dof and sensor size

[laptoprob]

Been wondering lately about how depth of field behaves with different sensor formats. I know about dof being relative to object distance and all that, but here is the thing:
It seems that my Digilux 2 small sensor not only shows a bigger depth of field, but also the out of focus areas seem sharper than a full frame photo at similar 50mm equivalent lens setting.

Is the in-focus optimum in larger format sensors (think middle or large format) steeper than with smaller sensors? Or is this just the same result of the true focal distance of the lens?

 

[peripatetic]

Here's a good place to start.

http://www.josephjamesphotography.com/equivalence/index.htm

And here's a good place to experiment with the DOF calculations for your own equipment.

http://www.dofmaster.com

 

[bmattock]

You can get all kinds of scientific (and pseudo-scientific) explanations from people, which usually involved something called 'Circle of Confusion' (CoC), at which point the discussion generally breaks down into something involving a religious argument.

However, in general, and from observation, you are right - all things being equal, to the normal eye on a normal sized print, DoF is made smaller by a larger sensor.

DoF is affected by these things:

1) Focal Length.
2) Distance to Subject.
3) Aperture.
4) Sensor or Film size.

For a practical demonstration, if you take a macro photo with your digicam, you'll see that it exhibits thin DoF; that is, things in the background are blurred out pretty well. This is because you've decreased DoF by decreasing the distance to your subject, without changing anything else.

You can also decrease DoF (to increase those out-of-focus effects on the foreground or background) by opening up your aperture (lower f-number), using a longer zoom (more focal length) -OR- increasing your sensor size.

If you want to play around with cause-and-effect, try DoF Master's free online calculator for fun.

http://www.dofmaster.com/dofjs.html

All you have to do is manipulate the four things I mentioned above - and not the resulting near limit, far limit, and total DoF.

Why are these things good to know? Well, from my point of view, understanding and intentionally controlling DoF to the extent that one is able given circumstances, is another way to add creative control to your photographs. Just like experimenting with focus, shutter speed, and aperture, this is something you can add to your bag of tricks.

The most common example of this is in portraiture, when it is generally desirable to have the background blown out into unfocused fuzz to some extent. A common digicam on a sunny day just can't do that - it is limited to deeper DoF due entirely to the small sensor. So a dSLR makes a better choice in that case - a film camera such as a 35mm, medium format, or even large format possibly even more so.

We work with what we have, but it helps to know how to control the tools we have to the extent they can be controlled.

 

[Nikon Bob]

Rob

Just from observation and not scientific study, I am thinking my C5050 has more DOF at any given f stop than my 35mm film cameras. Something like f4 on the C5050 is similar to f8 on a 35mm film camera. Like I said that is just my impression and nothing to prove it.

Bob

 

[Sonnar2]

Yes, if you photopgraph with Large Format you will notice there is also smaller DOF than with 35mm film.
A picture taken with a 6x9 cm folder camera and 105mm lens (which is about as "normal" in viewing field to a 55mm in 24x36mm) wide open at f/3.5 will have a similar small DOF area as a 55/1.8 or /1.4 lens!

And vice versa: I took all close-up pictures in my camera website with a 1/1.8" sensor cam (pretty small 7.2x5.4mm) at f/2.8-4 (the lens has f/2 maximum speed). Having made the pics with the same large DOF with 35mm film would require a stop of ~ f/32 at the corresponding focal length (about 85-100mm) -- and, of couse, a tripod.

cheers,

 

[laptoprob]

Thanks for all your replies.
One question remains: In equivalent circumstances (well, as eq. as they can be), is the measure of out-of-fucusness bigger, more blurred with a larger sensor?
Definetely seems like that.
This might explain why a larger format and fast lens makes subjects pop out more from the fore- and background. Fore- and back being more out of focus...

 

[ferider]

For the same physical focal length, a smaller sensor (crop) will cause DOF to DECREASE linearly with the crop factor. The same 50mm lens on an RD1 or M8 will have less DOF than on a film camera.

But typically one compares "equivalent" focal lengths across formats. DOF non-linearly decreases with increased focal length, and therefore a "normal" lens on larger format will have less DOF than a "normal" lens on smaller format. For example, a Hasselblad 6x6 80/2.8 has about the same DOF as a Canon 35mm 50/1.4. But the focal lengths are different.

Best is to play with dofmaster and see.

Roland.

 

[bmattock]

Thanks for all your replies.
One question remains: In equivalent circumstances (well, as eq. as they can be), is the measure of out-of-fucusness bigger, more blurred with a larger sensor?
Definetely seems like that.
This might explain why a larger format and fast lens makes subjects pop out more from the fore- and background. Fore- and back being more out of focus...

I understand your question, and I think I can answer it for you.

Depth of Field is the distance, in feet or meters, in front of and in back of a subject that you have focused on, which is more-or-less also in focus.

The further things are from that front distance or back distance, the more out-of-focus they appear.

So, if you play with DoFMaster, you see that larger sensors, with focal length and distance to subject and f-stop the same, have LESS depth-of-field. That means things outside that distance are FURTHER out of focus than with a similar but smaller sensor. They are further away from the borders (front or back) of 'more-or-less' in focus.

You know that a lens' focal length has to do with how 'wide' the image you see is. So a 'wide' lens shows you more of the area around you than a 'normal' lens or a 'telephoto' lens. So that part is easy to grasp, right?

Now imagine that there is a range from FRONT TO BACK of where you focus your camera that is in focus. That's the DoF zone. You can manipulate it to make it DEEPER or NARROWER - just like changing to a wider lens or a telephoto lens makes your scene wider or narrower. You're just thinking in a different direction. Front-to-back, not side-to-side.

A very shallow DoF makes objects outside of that area more out-of-focus than a deep DoF, for the same scene and same objects.

I hope that helps.

 

[ferider]

So, if you play with DoFMaster, you see that larger sensors, with focal length and distance to subject and f-stop the same, have LESS depth-of-field.

That is incorrect. I recommend you do play with it.

For instance, DOFmaster gives you:

35mm film, 50/2, 10ft distance, DOF = 1.45ft
6x6 film, 50/2, 10ft distance, DOF = 2.18ft

Your statement is only true when comparing the same FOV, corresponding to different focal lengths.

Roland.

 

[bmattock]

That is incorrect. I recommend you do play with it.

For instance, DOFmaster gives you:

35mm film, 50/2, 10ft distance, DOF = 1.45ft
6x6 film, 50/2, 10ft distance, DOF = 2.18ft

Your statement is only true when comparing the same FOV, corresponding to different focal lengths.

Roland.

You're quite right, of course. Mea culpa. I was indeed thinking of EQUIVALENT focal length, but stated it incorrectly as 'actual' focal length.

 

[CK Dexter Haven]

A simple article:

http://www.bhphotovideo.com/bnh/con...ge&Q=newsLetter/digi_photo_voodooDOFapr06.jsp

 

[tripod]

There was a thread on this a couple or few weeks ago on RFF. This thread (today's) is much more concise.

 

[Anupam]

All of this can be reduced to one simple principle. DOF is a function of MAGNIFICATION. The more magnification, the less DOF.

So larger film sizes/sensors appear to have less DOF because they need more magnification to achieve the same framing as a smaller film format/sensor. At the same magnification you have the same dof whether you are using an 8x10 or an APS-C sensor. Of course at the same magnification the 8x10 will probably show the whole room where the APS-C will only show someone's head, but they will have the same dof. By the time you zoom out on the APS-C sensor to the whole-room view, you are at a much smaller magnification, so dof appears more.

-A

 

[laptoprob]

confusing stuff...

confusing stuff...

A very shallow DoF makes objects outside of that area more out-of-focus than a deep DoF, for the same scene and same objects.

That is exactly what I was thinking. Therefore a smaller sensor camera will give not only less out-of-focus area but also less out-of-focus blur in that area.

The BH article tells this story too, thanks.

 

[Sonnar2]

All of this can be reduced to one simple principle. DOF is a function of MAGNIFICATION. The more magnification, the less DOF.

Nope. 10mm in 1/1.8" s equivalent to 50mm (24x36 film) "magnification" (means: angle of view) but has a lot larger DOF. 135mm in 3x4" Large Format has the same angle of view too, but if you might find a f/2.8 lens of this focal length, DOF will be a lot shallower than with any 50/1.4 lens for 35mm film...

cheers

 

[Anupam]

Nope. 10mm in 1/1.8" s equivalent to 50mm (24x36 film) "magnification" (means: angle of view) but has a lot larger DOF. 135mm in 3x4" Large Format has the same angle of view too, but if you might find a f/2.8 lens of this focal length, DOF will be a lot shallower than with any 50/1.4 lens for 35mm film...

cheers

Magnification does not mean "angle of view" - not even close.

It is the ratio between the size of the object on film/sensor to the real size of the object. I don't know how you came up with angle of view but in your examples the different dofs are the result of widely different magnifications required to achieve the same angle of view on different size sensors/film.

 

[Sonnar2]

Magnification does not mean "angle of view" - not even close.

It is the ratio between the size of the object on film/sensor to the real size of the object. I don't know how you came up with angle of view but in your examples the different dofs are the result of widely different magnifications required to achieve the same angle of view on different size sensors/film.

Different DOF on different sensor/ film sizes are best visible with lenses of equivalent angle of view. That was my point. Just that. Given an object, equally far away from a camera, picture taken with the same angle of view, different focus lengths are needed: 10mm for 1/1.8", 55mm for 35mm film, 135mm for 3x4" LF. What you call 'magnification' is 'focal length' in my scenario, probably directly related to DOF.

If magnification (and obviously: relative aperture) is the only deciding factor for DOF then close focus pictures of the same object/ same object size (say, a cat from 3 ft) taken with a 35 mm lens at f/2 and next time with a 100mm at f/5.6 (taken with 35mm film) would have the same DOF. I'm not sure abot that. From my feel, DOF with of the 100/5.6 should be smaller... who can calculate that?

 

[Anupam]

Different DOF on different sensor/ film sizes are best visible with lenses of equivalent angle of view. That was my point. Just that. Given an object, equally far away from a camera, picture taken with the same angle of view, different focus lengths are needed: 10mm for 1/1.8", 55mm for 35mm film, 135mm for 3x4" LF. What you call 'magnification' is 'focal length' in my scenario, probably directly related to DOF.

Well, your point is valid insofar as it is a convenient way of looking at things for you. But "magnification" is a standard photographic term with a standard definition - that is how I used it. If you want to call "focal length" or "angle of view" or whatever magnification then go ahead but you should say that you are now arbitrarily calling term X "magnification." I think a little precision about how we use terms and exactly what those terms mean would clear up much of the confusion that surrounds dof.

If magnification (and obviously: relative aperture) is the only deciding factor for DOF then close focus pictures of the same object/ same object size (say, a cat from 3 ft) taken with a 35 mm lens at f/2 and next time with a 100mm at f/5.6 (taken with 35mm film) would have the same DOF. I'm not sure abot that. From my feel, DOF with of the 100/5.6 should be smaller... who can calculate that?

No need to calculate. The proof of the pudding is to be found at this link.

Notice that "focal length" affects the "angle of view" and therefore affects perspective which changes wildly between these shots. But magnification does not change, so dof doesn't change.

-A

 

[ampguy]

practical answer

practical answer

The practical answer to your question is that with equivalent focal lengths, which will be shorter for smaller sensors, longer for larger sensors of FF film, the larger sensor will generally give you less DOF in your prints or web images.

For small digital cameras we generally have full frame (expensive), 4/3, APS-C, and for smaller digicams, we have 1/2.5" the tiniest and most DOF, 1/8, 1/7, 1/6 for mid-level digicams.

It's very difficult to get shallow DOF with a 1/2.5" sensor with the usual ~efl 35-105 type 3-4x optical zooms included on these.

It's very possible, using manual aperture settings and macro modes to get pleasing shallow DOF with 1/1.6 and larger digicam sensrs, IMHO.

http://tedmphoto.blogspot.com/2007/06/narrow-dof-macro-flower-photo.html

For full control of all this, a DSLR is best, with a nice fast prime of physical 35/50mm, f2 or faster, these all stop down to f16 or f22, that will give you DOF from inches to infinity and beyond.

Thanks for all your replies.
One question remains: In equivalent circumstances (well, as eq. as they can be), is the measure of out-of-fucusness bigger, more blurred with a larger sensor?
Definetely seems like that.
This might explain why a larger format and fast lens makes subjects pop out more from the fore- and background. Fore- and back being more out of focus...