Shallow DOF with 70-200mm at f/4

[Cathathome]

I recently went to see my friend's son play hockey (9-10 year olds) and had fun trying to catch the action with my 70-200. I don't use this lens often enough for various reasons, but love the images it can produce when I do something right.

Because of the lighting in the hockey arenas, I found myself using iso 800-1600 on my XT with the lens as open as it goes, f4. It doesn't have IS.

One of the things that I found was that many shots came out where only one player was sharp, and the others were out of focus, even if one or more were fairly close. Now I know that f4 is going to give me a shallow depth of field, but when I look at other peoples sports pics taken at 2.8, the focus and sharpness is spread more evenly across more than one player/person in the scene.

Is it just a condition of the focal length or am I not doing something right?

Thanks,

Cath

[lidarman]

DOF also varies with focal lengths. That means it's shorter at 200 mm than 70 mm. Plus it also depends on how close you are to the subject. The closer you are, the shorter it is.

Also, it's hard to know what "other photos" you are comparing yours to without all the conditions known.

One more issue to consider is your shutter speed and panning. Could any of your photos have the problem that you panned on a moving player, getting that shot ok, but the other player was motion blurred? Remember the min shutter speed is 1/focal length for hand-held static shots. This will get even faster for dynamic shots. Low light situations can be messy.

Set up some objects in the back yard and experiment with DOF in a controlled environment by shooting at different apertures, distances from the subjects and at different focal lengths. That will give you a good feel so you know how to handle it in a dynamic situation.

Those people shooting at f/2.8 and getting good shots are not doing any magic.

[Sushigaijin]

Cath,

absolute Depth of field is controlled by magnification and sensor size. F/4 at 600mm has the same depth of field as f/4 at 200mm or f/4 at 35mm. The rate at which objects fall out of view is different, which is why long focal lengths seem to offer less depth of field but this is really a product of field of view. Your friends shooting at f/2.8 will ALWAYS have less depth of field than you shooting at f/4, assuming your sensors are the same size and the subject is at the same magnification. Apparent magnification and actual magnification gets complicated for the same reason, but focal length really doesn't have anything to do with depth of field.

If they are shooting with a compact camera, the small sensor will give them a lot more depth of field for a given F/stop, so f/2.8 is like shooting at f/4.5 or so on your xt. Shooting with a larger sensor will do the opposite: shooting with a full frame canon 5d will give you less depth of field at f/4 than your xt will. F/4 on your xt will be more like f/6.3 on a full frame camera. These are just guestimates, I'm not sure what the exact conversion is, but hopefully it will illuminate the point.

The long and short of the matter is, to get more depth of field, reduce magnification. Make the players a little smaller in the frame and then crop to get the same composition. It isn't the focal length that's killing you, it's the magnification.

[Greg McCary]

Cath the IS isn't going to correct for a moving target, just a moving camera. Also F/4 is slow for low light sports. You will never get as many keepers as one using a 2.8. You just need faster glass.

[Cathathome]

One more issue to consider is your shutter speed and panning. Could any of your photos have the problem that you panned on a moving player, getting that shot ok, but the other player was motion blurred? Remember the min shutter speed is 1/focal length for hand-held static shots. This will get even faster for dynamic shots. Low light situations can be messy.

So by this am I right to understand that I might be attributing motion blur to DOF, and that this is really a condition of a too slow shutter? I ask because I went back and looked at the exifs for some of my shots and think that even some of the out of focus shots seem to be more evenly "sharp" when my shutter speed was closer to the (ratio?) of the focal length, e.g., 1/160sec or higher at 200mm.

Therefore, should I avoid shutter speeds less than 1/200 sec when using this lens?

Thanks,

Cath

[freygr]

Cathathome,

You have two problems, Low Light and Slow shutter speeds. A F4 lens just is not fast for action sports inside as to stop movement you need 1/500 of a second speed. Now DOF is determined by the actual focus distance of the lens not the 35mm equivalent. DOF is determined by the focus length, F stop, and distance from the lens. 1) the small F stop numbers you have less DOF, 2) the shorter the lens the more DOF you will have, 3) the closer to the subject the less DOF you will have.

To get DOF and sharp photos you have to get more light on the subject or increase the ISO so you can use the fast shutter speeds needed to stop the action.

[lidarman]

So by this am I right to understand that I might be attributing motion blur to DOF, and that this is really a condition of a too slow shutter? I ask because I went back and looked at the exifs for some of my shots and think that even some of the out of focus shots seem to be more evenly "sharp" when my shutter speed was closer to the (ratio?) of the focal length, e.g., 1/160sec or higher at 200mm.

Therefore, should I avoid shutter speeds less than 1/200 sec when using this lens?

Thanks,

Cath

Correct. and that is if you are shooting a still shot of someone standing. If they are moving, you need to to have an even faster. As other posters state, less than even 1/500th...depending on how fast the person is moving. If you pan, it helps.

If you want to get more into this, post some examples with the exif data.

[Skyman]

if motion blur is the cause you should be able to tell by zooming in on the out of focus parts of the image. You can normally see the blur lines and should be able to differentiate between blur from camera shake and blur from subject movent.

Also depth of field isn't a static thing, but a plane of sharp focus (with a varying depth) extending to the left, right, top and bottom of the point of focus. If the point of focus is a person and there is a person standing either closer or further from the camera than the point of focus then they are likely to be out of focus, if however they happen to be standing in the plane of focus they will also be sharp (assuming that motion blur / camera shake isn't playing a part). If part of the image is sharp motion blur / shake is less likely to be the culprit. You also need to realise that panning with a subject that is moving along the plane of focus is ok, but you can't pan something that is moving towards or away from the camera, even if the camera's autofocus is fast enough to keep up with the subjects movement, their size in the frame will change which will lead them to being out of focus. with a motion blur type effect.

[Cathathome]

Lots of very helpful info! I've been reading all your replies and looking at my images and their exifs. I also realized that my images are probably suffering from more camera shake than I had considered. I was taking some practise shots at home with the lens and it became evident that I have a lot of trouble keeping the lens still. I was at the camera shop today to pick up a little toy for my flash and asked about this issue. The guy at the store said that it was very common, and recommended a monopod. He mounted a camera with a 200mm lens onto one and showed me how to use it. Also gave me some tips on panning.

For the record, here is an example of one of my images that just didn't have the right DOF. I did try to take a better look by zooming in with my editor, but don't know how to tell motion blur from camera shake.

Focal length: 70mm; 1/100 sec; f/4; ISO 800

[Dylan8i]

if it was camera shake everything in the image would be fuzzy.... but since the netting and boards (static objects that don't move) are fairly sharp, i would say this picture is suffering from the players moving, and not having a fast enough shutter speed to freeze them.

the focus appears to be on the goalie and is fairly sharp to....

DOF is not the problem in this image (although the closest player may not be as sharp as the goalie (where the camera focused)) but that would be ok
(normal) for this type of shot.

[freygr]

Our sample photo the AF got the goalie and background in focus. My suggestion is to use MF as you can adjust the in focus area in the view finder. Look on the internet for DOF and Hypofocus charts.