Dof and focus point

ndancona · Sep 23, 2013

I recently bought a nifty fifty f1.8 and I'm enjoying playing around with shallow depth of field with the lens wide open. However, I am getting a lot of focusing issues. My line of focus tends to be extremely narrow. I've tried changing the autofocus settings and experimenting a little but can't seem to resolve this issue. Is this just a short coming of the lens or a drawback to shooting wide open? Any tips on how I can resolve this problem?

The image attached is an example of what I'm talking about. The guy is in focus, in fact his right shoulder seems to be the most sharp, but the child is out of focus. :grumpy:

jwbryson1 · Sep 23, 2013

ndancona said:
I recently bought a nifty fifty f1.8 and I'm enjoying playing around with shallow depth of field with the lens wide open. However, I am getting a lot of focusing issues. My line of focus tends to be extremely narrow. I've tried changing the autofocus settings and experimenting a little but can't seem to resolve this issue. Is this just a short coming of the lens or a drawback to shooting wide open? Any tips on how I can resolve this problem?

The image attached is an example of what I'm talking about. The guy is in focus, in fact his right shoulder seems to be the most sharp, but the child is out of focus.
View attachment 56280

Not sure I have all your EXIF data correct, but shooting with a Canon 600D at f/1.8 and about 8 feet from the subject, the near limit on the DOF is 7.75 feet and the far limit is 8.27 feet, for a total DOF of 0.52 feet, or about 6 inches. Looking at your photo, it appears more than 6 inches between the front right shoulder and the girl's eyes.

If you back up to 20 feet from the subject, the DOF increases to 3.3 feet, more than enough to get the whole image in focus. You could then crop down to this size.

From a distance of 5 feet, the DOF is less than 3 inches based on the above assumptions. That's shallow and no wonder you have focus issues. Stop the aperture down a bit and take a few steps back...

I'm sure somebody with more experience will correct me, but that is my best guess. Hope that helps...

Tinderbox (UK) · Sep 23, 2013

Unless it was dark increase the f-stop above f1.8 and get a deeper depth of field, your shutter speed was 1/400 so you must have had plenty of light just increase the f-stop or increase your distance to the subject to increase the depth of field that way and crop as suggested.

John.

ndancona · Sep 23, 2013

Thanks for the replies.

I had my aperture at 1.8 intentionally to blur the background as much as possible. This was achieved but with obvious consequences. If I were to take a few steps back then this problem will probably resolve itself but the background will come more into focus.

I 've seen work by other photographers who shoot with this lens and these settings all the time and are able to get really good results. For this reason I asked for some tips. I will continue to experiment and learn this lens.

gsgary · Sep 23, 2013

ndancona said:
Thanks for the replies.

I had my aperture at 1.8 intentionally to blur the background as much as possible. This was achieved but with obvious consequences. If I were to take a few steps back then this problem will probably resolve itself but the background will come more into focus.

I 've seen work by other photographers who shoot with this lens and these settings all the time and are able to get really good results. For this reason I asked for some tips. I will continue to experiment and learn this lens.

Probably but not with two subjects unless the are in the same focal plane which your are not, F4 would have been your friend and still have a blurred background

play18now · Sep 23, 2013

Unless the railing behind you is very close, it should still be plenty blurred at f/4. At the same distance at f/4 your DoF will still only be 18 inches or so.

SCraig · Sep 23, 2013

ndancona said:
Thanks for the replies.

I had my aperture at 1.8 intentionally to blur the background as much as possible. This was achieved but with obvious consequences. If I were to take a few steps back then this problem will probably resolve itself but the background will come more into focus.

I 've seen work by other photographers who shoot with this lens and these settings all the time and are able to get really good results. For this reason I asked for some tips. I will continue to experiment and learn this lens.

No, actually you haven't. Physical limitations such as depth of field are physical limitations regardless of who the person is. If the total depth of field is 6" for you then it is also going to be 6" for ANYONE shooting the same equipment with the same settings under the same conditions. You might have seen some good shots using f/1.8 but I guarantee that something was different; a different camera to subject distance, a different pose so that everything was on the same plane, SOMETHING was different.

table1349 · Sep 23, 2013

Just because your lens will go to f1.8 doesn't mean you should shoot at f1.8. You need to have a good working understanding of DOF. Here is a good resource. Understanding Depth of Field in Photography
You also need to understand aperture. Understanding Camera Lenses
Once you have a good working knowledge of these two you should understand how to achieve the results you want.

Gavjenks · Sep 23, 2013

increase your distance to the subject to increase the depth of field that way

This is not a very good suggestion.

Backing up would require a longer focal length lens to frame the shot the same way, which would usually end up yielding approximately the same or sometimes even a shallower DOF. For example, a 50mm lens at f/1.8 and 10 feet will have almost the same framing as a 100mm lens at f/1.8 and 20 feet. The 50mm lens on a crop frame will have a DOF of 0.81 feet, while the 100mm twice as far away will have a DOF of... drumroll... 0.81 feet. (at smaller apertures it actually becomes shallower at the longer distance). The background will LOOK much creamier with the long lens further away, but it won't help your eyes being in focus if you focused on the shoulder.

Backing up without changing lenses wouldn't help much, either. DOF is not a purely optical property. It has to do with a combination of optics + how well our eyeballs can resolve detail, which means it matters how you crop and size the image. If you step back 3x as far, and then just crop down to 1/3 the photo size later and blow it up to full, you are blowing up the slightly-out-of-focus parts and making them easier to detect, thus your DOF is going to be probably shallower again, because you just undid the benefit of walking back by blowing up the fuzz, plus you also threw away a bunch of megapixels, making your image that much blurrier overall.

The mere act of printing an image larger or cropping it more tightly (and then making it larger to be a full size image again) decreases its DOF.

If you want to maintain the same composition and increase your DOF, making your aperture smaller is the easiest and most convenient way by far.

ndancona · Sep 24, 2013

Excellent feedback. This is the main reason I joined this forum, thanks for sharing all your knowledge.

I will continue to experiment with your points in mind.
gryphonslair99 - thanks for the links, I will study the info.

Big Mike · Sep 24, 2013

Gavjenks · Sep 24, 2013

Notice, however, that the effects shown in the last two images in the diagram above tend to cancel each other out: Short distances to subject mean shallower DOF, but wider angles (which you would normally USE at short distances) mean broader DOF.

The result is usually that if you change focal lengths to give yourself the same framing as you had at a different location, it tends to be almost exactly the same DOF as before, due to these two forces competing with one another.

So in most normal situations, where you decide on your composition FIRST, and want to stick with it, aperture is going to be by far your most potent tool remaining to control DOF. Because once you've decided on composition, the last two effects on DOF are sort of locked to each other. Very rarely do we decide we want a certain DOF prior to deciding on the composition.

Designer · Sep 24, 2013

ndancona; if your camera will set the focus point, set the focus on the eyes, making sure both sets of eyes are at the same distance. This may throw the man's shoulder and the child's hands out of focus, but then you wanted a shallow DOF. Remember also that hair that is more than 6 inches behind the eyes (in this case) will also be OOF.

Gavjenks · Sep 24, 2013

Another factor to consider here is that DOF is not actually a very complete measure of how "creamy" a background is.

A 200mm lens at 20 feet away will have almost the same DOF as a 50mm lens at 5 feet away. However, despite DOF being almost identical, the 200mm shot will still look MUCH MUCH "creamier" the background.

Check out the image on this page I am linking:
Bokeh and Focal Length Test 2 12Jun2012 | Flickr - Photo Sharing!

Look down the photos on the left or right side. All of those photos on a given side have almost identical depth of field. The range of stuff in focus is about the same. That tree is pretty much equally out of focus in every one of the images (if you were to take the tree in the 300mm shot and scrunch it down in photoshop, it would be almost pixel-for-pixel identical to the image of the tree in the 24mm shot). BUT, the bokeh is completely different. In the 300mm shot, the background is so magnified that it looks much "Creamier" despite not being really any less in focus than it is in the 24mm shot.

So since the OP is interested in the portrait "creamy look," it is important to keep in mind that DOF is not the only thing that matters in achieving that look or not. DOF as determined by physics and calculators, etc. doesn't tell the whole story by any means.

So one way to achieve what he wants to do (get more of the people in focus while leaving the background as creamy as possible) would be to use a longer focal length with a slightly smaller aperture, and stepping further back. The result would be somewhat increased DOF due to the aperture (one-ish stop would be good, f/2.8 is close enough and available in many commercial longer lenses). So you could get the shoulder and eyes in focus. Then the background, even though technically more in focus than before, would still look creamy, possibly even creamier than it looks here if the change in focal length is much larger than the aperture change.

For example, 200mm at f/2.8 and stepping back 4x as far would probably do the trick perfectly. That would be one stop more DOF from aperture to help with the eyes and shoulder, but about "two stops' (to use the term very loosely)" more creaminess from background magnification, for a net increase of about "1 stop" of creaminess.

It requires having a $1000-2000 lens, though.