Depth of field

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milspec78

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Tristan
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I shoot mainly portraits, I shoot Canon and mainly use the 24-105 at f4.

I am finding myself craving shallower depth of field pretty much every time i shoot.

My question is this, say i was shooting a 3/4 length portrait outside at 105mm and f4. If i was to increase my focal length to say 150 or 200mm, but shoot from a further distance would f4 give me an effectively shallower depth of field?

Have had a look at the depth of field calculators online, but numbers dont mean much to me at this stage.

Any help understanding this would be appreciated
 
Okay. Assuming you are 10 feet from the subject with 105mm at f4 DOF is only 5 inches. Approx half of this will be in front of the focus point.
At 15 feet from the subject f4 200mm. DOF is only 3inches. Moving the subject back to 20 feet away at 200mm f4 gives DOF 6 inches. Moving the subject back to 25 feet gives DOF 9 inches

Sure you wanna be even shallower?
 
Thanks for the reply trevor, it seems that f4 is shallow enough for almost any situation. I guess then i must be getting my camera to subject to background ratio wrong. I will try clearing a larger space behind the subject and see if that helps!
 
If the subject remains the same size in the frame, depth of field remains the same, regardless of focal length.

But reading between the lines of your post I'm guessing that maybe what you want is that 'big bokeh' look when the background is blown out of focus. Depth of field is one aspect of that, and using f/4 on a crop format camera is not going to make things easy for you there (ideally, you want f/2.8 or lower on full frame) but the other apsect to it is field of view, and a third consideration is the distance between the subject and the background (the more distance, the more blur).

A long lens has a narrower field of view, because you have to move back to fill the frame the same with the main subject. So while the DoF might not have changed, the background is effectively enlarged, which makes smaller distracting details into bigger bolder shapes that 'look' more out of focus.

Try it with your 24-105 lens and frame up a portrait at 25mm, then move back to twice the distance at 50mm, then twice more at 100mm. The subject will be the same size in each image, and the DoF the same, but the perspective will be completely different and the background dramatically changed due to the narrower field of view as focal length is increased.
 
You are right Hoppy, I bought a 70-200 2.8 which i am going to start using more frequently but i find it a little long on a crop for portraits. Although i dont use it much i have held onto it for when i do go full frame.

Am keeping an eye on the classifieds for a used 5d mark 1 to try. I pretty much only do portraits and dont need the speed of a 7d or 60d, i think full frame is the way to go for me.
 
milspec78 said:
You are right Hoppy, I bought a 70-200 2.8 which i am going to start using more frequently but i find it a little long on a crop for portraits. Although i dont use it much i have held onto it for when i do go full frame.

Am keeping an eye on the classifieds for a used 5d mark 1 to try. I pretty much only do portraits and dont need the speed of a 7d or 60d, i think full frame is the way to go for me.
Click to expand...

Full frame and your 70-200 2.8 is a good combo for bokeh - stock in trade for wedding photographers. If shallow DoF portraits are your thing, that's a very good way to go :thumbs:

Another option is the 85 1.8 or 100 2 on a cropper. The 100mm will deliver the same DoF/bokeh as the 70-200 on full frame at 160mm f/3.2.
 
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