depth of field

Depth of Field...

Depth of field is governed mainly by three things...

1... Focal length of the lens... The longer the focal length, the shallower the depth of field for any given f/stop...

2... Aperture used... The larger the aperture (f/stop) the shallower the depth of field would be for any given distance... A large aperture would be something like f/1.4 to f/2.8... The smaller the aperture the greater the depth of field would be for any given distance... A small aperture would be something like f/16 to f/32...

3... Distance from the subject... The closer you are to the subject the shallower the depth of field for any given f/stop...

So try and think of the components of depth of field as a FAD, Focal length, Aperture and Distance...
To maximise the depth of field you should focus about 1/3 of the way into the image because depth of field covers about 1/3 in front of the point of focus and 2/3 behind the point of focus... This is most important in macro photography as the depth of field is very shallow, at close range it can be a matter of millimetres... In the two sets of images below a Sigma 105mm f/2.8 macro lens was used to take some images of £2.00 coins (sterling)... These coins were set into a box at a distance of about 6mm apart to stop them falling over and the coins are 2.5mm thick... The camera, a Canon 20D, was set on a tripod, and shutter was released via the camera's self timer to try and keep any slight movement to a constant level... The lens was focused manually to make sure there was no back or front focus by the camera's auto focus points so any discrepancies are due to my old eyes...
The first image, made up from three separate shots, which were taken from a distance of 840mm from the cameras focal plane, and the text in the image lets you know where I focused... You may have noticed that the middle sample where I focused on the third coin from the right that two coins behind the coin on which I focused are sharp and only one coin in front is reasonably sharp...

You may also have noticed that I said the depth of field covers 1/3 in front of the point of focus and 2/3 behind the point of focus, but wait, something strange is going on... Where I have focused on the back coin the depth of field seems greater (it covers nearly three coins) than when I focused on the front coin (it covers nearly two coins), how could this be, well I'll tell you... Remember another contributing factor to depth of field is the distance from the subject and I said that in macro photography that at close range this can be a matter of millimetres... That's the answer, by focusing on the back coin, even though the depth of field is still only 1/3 in front of that coin, that coin is a greater distance from the camera therefore has increased the depth of field both in front and behind that coin... You can't of course see the depth of field behind the coin on the right as there is nothing there to show it... Yes even though the coins are only 6mm apart that extra distance has made all the difference to the amount of depth of field...
Now for the second image which I hope will help explain the distance thing... This is exactly the same setup as the image above but I moved the camera 200mm closer... The top section shows one sharp coin only as the second coin is starting to blur even though the depth of field extends 2/3 beyond that coin... This is due to the coin being fairly close to the camera... The centre section shows two sharp coins because the point of focus is further away, and the depth of field covers the coin behind it, yet not the one in front of it, even though they are both 6mm away from the point of focus... The bottom section shows again that nearly two coins are in focus, this is due to the distance they are from the camera even though only 1/3 of the depth of field is in front of that coin... Yes in macro photography that 14.5mm (2 x 6mm gaps + 2.5mm coin) can make all the difference to the depth of field...

Conclusion:- By not focusing on the front of an object but about 1/3 in you can maximise the depth of field...

In these next set of images I used three different lenses, a Canon 50mm, the Sigma 105 and a Canon 200mm all set @f/22 mounted on the Canon 20D... Again the camera was tripod mounted, the self timer and manual focus were used... The six coins (2.5mm) and spaces (6mm) give a total distance from the front of the firs coin to the front of the last coin is 42.5mm... As you will see from the notes on the image that I moved the tripod away to keep the image the same size in the frame... You may also notice that again there seems a contradiction in the statement that focal length effects the depth of field because all three parts of the image seem reasonably sharp from front to back... The answer is again in the distance the subject is away from the camera... The depth of field of a 50mm lens on the 20D is approximately 63mm, the depth of field of a 105mm lens on the 20D is approximately 55mm, and the depth of field of a 200mm lens on the 20D is also approximately 55mm... Because the distance between the front and back coins is only 42.5mm all three lenses set @ f/22 have enough depth of field to cover all six coins...

Conclusion:- Regardless of the focal length of the lens if the aperture is kept the same, and the distance between object and camera are adjusted to keep the image the same size in the frame there will be very little difference in the depth of field...

	Freelance Photographer Seeing things differently A taker of Photographs - A creator of images
Kennymc.com

Depth of Field... Depth of field is governed mainly by three things... 1... Focal length of the lens... The longer the focal length, the shallower the depth of field for any given f/stop... 2... Aperture used... The larger the aperture (f/stop) the shallower the depth of field would be for any given distance... A large aperture would be something like f/1.4 to f/2.8... The smaller the aperture the greater the depth of field would be for any given distance... A small aperture would be something like f/16 to f/32... 3... Distance from the subject... The closer you are to the subject the shallower the depth of field for any given f/stop... So try and think of the components of depth of field as a FAD, Focal length, Aperture and Distance... To maximise the depth of field you should focus about 1/3 of the way into the image because depth of field covers about 1/3 in front of the point of focus and 2/3 behind the point of focus... This is most important in macro photography as the depth of field is very shallow, at close range it can be a matter of millimetres... In the two sets of images below a Sigma 105mm f/2.8 macro lens was used to take some images of £2.00 coins (sterling)... These coins were set into a box at a distance of about 6mm apart to stop them falling over and the coins are 2.5mm thick... The camera, a Canon 20D, was set on a tripod, and shutter was released via the camera's self timer to try and keep any slight movement to a constant level... The lens was focused manually to make sure there was no back or front focus by the camera's auto focus points so any discrepancies are due to my old eyes... The first image, made up from three separate shots, which were taken from a distance of 840mm from the cameras focal plane, and the text in the image lets you know where I focused... You may have noticed that the middle sample where I focused on the third coin from the right that two coins behind the coin on which I focused are sharp and only one coin in front is reasonably sharp... You may also have noticed that I said the depth of field covers 1/3 in front of the point of focus and 2/3 behind the point of focus, but wait, something strange is going on... Where I have focused on the back coin the depth of field seems greater (it covers nearly three coins) than when I focused on the front coin (it covers nearly two coins), how could this be, well I'll tell you... Remember another contributing factor to depth of field is the distance from the subject and I said that in macro photography that at close range this can be a matter of millimetres... That's the answer, by focusing on the back coin, even though the depth of field is still only 1/3 in front of that coin, that coin is a greater distance from the camera therefore has increased the depth of field both in front and behind that coin... You can't of course see the depth of field behind the coin on the right as there is nothing there to show it... Yes even though the coins are only 6mm apart that extra distance has made all the difference to the amount of depth of field... Now for the second image which I hope will help explain the distance thing... This is exactly the same setup as the image above but I moved the camera 200mm closer... The top section shows one sharp coin only as the second coin is starting to blur even though the depth of field extends 2/3 beyond that coin... This is due to the coin being fairly close to the camera... The centre section shows two sharp coins because the point of focus is further away, and the depth of field covers the coin behind it, yet not the one in front of it, even though they are both 6mm away from the point of focus... The bottom section shows again that nearly two coins are in focus, this is due to the distance they are from the camera even though only 1/3 of the depth of field is in front of that coin... Yes in macro photography that 14.5mm (2 x 6mm gaps + 2.5mm coin) can make all the difference to the depth of field... Conclusion:- By not focusing on the front of an object but about 1/3 in you can maximise the depth of field... In these next set of images I used three different lenses, a Canon 50mm, the Sigma 105 and a Canon 200mm all set @f/22 mounted on the Canon 20D... Again the camera was tripod mounted, the self timer and manual focus were used... The six coins (2.5mm) and spaces (6mm) give a total distance from the front of the firs coin to the front of the last coin is 42.5mm... As you will see from the notes on the image that I moved the tripod away to keep the image the same size in the frame... You may also notice that again there seems a contradiction in the statement that focal length effects the depth of field because all three parts of the image seem reasonably sharp from front to back... The answer is again in the distance the subject is away from the camera... The depth of field of a 50mm lens on the 20D is approximately 63mm, the depth of field of a 105mm lens on the 20D is approximately 55mm, and the depth of field of a 200mm lens on the 20D is also approximately 55mm... Because the distance between the front and back coins is only 42.5mm all three lenses set @ f/22 have enough depth of field to cover all six coins... Conclusion:- Regardless of the focal length of the lens if the aperture is kept the same, and the distance between object and camera are adjusted to keep the image the same size in the frame there will be very little difference in the depth of field...


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