Question Re: Focusing

futureal33

futureal33

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Hiya all,

This is probably a fairly noobish question... but I genuinly dont know the answer so Im hoping someone can help me out!

I recently bought a new f2.8 lens. I am taking pictures in low light. Lets use the below photo as a typical example for the question I am about to ask.
Basically... when I am focusing on the girls below, because there is a gap between their heads, I have to focus on one of the girl's head. If I focused on the gap, it would set the AF for whatever was in the distance behind them, this is not what I want. However... this is where I begin to get confused.

If I focus on the left hand girl for arguments sake, and take the picture with 1/10 ISO 800 and f2.8 then the left hand girl is in good focus, but the girl in the middle is a little soft and the right hand girl is very soft. They are all the exact same distance away from the camera lens as the left hand girl, albeit at a slightly different angle. So from this I have 2 questions...

1) Is the reason for this that I am using too wide an apperture? Is f2.8 ok for photos with 1 subject or where there isnt a gap between multiple subjects (thus making it like one big subject) but for photos like the attached I should be using a narrower apperture?

or

2) Is there a way on a Canon 400D to select multiple focus points, so that I could make all of the girls faces in focus.

I am happy with the background blur - that is the effect I wanted to achieve by using such a low f... but im unhappy that only the girl I focused on is in focus whereas the other 2 girls are not!

Please could someone explain/help?

Thank you!

examplephoto-1.jpg
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As an example at 75mm on your lens with the girls about 8 feet away, at 2.8 You will have an area of focus about 4 inches deep. As the 2 girls on the side are standing sideways a focus point on the middle girls eyes would put the others outside the focus zone.

Unfortunately no exif in your image so i don't know exactly how you shot it.
 
The depth of focus will vary depending on the aperture you use, the focal length you use & the distance you are from the subject.

You will find an easy calculator here.......

http://www.dofmaster.com/dofjs.html

try changing your f number and you will see.
 
futureal33 said:
Is there no way to have multiple focus areas?
Click to expand...

Yes... however the lens can only be physically focused on one thing, you need to stop down the lens to around f5.6 to f7.1 to increase the depth of focus of the lens, to get all of the girls in sharp focus.

A 1/8 second shutter speed has also added the possibility of introducing camera shake into the equation, this will cause blur.

Line up a couple of cereal packets slightly staggered, focus on the closest and fire off some shots at f2.8, then f4, then f8 and digest the results.

Understanding the depth of field and it's relationship to aperture and focal length is crucial to getting the results that you expect.


.
 
1/8 of a second is way to low. you don't want to be below about 1/60. which won't be a problem with the flash or possibly higher if your camera has high speed sync as an option.

Waits for a canon owner to confirm the last.

Play around with the dof link and experiment.

Also a good read is "Understanding Exposure" by Bryan Peterson.
 
I think there is a basic misunderstanding of focussing here.

Reading the original post and subsequent posts carefully you appear to be thinking that the camera focusses on the object at the focus point, and does not focus on objects to the side of the focus point used . . . . ?

The camera focusses on a flat plane perpendicular to the camera. The camera uses the individual focus point to set the focus distance and everything else at the at the same distance from the camera will be in focus. It does not worry about left or right or up or down, the objects which will be in focus are all on a theoretical flat wall infront of the camera. If you setup the camera facing a flat brick wall, and you are careful to make sure its square to the wall vertically and horizontally then in theory the whole face of the wall will be in focus no matter which focus point you use.


Then we hit the problems of depth of field.

The focus is perfect at the focus distance, but closer to the camera and further away from that theoretical wall the focus gradually gets worse as you get further away from the wall. As we move an object away from the wall (closer or futher) at some point our eyes can say that things are no longer in focus. The furthest and closest points of this are the depth of field. Have a read here: http://en.wikipedia.org/wiki/Depth_of_field

There are a lot of factors that determine how big the depth of field is. But first lets consider the human perception factor which is based on the resolution of the photo and how big it is displayed and the viewing distance. It basically comes down to how well our eyes can see the detail, the standard usually used for judgement is something like a normal photo quality print of A4 size held around about arms length away from our eyes. A big poster is fuzzy and blobby up close, but the details are crisp and the picture in focus when viewed from the other side of the street.

Using a reasonable standard like this people have constructed depth of field calculators, because what the camera smudges out of focus and how big the Depth of Field is depends on the distance to the subject, length of the lens and the aperture used (using a camera with a different sensor or film area will also affect it). Its all down to the optics and how it bends and mangles the light to arrive as a clear image on the camera's sensor or piece of film.

The Depth of Field decreases as the distance to the subject gets closer, and/or the aperture gets bigger (smaller f number), and/or the focal length of the lens gets longer (eg. 80mm end of the zoom range rather than 20mm).

At a distance of 2 metres with a 50mm lens at f/2.8 the depth of field (on a crop sensor camera like the 400D ) can be calculated as being 17cm total depth, with objects within the distance 1.92m to 2.09m from the camera being in focus.


Back to your example shot of 3 charming young ladies. If you are using a slow shutter speed such as 1/8th second then all the great optics, focus points and depth of field in the world cannot save you, as you will suffer from motion blur.
Image stabilisation will not help you with the movinement of the subjects, it only compensates for wobble of the camera.
You need to use a faster shutter speed to freeze the motion.
Flash will help as it is very brief and so acts like a very fast shutter, but only if its the predominant light source. If you are using the camera with settings that allow in a lot of light then the briefness of the flash will be negated by the movement illuminated by the ambient light on your subjects.


am I helping here?
 
Looking at that image, that's not a focussing error - it's camera shake and subject movement, though using f/2.8 is making life difficult for yourself with multiple subjects like this.
 
futureal33 said:
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examplephoto-1.jpg
[/IMG]
Click to expand...

The problem with this pic is incorrect focus. Nothing is in focus. It's not camera shake or subject movement, the girls are predominently lit by the flash which would freeze that, but still nothing is sharp.

Then there is the depth of field issue which means it's unlikely you will get all three sharp at f/2.8 even when the focus on one of them is correct, unless they are standing exactly in line. Which they are not - going by the eyes, the girl on the right is closer than the girl on the left who is closer than the one in the middle.
 
There are a lot of porkies being said here.
1/ 35mm f2.8 6 feet the DOF is 1.64 feet to inf.
2/ With a 35mm lens those girls are never 6 feet away.
 
was flash used. I shoot in night clubs and shoot between 0.3s and 1/15 depending on what kind of shot i want to acheive.
my flash is set to rear curtain sync so any movement in the camera deliberate or from shake adds to the blur of the background. the flash will freeze the people in the shot and they come out sharp. The flash throws a focus assist grid as well to make up for the poor light.
Aperture used is between f/5-f/8 depending on group size (canon 10-22mm). I know its a wide angle but space is restrictive in a packed club.
 
classcams said:
There are a lot of porkies being said here.
1/ 35mm f2.8 6 feet the DOF is 1.64 feet to inf.
2/ With a 35mm lens those girls are never 6 feet away.
Click to expand...

What DoF tables are you using Andrew? With a 400D and 35mm lens at f/2.8 and 6 ft distance, which looks about right to me, DoF is 0.95ft http://www.dofmaster.com/dofjs.html

TBH that pic just looks misfocused for some reason - it's quite a way out. Just guessing, but it might be caused by the focus-recompose technique the OP adopted. That is the method I use all the time but it is dangerous when working close at low f/numbers, and DoF is very shallow.

1) When you focus-recompose, you have to pivot the camera, moving it away from the actual shooting position. This changes the shooting distance, only by maybe half and inch or so but when DoF is measured in mms, maybe a close up head and shoulders at f/1.4, that can make all the difference.

2) What I think is more likely here is that the OP has locked the focus and held it there for a second or two, during which time either he or the girls have moved. If both he and the girls have moved back simultaneously while the group was positioned, that would easily explain it.
 
futureal33 said:
Flash was used.. but it is the standard 400D pop up flash unit.
I am not sure you can alter the flash settings when using a 400D - I have never found an option to select rear curtain sync?
Click to expand...

Flash exposure compensation, and rear/second curtain sync is in the menus and custom functions. You'll find it in the manual index.

TBH second curtain sync usually makes no difference, and can even be disadvantageous. A lot of club photographers use it as if it's some kind of magic bullet but all it does is dictate whether the ambient light blur occurs before the flash (second curtain) or after it (first curtain). In other words, it dictates the direction of the blur, which can be important with subjects that move in a predictable direction (eg car headlights with flash) but with random movement such as you get in a club situation, it makes no difference.

The drawback with second curtain sync is that if the shutter speed is quite long, maybe a quarter or half a second or more, then it becomes very difficult to predict and capture fleeting expressions and movements as by the time the flash goes off, the situation has changed.
 
HoppyUK said:
Flash exposure compensation, and rear/second curtain sync is in the menus and custom functions. You'll find it in the manual index.

TBH second curtain sync usually makes no difference, and can even be disadvantageous. A lot of club photographers use it as if it's some kind of magic bullet but all it does is dictate whether the ambient light blur occurs before the flash (second curtain) or after it (first curtain). In other words, it dictates the direction of the blur, which can be important with subjects that move in a predictable direction (eg car headlights with flash) but with random movement such as you get in a club situation, it makes no difference.

The drawback with second curtain sync is that if the shutter speed is quite long, maybe a quarter or half a second or more, then it becomes very difficult to predict and capture fleeting expressions and movements as by the time the flash goes off, the situation has changed.
Click to expand...

So does second curtain sync simply mean that the shutter opens for X amount of time before the flash fires? Im a little confused as I have only ever used the normal flash mode of press shutter half down to focus, press shutter fully down, flash fires, then shutter remains open for X and then closes. That is how all of my photos have been taken.

Must admit I quite like the sound of it working in reverse, pull the background lighting through before the flash fires, then allow the flash to illuminate the subjects... although couldnt that run the risk of making the subjects overly white as they wont have any background light on them (which in a club there would normally be some coloured light on them)?

Also, HoppyUk - there is a chance that could be the case yes. As I was using standard pop up flash I was struggling to auto focus and each time it did it the flash flickers like crazy - so once I had found focus I tended to hold it half way down so the shot was ready.... in the time between AF and the photo being taken I guess either I or the girls (or both, as you said) could have moved slightly - giving the out of focus characteristics of the photo above.
I took about 60 photos that night and that was the worst of all of them for focus so hopefully it was a one-off bad shot rather than an issue with my lens/camera.

I may try raising an F stop this weekend, to f3.2 - do you think this would help?

Thanks for the help so far, it has been really helpful!!
 
Shutter's too slow. If one of the three girls (ie the one most out of focus) moves during the exposure you've had it. DOF or not I would have said you had no chance here with that shutter speed.
 
The f stop you need depends on how the people are posed , as previously linked use the dof calculator and enter in some parameters to give you an idea of what you might need.
 
the shutter is actually in 2 parts or "curtains", one opens, the other closes. This way the area opened across the film /sensor is constant as its effectively a rectangular hole that moves across. If it opened like a door or a single shutter which opened and then closed then one side would receive more light than the other during the opening & closing.
Hence flash can use 1st curtain (the one opening) or second curtain (the one closing).

The advantage of first curtain is that there should be plenty of time to fire the flash before the 2nd curtain shuts. The disadvantage of first curtain is that the flash freezes the motion of the subject action at the start of the shutter being open, then the trails created from any ambient light move away from the frozen subject, which is the opposite of logical to our eyes when we look at a photo as we expect to see a frozen subject with the trail behind them, not leading away from them.

2nd curtain flash fixes this by using the ambient light first, then fires the flash. But this means there is very little time for the flash to operate as it is triggered as the shutter begins to close. This can be a problem for the studio and strobist guys using off camera flash fired by radio triggers that add a delay to when the lights fire, their shutter can be closed by the time the flash operates, or the shutter is half closed leaving half the photo blacked out.
 
futureal33 said:
So does second curtain sync simply mean that the shutter opens for X amount of time before the flash fires? Im a little confused as I have only ever used the normal flash mode of press shutter half down to focus, press shutter fully down, flash fires, then shutter remains open for X and then closes. That is how all of my photos have been taken.

Must admit I quite like the sound of it working in reverse, pull the background lighting through before the flash fires, then allow the flash to illuminate the subjects... although couldnt that run the risk of making the subjects overly white as they wont have any background light on them (which in a club there would normally be some coloured light on them)?

Also, HoppyUk - there is a chance that could be the case yes. As I was using standard pop up flash I was struggling to auto focus and each time it did it the flash flickers like crazy - so once I had found focus I tended to hold it half way down so the shot was ready.... in the time between AF and the photo being taken I guess either I or the girls (or both, as you said) could have moved slightly - giving the out of focus characteristics of the photo above.
I took about 60 photos that night and that was the worst of all of them for focus so hopefully it was a one-off bad shot rather than an issue with my lens/camera.

I may try raising an F stop this weekend, to f3.2 - do you think this would help?

Thanks for the help so far, it has been really helpful!!
Click to expand...

You're welcome bud :)

In case you were thinking that second curtain sync somehow makes the flash image more prominent, because it goes last and therefore kind of overlays the preceding ambient exposure, it doesn't. Light doesn't work like that and it makes no difference, despite what some people might think.

If you want the flash image to be clean and clear, then make sure the subject is against an area of black (unexposed) background. If, because there is movement during the exposure and part of the frame has ambient light blurs going on, they will be just as visible and overlay the flash exposure regardless of whether it is fired first or last.

The difference between first and second curtain sync is only in the direction of the ambient blur relative to the subject. For example, if someone in a club is holding coloured glow stick and moves it from high to low, with first curtain sync the coloured blur will appear below the flash image as they move their arm downwards during the exposure. If you then ask them to hold the glow stick down and move it upwards, and use second curtain sync, the two images will be the same.

On the depth of field thing, check the DofMaster link posted earlier to see exactly how much DoF you get at different f/numbers. The difference between f/2.8 and f/3.2 is very small. To change things significantly you could try f/5.6. Using a 35mm lens at 6ft as posted above, at f/5.6 DoF would be 1.94ft.
 
I think there is no ONE definite answer here, as none of us were there to watch the shoot we cant nail the exact reason for the softness..

I do think the answer could be a mixture of things..

1St, we have a very slow shutter speed.. slow enough to register movement of the girls.. movement of the shooter, and camera shake!!

2nd, theres technique, half pressing, then taking the picture with all the possibilities of movement.. (I use the AF ON button so i continually focus).

3Rd, there is the DOF problem.. To me, the girl on the left looks sharpish.. the girl in the middle looks slightly further away from the girl on the left and the girl on the right looks closest.. of course on a 2D image its hard to tell..

So.. to me at least, i think theres not just one reason, or at least not one that can be pinned down..

The other thing, about flash freezing the image.. if the ambient light at the shutter speed he used was just fast enough for the camera to choose a very small amount of fill.. then it may NOT have the ability to freeze the action perfectly...

This is all just my opinion of course.. :thumbs:
 
I'll try again tomorrow night - got a few ideas to go off though!
Unfortunately, im sort of stuck (sort of) with the f 2.8 as the club owner told me he loves my last photos espeically the background blur, and other than spend X hours in photoshop manually adding the blur, which I am not willing to do!, I am stuck with having a low(ish) f stop.

Just as an immediate cure, if the same sort of photo opportunity arises tomorrow night, which I know it will, ie 3 girls stood a little distance apart and at slightly different distances from the lens, what should I do? Raise the F and focus on the middle girl?
 
Using the dof link again you will see that as long as you have separation from the girls to the background you will still get the out of focus effect required anything further that 7 feet will not be in focus and the farther away it is the more prounounced the effect this using F5.6
 
futureal33 said:
I'll try again tomorrow night - got a few ideas to go off though!
Unfortunately, im sort of stuck (sort of) with the f 2.8 as the club owner told me he loves my last photos espeically the background blur, and other than spend X hours in photoshop manually adding the blur, which I am not willing to do!, I am stuck with having a low(ish) f stop.

Just as an immediate cure, if the same sort of photo opportunity arises tomorrow night, which I know it will, ie 3 girls stood a little distance apart and at slightly different distances from the lens, what should I do? Raise the F and focus on the middle girl?
Click to expand...

I'm not sure you are fully understanding depth of field. It will come with experience if you check your results against a depth of field calculator like the link posted earlier.

Meanwhile, don't worry about it. As posted previously, with that lens, on that camera, at that distance and at f/2.8, you have 0.95ft of depth of field to play with. Basically, if the girl's eyes are all within that zone they will be sharp and it will look great.

That's easy workable. In fact, in your sample pic if you/they hadn't moved while you did the focus-recompose thing and thrown the focus out, the shot would be good to go anyway. (And that is what the problem is there - it's simply out of focus, not camera shake or anything else.)
 
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