Focusing issue with an IR filter

[bnnrcn]

I have an infrared filter which unfortunatelly I haven't had the opportunity to try yet. Today I met a photography group in my new location and some of them told me that IR photography is difficult and I need an IR corrected lens to use my IR filter properly because the focus distance of a normal lens in visible light is different from the focus distance with infrared light. So, if I shot an IR image with an IR filter mounted on a normal lens, I would most probably get out of focus images. When I came back home I googled about IR photography and I read a couple of sentences indicating the focusing issue. But strangely I couldn't see many IR corrected lenses sold in Amazon or other websites. So, I just wonder if the focusing issue is really a big problem and do I really need to get an IR corrected lens to shoot sharp -well focused IR images? If so , where can I buy them? Or do you know any other solutions to the problem?

Thanks for your help in advance

[mikesan]

Don't know anything about IR corrected lens. In the "old days", when one focused using the distance scale on the lens, many lenses had an additional index mark for the focus ring to be used when doing IR photography. This corrected for the focusing error with IR light. If your camera is a SLR and you focus using either autofocus or using the viewfinder, I don't think you will encounter a focusing error. (caveat: I do not do any IR photography).

[Manfred M]

Binnur - Mike is correct and that was definitely the way we focused when shooting IR film on a DSLR. The longer wavelength of IR does focus slightly differently than visible light.

Focus manually and shoot at a moderate aperture so that the DoF will be the fastest and easiest fix for you. A lot of IR work tends to be landscape work, so f/8 or f/11 aperture settings should give you some decent results.

[Stagecoach]

Interesting Binnur,

I have just read up a bit on it but did not find anywhere where it actually 'quantifies' the amount out it would be, only that it theoretically would be

If I were you I would do some tests of a specific subject (say bush or tree) using a local scene/view at typical FL, distances and aperture you would with your sea/landscape photography.

[Dave Humphries]

Hi Binnur,

I did a Google image search for Infrared focus marks on lenses

By studying those we can determine that the IR line's offset (towards infinity) is greater at wider angles of zoom than when using longer focal lengths. Note that the offset is independent of focus distance (for a given focal length); e.g. if subject's optical focus is at 0.7m or 5m, it seems to be a set angular rotation of lens barrel that will fix the problem.

From those, you may be able to manually adjust focus AWAY from Infinity, after an optical focus has been achieved. Sure, it'll take practice, but might help when DoF doesn't give enough. On a prime lens, you may be able to scratch or paint an IR focus index line yourself.

Alternatively, if you can manually focus anyway on your camera, buy an old film era lens that has the mark on the lens barrel already. If following this route, be aware that some lenses have a 'hot spot' problem with IR 'light', which appears like a blob of flare in centre of lens, but I assume (or naively hope?) that lenses with an IR mark shouldn't be prone to this issue.

HTH, Dave

[Mike Buckley]

when shooting IR film on a DSLR.

The only way I can think of doing that is to use a DSLR to make a photo of a piece of IR film.

[xpatUSA]

I have an infrared filter which unfortunatelly I haven't had the opportunity to try yet. Today I met a photography group in my new location and some of them told me that IR photography is difficult and I need an IR corrected lens to use my IR filter properly because the focus distance of a normal lens in visible light is different from the focus distance with infrared light. So, if I shot an IR image with an IR filter mounted on a normal lens, I would most probably get out of focus images. When I came back home I googled about IR photography and I read a couple of sentences indicating the focusing issue. But strangely I couldn't see many IR corrected lenses sold in Amazon or other websites. So, I just wonder if the focusing issue is really a big problem and do I really need to get an IR corrected lens to shoot sharp -well focused IR images? If so , where can I buy them? Or do you know any other solutions to the problem?

Thanks for your help in advance

I'm not convinced that a specifically "IR-corrected" lens exists, Binnur, so I am not surprised that your search was fruitless!

Does your camera have Auto-Focus-Micro-Adjustment (AFMA)? That is a function which recognizes the lens model that you mount on the camera and lets you enter an AFMA value for that lens model so that you can correct one time for front/back focus for that lens for a particular setting/zoom.

With my Sigma SD1M DSLR (converted to IR) I can go to the field, focus and compose with the IR filter off the lens. Then put the filter on the lens, set the shutter speed and shoot. Point being that the AFMA setting substituted for that red line on old lenses.

Here's a post on another site about that:

https://www.dpreview.com/forums/post/57711667
.

[xpatUSA]

I have an infrared filter which unfortunatelly I haven't had the opportunity to try yet. Today I met a photography group in my new location and some of them told me that IR photography is difficult and I need an IR corrected lens to use my IR filter properly because the focus distance of a normal lens in visible light is different from the focus distance with infrared light. So, if I shot an IR image with an IR filter mounted on a normal lens, I would most probably get out of focus images. When I came back home I googled about IR photography and I read a couple of sentences indicating the focusing issue. But strangely I couldn't see many IR corrected lenses sold in Amazon or other websites. So, I just wonder if the focusing issue is really a big problem and do I really need to get an IR corrected lens to shoot sharp -well focused IR images? If so , where can I buy them?

On second thoughts, the members of your group might be referring to 'apochromatic' or 'super-achromatic' lenses both of which attempt to provide good focus at near-IR wavelengths:

https://en.wikipedia.org/wiki/Apochr...hift_plots.svg

see:

https://en.wikipedia.org/wiki/Apochromat

However, I have looked for such lenses and they tend to be long and expensive.

Since both appear to be veering off above 900nm (silicon goes up to about 1120nm), a lot depends on your camera's response to IR - do you intend to buy a modified camera?

It just occurred to me that sharper pictures might be taken using two lens filters one to pass IR from say 700nm upward and the other to cut IR above say 900nm. The combination becomes a narrow band-pass filter. As always, you lose something. In this case, more exposure is required due to the lower transmittance of stacked filters. Plus possible vignetting at very wide angles.

Another way around the IR focus problem is in post-processing. Software like 'Piccure+' can restore sharpness which has been lost due to poor focusing. I have it, and it works very well.

[inkista]

I have an infrared filter which unfortunatelly I haven't had the opportunity to try yet. Today I met a photography group in my new location and some of them told me that IR photography is difficult and I need an IR corrected lens to use my IR filter properly because the focus distance of a normal lens in visible light is different from the focus distance with infrared light. ... So, if I shot an IR image with an IR filter mounted on a normal lens, I would most probably get out of focus images .. I just wonder if the focusing issue is really a big problem and do I really need to get an IR corrected lens to shoot sharp -well focused IR images? If so , where can I buy them? Or do you know any other solutions to the problem?

Wow. I've never heard of that. And I think someone may have garbled some information. No, you don't need an IR-corrected lens to shoot well-focused IR images.


Canon XT/350D, adapted CY-mount Zeiss Distagon T* 28/2.8. Hoya R72 filter, tripod. 28mm, iso 40, f/8-ish, 120s.

IR does focus a little differently, because of the wavelengths involved, but given that a Hoya R72 filter pretty much blacks out your viewfinder and drops the exposure on a non-converted camera by about 10-12 stops, you're liable to be shooting on a tripod and using live view, anyway.

With liveview, you're actually seeing what the sensor's seeing, so you can use magnification and manual focus to make sure everything's focused.

The three main issues you should be looking into are shooting with an unconverted camera, possibly having a lens with an IR hotspot, and post-processing techniques.

Most digital sensors are covered by an IR/UV blocking filter (that's actually the glass you clean when you "clean" the sensor). This is because sensors are sensitive to light outside the visible range of frequencies. If there weren't a blocking filter, colors would be thrown off, since that infrared/ultraviolet light would be included in the results. Luckily, it's not 100% efficient, and some infrared can get through.

But when you put an IR-pass filter on the camera, between that filter, and the one over your sensor, you can only grab a little light at a time. So you'll have to do very long exposures. As I said, my Hoya R72 is almost the same as using a 12-stop ND filter for darkness. And it's not uncommon for me to have to shoot 15-30s exposures in sunny-16 conditions. This is why folks who do infrared intensively eventually get a camera converted--that is, they pay someone like LifePixel to remove the IR/UV blocking filter over the sensor and replace it, either with an IR pass filter (so you don't need one for the lens any more), or a full-spectrum non-blocking piece of glass (this can be useful for UV or astrophotography). If the IR filter was used, the camera is now IR-dedicated. If the camera is full-spectrum, it can now only be used for visible light photography with an IR/UV cut filter on the lens.

Secondly, lenses are designed to minimize internal reflections in visible wavelengths, but may not be so well-designed for infrared reflections. My old EF-S 18-55 II kit lens on my Canon XT was notorious for this, and sure enough, when I used it for IR, a big hot-spot showed up right in the middle of the frame.

Thirdly, of course, what you're seeing all over the web as infrared images are probably post-processed and look nothing like the images you'll get coming out of the camera. The Hoya R72 tends to give deeply red images that I have to either convert to B&W, or use channel-swapping to get some interesting false color. Sometimes I'll even combine with a visible light shot. For more information on this, see this old answer of mine on a different infrared thread.

[bnnrcn]

Thank you all , you have been very helpful as always and I feel relieved now My camera is a Canon 70D which has AFMA function. So, I will read the AFMA part in the user manual carefully. What I understand from the comments above is that if I focus to infinity the risk of getting an out of focus image is highest. Because I usually shoot landscapes with a wide angle lens , I usually try to focus near the hiperfocal distance to get sharpness till infinity. So if I focus by using a single focus point close to the hiperfocal distance and then switch to manual focus to lock the focus before putting the IR filter on,even if the focus point differs with the IR filter, the new focus point might still provide sharp images till infinity as the new focus distance might be near the hiperfocal distance. Well, I might as well do some test shots and see the results.

Another alternative way might be ( as my friend Rob Castro who is also a member of CinC suggests) focus stacking images which have been taken with different focus points at different focus distances for the same composition.

I hope I have understood your comments correctly Please correct my thinking if I'm mistaken as I'm not as experienced and knowledgeable as you.

[bnnrcn]

I think you posted your comment while I was writing my reply to the other posts Kathy , so I have seen it just now. Thanks for the long explanation

Could you please tell me how I can focus manually with liveview when the very dark filter is on? I can only see a black screen, so how can I focus?

PS. I tried liveview with the IR filter at night in my living room and I couldn't see anything on the LCD screen. Do you think the light wasn't enough in the room and that's why my camera's liveview can't see through the IR filter? Well , I will try again tomorrow in day light.

Wow. I've never heard of that. And I think someone may have garbled some information. No, you don't need an IR-corrected lens to shoot well-focused IR images.


Canon XT/350D, adapted CY-mount Zeiss Distagon T* 28/2.8. Hoya R72 filter, tripod. 28mm, iso 40, f/8-ish, 120s.

IR does focus a little differently, because of the wavelengths involved, but given that a Hoya R72 filter pretty much blacks out your viewfinder and drops the exposure on a non-converted camera by about 10-12 stops, you're liable to be shooting on a tripod and using live view, anyway.

With liveview, you're actually seeing what the sensor's seeing, so you can use magnification and manual focus to make sure everything's focused.

The three main issues you should be looking into are shooting with an unconverted camera, possibly having a lens with an IR hotspot, and post-processing techniques.

Most digital sensors are covered by an IR/UV blocking filter (that's actually the glass you clean when you "clean" the sensor). This is because sensors are sensitive to light outside the visible range of frequencies. If there weren't a blocking filter, colors would be thrown off, since that infrared/ultraviolet light would be included in the results. Luckily, it's not 100% efficient, and some infrared can get through.

But when you put an IR-pass filter on the camera, between that filter, and the one over your sensor, you can only grab a little light at a time. So you'll have to do very long exposures. As I said, my Hoya R72 is almost the same as using a 12-stop ND filter for darkness. And it's not uncommon for me to have to shoot 15-30s exposures in sunny-16 conditions. This is why folks who do infrared intensively eventually get a camera converted--that is, they pay someone like LifePixel to remove the IR/UV blocking filter over the sensor and replace it, either with an IR pass filter (so you don't need one for the lens any more), or a full-spectrum non-blocking piece of glass (this can be useful for UV or astrophotography). If the IR filter was used, the camera is now IR-dedicated. If the camera is full-spectrum, it can now only be used for visible light photography with an IR/UV cut filter on the lens.

Secondly, lenses are designed to minimize internal reflections in visible wavelengths, but may not be so well-designed for infrared reflections. My old EF-S 18-55 II kit lens on my Canon XT was notorious for this, and sure enough, when I used it for IR, a big hot-spot showed up right in the middle of the frame.

Thirdly, of course, what you're seeing all over the web as infrared images are probably post-processed and look nothing like the images you'll get coming out of the camera. The Hoya R72 tends to give deeply red images that I have to either convert to B&W, or use channel-swapping to get some interesting false color. Sometimes I'll even combine with a visible light shot. For more information on this, see this old answer of mine on a different infrared thread.

[inkista]

Could you please tell me how I can focus manually with liveview when the very dark filter is on? I can only see a black screen, so how can I focus?

Make sure your LCD/liveview is set to do exposure simulation. Then crank up the exposure settings until you can see what you've got. Most LCD/liveview will also let you magnify the view, so you can magnify on the portion of the image that's important to you, then use the focus ring on the lens to fine-tune (obviously, set the lens to manual focus, or half-pressing the shutter button may refocus the camera).

PS. I tried liveview with the IR filter at night in my living room and I couldn't see anything on the LCD screen. Do you think the light wasn't enough in the room and that's why my camera's liveview can't see through the IR filter?...

Probably. It could also mean your camera isn't set to do exposure simulation in liveview as well as that your camera exposure settings were too low to get a good exposure. I'd say, start at iso 1600, f/4-8, and maybe 10s and see what you've got. Remember, 12 stops isn't 12x less light. It's 2 to the 12th power (2^10 -> 4096)x less light.

BTW, here's a list of known good/bad lenses for IR in terms of hotspots.

[Mike Buckley]

Many cameras do not have an exposure simulation setting when using Live View. My Nikon D7000 doesn't have one. The only related information in the manual about that is as follows:

"Depending on the scene, exposure may different from that which would be obtained when Live View is not used. Metering in Live View is adjusted to suit the Live View display, producing photographs with exposure close to what is seen in the monitor."

I was actually surprised to notice that second sentence, as stopping down the aperture when using Live View instead reduces the amount of light just the same as if the camera had no automatic aperture.

[bnnrcn]

Hi again Kathy . You have been really helpful, thanks a lot Actually my exp. simulation was on when I tried yesterday but as I said before it was night and I had the lamps on in my room.So, I think the light wasn't enough and I didn't set my camera to get as much light as it can. I tried your suggestion this afternoon actually about 15 minutes ago outside in day light. Although it wasn't very sunny I was able to see something on the LCD screen when the filter was on. My camera was in Manual mode with f/2.8, ISO 12800 and 1/50 (exp.simulation doesn't work with exposures longer than 1/50 in my Canon 70 D). What I saw was still a dark image and the bright parts were red (as expected for an IR image). I think I might get a better view in full sunlight It seems that your comments helped to solve my problem. Thanks also for the list of the good/bad lenses for IR. I used my Tokina 11-16 for my trial this afternoon, but I'm planning to buy a good Canon wide angle lens soon so I will look at the list before buying my lens in order not to get a bad one for IR.

Make sure your LCD/liveview is set to do exposure simulation. Then crank up the exposure settings until you can see what you've got. Most LCD/liveview will also let you magnify the view, so you can magnify on the portion of the image that's important to you, then use the focus ring on the lens to fine-tune (obviously, set the lens to manual focus, or half-pressing the shutter button make refocus the camera).



Probably. It could also mean your camera isn't set to do exposure simulation in liveview as well as that your camera exposure settings were too low to get a good exposure. I'd say, start at iso 1600, f/4-8, and maybe 10s and see what you've got. Remember, 12 stops isn't 12x less light. It's 2 to the 12th power (2^10 -> 4096)x less light.

BTW, here's a list of known good/bad lenses for IR in terms of hotspots.

[Mike Buckley]

Considering how good Binnur is as a landscape photographer, I'm willing to bet that she eventually converts a camera to IR and does away with her IR filter.

[bnnrcn]

Thank you so much for your kind words Mike I actually thought to convert my Canon 1100D to IR but unfortunatelly they don't have the filter in Turkey to install into the camera . If I want to do such a thing I have to order the filter from abroad. So, I chose the simpler way and I bought an IR filter in Turkey. But if I love shooting and editing IR images , I might think of buying the filter from abroad and having my camera converted. As far as I remember the technical service told me that I might get some focus problem even if I convert my camera (or I might have read about it somewhere). I will of course inquire about everything if I decide to convert my camera to IR.

Considering how good Binnur is as a landscape photographer, I'm willing to bet that she eventually converts a camera to IR and does away with her IR filter.

[Mike Buckley]

If you like doing the IR photography enough to go to the hassle of converting your camera, consider shipping it to a company outside the country to do the conversion. Considering your artistic flair, IR photography seems to be such a naturally good fit for you.

[xpatUSA]

Thank you so much for your kind words Mike I actually thought to convert my Canon 1100D to IR but unfortunatelly they don't have the filter in Turkey to install into the camera . If I want to do such a thing I have to order the filter from abroad. So, I chose the simpler way and I bought an IR filter in Turkey. But if I love shooting and editing IR images , I might think of buying the filter from abroad and having my camera converted. As far as I remember the technical service told me that I might get some focus problem even if I convert my camera (or I might have read about it somewhere). I will of course inquire about everything if I decide to convert my camera to IR.

You might be better off converting your camera to "full spectrum" which means that they don't put an IR filter on the sensor. Then you have a big choice of filters to put on the lens. And shooting "full spectrum" can be fun itself:



Sigma SD1 Merrill, no filter inside or on the lens.

Just a reminder that all Sigma DSLRs come with an easily-removable and replaceable hot mirror in in the lens mount.

[bnnrcn]

Thanks Mike You might be right about shipping my camera abroad for the conversion if I want one day , as the facilities about IR photography in Turkey are limited.

If you like doing the IR photography enough to go to the hassle of converting your camera, consider shipping it to a company outside the country to do the conversion. Considering your artistic flair, IR photography seems to be such a naturally good fit for you.

[bnnrcn]

Thanks for the idea Ted

You might be better off converting your camera to "full spectrum" which means that they don't put an IR filter on the sensor. Then you have a big choice of filters to put on the lens. And shooting "full spectrum" can be fun itself:



Sigma SD1 Merrill, no filter inside or on the lens.

Just a reminder that all Sigma DSLRs come with an easily-removable and replaceable hot mirror in in the lens mount.