Depth of Field Preview button

[bloosqr]

After getting my 2nd fast lens, I've realized I dont quite understand what the Depth of Field Preview button is doing exactly on the camera..

Let me rephrase, I understand what it does literally, stop down the aperture .. but what I don't understand is what in practice this is doing. Allegedly if we hit it we will see 'what else is in focus' but it seems to me, that there is always more in focus via the view finder than is ultimately in focus on film..

that is, if one takes a fast lens say the 50/1.4 and focus on something not too far away and then looks at the background, it is also in focus, one doesn't get the 'bokeh' image that would ultimately show up if one takes a picture ...

this is with a film camera (elan 7e)

why is this? Before taking the photo isnt the aperture supposed to be completely wide? so shouldn't only the appropriate sliver be in focus?

I am guessing 'chimping' w/ an d-slr does not have this issue .. but I am not exactly following why I am seeing more than I think I should be ..

-best,
-avi

 

[wijnands]

You're quite accurately describing why I think it's the most useless feature ever to be put on a SLR (digital or analog)
--
Don't wait for the Nikon D-whatever, have fun now!
http://www.flickr.com/photos/j_wijnands/

 

[PeterQ]

IMO opinion it's not useless but rarely used.

The image you'll get with DOF preview is (most of the times) too dark to give an interpretable DOF view. But it really does what it's made for.
I think it competes with the Print button on the DSLR.

--
Enjoying to try making better images again and again and ...

 

[sherwoodpete]

that is, if one takes a fast lens say the 50/1.4 and focus on
something not too far away and then looks at the background, it is
also in focus, one doesn't get the 'bokeh' image that would
ultimately show up if one takes a picture ...

this is with a film camera (elan 7e)

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

 

[bloosqr]

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.

Gotcha, that is actually what I was worried was going on .. the odd thing here is this happens to me w/ all my cameras .. I have a used canon ae-1 from back in the day, a eos-3 and the elan 7e and I feel like they are all doing this, so is this issue a function of the person (a bit like those magic eye things (which i've never been able to see) ) or a function of the camera? That is, is there a camera that we could all buy and go, ahh lovely bokeh via the viewfinder?

Now would I be correct in thinking a pocket digital camera doesn't have this issue since the preview image is the image direcly from the ccd itself? Along these lines, if one made an SLR w/ an lcd viewfinder that showed only the appropriate image on the view finder glass plane, would this actually be better than a conventional SLR?

The reason I am asking this question is getting the depth of field w/ fast glass is really tricky for me (to the point where I've just memorized a few numbers at the

widest aperture) (i.e. 35/1.4 at 5 ft is 6 inches), but if the image was vaguely proper, one wouldn't have to do it this way .. i would think one could just eyeball it .. after all that is the point of an SLR right to see the 'proper' image?

 

[becksi]

hello bloosqr,

actually the DOF preview button is rather useful - if you know of it's qualities but also its limitations. Excuse me, if I'm eventually making wrong assumptions on your understanding of photography here and if I am too basic at times. (it's probably better to give too much info than too little - and others might enjoy the details too)

When your camera focuses it will focus with a wide open aperature, regardless of the actually selected aperature that you want to take the picture with. I.e. if you are taking a photo with you 50mm f/1.4 lens set to aperture f/22, via the viewfinder, you will still only see what is in focus as if you were photgraphing at aperature f/1.4. The DOF (preview) button closes the aperature diaphragm to the selected aperture in order to give you a preview of how this selected aperature will affect the DOF of the picture you are taking. The viewfinder also becomes dark(er), more so, the smaller the aperature you have selected is - the darker it gets. The difference between your lens wide open f/1.4 and f/22 is 8 stops (f/1.4, f/2, f/2.6, f/4, f/5.6, f/8, f/11, f/16, f/22), each stop cutting the amount of by half, thus making the image viewed at f/22, 256x (2^8) darker than the image at f/1.4. So far pretty logical right? Ok, before I veer off too much more, let me get back on track and explain why I went through this routine. Here - the getting dark - is where it gets tricky with the DOF previewing. As the image gets darker the quality of the viewfinder image deteriorates as and that is usually the first - and sometimes only - impression and thing people get and notice from depressing this button. They say: All it does, is make the image darker. Correct! And the explanation why, is further up. But there is more if you look more closely. It will actually give you an idea of how the DOF changes by having selected a specific aperture.

You can test this yourself, by choosing a rather extreme example and situation to learn how the DOF button works. Do this, take three glasses (or bottles, candles, etc.) and arrange them in a line on a table, each about 1 foot apart and each one being farther away than the other. (e.g: one at 1ft, the other at 2ft, the next at 3ft). Now take your camera and set your lens to f/22. Set your camera focus on the first glass and - without changing the focus - look at the 3 glass in your viewfinder. It should be blurred. Now press the DOF button and look at who the DOF changes. Do you notice the difference? Make sure that for this setting you have good light. You also get more background bokeh if your distance between the camera and the first glass is shorter and the distance between the glasses bigger. Change the settings yourself and see what the difference is between various settings.

This test should help you understand what to look for when you use the DOF button and will make it more useful when 'out in the field". It's not 100% perfect (especially because it becomes hard to tell DOF if lighting isn't good and the viewfinder gets too dark), but it will allow you to estimate better, what in your picture is in focus and what is not.

Another way of helping you get the best DOF for the situation your are trying to capture on film, is understanding hyperfocal distance. When you focus on a certain spot on a subject, depending on what aperture you select, not only this spot will be in focus, but a certain range BEFORE and AFTER if will also be sharp and clear. Now, since nowadays lenses hardly ever feature hyperfocal distance scales on the lens anymore, it is hard to know how big the foucs plane is. BUT there are charts and little wheels that you can print out (or order) and use in the field. This way you don't have to memorize all the different values and settings. (which I think would be a very hard thing to do anyways).

Follow this link for more info and instructions on how to use these tool, click on the following link (if that's not sufficient, I recommend google the term)

http://www.dofmaster.com/hyperfocal.html#focusing

Ok, I hope this helps
cheers
simon

After getting my 2nd fast lens, I've realized I dont quite
understand what the Depth of Field Preview button is doing exactly
on the camera..

Let me rephrase, I understand what it does literally, stop down the
aperture .. but what I don't understand is what in practice this is
doing. Allegedly if we hit it we will see 'what else is in focus'
but it seems to me, that there is always more in focus via the view
finder than is ultimately in focus on film..

that is, if one takes a fast lens say the 50/1.4 and focus on
something not too far away and then looks at the background, it is
also in focus, one doesn't get the 'bokeh' image that would
ultimately show up if one takes a picture ...

this is with a film camera (elan 7e)

why is this? Before taking the photo isnt the aperture supposed to
be completely wide? so shouldn't only the appropriate sliver be in
focus?

I am guessing 'chimping' w/ an d-slr does not have this issue ..
but I am not exactly following why I am seeing more than I think I
should be ..

-best,
-avi