|Lesson 1 - Depth of field
||[Jul. 31st, 2006|08:34 pm]
Enjoy a monthly photo assignment with critiques an
Most photo courses start with teaching people about exposure and exactly what an f-stop is an all the rest of that jazz. I realize that a lot of people leave most of their settings on automatic and are mostly happy with the results they get so I’m going to focus on depth of field before getting into the whole run of exposure with this series.|
Put simply, when looking at an image, depth of field relates to everything that is in focus in an image from the closest item you can make out to the furthest. In the center (actually a bit closer to you than the centre but I don’t want to get into math here) of the depth of field is the plane of critical focus. You could also think of this as the point in space that the camera has actually focused on.
*I wrote the following two lines a week ago and have not had a chance to shoot the pictures yet... use your imagination for now*
Just in case people have any questions about what this looks like here is a picture that has a very shallow depth of field. Notice how crisp the subject of the image is while everything else in the photograph is out of focus.
And I don’t want to show shallow depth of field without giving some time to an image with great depth of field. Notice this is the exact same subject but you can now see everything around the subject much more clearly.
There are three factors that affect depth of field; distance to subject, focal length and aperture opening.
The first one, distance to subject is one of my favourite to teach people because you don’t even need a camera to see how this works. Start out by holding your finger up at arms length away from you. You should be able to make out things behind and around your finger pretty well. As you move your finger closer to your eye (this works better when you have one eye closed) you will notice that everything else seems to go out of focus. The lesson here is that as you get closer to your subject the depth of field will become shallower.
Next, focal length is a bit trickier to explain but there are examples of how this works all the time. Using a shorter focal length will give you a greater depth of field while a longer focal length gives you less depth of field. If you ever look at skate board photos you will notice that there are lots of wide angle shots where you can see the skater pulling off some incredible trick while the world falls away behind him with crystal clarity. The opposite end of this is when you have a portrait where you can see the model sharp as a tack but everything else in the image is out of focus. This is usually accomplished by the photographer using a longer lens (usually around 150mm or so) and zooming in on the subject to achieve the effect they are looking for.
Finally, this is going to be the most technical portion of depth of field, the aperture. The aperture is that bit inside of the lens that they use in the opening scenes of all the classic James Bond movies. As the hole in the aperture gets smaller it lets in less light and it increases the depth of field.
This is where the math comes in to the whole thing.
With a perfect lens that doesn’t absorb any light it would have an aperture of 1.0. An aperture setting that lets in 50% of the light has an aperture, or f-stop of 1.4. 50% of that is 2.0. 50% of that is 2.8 followed by 4.0 which is followed by 5.6 then 8.0 and 11.0 and on and on forever and ever letting in half as much light for each f-stop.
Almost every camera lens (including disposable cameras) will have the aperture printed somewhere near the lens. Disposable cameras are typically between f8.0 and 11.0 with a fixed focus that uses the greater depth of field of a short camera lens to have everything from 4’ away to infinity in focus. This is how those cheap little cameras manage to still give decent photos. Consumer SLR zoom lenses will have a maximum aperture (by maximum I mean letting in the most light possible) of f3.5 to f5.6. Pro SLR zoom lenses will have a maximum aperture of f2.8. You can get fixed focal length lenses (they don’t zoom at all) that let in even more light such as a 50mm f1.4 but you can’t zoom in or out with these.
There are some SLR cameras that have a depth of field preview button on them. This can be confusing to many people if you don’t quite understand how cameras work. When you focus, the lens keeps the aperture wide open so you can easily see where the lens is focused and compose your photograph. When the camera takes the picture, the aperture momentarily will close down to the f-stop set for the exposure. The depth of field preview button allows the camera operator to look through the camera lens with the aperture set to the actual setting for taking the picture. If you do this at higher apertures it can be quite hard to see because of how little light is permitted through the lens but if you can see the image you will be able to see how much of your image will be in focus.
Now… let’s put this newfound knowledge to good use. This month’s assignment is to take a photograph with a shallow depth of field that involves a hand in some way as the subject. If you want to participate in this project then email a copy of the original image (this means no cropping or playing with the exposure after you take the picture) to photo_101 at szilvagyi dot com (you can figure that out) before August 15th. I will have all photos posted in the journal with critiques by the end of August 19th with advice for cropping and adjusting. If you use a particular program to edit your photos then please mention it so I can provide specific instructions in my critique. The photos will be posted anonymously and members of the community will be welcome to add additional constructive commentary.
After the critiques have been posted, I ask that people take up to 1 week to act upon the advice provided and email the edited copy of the image to me again so I can post them all and show the world the benefits that a little bit of post-processing can achieve.
Now I would like everyone to have fun taking this photo and I’m looking forward to the results. If anyone wants some pointers on how to do something with their camera for this assignment feel free to either email me or to post the question here in the forum for all to benefit from. I will be participating in this exercise as well and I will be using my cheap point and shoot camera so you can see what can be accomplished without needed professional equipment.