Focusing properly...

*DanLoc78* · 06-17-2008, 08:42 PM

I've become a little frustrated lately because I don't completely understand how to focus properly, especially when shooting people. One person...great...easy...more than one (in low light) and I'm scratching my head.

I'm assuming that the exact point of focus will be the sharpest point, and everything from that point to the edges of the focal plane will be less sharp, how much less sharp will depend on several factors including lens quality and aperture. At least that's how I understand it. Please correct me if I'm way off here.

I've been using the FA 50 f/1.4 lately, so I'd say lens quality is not my issue. To make a long post short...what is the best way to keep a group of people in focus in not so good light? Do I need to use a flash? Do I need to set a particular autofocus mode? Do I need to jack up the ISO and stop down? I'd rather not use a flash because I love the natural light look...and I don't have anything besides the pop-up flash.

Thanks for any tips.

*Canada_Rockies* · 06-17-2008, 08:45 PM

You need to stop down enough that the depth of field covers both the closest and farthest subjects you need in focus. At f/1.4 your depth of field probably won't hold the nose and the ears on one subject in focus. A good thing to use is the optical preview on your power switch. It takes a little while to get used to reading the dark image, but you can learn how. The darker the subject the harder it is.

*Mitch34* · 06-18-2008, 04:46 AM

the wider the aperture the smaller the depth of field.
see snapshot of DOF calculation.
for aperture 1.4 if your main subject is at 3 meters then in focus distance is only 33 cm (a bit more than a ft). could be even less if the resolution is better than 30 lines/mm.

if you go to Aperture 8 and the main subject is at 3 meters then the infocus distance become 210 cm (7 ft).
Try your flash in slow mode it can be a compromise to keep some natural light.
Exercise/try know your camera

screen capture from pocketpc software "DOF" ver 1.0 / Jonathan Sachs

**Peter Zack** · 06-18-2008, 04:54 AM

There's not much more to it than what CR and Mitch suggested. You should look into a flash or a reflector if the conditions don't allow enough light to stop down. An off camera flash fired wirelessly from the camera can offer a ton of possiblities and not ruin the natural light look. It's a matter of practicing with the flash and not overpowering the scene.
I'd much rather shoot with a flash than shoot high ISO's. The results will be better. What sort of scenes are you trying to capture? weddings and indoor events?

*DanLoc78* · 06-18-2008, 08:23 AM

yeah, i understand that DOF is razor thin at f/1.4 and much deeper stopped down. I just wasn't sure if there was more to keeping more subjects in sharper focus than that. I have a couple of recent pics where i was at f/2.8 and 3.5 and parts of my subjects were still somewhat out of focus. But that could also be due to the slower shutter speeds.

I wasn't sure if different autofocus modes were more beneficial in that situation or not. Thanks for the tips guys.

*Wheatfield* · 06-18-2008, 08:44 AM

Originally Posted by DanLoc78

I've become a little frustrated lately because I don't completely understand how to focus properly, especially when shooting people. One person...great...easy...more than one (in low light) and I'm scratching my head.

I'm assuming that the exact point of focus will be the sharpest point, and everything from that point to the edges of the focal plane will be less sharp, how much less sharp will depend on several factors including lens quality and aperture. At least that's how I understand it. Please correct me if I'm way off here.

I've been using the FA 50 f/1.4 lately, so I'd say lens quality is not my issue. To make a long post short...what is the best way to keep a group of people in focus in not so good light? Do I need to use a flash? Do I need to set a particular autofocus mode? Do I need to jack up the ISO and stop down? I'd rather not use a flash because I love the natural light look...and I don't have anything besides the pop-up flash.

Thanks for any tips.

Stop down the lens to get more depth of field. This will probably necessitate the use of a higher ISO, and you should be using a tripod anyway. If your people can hold still, it is possible to shoot fairly long shutter speeds and still get a sharp image of the subjects.
I presume when you are setting up a group that you are arranging them, to a certain extent in a line (perhaps tiers) across the frame. This may not matter, but it might. I'm pretty sure the FA50/1.4 is a curved field lens. What this means in practical terms is that it's point of focus follows an arc around the camera at the focussing distance.
What this means in practical terms is that you might want to consider moving the people at the sides of the frame a bit closer to the camera (the posing then follows the arc of the plane of focus).

*heliphoto* · 06-18-2008, 12:35 PM

Originally Posted by DanLoc78

yeah, i understand that DOF is razor thin at f/1.4 and much deeper stopped down. I just wasn't sure if there was more to keeping more subjects in sharper focus than that. I have a couple of recent pics where i was at f/2.8 and 3.5 and parts of my subjects were still somewhat out of focus. But that could also be due to the slower shutter speeds.

I wasn't sure if different autofocus modes were more beneficial in that situation or not. Thanks for the tips guys.

I know f/2.8 or 3.5 feels like a lot of stops when you have a 1.4, but it's still fairly wide open, and not giving you a whole lot of DOF. I think that's what you're seeing here.

travis_cooper · 06-18-2008, 01:28 PM

Is everybody forgetting that your aperture is just one part of DOF? You can still get a fairly large DOF with f/1.4 if you are standing far enough away. DOF is based on aperture, focal length, and distance to focusing point. Change any of those three things and you change your DOF. If you are standing 10 ft away from the focus point with your 50mm set to f/1.4 you have 2.75 ft to work with, that definitely isn't razor thin. It might not be enough for a large group, but for a couple of people it should be enough. One of these days I'm gonig to go find the actual formula so I understand which part plays the biggest role, but it seems to me that aperture is actually the smallest factor in the equation. From seeing people play with the kit lens for example, if you are standing 10 ft away from your subject and are wide open, the 18mm side has a max aperture of f/3.5 which gives you around 20ft in focus, now zoom into 55mm, which has a max aperture of f/5.6(this aperture, if nothing else changed, would give you a larger DOF), and now your DOF changes from 20ft to around 2ft. Anyway, you can adjust your DOF, while keeping your wide open aperture, by moving your feet. The further you scoot back the larger the DOF becomes. Here is an online DOF calculator if you want to play around with it.

Zewrak · 06-18-2008, 01:40 PM

Originally Posted by travis_cooper

Is everybody forgetting that your aperture is just one part of DOF? You can still get a fairly large DOF with f/1.4 if you are standing far enough away. DOF is based on aperture, focal length, and distance to focusing point. Change any of those three things and you change your DOF. If you are standing 10 ft away from the focus point with your 50mm set to f/1.4 you have 2.75 ft to work with, that definitely isn't razor thin. It might not be enough for a large group, but for a couple of people it should be enough. One of these days I'm gonig to go find the actual formula so I understand which part plays the biggest role, but it seems to me that aperture is actually the smallest factor in the equation. From seeing people play with the kit lens for example, if you are standing 10 ft away from your subject and are wide open, the 18mm side has a max aperture of f/3.5 which gives you around 20ft in focus, now zoom into 55mm, which has a max aperture of f/5.6(this aperture, if nothing else changed, would give you a larger DOF), and now your DOF changes from 20ft to around 2ft. Anyway, you can adjust your DOF, while keeping your wide open aperture, by moving your feet. The further you scoot back the larger the DOF becomes. Here is an online DOF calculator if you want to play around with it.

What about angle?

travis_cooper · 06-18-2008, 01:50 PM

Originally Posted by Zewrak

What about angle?

What do you mean by angle? I don't think the angle you are at has anything to do with this, it has to do with the distance from the lens. The only other factor is the circle of confusion, but the only control you have over that is by using a different sensor, which basically means using a different camera body.

*DanLoc78* · 06-19-2008, 08:49 PM

are you sure that 2.75 feet of depth is correct for using 50mm at f/1.4? Using a DOF calculator at 50mm and 75mm (not sure if it accounts for the smaller sensor) I got 0.33 and 0.3 feet of focusing distance....10 feet from subject.

Perhaps you meant 20 feet? I just typed that in and got 2.75 feet of depth.

*Wheatfield* · 06-19-2008, 09:38 PM

Originally Posted by travis_cooper

Is everybody forgetting that your aperture is just one part of DOF? You can still get a fairly large DOF with f/1.4 if you are standing far enough away. DOF is based on aperture, focal length, and distance to focusing point. Change any of those three things and you change your DOF. If you are standing 10 ft away from the focus point with your 50mm set to f/1.4 you have 2.75 ft to work with, that definitely isn't razor thin. It might not be enough for a large group, but for a couple of people it should be enough. One of these days I'm gonig to go find the actual formula so I understand which part plays the biggest role, but it seems to me that aperture is actually the smallest factor in the equation. From seeing people play with the kit lens for example, if you are standing 10 ft away from your subject and are wide open, the 18mm side has a max aperture of f/3.5 which gives you around 20ft in focus, now zoom into 55mm, which has a max aperture of f/5.6(this aperture, if nothing else changed, would give you a larger DOF), and now your DOF changes from 20ft to around 2ft. Anyway, you can adjust your DOF, while keeping your wide open aperture, by moving your feet. The further you scoot back the larger the DOF becomes. Here is an online DOF calculator if you want to play around with it.

I'm kinda presuming that the guy doesn't want to be 50 feet back from his subject matter just to secure adequate depth of field. An answer that doesn't adress the question is worse than no answer at all.

troyz · 06-19-2008, 09:44 PM

Originally Posted by DanLoc78

I've become a little frustrated lately because I don't completely understand how to focus properly, especially when shooting people. One person...great...easy...more than one (in low light) and I'm scratching my head.

I'm assuming that the exact point of focus will be the sharpest point, and everything from that point to the edges of the focal plane will be less sharp, how much less sharp will depend on several factors including lens quality and aperture. At least that's how I understand it. Please correct me if I'm way off here.

I've been using the FA 50 f/1.4 lately, so I'd say lens quality is not my issue. To make a long post short...what is the best way to keep a group of people in focus in not so good light? Do I need to use a flash? Do I need to set a particular autofocus mode? Do I need to jack up the ISO and stop down? I'd rather not use a flash because I love the natural light look...and I don't have anything besides the pop-up flash.

Thanks for any tips.

Photographing a large group of people in low light is hard!

If the subjects are moving, you'll need to increase the shutter speed (which will require you to increase ISO to use an aperture small enough to get an acceptable depth of field).

Here's a web site that generates very useful DOF tables:

Depth of Field Table

You'll need to enter your camera type (e.g. 35mm film or K20D) as well as focal length to get accurate results; the greater magnification used to print/display from a smaller sensor results in less depth of field for a given focal length.

Two rules of thumb I use when DOF matters:

-- When using a DOF scale on a lens designed for film (such as this one), use the markings one stop larger (e.g. for f/22, use the markings for f/16) to obtain near and far focus for an (APS-C-sized) cropped sensor

-- Focus (auto or manual) one third of the way between the nearest object of interest and the farthest.

emalvick · 06-20-2008, 10:33 AM

I saw someone mention the effects of different parameters on depth of field. I setup a basic excel worksheet (in the attached zip file) using the equations shown on the depth of field website so that if someone wanted to investigate things further, they could.

The sheet isn't anything special, so you could tweak it to your liking. The first sheet, "BASIC", is a basic DOF calculator that can be done in meters or feet. Yellow highlighted field are inputs. Note that if the focal distance, s, gets big enough (>H), the results for the furthest sharp distance will turn to inf (infinite).

The second sheet takes those same equations and sets them up in a table for those more comfortable with Excel. This can allow you to vary different parameters and setup charts or pivot tables to your liking... I've never gotten beyond setting it up. The table is setup exclusively in metric, but I provided a little converter at the top of the sheet in case you need the results in feet or to convert feet to metric.

The table is also setup so that instead of showing inf (as the previous sheet), the table shows 10000 when the DOF becomes infinite (Excel needs a finite number for charts and graphs).

Have fun.