Depth of field - friend or foe?
One great benefit of DSLR cameras over their compact brethren is the wider range of depth of field you have at your control; maybe they should write:
Attention: This camera has narrow depth of field
on the LCD screen when you first switch the camera on.My view is that this in itself is a likely cause for poor focus impressions; you have to have the depth of field cover the required subject. To check out how much or how little depth of field you have then take a look at the Online Depth of Field calculator. With depth of field influenced by the camera sensor size, the distance to the subject, the actual focal length of the lens and the aperture then photographers end up with a big shift in this one critical parameter. Understanding and control of it is essential.
Maths bit
Consider that with a PowerShot S5 IS at it's wide angle setting of 6mm (36mm equivalent), will give you a range from 2 meters to infinity in focus [f/2.8 aperture and 10m to the subject]. So you move to the EOS 1000D with it's standard 18-55mm IS lens, now set it to 22.5mm (36mm equivalent) and choose the wide open f/3.5 aperture and suddenly 4.5m is the closest point in focus and it runs to infinity. Now add in that the AF on the DSLR cameras usually has a fixed number of points, 3, 7, 9, 19, 45 etc and that in automatic modes the camera takes the one that finds a subject closest to the camera. That edge AF point picking up a fence post only 2 meters from your camera is going to hurt your pictures. The EOS 1000D above would focus at 2m and give you about 1.6m to 2.7m in focus, the PowerShot S5 IS would be 1.1m to 9.2m at f/2.8 and stopping down to the f/3.5 of the EOS camera that becomes 1m to more than 30m in focus. Suddenly out of focus pictures are here and you spent all that money on an SLR for better pictures.
I was talking with some other photographers about their pictures from the F1 test day at Silverstone. They were surprised on several accounts, the cars were louder than expected, the cars were faster than expected, they needed to use higher ISO settings than expected for a bright sunny day and that the cars got harder to get sharp the closer they got to the photographer. These photographers were fortunate in having access to some big lenses 400mm and 500mm and were using EOS-1D Mark III cameras - the tools for the job.
Shutter speed, depth of field and fast cars
Shooting at F/4 and with an F1 car approaching from 50 meters away the depth of field covers a range of less than 3 meters, as the car gets closer, say 20 meters that f/4 aperture now gives you less than half a meter of depth of field. Now add in that the car could be travelling at 150 miles per hour quite easily (67 metres / second) and that means the car will move closer by 0.06 meters during a 1/1000s exposure. As a rough estimate the distance from the front to the back of the drivers crash helmet is say 30 centimeters at least, the chance to get it sharp at 20 meters distance with a 400mm lens at f/4 and 1/1000s shutter speed is quite low, and requires the photographer to be really good at keeping the focus point in the right place all the time. The AF system is also having to judge the AF, the speed of change and people just think it's easy to shoot DSLR cameras.
People and depth of field
Sure we'd all like to have an 85mm f/1.2L lens and EOS 5D, but they are tough to use. Your beautiful model is posed perfectly some 2 meters in front of you it's a tight crop and they are turned slightly so one eye becomes further from the camera than the other. You focus on the front eye and the depth of field wide open is 4 centimeters, you're both rocking slightly and now you struggle to get both eyes in focus. Extreme... yes, but what if you're a new DSLR user with an EOS 450D and a 50mm f/1.8 in the same situation. You may be a bit lucky since you get a whole 11 centimeters of depth of field to play with. Better hope that you choose a single AF point, otherwise your camera might decide that the models jewelery is a better subject than their eye.
Tilt-shift, shake-rattle and roll
In the past I rented a Canon tilt shift lens the TS-E24mm f/3.5L and tried it out without much success. I was trying to explore the tilt and shift, but my selected aperture of f/8 pretty much masked all the results. I've since tried the TS-E90mm f/2.8 and used it at f/2.8 to get a more visible result.
With all the above considered is this the reason for a lot of new DSLR users struggling to get pictures that were a snap with their compact cameras?
Knowing all the above the real benefit of this depth of field range is the chance to apply it selectively. Some pictures look great with narrow bands of focus, some need a bit more and if you've been looked after by the compact camera's big depth of field then DSLR may come as a bit of a shock.
- p4pictures -
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