some of their lenses to be able to focus beyond Infinity.

(N.B. There are other reasons for lenses being able to focus beyond Infinity).

As a general comment, this feature is usually found on:
> Long FL Lenses
> Large diameter barrel lenses, especially those with a Fluorite Element.

(Noted also, nowadays, some W/A lenses are an inverted telephoto design and many of these lenses are designed to focus to beyond Infinity.)

I haven't used Nikon for a while, but a document easily to my hand now, from Nikon, also mentions this design factor, though does not specify the different rates of expansion of various lens elements:

“The reason for this [a lens being able to focus beyond infinity] is that in extremely hot or cold conditions the effective focal length of the lens may change due to the effect of the temperature on elements in the lens, which causes expansion or contraction compared to optical performance at regular temperatures. Depending on the construction of the lens elements, some lenses will be affected more than others. To compensate for this, the option to focus with the lens beyond the infinity focus position is available on some lenses.”

REF: Nikon Support Services; Article ID 000027772; Published 12/21/2016; “Why is it possible to focus manually beyond infinity on some lenses?”

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My Post #33, mentioned Manfred's maths which referred to Aluminium (i.e. Lens Barrels).

In my Post #33, an implicit conclusion could be made that change in the size of the Aluminium lens barrel being so insignificant, was the reason for my disagreeing with "refocus every 30 minutes or when temperature change is 1°C"

Such a conclusion, is slightly flawed.

Specifically, I disagree with "refocus every 30 minutes or when temperature change is 1°C"
because:

1. a change of 1°C in the Ambient Temperature will have no real world effect on expansion and contraction of anything in a lens.

2. in reasonable shooting conditions, there is absolutely no reason to refocus every 30 minutes.

WW

I had overlooked the 'infinity' position Bill which has been mentioned for years as being affected by temperature by the manufacturers. Clear confirmation that they have undertaken testing.

Interestingly I found reference to one of the sky stacking programmes having a 'focus/sharpness' score for each frame and the use of the difference of this for showing focus accuracy over time and temperature change.

[William W]

I am indebted to the two engineers who attacked their slide rules and log tables and attended to the arithmetic . . .

I can't imagine a focus mechanism that can focus at that level of precision. In fact, I suspect the overall manufacturing tolerances for the various machined or moulded parts would at best be in the order of ±0.03mm / 0.001" as a best case. In fact I would not be surprised if the manufacturing tolerances are not a lot looser than that.

I’ve a few lenses from various manufactures; with delicate touch one can feel ‘play’ in many of the parts, not the least of which is the bayonet camera to lens mount: I don’t have a measuring tool accurate enough for the task, but I’d bet a Mars Bar that many of my lenses wiggle “a thou o’ an inch”, yet they all seem to make sharp photos, (provided I get the focus correct).

That bit in the brackets, the possibility of human error, is the important bit and is often the bit that is least investigated when things go belly up.

***

As a general comment, not associated to any comment in this conversation nor directed at any contributor.

I think that sometimes a snippet of information gets loose and Chinese Whispers develop . . . it’s always been that way: people usually feel good when they are expounding.

Especially with the advent of the www and self-publication, reinforcement of an original fact or rationale can quickly become a factoid laced with embellishment and presented as a decree. The present day resistance to, or ignorance of, confirming what is written with primary and secondary sources, only adds to this mess.

WW

[xpatUSA]

That being said; the operational specs on my camera body say that it can be used from 0 °C - 40°C / 32 °F - 104°F; we are looking at expansion of 40 x 0.4 = 16 µm; still a very small number.

Small indeed and, with the 18mm lens set at f/5, that 16um begets a blur circle of a mere 3um at the sensor. However, I could imagine the expansion being significant for a telephoto lens set with a large aperture.

[William W]

Referring to my Post #38, here is one of the source documents in my library, published by Canon:

"Lenses contain glass elements. These expand with heat. This is not usually a problem with compact lenses − the amount of expansion is small. But large lenses contain large elements and here expansion can bring a lens close to the limits of its design tolerances. A white surface reflects sunlight, helping to keep the lens cooler.

Incidentally, you will find that the manual focusing action of some black lenses allows focusing past its infinity setting (sometimes called over-focusing). This is also to allow for expansion. If the lens is used in hot conditions, infinity focusing will be closer to the end of the travel."

REF: Canon Professional Network; Infobank; "Lenses"; (re)published circa 2017; my library.

WW

[Manfred M]

Small indeed and, with the 18mm lens set at f/5, that 16um begets a blur circle of a mere 3um at the sensor. However, I could imagine the expansion being significant for a telephoto lens set with a large aperture.

Agreed Ted, but the initial inquiry dealt with shooting with an 18 - 55mm lens with a star filter used as a focus assist. That's why I suggested (and continue to insist) that the proposed approach is overkill. Put a large chunk of plastic. metal and glass together using a long focal length; that is an entirely different problem.

The discussions on temperature assume an equilibrium state where all internal mechanisms in the lens have stabilized at the new ambient temperatures, rather than the dynamic state where different components are heating up at different rates, dependent on the thermal properties of the specific lens component. Metal components are conductors and will do so rather quickly whereas glass and resin components are insulators and will do so more slowly.