Can you really see DOF?

[pkallos]

This question is not as stupid as it sounds. I have two f/2.8 canon lenses,

and more than once I've been burned by shallow DOF. I'm taking the pictures,

I'm on f/2.8 and everything looks great through the viewfinder. Then I get

home to process the photos and I notice the two people in the photos are offset

from each other, so one of them is in focus, and one is not. But it looked

fine while taking the pictures! So can people really see this while they're

taking the images? Or do people figure out what the DOF is going to be for

every shot and see if they have enough. Or do people stay away from f/2.8

unless they have only one subject in the photo?

[rcox2]

Are you pushing the DOF preview button?

[colinsouthern]

You can see it, but it's something you have to consciously look for. Personally, I steer away from using the DOF Preview button, as I find that once the lens is stopped down it generally gets too dim to be able to see DOF issues accurately.

 

As to the part about "avoiding" F2.8 etc ... no, but you do have to be conscious of just how narrow it is when you're up close (just a few millimeters up close with a 200mm lens).

[Matt Laur]

If I'm not mistaken, what you see through the viewfinder (unless you press the DoF preview button AND have the lens set to stop down) IS the lens at f/2.8 (or whatever its widest aperture happens to be). That's so that you have the most light in the finder, and so the camera has the most light for its autofocus sensors to work with. So, the "looks fine" part that you're seeing is already out of focus in the finder, you just can't tell because your eye isn't resolving it very well. Knowing - either very technically with each lens/focal length, or just gutwise from experience - how tight you need the lens given the scene you're shooting just comes from doing a lot of it or carrying around a chart... OR: using your camera's display (presuming a DSLR) to zoom down to an eyeball or edge or other delicate area on your subject(s) to see if, while you're standing there, you got the depth you needed.

 

Personally, I know how shallow a working focal range 2.8 gives me on the lenses I like, and just try to be a little strategic in how/whether I use it. Because, like you, I can't truly see - on the fly while shooting busy scenes - just how much I'm getting in focus, plus or minus a few inches. But I KNOW how little will be there from experience, and it just sort of grows on you.

[mark u]

The apparent DoF depends on how big you view the image. The viewfinder image size is pretty small with many of Canon's DSLRs, which makes it harder to appreciate what a print may look like. One thing that can help is to use a different focussing screen. Focussing screens that are designed for manual focus (i.e. with rougher surfaces) will give a much clearer indication of the actual DOF at a normal print size, but often at the expense of brightness of the OOF areas.

[pkallos]

I know all about the DOF preview button, but WYSIWYG when shooting at the maximum aperture of the lens. I'm talking more about what Matt said. I guess I could bring along my palm pilot that has this nifty program called "DOF Master" which will calculate it, but that is kind of a pain to sit there at every different focal length and aperture and distance to subject and re-calculate. Also, that won't really be possible with non-studio work. Hey duck, don't move really quick while I calculate exactly what the DOF is going to be so I can make sure your whole body is in focus.

[hjoseph7]

Brian Peterson in his book "Understanding Exposure" explains the DOF preview button much better than what you can find in Manuals.

[rainer_t]

As Peter himself and Matt have already pointed out, pressing the DOF-preview-button on a f/2.8 lens used at f/2.8 is completely pointless.

 

But there is indeed a trick that can be used ... stop down to f/8 (or f/5.6 if its dark) and press DOF-preview ... if sharpness is "getting better" somewhere ... this spot isn't within DOF at f/2.8. Still, you have to look very carefully with the small viewfinders of most digital cameras. ... Don't forget to open up to f/2.8 after this (since you wanted to shoot with f/2.8).

[digitmstr]

It depends on the focal length of your lens: if it is a 24mm then, f/2.8 might be enough to keep two people (who are on different planes) in focus. If it's a 200mm that is not going to happen.

 

Generally speaking, if you are taking group shots you have to either pose people so that they fall within the same plane or use a smaller f.stop...or not be so picky about focus...or use a wider lens and make sure the aperture allows everyone to be within the DOF.

[arie_vandervelden1]

What Mark U says. A small viewfinder makes it difficult to judge how much is in focus and how much is slightly oof. Difficult - but not impossible if you look very closely. Be sure you have your diopter set correctly. Good lighting helps too.

 

I find that the dof-preview button is not very useful to judge slight variations in dof. I mainly use it to get an idea of edge/foreground sharpness in landscapes where I stop down to f/11-f/16. It is also handy for judging background bokeh and the size of the circle of confusion when shooting macro.

 

It is certainly possible (but not easy) to get two faces in focus at f/2.8, even in the 50-100 mm range. Imagine a plane running though the two faces, and position your camera perpendicular to the focus plane. f/4 is certainly more forgiving.

[bloosqr]

I asked a similar question to this on dpreview a while back when I had just bought my 2nd

fast fixed lens. I am not sure if it is "person" dependent but I also see a much wider depth

of field through the viewfinder than I do on the print. (Which also brings up the obvious

question, what is the point of the DOF button). What apparantly is going on is the eye can

focus on the image in front of and behind the ground glass of the view finder and not

simply the image that is directly on the ground glass.

 

Here is the response I got from Pete Sherwood who explained it more clearly than I ..

 

"A long time ago, the SLR viewfinder screen was a plain piece of ground glass, on which

the image was focussed, by manually adjusting the lens.

 

If it didn't work, the whole concept of the SLR would never have caught on at all. However,

over the years, many variations of the original viewing screen have been introduced,

initially to help with manual focussing, such as the split-image and microprism devices.

And at the same time, in order to improve viewfinder brightness, the viewing screen has

been replaced with a more transparent arrangement, usually combined with a fresnel lens,

to better distribute the illumination from the corners.

 

The end result is that some viewfinder screens are a little too transparent, enabling the eye

to focus, not on the image on the surface of the screen, but instead on an "aerial" or

floating image, which may be in front of or behind the main image on the screen. Different

parts of the subject at closer or more distant distances from the camera will form an

image either in front of or behind the screen, and you may be unintentionally viewing

these.

 

Viewing the aerial image in this way is exactly what takes place when using a pair of

binoculars. In theory a flat ground-glass screen could be inserted into the light path inside

the binoculars and they would still work - in a fashion. Though of course there is

absolutely no reason to want to do this.

 

Regards,

Peter

"

[jmichaelc]

Peter, in my experience, the answer to your questions is YES and NO.

 

NO - i don't think you really can through the lens, effectivlly that is.

 

YES - after years of using focal length/aperture settings, I do think that one accurires the ability to mentally see DOF and know in advance what to expect after the press of the shutter.

 

That is where i am however. A good example (and practice)would be to focus on an object, like a point on a brick wall, at a 45 degree angle. Practice at all different focal lengths and at all different aperture/focal combinations. Now....let's jump forward....lets say you did this for a year constantly everyday. I guarentee you after one year of practice you would be able to mentally envision your DOF range and know almost exactly where you need to be to accomplish your desired image.

[dave.englund]

I agree with Jammey's approach - purposeful practice. I have been wavering between using Hyperfocal distance or Infinity focusing for landscapes (admittedly a little off subject), and it wasn't until I got out with the camera and lens this last weekend and shot at all major focal lengths over a spectrum of f/stops with both focusing techniques that I finally identified which technique works best for my Tamron 17-50mm lens. Turns out it depends on what focal length I'm using at the time. Anyway, up to now I've had to shoot tons of images using both techniques to make sure I got a good one. Now, I can zero in on the best focusing technique to use at a given focal length and save my memory card and energy for additional shots. A similar testing approach could be applied to identifying specific DOF at a given f/stop. It's a pain in the butt to be sure, but once done you KNOW.

[Vlad Soare]

Peter, I know what you're talking about. The viewfinder is too small to give a clear indication of what's in focus and what's not. There are two ways of coping with this: one is to use a high-end camera, like 1V or 1Ds, whose viewfinders are as big and bright as it gets. Another one is to use a focusing screen with a manual focusing aid. I have a 1V with the Ec-B focusing screen (horizontal split), and although I sometimes fail to see in the viewfinder a clear DOF difference between two not-too-offset subjects, this happens very rarely, and even if it happens the split-image screen does a very fine job. A split-image screen is extremely useful tool, and they are available from third parties even for cameras which officially don't have replaceable screens, like the 30D. Maybe you can find one for your camera.<br>

To answer your question, no, I never stay away from f/2.8. The reason I bought 2.8 lenses is that I really wanted to use them at f/2.8. :-)

[bloosqr]

Incidentally take a look at this old thread:

 

http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00GrRY

 

My experience with this problem is w/ the EOS-3 (w/ the Ec-N screen). There is clearly a

trade off between the brightness and DOF of the screen. I am not sure what screen gives you

a 'proper' DoF at low apertures <2 ), perhaps someone can clarify?

[lester_wareham]

Comments on DOF Preview button and low magnification through the viewfinder are all relevant. Also, modern focus screens on AF cameras are optimized for brightness and do not show DOF and critical focus so clearly.

 

DOF preview may work better if you have one of the manual focus ground glass screens on those cameras that screens can be changed. The downside of this is the screen will be even darker than the normal screen when using stopdown preview.

[ben_goren]

It's worth noting that, while you can't necessarily tell whether or not a part of the image is

within acceptable focus just by looking through the viewfinder, you *can* tell what parts

are *not* in focus. That's most useful for situations such as, ``I'm gonna stop down a bit

to make sure that this one flower is entirely in focus, but that bush twenty feet away

darned well better be blurry!'' While the DOF preview button might not tell you that you've

got the whole flower in focus, if you press it and you can see the tree clearly, you at least

know that you've stopped down too far.

 

Cheers,

 

b&

[keith_lubow]

If you are shooting wide open, what you see is what you get, period.

 

Keith

[William Michael]

Regarding the practical part of the question:

 

When working at F2.8, and if it were `critical`: or even if not `critical` but professional, i.e. for money, I would normally use a DoF chart or similar aid.

 

But when working with a lens I know, at distances I am familiar with, I would more often `know` the limits, or close to them.

 

I do not rely on DoF preview for work such as shallow DoF portraiture or similar work especially where there is a relatively short Subject to Lens distance with longer FL lenses.

 

As pointed out DoF Preview is irrelevant when the lens is wider open.

 

WW

[Vlad Soare]

<i>"If you are shooting wide open, what you see is what you get, period."</i>

<br><br>

Not necessarily, Keith. The viewfinder is relatively small, and the eye cannot clearly distinguish between a subject that is in focus and one that is veeeeery slightly out of focus. It may appear to you that both subjects are in focus, only to discover later in the finished print that one of them is blurred.<br>

That's what Peter was talking about. That's why a split-image screen (or any other kind of focusing aid for that matter) is very useful, because you can focus on one subject and then check if the other one is in focus as well, by looking at it through the center prism.

[pkallos]

Thanks for all the responses. I think most of the people who responded got the meaning of my question. So I'm not alone and the only one who has trouble with the viewfinder and how things "seem" to be in focus, but then on inspection later you find out that the shot is marginally in focus, if not totally garbage depending on how much you really like the picture.

[keith_lubow]

"Not necessarily, Keith. The viewfinder is relatively small, and the eye cannot clearly distinguish between a subject that is in focus and one that is veeeeery slightly out of focus. It may appear to you that both subjects are in focus, only to discover later in the finished print that one of them is blurred."

 

Quite necessarily! That is the main strength of an SLR. If you are getting something significantly different than what you see when shooting wide open, you need to pay better attention, get your eyesight checked, or both. Slow down and pay attention to detail and you will be fine.

 

Keith

[mike dixon]

<i>Quite necessarily! That is the main strength of an SLR. </i><P>

The transition between "in focus" and "out of focus" depends on the design of the focusing screen. You can change the viewfinder's apparent DOF by changing focusing screens. <P>

Depth of field is also dependent on the magnification of the photos. It's absurd to claim that the apparent DOF of an image a postage-stamp-sized piece of plastic or glass (which can have varying degrees of "roughness" etched into it) perfectly and accurately shows the DOF on an enlarged print.

[keith_lubow]

If your eye is right in it, size matters not. You end up with the same magnification you have when viewing a print from a distance equal to the diagonal, if not greater. This has not once been a problem for me...I don't see what the issue is. Perhaps different focusing screens render out of focus areas slightly differently than others, but if something is not sharp, it doesn't look sharp in the viewfinder, and if something is sharp, it looks sharp.

[Vlad Soare]

<i>"Quite necessarily! That is the main strength of an SLR."</i>

<br><br>

Of course it is, but that only works to a certain extent. There's a limit to how accurately the eye can detect a blurred subject in the viewfinder, and this limit has got higher and higher in the latest years, because of customers' expectations in regard to viewfinder brightness. Viewfinder brightness comes at a cost.<br>

Why do you think they invented microprisms and split-image focusing aids? :-)