DOF question

J

joescrivens

Suspended / Banned
Messages
15,052
Name
Joe
Edit My Images
Yes
Say I was using a 24-135 f/2.8 lens. What would result in a sharper portrait?

30mm focal range 1m away from my subject at 2.8

Or

85mm focal range 3m away from my subject at 2.8?

the total depth of field here is practically identical according to an online calculation I did, but how would the images differ in sharpness? Or would they be the same?
 
http://www.dofmaster.com/dofjs.html

There is a calculator and tables on the website so you can calculate where exactly your DOF is for each lens and distance.

Have fun :)

other than that you'd have to resort to some very fancy physics tests and once you start doing that there really IS no hope. Just go and take pics!! ;)

I suppose out of those two I'd plump for shooting at 85mm. Why? 85mm is a great length and widely recognised as a portrait length lens due to the effects of perspective. You get more distortion at 30mm due to the curvature of the lens elements. Then again, it depends which camera you are talking about too with different crop facors. But on FF I'd shoot at 85mm :)
 
thanks, I did use a calculator and the dof is practically the same which was why I asked my question, what other difference would I see if the dof was the same.

Your 2nd point explains a bit more, so 30mm you get more distortion. Interesting.

I'm talking about on a 1.6x crop camera
 
The DoF would be the same as you are using the same aperture in each example and maintaining equal magnification ratios.

In all ordinary circumstances though, I would expect the 85 lens to offer the sharper and more pleasing looking frame. For the reasons that the lovely miss melons outlined. :)
 
I'd favour the 85mm on the basis that the 30mm at 1m is likely to exaggerate perspective and so draw out features such as the nose which would be unattractive in a portrait. This effect is the same irrelevant of sensor crop.
 
Have a look

http://en.wikipedia.org/wiki/Perspective_distortion_(photography)

http://www.statemaster.com/encyclopedia/Perspective-distortion-(photography)

Your 30mm lens x you crop factor gives you a 48mm equivalent which you can get away with but I'd recommend it for full length shots rather than tight head shots, all to do with distance from subject and perspective. Your 85mm will give you 134mm and if you look at Canon's lens line up there is a 135mm lens in there for a very good reason. It's a cracker for portrait work. :)

I'd say that my lens selection in your shoes would be a lot less to do with sharpness and a lot more to do with perspective control. How you hold the camera would have more effect on sharpness than anything :)
 
joescrivens said:
thanks, I did use a calculator and the dof is practically the same which was why I asked my question, what other difference would I see if the dof was the same.

Your 2nd point explains a bit more, so 30mm you get more distortion. Interesting.

I'm talking about on a 1.6x crop camera
Click to expand...

you get wide angle distortion and telephoto compression (at say 11mm and 300mm these are v noticable) telephoto compression tends to give pleasing portraits but wide angle distortion can look v funky if used right to say elongate a foot or arm or something, it can help you make something in an image stand out if you want it to
 
thanks for all the explanations, this makes sense.

Would the same be true of primes? So a 30mm prime compared with an 85mm prime or is it just to do with the way zooms are built?
 
Wide primes tend to show a little less distortion than zooms but the effect they have on perspective remains the same.
 
Its likely also the 30mm focal length at 2.8 would be softer than the 85mm at the same aperture. The lens maker would find it easier to make a 'sharp' lens in the middle of its focal length when wide open rather at the ends of the focal lengths.
At 24mm distortion etc would be a given when wide open, but most people wouldnt care/notice because of the type of shots being taken (quirky portraits/landscapes etc) at the 135 length you could use the 'softness' to get a sf type lens (ideal length on FF for portraits), so manufacturers tend to not go overboard in trying to make these zoom lengths ultra sharp at the focal extremes, but would try to get mid-range sharp. There are physical reasons also of course.

Matt
 
joescrivens said:
Say I was using a 24-135 f/2.8 lens. What would result in a sharper portrait?

30mm focal range 1m away from my subject at 2.8

Or

85mm focal range 3m away from my subject at 2.8?

the total depth of field here is practically identical according to an online calculation I did, but how would the images differ in sharpness? Or would they be the same?
Click to expand...

sharper is the focus in itself...you dont need dof to do focus...it brings in other parts of the shot which appear to be focussed...or sharp that is

for a portrait ...to avoid distortion and get compression...always use a longish lens...85 as you mentioned...and dont get sidetracked into technical dof considerations

you dont want your subject to look like pinocchio with big eyes
 
joescrivens said:
Say I was using a 24-135 f/2.8 lens. What would result in a sharper portrait?

30mm focal range 1m away from my subject at 2.8

Or

85mm focal range 3m away from my subject at 2.8?

the total depth of field here is practically identical according to an online calculation I did, but how would the images differ in sharpness? Or would they be the same?
Click to expand...

If the subject is framed the same, ie the same size in the picture, and you use the same f/number, then depth of field will be the same. As you have found out. (DoF changes with different format cameras though.)

It is a common mistake to say that long lenses give shallow depth of field and wide-angles give more. What is happening is that magnification is changing, and that alters depth of field. Same as when you move closer or further away. If you fit a longer lens but also move back to maintain magnification, as in your example, then the two cancel out and depth of field is maintained.

joescrivens said:
thanks, I did use a calculator and the dof is practically the same which was why I asked my question, what other difference would I see if the dof was the same.

Your 2nd point explains a bit more, so 30mm you get more distortion. Interesting.

I'm talking about on a 1.6x crop camera
Click to expand...

You would see two other differences, and both are the result of shooting distance - they both have nothing to do with lenses or focal length. The reason why wide-angles for example appear to exaggerate perspective is because they allow you to move close and yet still include a lot in the field of view.

If you shoot close, perspective is exagerated - objects close to the camera are large, and those only a little further away are much smaller. Obviously the opposite if you shoot from distance, and it is generally believed that a distance of 6-8ft gives best perspective for portraits.

The other thing that changes is field of view, and IMHO this is the most important thing. More distant shooting position means a longer lens which has a narrower angle of view (same thing as field of view) ie there is less background visible behind the subject.

The reason why this is significant is because it often looks a bit like shallow depth of field. The background is often less cluttered, cleaner, more simple, and so the subject tends to stand out more.

joescrivens said:
thanks for all the explanations, this makes sense.

Would the same be true of primes? So a 30mm prime compared with an 85mm prime or is it just to do with the way zooms are built?
Click to expand...

Zooms, primes, makes no difference.
 
You must log in or register to reply here.
Back
Top