One of the first principals of photography that must be learned is, Depth of Field (DoF). You may say, no the Exposure Triangle is, but Depth of Field is so entrenched in that, that we can’t separate the two. People say they understand it and in the most basic way most photographers do. ” It’s when your subject is In-Focus and the background isn’t”. Sounds simple enough, but unfortunately it isn’t.
I see it all the time, especially in portrait work where many proudly exclaim ” I always shoot wide open” and then I know…they don’t really get it.
In most instances, we can get away with having a rudimentary understanding and it won’t hurt our images. But when the camera get’s close, when the magnification increases, that’s when that rudimentary knowledge falls all apart. Macro enthusiast know it far too well, how trying DoF becomes and how much beyond a basic understanding of DoF it takes to produce good work. I’m not going to get into pure Macro work here because it is an article all to itself and I’m not trying to reach those people. Instead I just want to concentrate on the Close up, shooting at or near your lens Minimum Focus distance.
So for this lesson, I set up my camera in a fixed spot and took an overhead image of it and the subjects of the shoot (three beverage bottles) and then our camera settings and the corresponding image to try and get a better understanding.
Here is the set-up. I have a Canon 6D MKII on a tripod with a Canon 85mm 1.8 lens attached. The minimum focus distance of that lens is 2.79 feet. I had it set to 3′ 2″. Remember this distance is from THE SENSOR or FILM PLANE, NOT the front of the lens.
Here is the overhead shot so you get an idea. The first Diet Coke bottle is 3′ 2″ from the Sensor Plane. The Water bottle is 2″ behind that bottle and the Sunkist bottle is 4″ behind the Diet Coke.
NOTE: Because I used a wide-angle lens and a short step stool for the overhead shot, there are some distortions and the camera and bottles both “bend back” so the measured distances are to the bottom of the bottles even though the label, laid back in apperance, is on the same plane.
Remember this one very important point: There is only ONE point of focus, that is the only point that is absolutely sharp (within the ability of the lens) everything else is within the “Field of Acceptable Focus”, which is different, I’ll touch on that later. But let’s define Depth Of Field. “The Area, near to far, in front of and behind our point of focus, that is within the filed of acceptable focus.”
Field of Acceptable Focus is, looking at a image under normal viewing conditions the object looks reasonably in focus. So looking at these images on your screen would be normal viewing so what you see that is reasonably in focus is with in the field of focus. If you click the image and make it full size and zoom in, you are no longer looking at it in a normal way and things will appear more un-sharp but will mislead you in the meaning of DoF
Here is our first shot at f/2.8 as you can see that gives us a DoF of ONLY .79 inches!, (The DoF is indicated by the white lines) remember about 1/2 of that is in front of the point of focus and 1/2 behind it. I’ve included the D0F app so you can see the exact numbers.
Here is what the shot looks like
As you can see the Coke Label at the Point of Focus is perfectly sharp. We can see, even without blowing it up, that the water bottle is out of focus only 2″ back, the Sunkist bottle more so. If we were to blow up the image ( go ahead you know you want to you would see on the left side that the UPC abel on the side of the Coke bottle is already falling out of focus. That’s that .4″ !
Here are the particulars for this shot. And NO, I don’t do some amazing feats of math, I use a DoF Calculator app. This one is Simple DoF.
The next image I moved the aperture up two stops to f/5.6
The resulting image has a Total DoF of 1.57″
The Water Bottle changes somewhat but still isn’t something you would consider in focus, It’s still about 1,25″ behind the DoF Limit
I now jumped up another two stops to f/11 giving us a DoF of 3.15″. Notice we are taking Huge Aperture swings but not gaining a lot of DoF at this close focus distance
The resulting image shows that the edge of focus is just almost touching our water bottle but you can see here, viewing it at this size it appears to be within the field of acceptable focus even if it just slightly out. The Sunkist bottle is still a bit fuzzy.
And finally we are at f/16 the maximum we want to go before “Defraction sets in (defraction is an optical condition that causes softening of the image)
And as you can see we are now at a whopping 4.48” of DoF!!. Not much but it may be just what you need to get all of that flower you are trying to photograph in acceptable focus while still leaving enough drop off to the background ( it’s important to notice the effect on the background throughout all these images. The Grill was about 8′ from the camera. So you can take from that having a a background very far away helps if you ARE trying to isolate a subject.
And our final image with the maximum DoF possible this close up
The water bottle is FINALLY with in the Field of Focus, the sad Sunkist is not but again, it appears at this size and viewing distance to be close to in focus. You can see in the background the Grill even though outside the Field of Focus starts to become recognizable
And out final Numbers from the DoF app
Pretty neat huh? Shows you how much work it actually takes to get what you need in focus. It certainly isn’t a “whatevah and forget about it”.
I know, a few of you are saying, “Simple I would have just backed up and zoomed in, others are saying, “I would have just used a wide angle lens cuz they have more DoF.” and you would of course…be wrong as we learned in this article I did on just that subject. That focal length and distance cancel each other out when we frame our subject the same because they become the same Magnification.
I tried to keep this one simple so more can grasp it, but I know as usual I get carried away. I hope the images and graphic help.
NOW, think about the challenges the Macro people have to deal with. (what with them being so small…hehe..macro people)