The Depth of Field Preview (DOFP) button is probably one of the lonelier buttons on our cameras. Indeed it's probably only beaten to the top of the "least used button" table by the inclusion of the direct print button.
Yet I find it can be really helpful so I thought I'd share a few thoughts about it.
Most of the time when you are using your camera your lens is operating at its maximum aperture (f/2.8, f/4.0, etc). The reason for this is simple - at its maximum aperture the lens allows the most light into the camera. This makes the viewfinder as bright as possible.
When we look at a scene using our camera we don't see how it will be rendered in our final photograph - for landscapes we might typically be shooting in the f/8 - f/16 range. The challenge we have is knowing how much of the scene will be in focus. We can see the point that is in focus, it will be sharp in the view finder or in LiveView, while the rest of the scene is out of focus due to the wide aperture.
Often we rely on experience and intuition or precise depth of field calculators to work out how much of the scene will be "sharp enough" in the final image. I talk about the principles and some of the tools in this post.
Depth of Field Preview
What the Depth of Field Preview button does is close down the lens opening to the actual aperture you have selected for the shot. If you've chosen to shoot your landscape at f/16 with your f/4 max aperture lens then if you press the DOFP button it will change the aperture from f/4 to f/16. This has two effects.
The first is that you will see the depth of field what will actually be captured in the final shot. If something is sharp using DOFP it will be sharp in the final shot. If it's blurry, it will be blurry in the final shot.
The other effect is that the viewfinder becomes dark - very dark. This is because the lens is now letting in a lot less light. For example stopping down an f/4 lens to f/16 basically makes your viewfinder 16 times darker. This might be just about usable in bright conditions. But on the fringes of daylight where the viewfinder is probably already dark, losing that much light makes DOFP practically unusable especially considering the small size of the viewfinder.
Where Depth of Field Preview really becomes useful is when using LiveView. As I've previously shared (here) I use LiveView pretty much exclusively. Using the LCD to compose and see the image come alive gives me a much better connection than using the viewfinder which I normally only use when "roaming" for a shot or when the sun makes the LCD unusable (and if it's that sunny I've probably given up! :)).
One benefit of using LiveView is that the picture that you see on the LCD is amplified. This means when it's getting dark, you can use LV much longer to see your composition better than might through the viewfinder.
This also means that when using Depth of Field Preview you can still see a usable image on the LCD. The upshot is that by using LiveView you can use DOPF to see what your actual depth of field will be where otherwise the viewfinder may be black.
How I Use DOPF
The most obvious use of Depth of Field Preview is to check your focus. But you can take this further.
Whilst holding the DOPF button you can still use the camera controls and see the results immediately. For example you can turn the lens' focus ring and see the changes on the LV monitor, or you can dial the aperture up or down and see how that changes sharpness in the final image.
I think this makes getting your depth of field right less about guesswork, as well as giving you a better appreciation of how depth of field works. If DOFP is new to you then here's something of a workflow to get you started:
- hold down DOPF (with LiveView on)
- zoom in to 10x on LiveView on the nearest point you want in focus. Is it sharp enough?
- zoom in to the far point you want in focus. Is it sharp enough?
- if you're happy that the near and far points are sharp enough, take the shot
- if you're not happy, keep the LV cursor on the far point and keep holding DOPF down
- start moving the focus further away to start to bring your far point into focus
- as soon as it becomes acceptably sharp, stop. If you go too far, bring the focus point closer so that the far point is only just in focus.
- move the LV cursor to the near point. If this still looks acceptably sharp then you can take the shot knowing that near and far points are in focus
- if the near point isn't sharp then you need to narrow your aperture. Whilst holding down DOPF narrow your aperture one stop at a time and watch as the near point becomes sharper.
- If the near point doesn't become sharp with a narrower aperture, then you aren't able to keep both the near and far points in focus. You need to make a compromise - decide which point is more important, change focal length, or change your position.
Unless the far point is wildly out of focus I tend to favour getting the near point sharp. A little softness in the distance is acceptable (perhaps even expected) but having a soft foreground can be visually jarring. But it really depends on the particular image is and what your intention is that informs your decision.
In the field I don't really follow a particular workflow. It's more like a fluid "dance" checking the near and far point, playing with the focus and aperture to try and bring both points into focus. But hopefully it helps to explain what I'm trying to achieve and give you a basis for adopting your own approach.
I don't use Depth of Field Preview for every shot, normally I use knowledge of the hyperfocal distance of my lenses and my intuition. But when I think my depth of field may be lacking for a particular shot it really helps me to make adjustments.
Note that I use a Canon 5D Mark II. Whilst the above should apply more widely there may be differences between different manufacturers and models. Indeed, some don't even have a DOPF button! :)
Want to Learn More?
I'm currently preparing a workshop schedule where I will pass on my experience of this and other topics. If you'd like to be among the first to hear when I release these workshop dates and special offers then please join my mailing list.