How to Use the DOF Calculator

Today I'm going to answer another question I've received in a comment from a reader. I try to answer all comments and emails either individually, or in cases like this one - where I think others might find the  question and answer helpful - in a blog post. It's easier to explain this way than trying to keep it to a few sentences!   

So, here's the reader's question......

"Im loving my 50mm 1.8 lens but I find that when I try taking pics of both of my kids one of them is out of focus, what aperture would you use to get them both in focus? Also often only one eye (the eye that I focus on) is super sharp while the other is a bit softer how do you get them both sharp? It's not always possible to have both eyes on the same focal plane.. is there anyway around this?"

This is all down to the aperture you are using.  Aperture controls the depth of field - in other words, how much of your scene is going to be in focus. A smaller F number means you will have a smaller depth of field (less in focus) and a bigger F number means more will be in focus.  (I always used to remember it like this:  "Small F = small area in focus, Large F = large area in focus)

You have asked the question about what aperture you would use to get both eyes in focus, or both kids in focus, and the answer is.....it depends on the camera model you have, the lens you are using, how far away you are from the subject and the distance between the two things you are trying to get in focus! Eventually, you will start to instinctively know your camera and what aperture you need to use in different situations, but for now, you can actually use a calculator to find out exactly what aperture you need. 

Here's how you do it. 

1) Open the DOF Calcuator and choose your camera model from the drop down list, add in the focal length of the lens you are using, an example F stop, how far you are away from your subject. In my example here, I'm saying I have a Canon T3I, and I'm using a 50mm lens, with an F stop of F2.8 and I'm about 100cm (1 Metre) away from my subject - so pretty close. 

2) The calculator tells me exactly how much of my scene is going to be in focus. You are looking at three things - the "Total", "In front of Subject" and "Behind Subject" numbers.  You can ignore the rest for what we are using the calculator for today.

"Total" shows me just how much of the scene will be in focus - with an F Stop of F2.8 only a total of around 4cm will be in focus.   That's a very, very tiny area! 

Of that 4 cm, 2cm in front of where I put my focal point, and 2cm behind my focal point will be in focus.  Let's look at this with your example of getting both eyes in focus. If I focused on my subjects eye nearest me (as you should) then that eye will be in focus, but everything just two centimetres behind that will be soft or out of focus.  Therefore, if your subjects eyes aren't on the same focal plane, the second eye will be soft. 

3) You can increase your F number until you get to a point where both eyes will be in focus.  If I increase my F number to F4.5, you can see that the area that will be in focus increases but not by that much.  It's now saying over 6cm will in focus, with over 3cm of that behind. If that's still not enough (only you know the distance between the two eyes from where you are standing) you would keep going for a higher F number. 

4) The other option is to stand back from your subject - moving back by half a metre gives me a larger area in focus. The total is now around 9cm, with nearly 5cm behind.   You can see that if you were trying to get both eyes in focus, that would probably do it.  If you are trying to photograph two things that have a greater distance between them, say another child behind the one you are focusing on, you are going to have to increase your aperture so it is much higher, or stand further back (or a combination of both) 

Now, I know that you are not going to whip out a DOF calculator every time you want to take a photo. But what you can do is use it to get some idea of what aperture you need to be using in different situations. Eventually, experience will tell you what aperture you need to use, but for now, have a play with the calculator and see how the various elements change your depth of field. Your lens focal length, aperture and distance from subject all work together to determine the depth of field, so change them on the calculator and watch the difference that it makes.  Understanding this is the key to getting everything you want in focus. 

When you are next taking a photograph hopefully you will be able to choose an aperture that is somewhere in the region it needs to be because you'll have a much better idea of what's needed.  (If in doubt, I tend to use a higher aperture number so I can be sure everything I want to be in focus will be) If you have the time, you can also take a picture and review it on playback. Zoom in and check your focus, if the back eye is soft, increase your aperture or stand further back and take another. 

I know you were probably hoping for a "use F3.5" type of answer, so sorry for this long-winded version instead - I hope it helps though!

Other Posts You Might Like

Getting Creative with Aperture 
Tips for Using the Canon 50mm F1.8 Lens 
How To Get Sharp Eyes in Photos


Jan said...

i love the way you explain things so perfectly. thank you!

here's another scenario...let's say you're taking a picture of your son on the beach and you want your son to take up a smaller portion of the picture (you have a lot of scenery showing). do you need to close down your aperture because he's further away from you to retain detail? or does focusing on your son do the trick and your aperture setting doesn't matter as much because your son isn't close in the picture. does that make sense? he's not close to you...a small part of the picture...so does aperture matter in that case?

Audrey said...

Hi Jan, you've got me scratching my head here! If I am understanding you right - you are fr away from your subject and it is making up just a small portion of the entire picture. If that's the case, your aperture is still important but it's less critical as the depth of field is wider the further away you are. However to get everything sharp you would be looking at a higher F number - depends really on the background.. It's probably best to show with examples so I'll see if I can dig some out!

menucha said...

Hi Audrey! I started reading your post and I was like hey that sounds just like my comment! Oh wait it is my comment! feel honored ;) Thank you for answering my question! I found all of this information very helpful and I will definitely try out the calculator. I got some really nice pics of my 2 kids together, (first time I took pics of them together) and when I looked at them on the computer I realized that one of them was slightly out of focus in almost all of the photos ;( My aperture was way too low. Oh well I"ll know for next time!

PS I absolutely love your blog! It is so nice to look at, it's bright colorful and neat, so well written and most importantly so much valuable information! Thank you ;)

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