dof question

trican

I have a simple question: If I use a wide aperture, lets say f/1.8 and focusing is at infinity - whats in sharp focus?

Is this anything to do with hyperfocal distances?

ThOnda

More is here.
Be aware, that you can focus further than infinity, but nothing will be in focus.
Depth of Field is range in front and behind the focused distance. And it is easy to get it to the infinity, depending on focal length and Aperture.

AnCatDubh

The dofmaster is a great tool to study.

It depends on a number of factors including focal length, aperture, subject distance. Have a look here - plug in your numbers and see what it gives you.

Thirdfox

One other thing to note - the DOF scales on the 50mm f1.8 (and any other FF/FX/35mm equivalent lens) are completely wrong if you're using the lens on a 1.5/1.6x cropped sensor.

So while my old 50mm f2 tells me I can hyperfocus from infinity to, say, 3 metres at f/8. The truth is that is only possible on a 35mm film or with full frame. With my DX camera I may only get infinity to 5 metres etc.

Just something to keep in mind.

trican

Thanks for that very useful information. I've skirted around the dof issue since taking up photography a year ago, but I'm noticing my quite a few of my pictures where the key subject is a tad soft, having fallen outside the dof.

And one final stupid question.... infinity as applied to photography means as far as the eye can see (the horizon I guess?)??

Thirdfox

Infinity as far as I understand means "infinity" as in "endless distance". Hence if you want to take photos of celestial objects like the moon, Mars, Jupitar etc you would use infinity.

We use infinity back here on Earth too as some distances are so great to be effectively "endless" to the camera I suppose.

AnCatDubh

Don't forget that a 'soft' appearance in a photograph may well be camera shake (even really minute), motion blur, SR/VR when camera positioned on tripod, too slow shutter speeds, etc... It happens us all so if it is this then don't worry about it.

It also can be out of focus / dof issues that you have described and to my mind it is worth studying and getting a deep understanding of depth of field.

trican

Here is a prime exampe of what I seem to be doing on a semi regular basis, especially with protraits. The face is soft, yet the ear ring is sharp. (It probably works for this photo though)

[IMG][/img]

AnCatDubh wrote: »

Don't forget that a 'soft' appearance in a photograph may well be camera shake (even really minute), motion blur, SR/VR when camera positioned on tripod, too slow shutter speeds, etc... It happens us all so if it is this then don't worry about it.

It also can be out of focus / dof issues that you have described and to my mind it is worth studying and getting a deep understanding of depth of field.

AnCatDubh

Just wondering what the exif was on this (stripped from pix.ie) ?

The other question is if it is a crop of a larger image?

CabanSail

Thirdfox wrote: »

One other thing to note - the DOF scales on the 50mm f1.8 (and any other FF/FX/35mm equivalent lens) are completely wrong if you're using the lens on a 1.5/1.6x cropped sensor.

So while my old 50mm f2 tells me I can hyperfocus from infinity to, say, 3 metres at f/8. The truth is that is only possible on a 35mm film or with full frame. With my DX camera I may only get infinity to 5 metres etc.

Just something to keep in mind.

I do not see how this can be true. The size of the sensor/film is irrelevant to the optics of the lens in front. The Circle of Illumination of that lens will be the same regardless of the size of the material it is exposing. When you use a Full Frame lens on a Cropped Sensor then all that is happening is that you are using less of the available image. It's very similar to the way that Digital Zoom operates.

If the DOF varied depending on the sensor it was exposing then if you exposed a Full Frame Sensor & then later cropped it to use just the centre 60% then DoF should then vary, which it doesn't.

DaireQuinlan

CabanSail wrote: »

I do not see how this can be true. The size of the sensor/film is irrelevant to the optics of the lens in front. The Circle of Illumination of that lens will be the same regardless of the size of the material it is exposing. When you use a Full Frame lens on a Cropped Sensor then all that is happening is that you are using less of the available image. It's very similar to the way that Digital Zoom operates.

If the DOF varied depending on the sensor it was exposing then if you exposed a Full Frame Sensor & then later cropped it to use just the centre 60% then DoF should then vary, which it doesn't.

This had to start at some point

Its all about COC ... circles of confusion. Generally, your DOF is the range of what is regarded as being in 'acceptable focus'. IE there will be a central plane, the focal plane, that is perfectly in focus, then this will tail off either side of the focal plane until we get to the point where we arbitarily decide that the image is no longer acceptably in focus. These two points either side of the focal plane are the two limits of our DOF (ie 5 metres - > infinity, or 1 metre -> 10 metres or whatever).

This 'acceptable focus' is decided be using these circles of confusion, its basically the circle that an out of focus point of light will make on the film plane or sensor. IE, at the focal point a point source will form a point. As the point source moves progressively further away from the focal plane it becomes blurred into a circle (at least assuming an ideal lens). At some point this circle will become large enough that we don't consider it acceptably sharp anymore, and we arbitarily declare it then do be OOF and therefore outside the DOF.

Sooooo, for 35mm this is generally .025, this is what is used for DOF scales on lenses. I personally often use .020 instead for hyperfocal stuff for landscapes. Crucially, because of the smaller size of the cropped sensor , the COC is correspondingly smaller wikipedia gives 0.018 for an APS-C. This results in a smaller DOF for a given lens if you're taking into account what people generally regard as images being acceptably in focus.

Thirdfox

CabanSail wrote: »

I do not see how this can be true. The size of the sensor/film is irrelevant to the optics of the lens in front. The Circle of Illumination of that lens will be the same regardless of the size of the material it is exposing. When you use a Full Frame lens on a Cropped Sensor then all that is happening is that you are using less of the available image. It's very similar to the way that Digital Zoom operates.

If the DOF varied depending on the sensor it was exposing then if you exposed a Full Frame Sensor & then later cropped it to use just the centre 60% then DoF should then vary, which it doesn't.

DoF does vary (as does perspective).

See here:
http://www.bobatkins.com/photography/technical/digitaldof.html

Relevant paragraph is the last one about DoF scales on FX lenses:

"Now let's look at the case when this lens is used on a Canon APS-C crop sensor camera. As the article indicates, if you use a given lens on a smaller format, depth of field is reduced and the angular coverage ("effective 35mm equivalent focal length) decreases. In the case of a Canon APS-C crop sensor camera it's reduced by a factor of 1.6, so it gives the same angular coverage (field of view) as a 27mm lens on a full frame 35mm body. The circle of confusion value for the Canon APS-C crop sensor camera is reduced by a factor of 1.6x and what this means in terms of DOF scales is that you need to use the markings for about 1 1/3 stops wider aperture in order to estimate the DOF. So with the lens set to f16, we need to look at the DOF scale markings about 1/2 way between f8 and f11 as shown below. In this case, if we want infinity to be at one end of the depth of field, we have to focus at 1m. This gives us a DOF extenting from just over 0.5m to infinity."

Or here:
http://www.naturescapes.net/docs/index.php/category-photographic-technique/41-photographic-technique/57-the-mathematics-of-depth-of-field-part-two-crop-factor-magnification-and-the-13-myth

Relevant paragraph
1 If we shoot from the same location, but use a shorter lens on the 1.6x DSLR, we will have more DOF (about 1.6x more than the full frame).
2 If we shoot from the same location with the same lens, we will have less DOF on the 1.6x DSLR (the full frame will have 1.6x more).
3 If we shoot with the same lens, but further back with the 1.6x DSLR to get the same subject size, we will have more DOF (1.6x more than the full frame).

Similarly, most flashes do not use the correct zoom distance when a DX/cropped lens is mounted. The newer Canon and Nikon flashes distinguish between this. You should manually adjust your flash so that extra power is not being wasted to the sides e.g. mount a 50mm on a DX camera and set your flash zoom to 75mm not 50mm.