DOF question

Devrij

Devrij

Devrij

Soldato
Joined
23 Jul 2009
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14,128
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Bath
I understand how focal length, distance and aperture all have an effect on DOF, but which has the greatest effect?

For instance, to get someone's face to fill the frame I could either step further back and use a longer lens (let's say from a 50mm f1.8 to a 100mm f5.6) , but being a slower lens and further away from the subject, which would have the narrower DOF?

I know you can use calculators to work it out, but I'd like to know in general which of the three has the strongest effect. Cheers!
 
I'd love to know the definitive answer to this but I'd opt for distance. The closer I get to something, the thinner the focal plane becomes regardless of aperture or focal length changes. This is using my 150mm sigma macro lens as an example btw. The background melts away at F2.8 at very close range to focusing on an object, even if the background is quite close, probably due to the relative distance between the background and my focal point being quite large.

Like I said, I'm not 100% sure, but with my own findings I'd hedge the bet on distance being the biggest player out of the 3 choices.

If I get further away, the DOF also increases even at the same aperture. Can test this with a 50mm prime and something like a raynox macro adaptive which clips onto it. The DOF at 1.8 on the 50mm is nice and quite easy to use, yet add the raynox macro adapter and it becomes about 2mm thick whilst closing the distance to the focal point massively (to a few cm away from about 30cm of the original lens in this case).
 
Yeah, that's what I'm finding though. Taking any of the above to extremes always minimizes the effect of the other two, but I still don't know which has the most sway. An 18mm lens, for instance, has pretty epic DOF even at reasonably large apertures. I know my kit lens gets really soft past f11, and very often I can shoot landscape at f8 if I get my focus right. With my 50mm, which is not exactly a long lens, f8 is still pretty short DOF at the same subject distances. I dunno.
 
The longer focal length at a smaller aperture can outweigh a shorter focal length with a wider aperture. If I could be bothered I guess I could map it in Excel.

Like, 200mm @ f/5.6 from 20 feet has a 0.63 foot DOF of where as a 50mm f/1.8 will give you a 3.3ft DOF.

To get the same DOF as the 55mm from 200mm @ 20 feet you need to go to f/28... which I am guessing a 200mm lens won't even go down to? This is all according to my good friend http://www.dofmaster.com/dofjs.html that I've been using recently. :p
 
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