IR focus shift and hyperfocal distance

I'm sure this may be simple or obvious (for some of you) but I've been trying to figure out how to calculate the hyperfocal distance when capturing IR images (~600nm and #720nm. Given that there is a focal shift when moving into the IR spectrum then I assume that the hyperfocal distance also shifts.....but does it and by how much?

Thanks in anticipation of my further education.....

Bob

You can't calculate the shift since it depends on the particular optical formula (some lenses have no shift) or at least not without a ray tracing program.

You can use the IR index on your lens and imagine the whole scale of apertures used for hyperfocal visualisation translated similarly.

Thanks Jerome. If I understand you correctly, I can focus normally at my chosen hyperfocal distance and then simply offset the focussing ring by the distance between the normal reference mark and the IR mark. I should now be at the hyperfocal distance for IR.

Bob

Bob Latham said:

I'm sure this may be simple or obvious (for some of you) but I've been trying to figure out how to calculate the hyperfocal distance when capturing IR images (~600nm and #720nm. Given that there is a focal shift when moving into the IR spectrum then I assume that the hyperfocal distance also shifts.....but does it and by how much?

Click to expand...

Hi Bob,

It's not obvious at all, because it varies by lens design. However, since there is often an IR indicator dot on the focus scale, you can draw some clues from that.

Because the IR image plane usually focuses behind the sensor plane, we need to add a bit of lens extension to achieve focus by focusing a bit closer. The focal length for IR is a bit longer.

Since the Hyperfocal distance is calculated from the focal length, the Aperture, and a CoC criterion, we can only adjust the focal length in the formula to be 'a bit' longer.

vHyp = f^2/(NxC) + f , where f= focal length in metres, N= aperture number, and C is the COC. If you keep the aperture number and CoC constant, you can try different focal lengths and see if the vHyp changes much.

How much longer the focal length (f) must be to achieve the required bit of additional lens extension, and how much extension, is lens dependent.

What is the regular focal length, and what aperture number are you considering?

Cheers,
Bart

Bob Latham said:

Thanks Jerome. If I understand you correctly, I can focus normally at my chosen hyperfocal distance and then simply offset the focussing ring by the distance between the normal reference mark and the IR mark. I should now be at the hyperfocal distance for IR.

Bob

Click to expand...

Hi Bob,

Yes that should work, assuming you can dial in the Hyperfocal distance accurate enough.

Cheers,
Bart

Hi, Bob,

Bob Latham said:

I'm sure this may be simple or obvious (for some of you) but I've been trying to figure out how to calculate the hyperfocal distance when capturing IR images (~600nm and #720nm. Given that there is a focal shift when moving into the IR spectrum then I assume that the hyperfocal distance also shifts.....but does it and by how much?

Click to expand...

There are two distinct issues here.

The actual hyperfocal distance will only change by virtue of the focal length of the lens changing (which it does slightly at IR wavelengths), but the effect of that will probably be negligible.

The second issue then comes into play: that of the shift of the focus distance scale because of the difference in back focal length at the IR wavelengths.

This is the same issue we have for focusing at any desired distance when operating at IR wavelengths.

If the IR reference mark gives a good accommodation of that for other situations, then it should do equally well when you are trying to set the focus distance to the hyperfocal distance.

Best regards,

Doug

Bob Latham said:

Thanks Jerome. If I understand you correctly, I can focus normally at my chosen hyperfocal distance and then simply offset the focussing ring by the distance between the normal reference mark and the IR mark. I should now be at the hyperfocal distance for IR.

Click to expand...

That is what I was suggesting. It is an approximation, but close enough for normal photography practice.

Bart_van_der_Wolf said:

What is the regular focal length, and what aperture number are you considering?

Click to expand...

Typically between 15-50mm and f/5.6-8

Bart_van_der_Wolf said:

Hi Bob,

Yes that should work, assuming you can dial in the Hyperfocal distance accurate enough.

Cheers,
Bart

Click to expand...

Please see below Bart

Doug Kerr said:

There are two distinct issues here.

The actual hyperfocal distance will only change by virtue of the focal length of the lens changing (which it does slightly at IR wavelengths), but the effect of that will probably be negligible.

The second issue then comes into play: that of the shift of the focus distance scale because of the difference in back focal length at the IR wavelengths.

This is the same issue we have for focusing at any desired distance when operating at IR wavelengths.

If the IR reference mark gives a good accommodation of that for other situations, then it should do equally well when you are trying to set the focus distance to the hyperfocal distance.

Best regards,

Doug

Click to expand...

Thanks Doug....see below

Jerome Marot said:

That is what I was suggesting. It is an approximation, but close enough for normal photography practice.

Click to expand...

I'm heading in the right direction now Jerome...thanks


It's perhaps necessary that I explain my approach to using hyperfocal focussing for visible light and what was to be my approach for IR.

I have a clutch of manual focus lenses (Zeiss Distagons and Planars). Once I have chosen my aperture for the shot (typically f/11 unless I need more light), I then use Liveview to manually focus on an object at the appropriate hyperfocal distance (either in the scene or by choosing something in the vicinity).

With IR I generally use f/5.6-8 to cover for the effects of diffraction kicking in a little earlier. I had presumed that I could perform a similar operation if I had the "IR hyperfocal distance" to refer to. This is an IR converted (720nm) 5DMkII.

I hope that made sense.

Bob

Bob Latham said:

I have a clutch of manual focus lenses (Zeiss Distagons and Planars). Once I have chosen my aperture for the shot (typically f/11 unless I need more light), I then use Liveview to manually focus on an object at the appropriate hyperfocal distance (either in the scene or by choosing something in the vicinity).

Click to expand...

If you use Liveview on a stopped-down lens, you will see what is sharp in IR and what is not. There is no need for calculations or approximations, the screen shows the scene as it will be recorded in IR.

Jerome Marot said:

If you use Liveview on a stopped-down lens, you will see what is sharp in IR and what is not. There is no need for calculations or approximations, the screen shows the scene as it will be recorded in IR.

Click to expand...

Thanks Jerome. I find it quite difficult to assess more distant objects (tending towards infinity) using the LCD screen even when it's zoomed in 10x. With my wider lenses (15, 18 &21mm) it's the issue that I'm trying to overcome whilst 35-135mm presents less of a problem.

Now that I know it is very much lens dependent and not simply focal length and aperture I can perhaps experiment by shooting the moon with a moveable (and distance defined) foreground object to produce a quick reference chart for some of the lenses.

Bob

Hi, Bob,

Bob Latham said:

Now that I know it is very much lens dependent and not simply focal length and aperture . . .

Click to expand...

What does that mean?

Maybe I don't recognize what "it" is.

Thanks.

Best regards,

Doug

Doug Kerr said:

Maybe I don't recognize what "it" is.

Click to expand...

Hi Doug,

'It' is the amount of IR focus shift. 'It' varies from lens design to lens design.

It would require an apochromatic lens design to have multiple wavelength bands (R+G+B) focus in the same plane. Only few lenses are designed that way. See Q31 and here for some more info.

Cheers,
Bart

Hi, Bart,

Bart_van_der_Wolf said:

Hi Doug,

'It' is the amount of IR focus shift.

Click to expand...

Oh, of course. Thanks.

Best regards,

Doug

I've never used a digital camera but I've shot a lot of infrared on film and some things come to mind:

The amount of focus correction for IR is dependent on the wavelength. Lenses I've used that are corrected for visible light show negligible shift at 720nm.

The same lenses exposing Kodak High Speed Infrared (HEI) out near 900nm show significant focus shift.

I suspect (but cannot prove) that the red IR focussing dot on lenses was traditionally calculated using Kodak HEI as the de facto standard challenge.

Now that HIE is long discontinued I wonder how IR focussing marks are calculated by lens makers.

Venerable authorities were guessing like everyone else. A popular rule of thumb for unknown lenses was to advance the focussing movement by 0.3% to catch the IR focus. Ansel Adams suggested +0.75%. No body specified the actual IR wavelength but all suggested stopping down generously so depth of field would swallow the focussing error. This I still do.

Just for fun here's a recent effort with no focus correction but with the lens stopped down to f22:


Gelatin-silver photograph on Ultrafine Silver Eagle VC FB photographic paper, image size 16.3cm X 21.4cm, from an Efke IR820 rollfilm negative exposed in a Mamiya RB67 single lens reflex camera fitted with a 50mm f4.5 lens and IR680 filter.
Titled and signed recto, stamped verso.

Hi, Maris,

Maris Rusis said:

I've never used a digital camera but I've shot a lot of infrared on film and some things come to mind:

The amount of focus correction for IR is dependent on the wavelength. Lenses I've used that are corrected for visible light show negligible shift at 720nm.

The same lenses exposing Kodak High Speed Infrared (HEI) out near 900nm show significant focus shift.

I suspect (but cannot prove) that the red IR focussing dot on lenses was traditionally calculated using Kodak HEI as the de facto standard challenge.

Now that HIE is long discontinued I wonder how IR focussing marks are calculated by lens makers.

Click to expand...

Knowing the way these things go in photographic "practice", I would not be surprised it were done the same way! There is a great tendency to treat empirical practices as somehow coming from laws of nature and thus deserving immortality.

Venerable authorities were guessing like everyone else. A popular rule of thumb for unknown lenses was to advance the focussing movement by 0.3% to catch the IR focus. Ansel Adams suggested +0.75%. No body specified the actual IR wavelength but all suggested stopping down generously so depth of field would swallow the focussing error. This I still do.

Click to expand...

Very interesting discussion.

Thanks.

Best regards,

Doug