Shooting IR

[the_macman]

I have a question which may sound stupid. I know infrared film requires a slight

adjustment of the focus. Unfortunately some of the lens I own do not have the mark

indicating IR. However I plan to shoot at very large DOF (f/11, f/13). Do I still have to care

about the adjustment or the new focus for IR will fall anyway into the DOF interval?

[bruce_karnopp1]

If this is landscape, I suggest using the hyperfocal distance for the lens. While with Infrared Film, this won't be the hyperfocal distance, it will take care of the focus adjustment and then some if you are using f11 or smaller.

[mark_bouquet1]

What film format and focal lengths are you using? It's probably fairly safe to assume that the required extension for a given focal length lens from one manufacturer would work well for that same focal length from another lens manufacturer. With that in mind, the infrared focus marks for my Olympus lenses are as follows.

 

For my 100mm/2.8, the infrared focusing mark corresponds with the depth of field mark at f/11.

 

50mm/1.4 -> f/4

 

28mm/2 -> f/4

 

21mm/3.5 -> f/3.5

 

The trouble with using the hyperfocal distance mark is that there's a different one for each aperture, while there's only one correct offset for infrared focusing. If, Bruce, you meant that you use the hyperfocal distance for the aperture you're using, that won't work, particularly with short focal length lenses and small apertures, because you would be way over compensating.

 

Perhaps some others could report the values from their own lenses and we could find out if there's a typical extension for each focal length?

[mike_elek]

If you're shooting at f/11, the depth of field for normal to wide-angle lenses should take care of any focusing differences.

 

You might have an issue with telephoto lenses longer than 135mm for close-up work, but f/11 or smaller will help.

 

That generally has been my experience.

[stephen hazelton]

If you're using a red, orange, or no filter, then a good bit of the image is actually made with visible light. Shifting the focus there would tend to focus the IR and unfocus the visible, so it's really somewhat pointless.

 

With the opaque filters, it makes more sense, but just using a smaller aperture will probably do the trick. Kodak recommends a starting shift of some small percent of the focal length (look up the HIE information sheet on their website).

 

The shift is NOT necessarily the same for all lenses of a given focal length. It depends on the degree of correction built into the lens, and is zero for some well-corrected lenses.

[kelly_flanigan1]

The edge of the DOF tick marks on 35mm still camera lenses has the COC criteria of usually 1/30mm. Thus a star will be roughly 1/30mm diameter; instead of smaller.<BR><BR>Are you shooting ONLY in IR; or both IR and visible?<BR><BR>When shooting capturing both IR and visible; you may want to use a focus point between the visible and IR best focus points; and stop down too..<BR><BR> Here when I use a digital back on a 4x5; without the IR filter; I refocus; after a trial scan. Most lens do shift abit ; when focusing visually; and then shooting mostly in IR. Older books throw out a 1/4 percent shift in focal length as a starting point. Aerial lenses and scientific lens are often scribed with the actual measured focal lengths; or the Focal Distances; and/or both. Here my 6 inch F2.5 Kodak Aero Ektar is scribed FD 5.512 IR FD 5.521 RR 1XXX ; Here the FD is the focal distance; NOT the focal length. The FD is measured from the lens mounting surface; to the infinity image plane. FL is focal length. Here a Nominal 6 inch focal length Aero lens has a focus shift of 5.521 -5.512 = 0.009 inches; or 0.009/6 = 0.15 percent. <BR><BR>The percentage of focus shift depends on how the lens was corrected. The Aero lens example shifted less than 1/4 percent; because it is designed for IR. A NON IR typical photographic lens shifts more; this is why the 1/4 percent number is sometime used; if IR tick marks are not on your lens. <BR><BR>You can do some focus tests; and scribe/mark your lens with a dinky IR mark; by doing some actual focus tests. The IR focal length is longer; ie infinity focus in IR requires a setting so a 50mm lens is set to say 50 or 60 ft.

[kelly_flanigan1]

MACMAN; I posted some digital IR images on <a href=" http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=008J6U">this thread; I used a PC; PIII, 667Mhz; PS 7.0 </a>