[adifrank]

Hi. I'm reading through the manual of my new Canon 5D mark ii.



In the chapter on Av camera mode there is a short and not so clear explanation on something called a Depth of Field Preview Button. I found the button. When I press it the image in the viewfinder gets a bit darker.



The manual says:

Press the depth-of-field preview button to stop down the lens to the current aperture setting. You can check the depth of field through the viewfinder.

What does it mean by - "stop down the lens to the current aperture setting"? And how exactly do I check the depth-of-field through the viewfinder?

I just can't figure out what exactly this preview is and what it has to do with depth of field.



Can anyone please explain this to me?



Thanks!

[Rainer]

Well depth of field is controlled by several parameters ...

distance to subject, focal length and aperture (plus sensorsize).



Since with modern SLR cameras you see the viewfinder image

through the wide open aperture, you do not see the image under the same

circumstances as the camera will take the image (given that you do not

shoot wide open).



Make a small test:



- Use the longest focal length available on your standardzoom.

- Focus on something near ... very near ... as near as possible.

- select f/11 or f/16 as aperture. (edited to f/11)



Now look through the viewfinder and press and release the

little DOF-preview button several times ... and look on those

areas of the image that are NOT in focus ... that are before or

behind the thing you focussed on. You should see that (besides

getting darker) these parts are also getting sharper when the

aperture is stopped down.

[popo]

The camera normally idles with the aperture fully open. This lets most light in to the viewfinder, but the DoF shown may differ from the selected aperture you want to take the photo in, as the aperture is one of the contributing factors. Pressing the DoF preview button temporarily sets the aperture to the selected value so you can see what impact it has. As the aperture can only get smaller, it lets less light through and the viewfinder image gets darker.

[wim]

Rainer and popo are both right.



There is one caveat with modern cameras however, although this was also valid to some degree in the film age. Unless you use an -S type focusing screen (for Canon anyway), an Eg-S for the 5D Mark II, which is also called a high precision matte, or something similar on any other camera, you will perceive a DoF through the view finder as if the lens is stopped down to approximately F/5.6.



This is caused by the fact that standard focusing screens for dslrs are relatively coarse grained mattes, to let through as much light as possible, making the VF image appear brighter. Because coarser screens have a greater thickness for capturing the image, DoF appears different than what the camera will record, unless you take a picture at F/5.6. The DoF preview button therefore only really helps from F/5.6 on into the smaller apertures, which is probably why popo suggested F/11 and F/16.



A high precision focusing screen has a much finer structure, hence is much thinner in the image capture area, and therefore displays more accurately how the actual DoF looks like.



Kind regards, Wim

[adifrank]

Understood. Thanks for the replies. This is really a great forum.

Although I haven't really had the chance to really use my new dslr in practical use or print out an image taken with it, I'm having lots of fun learning about it, testing it and getting accustomed to it.



Wim said:

A high precision focusing screen has a much finer structure, hence is much thinner in the image capture area, and therefore displays more accurately how the actual DoF looks like.

Would this be, in your mind, something definitely worth purchasing for my camera? For the reasons you mentioned and also for better manual focusing?



Thanks!

[wim]

[quote name='adifrank' date='19 July 2010 - 11:40 PM' timestamp='1279575600' post='1138']

Understood. Thanks for the replies. This is really a great forum.

Although I haven't really had the chance to really use my new dslr in practical use or print out an image taken with it, I'm having lots of fun learning about it, testing it and getting accustomed to it.



Wim said:





Would this be, in your mind, something definitely worth purchasing for my camera? For the reasons you mentioned and also for better manual focusing?



Thanks!

[/quote]

In one word: Absolutely! <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='' />



It is not expensive (less than $30 / €30), the light loss is minimal, and MF with it is a doddle, unlike with the standard focusing screen.



Offcially, you're supposed to use it only with fast, i.e., wide aperture lenses, F/2.8 and faster, but in real life the light loss is so minimal that you can use it with much slower lenses too, or for macro, where you lose a lot of light due to the magnification factors involved. I have used the -S types of focusing screens since getting a 40D, followed by 5D and now since a year wih the 5D II, and I have never since mounting it, exchanged it with the standard focusing screen. I find there is no need, not even with the 100-400L.



The nice thing that it is easy to focus on any part of the screen, so what I generally do for fast work with reframing is let the AF do its thing first (centre AF point only with the 5D II for me, except when shooting sports), which is much faster than I can do it manually anyway for a first approach, then reframe, and adjust if necessary with a little MF. With MF only lenses I don't use AF of course, just MF <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='' />.



I've had screens like these in the past with my analog cameras as well, and I always enjoyed using them.

For the 40D I also had a Katz-Eye OptiBright with split prism wedge, but I preferred the Ef-S screen for it.



For the 5D Mark II you'll need the Eg-S, if I didn't mention that already.



One thing: don't forget to set the screen type on your camera to the correct one. That's C. Fn IV Option 5. The Eg-S is the third one in the list. Because it is allows for slightly les sligth to pass through, the camera needs to know about it, as the light cells for the light meter are situated in the optical path behind the focusing screen, in the prism housing. These settings compensate exposure for the light readings with the different screen types. If you don't set the screen type, you may end up with slightly overexposed images.



Kind regards, Wim

[Guest]

[quote name='wim' date='19 July 2010 - 11:28 PM' timestamp='1279574890' post='1136']

Rainer and popo are both right.



There is one caveat with modern cameras however, although this was also valid to some degree in the film age. Unless you use an -S type focusing screen (for Canon anyway), an Eg-S for the 5D Mark II, which is also called a high precision matte, or something similar on any other camera, you will perceive a DoF through the view finder as if the lens is stopped down to approximately F/5.6.

[/quote]



Surely what it looks like depends on what the maximum aperture of the lens is? To be extreme, I doubt an f/1.2 shows the same DOF as an f/5.6, and I'm pretty sure an f/8 (e.g. mirror lens) will not look like an f/5.6. Wouldn't it be more correct to say that it looks like about two stops slower (assuming you were talking about f/2.8 lenses?



-Lars

[wim]

Hi Lars,

[quote name='larsrc' date='20 July 2010 - 09:42 AM' timestamp='1279611766' post='1143']

Surely what it looks like depends on what the maximum aperture of the lens is? To be extreme, I doubt an f/1.2 shows the same DOF as an f/5.6, and I'm pretty sure an f/8 (e.g. mirror lens) will not look like an f/5.6. Wouldn't it be more correct to say that it looks like about two stops slower (assuming you were talking about f/2.8 lenses?



-Lars

[/quote]

No, it actually does, and I did see the calculations for this a few times, somewhere. Unfortunately I don't hav eth elinks for thsi anymore, so I can't show those to you. It went a little beyond me anyway, the calculations that is <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='' />. It may be a little less with FF than with APS-C, but it is roughly F/5.6 with the standard focusing screens. If this wasn't the case, it would make no sense to have focusing screens with a much, much finer matting structure for use with large aperture lenses.



In a way you can compare this with film/analog. The emulsion thickness of film, 0.2 mm, provides more DoF than digital does, with a sensor thickness of 0 mm. This is a similar effect, not entirely, but similar anyway.



Kind regards, Wim

[adifrank]

In my specific case - the only lens I currently own for my 5D mark ii is the Canon 24-70mm 2.8 L. So I guess I'm in the "safe" zone.

I've been reading several threads from different forums about the pros/cons of using the EG-S focus screen. It seems that most opinions range between those who moderately caution using the EG-S with slow lenses to those who see the issue as almost negligible.



I suspect the difference in opinions is due possibly to different needs, styles and types of photographic work.



Since it is one of the less expensive accessories you can get for this camera - I think I'll just try and see for myself.



Thanks!

[Guest]

[quote name='wim' date='20 July 2010 - 10:36 AM' timestamp='1279614986' post='1148']

Hi Lars,



No, it actually does, and I did see the calculations for this a few times, somewhere. Unfortunately I don't hav eth elinks for thsi anymore, so I can't show those to you. It went a little beyond me anyway, the calculations that is <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='' />. It may be a little less with FF than with APS-C, but it is roughly F/5.6 with the standard focusing screens. If this wasn't the case, it would make no sense to have focusing screens with a much, much finer matting structure for use with large aperture lenses.



In a way you can compare this with film/analog. The emulsion thickness of film, 0.2 mm, provides more DoF than digital does, with a sensor thickness of 0 mm. This is a similar effect, not entirely, but similar anyway.



Kind regards, Wim

[/quote]



I believe my own eyes over those calculations. If the viewfinder showed f/5.6 regardless of aperture, you wouldn't see any DOF effect of using the DOF preview button. I see a very clear effect. I'm guessing those calculations talked about using an f/2.8 lens.



-Lars