DOF Preview Problem

Hi All,

I just got a new nikon 50mm f1.4 that I was testing wide open on my D300 (I have several film bodies on the way). I noticed that on close focus what looks to be in focus in the view finder is completely out of focus in the picture. So it looks like I have more aparrent DOF in the viewfinder than in the actual picture. The actual focus point looks sharp. Can someone explain this? Is DOF preview less accurate at close focus?

Thanks
Chris

There are many on this forum who can explain this to you in much depth. I can only explain casually.

Depth of focus has a lot to do with the size of image and viewing distance. If you are looking at it through D300 which is a cropped sensor camera with correspondingly small view finder, you are really looking at a small image. Compare to this, if you are looking at the same image on your display, you are looking at larger than normal print size image. What's out of focus look less obvious on smaller image.

One of the trick I use a lot with accidental out of focus negative is to print it small. Same thing....

Lens may be back or front focusing? check with a ruler and aim for a specific measurement. Also you camera is a digital crop, those DOF markings on the lens dont apply anymore. and its important to remember the closer you focus the more narrow the dof, the larger the aperture the more narrow the dof.

try again with live mode, its quite hard to focus digital manually.

I don't know about Nikon, but there are many Canon DSLRs out there with a focusing screen limited to f/2.8. This means that, if you open up more than that, you won't notice a difference in brightness or DOF in your viewfinder.

AnselAdamsX said:

Hi All,

I just got a new nikon 50mm f1.4 that I was testing wide open on my D300 (I have several film bodies on the way). I noticed that on close focus what looks to be in focus in the view finder is completely out of focus in the picture. So it looks like I have more aparrent DOF in the viewfinder than in the actual picture. The actual focus point looks sharp. Can someone explain this? Is DOF preview less accurate at close focus?

Thanks
Chris

Click to expand...

Is the point of focus on the negative in front, or behind the focal point seen in the view finder? If you have film bulge in the film gate, the point of focus on the negative will be behind the point of focus in the viewfinder.

If the focus screen is too close to the lens (needs to be shimmed back), the point of focus in the negative will be in front of the point of focus of the viewfinder.

waltereegho said:

I don't know about Nikon, but there are many Canon DSLRs out there with a focusing screen limited to f/2.8. This means that, if you open up more than that, you won't notice a difference in brightness or DOF in your viewfinder.

Click to expand...

I think I read the same thing on Ken Rockwell's site about the newer Nikons. When I get home I will try varying the fstop to see where it agrees with what I see through the view finder. I've never had a lens faster than f2.8 before so I would have never noticed. I suspect the N80's I'm getting for film will have the same issue.

THanks
Chris

The D300 is designed for autofocus lenses, and as such focusing manual focus lenses is a pain unless you change the screen. I suspect the N80 will be better. Your eye can adapt to seeing the aerial image in focus when it really isn't. Which is why focusing screens have a matte finish that the image is projected on. You can see the same problem looking thru one half of the split image in some screens, that small semi-circle never appears to be out of focus.

I agree with Bob-D659. Autofocus cameras have different focusing screens than manual focus cameras. Focusing screens of autofocus cameras are optimized for light transmission. They are bright. Focusing screens of manual focusing cameras are optimised for focusing: they allow better detection of what is in focus and what is not. Those two qualities are conflicting. Given a certain manufacturing cost, there is a trade-off between light transmission and ease of focusing.

With AF cameras the focusing screen is not a good aid in judging fine focus and as a consequence is also a bad aid when judging depth of field. If the AF camera is a "cropped" one this makes things worse as the entire image is smaller, as said by Newt_on_Swings.

Possible cures:
- If it is a digital, use live-view if you have it, especially if there is a magnifier (2x no good, 10x good).
- With any camera: a separate magnifying loupe applied to the viewfinder might help in judging focus;
- IIRC some cameras have a function, a "focus priority", whereby you focus the nearest point you want in focus, the farthest point you want in focus, and the camera will set the aperture for you. This presumes that the circle of confusion acceptable for the camera is acceptable also for you, otherwise you take note of what the camera suggests and close one stop more.
- DoF tables, calculators etc. might be of use in such situations.
- "proper" manual focus cameras allow a more thought about, albeit slower focusing. As ever, the price to pay for automatism is control.

Contact the camera seller or Nikon with your question.

Steve

You will want one of these to go with that 1.4 on the D300. http://www.katzeyeoptics.com/item--Nikon-D300-Focusing-Screen--prod_D300.html

This question applies to all AF cameras, not just digital cameras.

Laser cut, as opposed to ground glass, screens usually show the depth of field at f/2.8 maximum, regardless of how fast your lens is. Only recently have a few screens been introduced that can show D of F at f/1.8 maximum (still inaccurate for your lens, though less so), and they are only available for a few cameras. There are also screens specifically for slow lenses (e.g. f/5.6 max). But 98 percent of all AF screens show D of F at f/2.8.

I would just learn to use the focus confirmation dot religiously.

You can also get aftermarket screens installed, e.g. Brightscreen or Katzeye. However: I use nothing but fast manual focus glass on my new (to me) Nikon digital. I thought for sure I would get an aftermarket screen. But after using the camera, I decided that the stock screen is fine.

P.S. you can find the "cutoff" point by opening up your lens all the way and then holding down the D of F preview button. Then stop down your lens bit by bit while keeping the D of F preview button pushed. The f stop past which you first see a darkening of the viewfinder as you stop down is the f stop for which you have full time D of F preview. I am almost certain that you will not see your screen get any darker until you pass f/2.8. (Unfortunately, I think that this also means that fast lenses do not make your screen any brighter than an f/2.8 lens would.)