This macro FAQ post is going to focus on depth of field (DOF) in macro photography.
Question 4: DOF and how you will need a narrower aperture than you might think to achieve the desired amount of bokeh
If you have played around with any online DOF calculators (or an iphone app, there are several for this) you will see that once you enter the range of shooting 1:1 your DOF is incredibly narrow. For example, on my 5D mark ii, with my 100mm lens, if I am shooting at 1:1 (ie the minimum focusing distance of my lens, 31 cm) my DOF would be as follows:
f/2.8 DOF = 0.11 cm
f/4.0 DOF = 0.16 cm
f/5.6 DOF = 0.22 cm
f/8.0 DOF = 0.31 cm
f/11 DOF = 0.44 cm
f/16 DOF = 0.63 cm
f/22 DOF = 0.88 cm
f/32 DOF = 1.25 cm
That is insanely narrow!
So if you have a little flower and the stamen are about 5mm tall you would need an aperture above f/11 to get the whole center of the flower in focus shooting from the top down. I shot this at f/2.8 and you can see that the whole center of the flower is not in focus.
You can see how much more is in focus when I stopped down to f/6.3
But the whole center of the flower is still not in focus.
The best way to really get a grasp on this in your own mind I think is to do an little shooting experiment. Start about 3 feet back from your subject and start shooting at f/2.8 and progress up all the full stops of aperture (listed above) taking 1 shot at each. Ideally you would use a tripod for this and just put your camera on ISO 100 and aperture priority, if you don't have a tripod up your ISO as need to keep your shutter speed reasonable. You lens might not go as high as f/32, I think only my macro does. Now move in to 2 feet away and repeat, and then to the minimum focusing distance ~1 ft and repeat. When you go back and look at those images you will see how much narrower you DOF was for the same aperture at 1 ft away vs 3 ft and how narrow of an aperture you can go to and still have bokeh.
Question 5: Do you shoot wide? I am so used to shooting pretty wide for portraits, that closing the ap is weird to me.
All the information in question 4 aside I do still tend to shoot really wide open most of the time. I love the super creamy, dream like quality this gives to images. But I think that is a style thing and you really need to experiment with what works best for you. When I can get my subject and background separated enough I will stop down more to have more of the subject in focus.
This is an old shot, but I was shooting at f/8 and since my background was across a 3 lane road from the flower I was shooting, my background is still super creamy but I have a decent part of the flower in focus.
Here is another that was shot at f/7.1
Question 6: When dealing with the much shallower DOF, what do you tend to place your focal point on?
Where you chose to place your focus I think is part of what defines your style. In portraits we know that 99% of the time if the eyes are in the image that is where our focus should be. With flowers or other macro subjects (other than bugs, which have eyes so I assume that the eye rule would apply) you have a lot more flexibility. If the general flower is my subject I tend to focus on the stamen or the center part of the flower.
However there are times when a different part of the flower is my focus, like a petal and the texture of it or the light shining through it so that will be where I chose to place my focus.
Here is an example where I think it would have been better if I had focused on the stamen instead of the petals.
That said I think the key thing to think about is what is the subject of your image, what part of the image do you want to draw the viewer into? That should determine where you put your focus.
Question 7:What do you choose to place in the background (ie shoot against blank space, or do you prefer context in the bokeh?)
I actually like to play with both something that will appear to be just a smooth buttery background,
and one that will provide some context.
I think again it depends on your subject. Are you trying to single out a specific flower or shoot more of a "flowerscape"? If I am doing the later I will try to include more context. I think these types of images are harder to shoot and I am still working at perfecting my skills with them. Martin Bailey is pretty much the master at them.
Another consideration to think about when picking your background is contrast. You want to try to provide a contrasting color to highlight your subject in the bokeh. So if you are shooting a white flower I look for what I can find around it to provide a darker background.
as they would be lost on a white background
I was very careful in the following image to use the green to separate the purple petal that is the subject from the purple petals that made up the bokeh.
I often shoot flowers where I find them and I rarely if ever bring something with me to provide a "blank" background so I look around at the enviroment I have to determine how best to use it to highlight my subject.
Bonus Question: DOF Preview Button
Do you know where the DOF preview button on your camera is? If not look it up in your manual. Did you know that when you look through the viewfinder what you are seeing is what the image would look like if you were shooting wide open? This is great when you are manually focusing and you want to be sure you get your focus just where you want. But with macros or anything where you are shooting with a much higher aperture this can be deceiving. This button is a great way to see what the background will really look like before you shoot, and show you approximately how much of your subject you will end up having in focus after you snap the image. Test it out next time you are out shooting.
I used it on this one to decide that I did really want to shoot wide open.
Next time we will discuss lighting, specifically using natural light.