Question about Lenses and Depth of Field

kry10 · Apr 18, 2012

Hi everyone, I am a returning hobbyist and I have bought myself a Canon EOS 1100D with Canon EF-S 18-55mm Standard Lens and Tamron AF70-300mm F/4-5.6 Di Lens.

What I want to know is pretty basic, I am aware that different lenses have different minimum and maximum aperture sizes, but, what I want to do is some fancy stuff like
Macro and Portrait stuff, so, need a short depth of field, but, the lens that came with the camera goes from f4.5 to f32, so, is it possible to get the effect where my subject
is sharp and the background is blurry or do I need a lens with a better aperture range, such as something that goes to f2.8 or larger ?

Derrel · Apr 18, 2012

For portraits, keep the subject CLOSE to the camera, and use a wide aperture like f/4 on the tele-zoom, or as wide-open as you can get on the 18-55, with as I stated the subject CLOSE to the camera, and the background fairly far behind the subject. The closer you are to the subject, the shallower the depth of field will be. There is a fine line between focal length used with a 70-300 consumer-speed lens and exactly how FAR AWAY the subject is from the camera, and how far the background is behind the subject.

You will in general, probably find it easiest to get shallow DOF on portraits at around the 70-85mm focal length range, with the aperture wide-open at f/4, from relatively close range. At longer distances, the DOF behind the subject starts to increase, and even zooming in will not throw the backdrop out of focus as well as shooting from a shorter camera-to-subject distance.

In macro and close-up shots, the background is going to be out of focus if it is more than a few inches behind the plane of focus.

TCampbell · Apr 18, 2012

Lower focal RATIOS (f-stops) narrow the depth of field (DoF) and increase the background blur. Higher focal ratios increase the DoF so more of the image will be in-focus.
Low focal LENGTHS increase the DoF. Longer focal lengths have a compression effect which narrow the DoF.
Closer focusing DISTANCES decrease the DoF. Longer focusing distances increase the DoF.

This means that with the lenses you have, when you use longer focal lengths your lenses will have to use the higher end of your variable focal ratios (f/5.6). Although 70mm at f/4 is pretty good.

You could take a portrait with the Tamron 70-300 at 70mm focal length and f/4, put the subject... say 20' away, and you'd get a DoF of just under 4'. If the ONLY change you make is to change the focal length to 100mm and the DoF will drop to just shy of 2'. The problem is you probably won't be able to use f/4 as soon as you zoom off the 70mm focal length. You may have to drop to f/4.5... which would increase the DoF fractionally to just above 2'.

Lenses used to have depth-of-field index marks to make this easy to check. But not many lenses bother to include these anymore. You can look up the DoF using a website such as this: Online Depth of Field Calculator Having to look up DoF at a website isn't necessarily handy when you're out shooting. But if you have a smartphone, you can usually get an app that will provide this info.

Sherm · Apr 18, 2012

I use the "Lens Lab Guide" app.

It has a slider that lets you change the variables that alter DoF.

So far it has been pretty helpful.

DiskoJoe · Apr 19, 2012

You could get an okay shallow depth of field at f4 but its easier at f2.8 of wider. Go get you a 50mm f1.8 for some portraits or maybe a 50mm f2.8 that will do macro as well.