DOF control

[tricky]

I consider myself to be reasonably knowlegable about photography (don't ask me any difficult questions though!), but I never seem to get expected results when trying to force out some exaggerated DOF effects with vraycameras.

In a real camera, I'd use, for example, my 50mm F2.0 lens and be able to lock focus on something about 5 feet away and get a really stong DOF effect. If I use similar settings on a vraycamera, I don't get much in the way of a DOF effect. I want to achieve a really shallow DOF.

Clearly I am missing something. What should I be looking at?

[tricky]

I may be getting somewhere. I think its to do with scale. I have my building model which is built in meters (but my display units are mm). I need to either scale the building down or use a much smaller s-number value for the camera....I think...

[giraffe]

can you please explain me this problem about units and scale?
I am having problems too. And I didnt clearly understand your explanation. Thanks

[tricky]

I could be completely wrong, but I always model (as most/all do) at 1:1 scale. Say I have a 5-storey building in a high street for whichy I am producing an image. I use a 50mm lens and have to set the vraycamera a good distance away to get the whole building in. I choose 50mm because it is a pleasing FOV to use.

Now I don't know what the calculation is (somebody on here will know), but my knowledge of photopgraphy tells me that I can not achieve a shallow DOF with a real camera when stood a good distance away from the subject, even with F2.0 or F1.4. The stronger the DOF effect I want, the closer the targeted subject must be to the lens.

Back to my 3d model and the vraycamera: even if I set the 50mm vraycamera to F1.4, it still isn't going to give me a particularly strong DOF effect. I must, in effect, move the camera much closer to the building, say just targeting on a window detail with the rest of the facade receding into the distance. Now, with the F-number set to something like F1.4 I will (or should) get a result as I expact with real life camera equipment.

The way to achieve a strong DOF effect with this scene, yet get the entire model into shot and continue to use a 50mm lens (as an example) is to shrink the size of the model to be a different scale - say 1:100 scale. The 5-storey, 20 meter high building now is a 20cm high building 'model' and can be photographed using 'standard vraycamera optics' to achieve narrow/strong DOF effects.

I assume that another way around this would be to have an F-number very small, like F0.002 or something, but then he relationship between ISO, shutter speed etc of the vraycamera and a realworld camera gets a bit silly.

[mdi]

its totaly todo with scale. Change the scene scale and your DOF will chage too.

[Neilg]

The way i've always done DOF in vray is to use the roll out in render settings and a standard camera. It makes it so simple it's ridiculous.

Get the focal distances matching between the two settings (vray rollout and standard cam DOF section), get the sample radius/aperture the same, then go to the standard camera and click 'preview' - viewport version of exactly what it's going to look like in a second. Turn that off at rendertime and turn the vray one on and you're sorted.

[giraffe]

tricky: thanks for goodexplanation.
but in real life, when you are shooting a photo of a building with 50mm, you need to walk away from the building a lot and then when you focus on the building you will not have much blur- right?
so I think it behaves like a real world camera. no?

on photography sites, they use word DOF as having all things in focus till horizon.
and in cg terms we use this word specificly to have the focused object in focus and rest of the image in blur.
and sometimes both terms get mixed up for me
but I guess you used the term shallow DOF for the second - am I right?

[glyph]

dof is "depth of field" so it means the depth (or extent) of the area that is in focus

its the lens /aperture combo that has the attribute of depth of field, not the image - that's just blurry or not

but like all terms it gets loosely applied!

[tricky]

Yes, you have to be careful with how you use the term. The nice 3D/deep effect where the background is blurry (nice bokeh effects - another term for ya!) is created by using a large aperture/small f-stop number. This is called a shallow depth of field. It is one of the characteristics of SLR cameras as opposed to most compact cameras which have a very large depth of field, where everything is in focus beteeen the target and the horizon, and is a factor of the aperture of the lens and the sensor size (I think!!!)

[MikeHampton]

DOF in post

Any reason why you wouldn't render out a zdepth element and use this to do DOF in post?

[giraffe]

Yes I know the meaning and bokeh and how is related with aperture size
I tried to mean that when somebody is using the word "DOF", u never feel sure if he refer shallow depth or deep depth. On most photography sites, if you click under category of DOF you usually see landscape photos with deep depth where everything is in focus till horizon. But on a cg site, when someone is using term DOF, he usually mean the shallow DOF.

[Dschaga]

I also wonder why the control of the physical camera is so difficult, because it would be more easier if there are little helpers like a point with the mouse on the area of interest or make only sharp areas visible.

If you like to play around with the normal max cameras and a real dof toy, which gives you also the accurate apertureopening you still can try this one:
http://www.cgtechniques.com/goodies/doftoy/index.php

[tricky]

Any reason why you wouldn't render out a zdepth element and use this to do DOF in post?

Just practicing/learning...

[Shoey]

Any reason why you wouldn't render out a zdepth element and use this to do DOF in post?

There are lots of reasons you'd want to do DOF in the camera instead of in post, the same as why you'd want to do motion blur in camera rather than in post.

For starters an object that is out of focus in front of another, reveals more of the object in true DOF than in post DOF. Post dof just smudges the image to look like it is out of focus, but it can't retrieve the data from befind the object.

I'm probably explaining this very badly. But in real life terms, if you stand in front of a wire fence, focus past it and take a shot, your photo will have no trace of the fence (with a shallow enough DOF), it will only desaturate the image. If you did this in 3D with post, the fence would still be there, just blurred, you'd never get the data back that is behind it.

Hope that makes sense

[Shoey]

On another note.

Another reason for getting less DOF in the vray camera is the conversion factor of Crop SLR cameras. If your SLR isn't a 35mm equivalent (full-frame), then the lens settings you are putting in to vray will need a conversion factor applying to them.