Out of focus
October 26, 2014 9:59 AM   Subscribe

I have been shooting with a DSLR for five years and apparently I still don't get focus, nor can I find anyone IRL to explain it to me. Halp?

So last night I went out with a group to try to shoot star trails against a lit bridge. Poorly focused photos ensued despite my shooting on a tripod, often with autofocus on, with a remote trigger.

I was given two pieces of advice:

* Focus on infinity (whut)
* Autofocus on a light 100 feet away, then put the camera on manual focus to take shots of the bridge & sky (double whut - doesn't putting the camera on manual focus get rid of the autofocus settings?)

Before the shoot we were given a link to this depth of field calculator. I don't even know how to interpret the results. Hyperfocus? Circle of confusion? How does this information help me in the field?

I get shutter priority, aperture priority, ISO, and the concept of depth of field in general (thanks to reading a bunch of classic film blogs over the years). And believe it or not I've actually been able to take some memorable shots.

I have an entry level DSLR (Nikon D40) and a 18-55 mm kit lens. There is no "infinity" indicator anywhere on the camera or the lens. I also have a Tamron 70-300 telephoto that has an infinity symbol. I tried setting the lens to the infinity setting this morning with laughable results.

Obviously a mentor working with me in the field would be the best bet but there is no one who will reliably fill that bill right now. I might eventually spring for a weekend away with some professional photographers - lots of weekend "safaris" around. Till then I am looking for a good book or online explanation.
posted by Sheydem-tants to Sports, Hobbies, & Recreation (15 answers total) 15 users marked this as a favorite
 
Autofocus on a light 100 feet away, then put the camera on manual focus to take shots of the bridge & sky (double whut - doesn't putting the camera on manual focus get rid of the autofocus settings?)

No. If dial in your focus while in AF, and then switch over to manual focus, the lens will stay focused on that same point in the distance.

Even though that light 100 feet away wasn't going to be the ultimate subject of your shot, the ultimate subject of your shot had the same focus requirements as the light 100 feet away. You would have just been using the light as an actual thing in the world that you could make the camera autofocus on, before you switched over to manual to take a picture of not-an-actual-thing-in-the-world.
posted by mudpuppie at 10:09 AM on October 26, 2014 [2 favorites]


* Autofocus on a light 100 feet away, then put the camera on manual focus to take shots of the bridge & sky (double whut - doesn't putting the camera on manual focus get rid of the autofocus settings?)

The focus setting is mechanical and does not immediately get wiped when you go to manual, in most DSLRs. In (a lot of?) Nikons, half-pressing the shutter to autofocus also allows for manual focus override: continue holding the shutter half-way and manually turn the focus ring.

re: focus on infinity, this is a good intro to the whys and wherefores. Unfortunately, I am unfamiliar with the D40 and your lens and cannot help you with how to focus on infinity.

The hyperfocal distance is the distance at which, for your lens/camera/aperture combo, anything from X/2 to infinity is in focus.
posted by flibbertigibbet at 10:11 AM on October 26, 2014 [2 favorites]


As you yourself point out, you really need to go to a library and check out some books on photography, or buy some, or start reading some articles online.

Cambridge in Color is a great FREE resource for new people; there are other great resources for more advanced students, but you should start there. Other major photography websites have great tutorials.

First of all, for night shooting, do not use autofocus. Some high end cameras have decent autofocus systems that handle it well, but a D40 is clearly not one of them. And autofocus is the wrong tool for this job anyway. If you can handle manual focus at all, autofocus is not the best tool for static subjects. Most autofocus systems struggle in low light.

Focus on infinity means to have the lens focused on the "infinity" distance, that is, as far at the end of its focus ability. At that point, everything beyond the normal focus range of the lens will appear to be in focus. Infinity is actually marked on the focus ring/display of your lens as the infinity symbol. The autofocus on a light 100 feet away is to get your lens focused on an infinity distance, then switching off the autofocus so the lens doesn't go off infinity focus. It's basically a way to get to infinity while you're still on auto. A way to do it, not necessarily the best way.

On preview, some people are covering the details on hyperfocus, etc. Don't worry about hyperfocus that much. It's not as useful a technique as people talk it up to be. Circle of confusion, too, is something to worry about when you're operating at the technical limits of what you're trying to do. Star trails and the lit bridge are likely to be so distant that they will both appear to be on the same flat plane as far as your lens/camera are concerned - if you're focusing at infinity because they're that distant, depth of field is not terribly relevant. Many people shoot star trails at f/2.8.

My advice? Learn how to go full manual on everything. Learn about how focus confirm works - in Nikon viewfinders, there will be a little green indicator (on my camera, it's a circle, or arrows indicating that I'm on focus, or not quite there). If you're capable of full manual focus, do that here. It may be tough to see at night, though. Newer cameras with live view can help here, or alternatively, a magnifier on your viewfinder.

Hope that helps.
posted by Strudel at 10:16 AM on October 26, 2014 [7 favorites]


Some good advice above.

Another comment though - you don't need an infinity mark to focus on infinity. Infinity is going to be in focus at the place where your lens can't focus any further out - there's not something to focus on that is further away than "infinity". So if you have a focus ring on your camera, one direction is going to focus close, the other direction focuses far. Turn the ring in the "far" direction as far as it goes and it's focussed on infinity.

The hyperfocal distance thing is for when you are doing landscapes and want to have some stuff in the "foreground" in focus also. It'll help you find an f-stop and focal distance to get everything from distance X to infinity in focus.
posted by RustyBrooks at 12:29 PM on October 26, 2014


(maybe I'm wrong about lenses with no infinity mark - but are there actually any lenses with that mark, where you can focus "past" it?)
posted by RustyBrooks at 12:51 PM on October 26, 2014


I like to keep it simple... if I was shooting stars, I'd put my camera on manual focus and turn the focus ring to focus to the farthest distance possible. And leave it there.
posted by kdern at 2:43 PM on October 26, 2014


Rusty and kdern, I take it you don't shoot Nikon? Many of the newer "G" type lenses - even professional grade ones - are not absolutely mechanically linked to the focusing mechanism. It is entirely possible to keep moving the focus ring past where infinity would be - though it is still marked. Older lenses, for example, my AF-D 50mm f/1.4, are mechanically linked and won't go further. But my AF-S 24-70mm f/2.8G can. The newer lenses, of which the OP's almost certainly is (because I think the D40 didn't have the in-body AF screw drive to autofocus older lenses) have this issue. Also, for some lenses, moving the ring past the infinity mark won't necessarily leave you at infinity. I think this varies by lens.

I don't know how widely this applies to other lens systems - I suspect that Pentax, for example, would not have this issue. But it may apply to Canon as well.
posted by Strudel at 2:50 PM on October 26, 2014 [2 favorites]


On non-mechanically linked lenses there is STILL going to be a spot where the lens stops focusing further.

If you focus past infinity, what, exactly, are you focused on?
posted by RustyBrooks at 3:07 PM on October 26, 2014


Just to make you feel better, getting sharp star photos is remarkably difficult with a DSLR. You can't really see well enough through the viewfinder to manually focus. Autofocus doesn't work well on stars. And since you're taking images of incredibly high-contrast objects any blurriness is immediately apparent. And you absolutely can focus past infinity on my DSLR lenses; both my Tamron 24-75mm and my Canon 28mm (EF-S mounts), to start. You can't just rack the focus all the way out.

Serious astrophotographers go through a lot of trouble to dial in good focus. The cheat I use is to set my camera up on the tripod and turn on autofocus. Then find some very far away object to focus on. The edge of the moon is perfect if it's visible. Far away buildings or trees will work, or if you get lucky an actual star. Autofocus, look at a couple of samples to be sure it's right. Then turn the lens to manual focus and don't touch the focus control. It will probably stay in focus for star photography as long as you don't jolt the lens and as long as the temperature doesn't change too much.
posted by Nelson at 3:23 PM on October 26, 2014 [4 favorites]


Turn the ring in the "far" direction as far as it goes and it's focussed on infinity.

Watch out OP, many if not most (certainly the vast majority of lenses I've used on many different systems) actually focus past infinity, so it's often the case you turn the focus add far as you can go, and then turn it back a little ways.

When I was experimenting with this type of photo, I would be using lenses I knew very well, so I could manually focus to infinity. You could try experimenting in the afternoon, where good light and visibility makes it easier to find infinity focus, then, keeping to focus manual, turn the camera off until that night. Alternatively, focus on the moon then switch to manual is the easiest way; it's bright enough that virtually every camera can lock on to it.

Astrophotography is very challenging, op, star trails even more so. Expect a high failure rate.
posted by smoke at 3:31 PM on October 26, 2014 [1 favorite]


Assuming you have this particular 18-55mm lens (since it's the one that came with the D40) let's plug some numbers into that DoF calculator. If you zoom it all the way out to 18mm for maximum field of view (to get the bridge AND the sky), and select an aperture of, say, f/8 (I don't have any personal familiarity with that lens so I'm just picking something standard) that calculator says that the hyperfocal distance is 6.7 feet. This means at f/8 (important) if you focus the lens at exactly 6.7 feet that everything from 6.31 feet away to infinity will be sharp. You'd then set your ISO and shutter speed to get the exposure you want. If we go from f/8 to f/22 (a much smaller aperture) the hyperfocal distance goes to 2.41 feet (focus on something 2.41 feet away and the total range of things-in-focus is 2.31 feet to infinity). On the other end of the spectrum, at f/3.6 (about as wide open as that lens will get at that focal length) the hyperfocal distance is 15 feet (range: 13.3-∞).

On the other end of the scale, if you zoom in as much as that lens will go (55mm), at f/5.6 (as open as it will get) the hyperfocal distance is 87.9 feet (range: 51.5-∞); at f/8 it's 62.2 feet (range: 41.4-∞); at f/22 it's 22.1 feet (range: 18.7-∞).

In short: the wider the aperture, the tighter the depth of field; the smaller the aperture, the deeper the depth of field. And since aperture and exposure are related, if the ISO is held constant, the smaller the aperture is, the longer the exposure must be (and vice versa). If you hold both the aperture and shutter speed constant, you'll have to adjust the ISO to get your exposure dialed in. This Petapixel guide suggests a 30 second exposure and has a handy chart of aperture and ISO (e.g. for 30 seconds at f/4, you'd need ISO3200).

Since that lens isn't really one you're going to want to focus manually, your best bet is to make notes of hyperfocal distances at various combinations of focal length and aperture (perhaps print out a spreadsheet). Then, carry a tape measure and have somebody stand at the hyperfocal distance, then turn on AF, focus on them (or on a target they're holding for you), then turn the AF off. As long as the camera is stable on a tripod and you're not touching the lens for any reason it should continue to hold the focus that was set before you switched AF off. Note that any time you change the zoom you should redo your focus (zoom tends to cause focus to drift).

For this purpose you might be very well served by buying an old manual focus prime lens. It will have focus and aperture guide marks on it, and would be easier to deal with at night (and you wouldn't have to worry about zoom drift). Set everything the old fashioned way and then just worry about exposure times. Plus with an old prime lens you can get higher apertures (up to f/1.4 or f/1.8 depending on the glass) and lower the ISO for finer grain in the finished product.

Also I wasn't there and I don't know how brightly lit the bridge was, but properly exposing the stars in the sky would probably result in overexposed foreground objects, if those objects are well lit. Presumably in that case you'd also need to use some HDR techniques to composite in a properly exposed frame of the bridge that you remembered to take before you dialed in your exposure for the stars.
posted by fedward at 6:22 PM on October 26, 2014 [2 favorites]


I'd really like someone to tell me: if when you focus at the "infinity" mark on your camera, it's basically focused on anything from about 100 feet to... infinity feet. If you focus "beyond" that, what distance is in focus? I can see 3 scenarios:
1. there is no plane in focus
2. "infinity" is still in focus
3. something closer than infinity is in focus

I forgot to mention, this is a spot where live view focusing probably works pretty good, if it's available on your nikon (I'm guessing it probably is, every canon DSLR has it). You get into live view mode, make sure you don't have exposure simulation on. With exposure sim off, it will boost the live view image to the limits of it's abilities. On canons you can zoom in up to 10x or so and do fine (manual) focus. I've never tried it on stars but I've used it in very very dim lighting many times. The mirror is also locked up in this mode so you can take pictures without having to enable mirror lockup.
posted by RustyBrooks at 7:13 PM on October 26, 2014


To answer your question, it's the first scenario: nothing is in focus.
posted by smoke at 8:11 PM on October 26, 2014 [1 favorite]


I've also been trying to figure out taking pictures of stars as well on my own using some really pedestrian cameras - a $299 compact and a $500 entry level DSLR with a kit lens.

Not sure what your question is: you've obviously been doing fine all these years, and I suspect will do fine without knowing the mathematics behind focus. If you want to know about star photos specifically...

Most newer lenses can focus "past" infinity so this is hard. What I do is use the 10x live view feature (all DSLRs should have it) and find a good star - Antares / Alpha Centauri / Southern Cross in the Southern Hemisphere - not sure what you guys see up north - and adjust your focus based on that. If you're not in a dark sky location you probably have something on ground level you can focus on that's far away enough. You shouldn't have the moon up if you're shooting stars anyway...

Ironically it was much simpler taking star pictures using my compact camera as you can specifically set "infinity" as a focus point in the camera, and the results are still good if a bit noisy. Like so, on my LX7. Sometimes the simple solution really is better... sigh at my DSLR. You're certainly not the first person to complain...

Rustybrooks - the newest lenses from Canon, the STM ones, have a completely free-wheeling focus wheel that doesn't stop anywhere and just spins infinitely, and consequently there's zero markings on the lens.
posted by xdvesper at 10:05 PM on October 26, 2014


The D40 most certainly does not have live view. It's much older now, it's a little younger than the D80 I bought in 2006. I don't know if Canon had Live View back then, but Nikon definitely did not. Rusty, not sure how old your DSLR(s) are, but there are actually a lot of features we take for granted now that just did not exist on DSLRs back then. Like video. Keeping in mind that this is back when 10 megapixels was as much as you could get on a DSLR that wasn't the flagship model. Compact cameras did less.

And smoke is correct. When you get beyond infinity focus, nothing is in focus. Xdvesper confirms my suspicion that newer Canon lenses are like the Nikon G-type lenses: as they're not mechanically linked, you can keep going.

I'm not terribly convinced hyperfocal techniques will help the OP here. Just worry about getting the important plane in focus. Once you've really nailed down that basic part of the night shots, then you can start fiddling with hyperfocal techniques.

For what it's worth, I don't typically find them to be more useful than other options a landscape photographer has.
posted by Strudel at 7:59 PM on October 27, 2014


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