Depth of Field for Faces - Rule of Thumb

[Suzan J]

Hello Everyone: I have a new lens! It is a 105 mm 2.8 Nikon micro which I want to use for shots of my garden. Alas, spring is nowhere to be found in my part of the world, so I enlisted my favourite model for a few shots around the house. Anyway, that got me trying to gain a better understanding of DOF. I took the shot below by focusing on the dog's right eye. I have Aperture 3.4 and it shows the single focus point to be just under the eye. That eye seems to be in focus. The dog's left eye is not. I used the DOF calculator and my (very rough) calculations show that there would be about a 3 inch area that would be in focus. I guess that would be about right as the eyes would be that far apart on the angle of the shot??

What is the general rule of thumb for shots of people or pets? Should both eyes usually be in focus? This is a bit tough to do with such a shallow depth of field. The aperture in this shot was 6.8 and I only got a fairly small slice of focus. If I open up the aperture to isolate the subject, then the DOF becomes even smaller. If I stop down, then the backgrounds would become too prominent and the shutter speeds too slow?? UGH!

Also, I have no idea how at least part of this shot was in focus as the SS is only 1/15. (I did sit on the floor and propped the camera on my knee.)

The DOF calculator did not have a drop down selection for aperture of 6.3. I believe the closest choice was 5.4. Sorry for the long winded question, but I really want to understand DOF before I can progress. My dog is also feeling somewhat glum. It snowed yesterday and we all have cabin fever.



_DSC0058 by Soo J, on Flickr

[Shadowman]

I take it your dog isn't allowed outside when it's snowing.

Using a shallow depth of field will soften areas not within the focus range. With the tilt of your dog's head, the left eye is not really within the focus range.
Nice image.

[shreds]

Hi Soo,

What camera are using (crop frame or otherwise), and how far where you from your 'model'? This 'eye' conundrum is not unusual and other than improving the light conditions, I would suggest that you work on the closest and most prominent eye, as that is where any viewer is going to look first.

(you could also open up the lens, then blur your background in PP but then you are not usually getting the best from the lens and I have to say I am a fan of this particular bit of glass).

[Suzan J]

Hi there: Actually, my buddy loves the snow. It's me that has had enough!

I was using a D3100 which has a crop factor of 1.5, I believe. I was 6.2 feet away from the model, so according to the DOF calculator, I should have about 3 inches in focus. The way I understand it, the DOF should be slightly ahead and behind of the focus point, with a bit more in focus at the rear of the point. Is that right? So, as you mentioned, it seems this eye thing is something that everyone has to contend with. I suppose it is a question of compromise as facing the camera flat on would not seem to result in too many flattering portraits. Also, do you know why the DOF calculator does not give F6.3 as an option in the drop down menu? Thanks

[spence]

Hi Susan J
Re the depth of field site,at best it is just a guide, do not take it as true. DOF works on the principle of third in front and two thirds behind point of focus. It is something that you have to play with and understand for your camera and lens. However nice image keep it up.

[arith]

I love it, and how clever are you to get spot on focus. I've done a spreadsheet just for fun, well actually just to get the basics and then as usual I stop. But you can work out a distance to get the dof with the required aperture, it is not exactly right because it was done in my head. You have to know the hyperfocal distance for the aperture your using or use the second line to get the hyperfocal distance. It is imperfect, you do have to allow macro's and it took along time to grasp the crazy VBS.

http://arismetique.com/basics/Book1.xls

but you might need a few more things, certainly need Excel. I don't know.

[Polar01]

Susan: the reason that is does not have f/6.3 is that the CIC DOF calculator goes up in 1-stop steps where as our cameras are setup in 1/3 stop steps.

Cheers:

Allan

[arith]

I guessed the camera type and put some numbers in assuming you used all 105mm and crop ratio was 1.5.

I thought about a quarter of a meter DOF or about 10 inches would be nice and got about 4.5 paces at f3.2.

[Mike Buckley]

Gorgeous model well captured!

I disagree that the left eye is out of focus due to depth of field; I think it's due to motion blur while using a slow shutter speed. Notice that the nose and right eye are very sharp. Considering that the software shows that the focus point was beneath the right eye and that the nose is sharp, I would also expect the left eye to be sharp if there is no motion blur.

If the left eye is soft due to depth of field, perhaps the software is reporting an incorrect focus point. If you focus on a particular point and then recompose, the software probably will not know where you focused.

If I wanted both eyes in focus in this shot, I would probably focus in between the dog's eyes.

Depending on the various factors, the depth of field can be 1/3 in front of the point of focus and 2/3 beyond it, as you mentioned. However, it can also be about 1/2 in front and 1/2 in back.

[RustBeltRaw]

There are several good points in this thread that I'm going to collect and re-state for clarity.

• spence - 1/3 forward, 2/3 back: Spence is saying that the focal plane does not mark the middle of your depth of field. These numbers are a non-precise example, but if your focal plane is located 6ft away and your depth of field is 6in, roughly 2in will be between you and the focal plane, and roughly 4in will be behind the focal plane. So you can frequently make better use of your depth of field by shifting slightly away from the autofocus point. Bringing the plane back toward you will add background blur, and sending it away will keep more of your subject sharp. This is why standard advice is to focus on the closest eye.
• Focal Plane vs. Depth of Field: Sometimes when people say depth of field, they mean focal plane. This borders on pedantic, but when you autofocus, you're only deciding where the focal plane goes. Your aperture and focal length determine the in-focus range around the focal plane.
• Effect of stopping down: If you went for more sharpness on your buddy's back eye by stopping down without changing your autofocus point, that would be a pretty good response, since 1/2 forward, 2/3 back dictates that stopping down will add more DoF at the far side of the focal plane than the near side.

[Stagecoach]

Hi Suzan,

Interesting question, rule of thumb DOF for people and pets.

You obviously have a good understanding of DOF so I will not repeat what has already been covered regarding this or when used for people portraits.

With respect to animals they have far more variable face shapes than humans, a simple example would be the relative distance between a rabbits eye and its nose and that of a horse. Taking your example of the dog it may have been possible to get the left eye more in focus at that aperture BUT at a cost of losing the sharpness on the nose.

In my opinion, whatever the 'rules' you have captured this perfectly and I believe the picture would have been less appealing if the nose had not have been so sharp along with the closest eye. There is of course the option to do some local sharpening of the left eye if felt really necessary.

Grahame

[William W]

The "Rule of Thumb" that the DoF is always ⅓ in Front of and ⅔ Behind the ‘Plane of Sharp Focus’ is flawed.

For general Photography, the (Near and Far) ‘Limit of Sharp Focus’, move closer to being ½ and ½ as ‘The Shot’ becomes tighter.

For example: for a Tight Head Shot, similar to the dog in the example above , the LoSF will be at ½ in Front of and ½ Behind, the Plane of Sharp Focus.

If the dog were in a group of twenty dogs, like a School Photo, arranged in three rows, and it were a Wide Shot, then the DoF would be spread closer to ⅓ in Front of and ⅔ Behind.

Note that ‘Tight Head Shot’ and 'Wide Shot' are NOT describing Lens’s Focal Lengths but are descriptions of ‘The Shot’ or the ‘Framing’

***

For more technically specific data – the example shot was made using a 105mm lens on an APS-C Camera. It's reasonable to assume the image is a full frame crop, then the Horizontal FoV (Field of View) is about 750mm.

Therefore the SD (Subject Distance) is about 3600mm.

The shot was pulled at F/6.3 @ 1/15s @ ISO400.
The DoF is 227mm. (Circle of Confusion used = 0.016mm)
The Near LoSF is at 3490mm and the Far LoSF is at 3717mm
That is ratio of 110:117.
Which is very close to ½ and ½.

***

Regarding the image itself – none of it appears tack sharp to me.

I concur that there is Subject Movement.
But I would also argue that the far eye is also in less sharp focus, than the near eye.

WW