infinity focus

[GhostIV]

Hi All,

Ive had my Nikon D3100 fora bit now and have 2 lenses, the stock 18-55mm VR and the 55-200mm VR. However given that these are relativly basic lenses there is no focul lenght symbol on the camera, is there a trick to setting it to infinity focus??


Thanks

[ik631]

I had the same problem with my Canon kit lenses. Don't know it will work with yours but I aimed at the sky, or distant object pressed the shutter halfway and it focused to infinity then turned off AF and moved the focus ring to determine which end of the focus was infinity (moving all the way clockwise or counter clockwise). Hope that helps.

[Doug Sundseth]

ik631's suggestion will work just fine. I'd recommend setting to hyperfocal distance for the focal length and aperture, though, especially for short focal lengths.

For example, if you're set to f/8 at 18mm, you can focus at the hyperfocal distance of 6.7 feet (about 2m for the metrically inclined) and get everything from 3.35' (about 1m) to infinity in focus. For more information about hyperfocal distances, there are apps for smartphones or you can go to DOFMaster.

[ik631]

I understand how to do that on lenses with a distance scale but how would you do that with the kit lenses where there is no distance scale? This is one reason I want to upgrade my kit lens.

[ravncat]

It's quite difficult without a distance scale.

Most modern lenses without distance scales focus beyond infinity. (this helps with zoom lenses that have different "infinity focus" at different levels of zoom, it helps with lenses that tilt and shift because it makes more sense when the focal plane is tilted, it helps with build tolerances because the phase detect system will autofocus at infinity...)

This means manual focus at night on infinity can be pretty tough. you've a few options, you can guess. Depth of field at infinity is usually pretty large - even with a wide open lens. Chances are if you've a zoom lens without a distance scale it's like to have a fairly slow wide open aperture - 3.5 or 5.6 or so. So - what you would do, is manually focus all the way to the right or left. - then back off a little bit. You can learn it by feel. Practice in the daylight - it's always the same amount.

This depends on your lens (and usually on lens brand, Nikon is usually to the right (Ie clockwise as you look from the back of the lens to the front)

- There are a few other tricks - some lenses have an automatic infinity setting - with the camera. For example the olympus ep-1 micro four thirds, when using a lens has a setting in the menu to either forget - or remember the last focus position. So you can tel it to forget - and then turn the camera off and on - and when you do the lens will be focused at infinity - so if you switch to manual mode, you'll have your lens at infinity.

I think there was a trick like this with canon, but I don't recall. (it involved using the AF - if I recall correctly, I think I remember reading it but I could be mistaken, so a google search may help)

- you can use a high powered laser. Point it at a distant object and it can sometimes be bright enough for the AF system to catch depending if you can find something far away and reflective. (or find some other bright far away object - the moon works for this alot) and then switch your camera to manual.

- You can use live view if you have it at full ISO and try to focus manually using the screen as a guide - this works best if you can zoom in on the screen.

[Doug Sundseth]

Originally Posted by ik631

I understand how to do that on lenses with a distance scale but how would you do that with the kit lenses where there is no distance scale? This is one reason I want to upgrade my kit lens.

It requires knowing the hyperfocal distance and being able to guess distances moderately well. For a short lens, it's pretty easy to find something to focus on that's about the right distance, since the distance is usually close enough that you're not likely to be badly off. For longer lenses, it's trickier.

Another option on many cameras is to press the depth of field preview button and look for yourself what's in focus. This doesn't work well in low light, since this is done by stopping the lens down to the desired aperture and that can be pretty dark.

Or you could go old school and focus about 1/3 of the way into the scene you want to capture while choosing an aperture that will give you sufficient DoF (determined by experience or by using a DoF calculator app).

It might be easier to help if you explain why you need to set focus to infinity rather than using some other focusing method. I'm normally shooting landscapes when looking to have things focused at infinity, so my goal is usually to get deep focus and I can pace off a distance if I need to. For astrophotography, keeping near objects in focus isn't usually an issue, it's hard to tell what is in focus and what isn't, and the hyperfocal distance can be very, very long, so advice could change.

[ik631]

[QUOTE=Doug Sundseth;1311279]Or you could go old school and focus about 1/3 of the way into the scene you want to capture while choosing an aperture that will give you sufficient DoF (determined by experience or by using a DoF calculator app).[QUOTE]

Thanks I just downloaded a DoF calculator app that also calculates hyperfocal distance. Playing with for a little while and going over some photos explains why some are off. This is going to be really useful.

[ik631]

[QUOTE=ik631;1312177]

Originally Posted by Doug Sundseth

Or you could go old school and focus about 1/3 of the way into the scene you want to capture while choosing an aperture that will give you sufficient DoF (determined by experience or by using a DoF calculator app).

Thanks I just downloaded a DoF calculator app that also calculates hyperfocal distance. Playing with for a little while and going over some photos explains why some are off. This is going to be really useful.

[OsmosisStudios]

Besides using hyperfocal distance, it's actually pretty easy to set infinity focus on those lenses as they use a simpler version of the AF-S motor. Just set the lens to manual and turn the front ring (the focus ring) until it stops. One way will get you nearest-most focus, the other will get you infinity.

[ravncat]

Originally Posted by OsmosisStudios

Besides using hyperfocal distance, it's actually pretty easy to set infinity focus on those lenses as they use a simpler version of the AF-S motor. Just set the lens to manual and turn the front ring (the focus ring) until it stops. One way will get you nearest-most focus, the other will get you infinity.

Unfortunately this is not likely so for the reasons I mentioned above. most modern lenses focus "beyond" infinity. The older lenses often had hard infinity stops, modern lenses tend not to. While you will get nearest focus in one direction by turning as far as you can - you won't get infinity focus turning the opposite direction as far as you can. The effect is quite noticable on stars, where turning beyond infinity will defocus them. (I'm only talking about overshooting infinity usually by a couple milimeters, but it is enough.) Each and every one of my lenses does this.