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Connection between speed of lens and DOF
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PostPosted: Fri Feb 24, 2012 8:07 pm    Post subject: Connection between speed of lens and DOF Reply with quote
I am sure this is well explained somewhere but i'd like to point to specific question. F-number means evaluation of how much light intensity is delivered to the focal plane. But does it strictly say anything about DOF?

I will exaggerate: you can have a F2.0 fast lens that gives you wide (deep) DOF like F8.0 lens. On the other hand you can have F4.0 slow lens that has thin DOF like F2.0 lens. Is that possible?

I know there are differences in DOF with different 2.8/28mm lenses. I guess it is a product of optical design. It's definetly harder to achieve thin DOF when object is far. Any similar experiences?


PostPosted: Fri Feb 24, 2012 8:57 pm    Post subject: Reply with quote
DOF depends on focal length, aperture, focussing distance and "circle of confusion", but NOT on the optical design.

See:
Wikipedia: Depth of field
DOF-Calculator


PostPosted: Fri Feb 24, 2012 9:01 pm    Post subject: Reply with quote
I'm not sure I quite understand your question, but here goes.

Depth of field is a product of three things: focal length, aperture and focal distance. These are physical laws. As I understand it, any perceived difference in DOF between lenses where all three factors are nominally identical can only be due to slight variations in the true focal length, aperture calibration, rendering characteristics of the lens and so on. In practice I cant see this making much more than a tiny difference.

edit: was typing this during the above reply.


PostPosted: Fri Feb 24, 2012 9:10 pm    Post subject: Reply with quote
I am certainly not an expert, however as circle of confusion also seems to be affected by lens properties (see for instance http://en.wikipedia.org/wiki/Spherical_aberration) this (and with it depth of field) would be affected to some degree by lens design/glass type?


PostPosted: Fri Feb 24, 2012 9:18 pm    Post subject: Reply with quote
Ymmot wrote:
however as circle of confusion also seems to be affected by lens properties

No, not at all. The circle of confusion in this context is the "amount of unsharpness" that is perceived as sharp. A point out of the focal plane becomes a circle on the image. Depending on image output/printing size and viewing distance these circles may still be perceived as sharp points.

See:
Wikipedia: Circle of confusion


PostPosted: Fri Feb 24, 2012 9:58 pm    Post subject: Reply with quote
Well, the F-Stop actually specified the DOF (in context of focal length of course)
but the T-Stop actually gives the speed (i.e. light on the focal plane), but it's usually not specified for photographic lenses as it's quite close anyways. Cine lenses usually get a T-stop rating

http://en.wikipedia.org/wiki/T-stop#T-stops

--> F-Stop defines the look, T-stop the speed.


PostPosted: Fri Feb 24, 2012 10:07 pm    Post subject: Reply with quote
In that aberrations affect the perceived image, yes, they may affect what we see as 'in focus'. If the most-in-focus point is not incredibly sharp, it may not be as distinguishable from the nearly-in-focus?

One common situation is with e.g. wide angle lenses when we're going for extreme DOF: we trade some absolute sharpness (due to diffraction) for the greater depth of perceived sharpness... quality for quantity.

This is of course in perceptual, utilitarian, terms, not in precise optical ones (i.e. I'm not overthrowing optics here Shocked )


PostPosted: Fri Feb 24, 2012 10:29 pm    Post subject: Reply with quote
Tilt (changing focus plan) makes the DOF seem thicker.

nikkor 35/1.4 at f1.4, about 60cm distance:


nikkor 35/1.4 at f1.4, about 40-45cm distance:


PostPosted: Fri Feb 24, 2012 10:46 pm    Post subject: Reply with quote
One has to distinguish the result and how (with what) this result is achieved. You can say to have 2m of sharp area / DOF; after and before that it slowly degrades (circle of confusion), using 28mm lens (angle of view) and at the focal distance of 5m. Plus F2.8 (absolute value).

But using some other 2.8/28mm lens wide-open, and at focal distance 5m, you get 3m thick DOF. That is 1m deeper then previous lens. I think it's possible. Is it? Why? Lens properties has to be the factor.


PostPosted: Fri Feb 24, 2012 10:57 pm    Post subject: Reply with quote
Pancolart wrote:
You can say to have 2m of sharp area [..] But using some other 2.8/28mm lens wide-open, and at focal distance 5m, you get 3m thick DOF. [..] I think it's possible. Is it? Why? Lens properties has to be the factor.

No, this is not possible if you use the same camera (film/sensor size) and same image output size. The lens design itself has no influence on DOF (unless you count lens flaws on the DOF side).


PostPosted: Fri Feb 24, 2012 11:53 pm    Post subject: Re: Connection between speed of lens and DOF Reply with quote
Pancolart wrote:
I know there are differences in DOF with different 2.8/28mm lenses.

Do you have particular examples in mind ?

Quote:
I guess it is a product of optical design.

DOF calculations assume a flat field. In practice lenses might have curved fields, with two different lenses having different degrees of curvature. Field curvature will reduce DOF so these two lenses will, under similar conditions, give different DOFs. I don't know how big the difference might be. However, for example, the Nikon Nikkor 28mm f/2.8 Ais is often reported as not being suitable for landscapes because of softening, in the corners, attributed to to field curvature. The Nikkor 28 mm f/3.5 Ai(s) is thought to be more suitable - presumably due to its flatter field.


PostPosted: Sat Feb 25, 2012 12:14 pm    Post subject: Reply with quote
I opened this topic due to my experience with PB Prakticar 2.8/28mm. When shooting i remember checking many times if aperture didn't close accidentally so very deep DOF it seemed after i crossed 1m focusing barrier.

Finally i can show my assumptions make sense: Yashica DSB 2.8/28mm & PETRI (Cosina) 2.8/28mm & Prakticar 2.8/28mm with it's DOF scale.
You can see how much DOF you will get with the same adjustments of focus and aperture. It's way different.

Yashica & PETRI


Prakticar & Yashica


With selected aperture F8.0 and focusing distance 2m (the same on all lenses) you will get approximate DOF values:

PETRI: 1.75m - 2.3m = 55cm
YASHICA: 1.2m - 4.2m = 3m
Prakticar: 1.2m - infinity = very deep


PostPosted: Sat Feb 25, 2012 2:16 pm    Post subject: Reply with quote
Pancolart wrote:
With selected aperture F8.0 and focusing distance 2m (the same on all lenses) you will get approximate DOF values:

PETRI: 1.75m - 2.3m = 55cm
YASHICA: 1.2m - 4.2m = 3m
Prakticar: 1.2m - infinity = very deep


A DOF calculator gives the following for 28mm, F8, 2m distance:
Near limit 1.25 m
Far limit 5.05 m
Total 3.8 m
This is about what the yashica and prakticar show.

For me It looks like the petri has the dof scale of a 50mm lens:
Near limit 1.68 m
Far limit 2.46 m
Total 0.78 m


PostPosted: Sat Feb 25, 2012 2:36 pm    Post subject: Reply with quote
In the first picture, the lens on the left (Yashica) is set at 2 meters, whereas the lens on the right (PETRI) appears to be set at 2 feet.

EDIT In the second picture the scales are similar if not identical. Maybe it's possible that the two manufacturers (one Japanese, one German) used different values for the Circle of Confusion (CoC) when calculating the scales. Remember that the CoC is somewhate arbitrary. If you do a search you will find debates about which value to use.

Last edited by sichko on Sat Feb 25, 2012 2:44 pm; edited 2 times in total


PostPosted: Sat Feb 25, 2012 2:38 pm    Post subject: Reply with quote
sichko wrote:
In the first picture, the lens on the left (Yashica) is set at 2 meters, whereas the lens on the right (PETRI) appears to be set at 2 feet.


Good point! DOF for 2 ft distance:
Near limit 1.84 ft
Far limit 2.2 ft
Total 0.36 ft


PostPosted: Sat Feb 25, 2012 4:00 pm    Post subject: Reply with quote
The transition of sharp and unsharp is with some lenses very harsh while with others it's very smooth causing a sligthly wider looking DOF. This has something to do with the bokeh which depends on the optical design.


PostPosted: Sat Feb 25, 2012 4:10 pm    Post subject: Reply with quote
hoacker wrote:
DOF depends on focal length, aperture, focussing distance and "circle of confusion", but NOT on the optical design.

See:
Wikipedia: Depth of field
DOF-Calculator

The circle of confucion depends on the magnification/display size of the captured image, so in that sense you could say that DOF does depend on optical design, because different designs can have very different amounts of background blur and rate of focus transition. For example, the Zeiss Makro-Planar has been shown to have significantly less background blur than the Planar 50/1.4 at the same apertures. In web sized images, the DOF of the Planar definitely looks more shallow, because it has a faster focus transition.

Edit: So I agree with Forenseil.


PostPosted: Sat Feb 25, 2012 4:56 pm    Post subject: Reply with quote
ForenSeil wrote:
The transition of sharp and unsharp is with some lenses very harsh while with others it's very smooth causing a sligthly wider looking DOF. This has something to do with the bokeh which depends on the optical design.


+1 my thinking too. I still wonder about effect similarity to focal length change -- perhaps two 28mm lenses are really 27.5mm and 28.4mm, to account for different dof measurement.

Field curvature can cause dof measurement error -- dof is same for both lenses, but in the region measured, the two dof are not in the same place.


PostPosted: Sat Feb 25, 2012 9:03 pm    Post subject: Reply with quote
Sorry about Petri mistake. Darn. It seemed too easy to be true. I'll check other 28mm tomorrow. And then give up my case Smile.


PostPosted: Sun Feb 26, 2012 1:02 pm    Post subject: Reply with quote
visualopsins wrote:
I still wonder about effect similarity to focal length change -- perhaps two 28mm lenses are really 27.5mm and 28.4mm, to account for different dof measurement.

Field curvature can cause dof measurement error -- dof is same for both lenses, but in the region measured, the two dof are not in the same place.

The Zeiss MP50/2 is 51.6mm and the P50/1.4 is 51.7mm, so no significant difference in my example of these lenses. Field curvature has an effect but not in the center, only near the edges and in the corners.
Anyway, I wasn't talking about DOF "measurement" -- the images from these lenses can look radically different w.r.t. background blur and perceived DOF.
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