7D viewfinder DOF doesn't match image DOF?

Hi folks.

I just got my 50 1.4 and my 85 1.8 and have been testing the 85 especially, on the 7D. I've noticed that the viewfinder seems to show a larger(deeper) DOF than what actually shows up(shallower) in the image when I review it on the LCD screen. Has anyone else noticed this phenomenon?

I am focusing back and forth between a label on my desk lamp light socket, and the corner edge of the wood work of the lamp itself. A difference that's ~4 inches apart. The edge of the wood is about 3.5 ft from the sensor plane. And the label is 2 inches behind the wood edge.

I did check my diopter. At first I thought this was the issue, however I can focus on my computer monitor, which is about 3 ft from the sensor plane and I get sharp focus in my VF and in the resulting image. I can then focus on my paper shredder box which is about 8 ft from sensor plane and I get a sharp image both in VF and on screen. These are relatively parallel to the sensor plane. The wood edge and label are not. The label sits about 3 inches higher than the edge and about 1.5 inches back. Think of a triangle with the one corner being the edge and the other corner where the label is. The 90 degree angle is at the center of the lamp column where the socket starts.

Originally I had the AF in full auto mode. I would lock focus on the wood edge and the label showed up fairly legible in the VF, but was blurred in the bokeh in the image. If I did some manual focusing override, getting the text of the label slight sharper in the VF, it was sharp in the LCD image.

Switch over to manual AF point selection. I use the bottom most point to focus on the wood edge so I can keep the label in the center of the VF and in the resulting image (this helps with flipping back and forth for comparison on the LCD). I get focus confirmation on the edge, label is still readable in the VF, take the shot, label is lost in the bokeh. Switch to the middle AF point, get focus confirmation on the label (lens motor does make noise and adjusts focus slightly in VF, label does become sharper), take picture, and label is now in focus on review image.

I think my AF is achieving accurate focus. I'm just led to believe that somehow the VF doesn't accurately display the true DOF. Which seems odd since I'm using an aperture of f/2.0 for the shots and I don't quite understand at the moment how the VF could get more DOF than what you should see through-the-lens, which is wide open at f/1.8 before a photo is taken. If someone can offer an explanation I'd love to read it.

Thanks!

Thats because when you aren't shooting your lens is always wide open. If you want accurate DOF then you need to press the DOF preview button. On both my 500D and 5D it is located below the lens release button.

themadman wrote in post #9833773
Thats because when you aren't shooting your lens is always wide open. If you want accurate DOF then you need to press the DOF preview button. On both my 500D and 5D it is located below the lens release button.

But how can the view finder show more DOF at f/1.8 than what I get in my images. The images have LESS DOF, not more. I'm not holding down DOF preview when I focus. I leave it untouched. So the VF should be representative of the DOF at f/1.8. But it's not. My images have less DOF than what the VF shows me to have, even wide open.

It shouldn't give you more than your lenses' max aperture... maybe someone who knows can chime in, I can only guess that it is something related to the fact that a camera sensor is different than the human eye.

themadman wrote in post #9833790
It shouldn't give you more than your lenses' max aperture... maybe someone who knows can chime in, I can only guess that it is something related to the fact that a camera sensor is different than the human eye.

That's what I would think too. I don't think i'm going crazy, at least not just yet . I tried playing with the diopter again to see if I could set it such that the OOF areas match how my images look, but when I try and focus on some of those blurry areas, they get a little less blurry, but still blurry, so i put the diopter setting back to the way it was.

oh, I understand OP's observation. Viewfinder give larger (deeper/more) DOF than the same picture on computer screen (shallower/less). This is because you enlarge the picture more on computer screen. I don't want to go into technical details, but the more you enlarge a picture, the preceive DOF is shallower. Try this, take two pictures of the same thing, one in focus, on slightly OOF. Resize both from 18MP to say 0.5MP, look at them side by side on screen. They will both looks like in focus shots.

This is why I hardly post so called "sample photos" on web to demonstrate "oh, how sharp is xxx body or xxx lens!!!" Because by the time the "sample" is resized to 800x600 displayed on web, anything will be sharp,

Modern focusing screens are brighter but show more DOF. On some Canon models you can go for "S" focusing screen which shows more accurate DOF, but it's darker when used with slow lenses.

tiha wrote in post #9834016
Modern focusing screens are brighter but show more DOF. On some Canon models you can go for "S" focusing screen which shows more accurate DOF, but it's darker when used with slow lenses.

+1

When I asked a similar question, I was advised that the ground glass of the focusing screen is not up to the task. In an attempt to get a brighter (or rather, in a forlorn attempt to be only a hundred miles behind full frame) image, modern crop frame dSLR's use ground glass that is smoother and less able to show DoF accurately.

And I would not want to trust anything the LCD gave me other than the histogram and that I might have recorded an image.

Lowner wrote in post #9834073
When I asked a similar question, I was advised that the ground glass of the focusing screen is not up to the task. In an attempt to get a brighter (or rather, in a forlorn attempt to be only a hundred miles behind full frame) image, modern crop frame dSLR's use ground glass that is smoother and less able to show DoF accurately.

And I would not want to trust anything the LCD gave me other than the histogram and that I might have recorded an image.

Are you able to explain this to me? Sorry to hijack the thread but as a newbie I am curious, why can't you trust what you see on the LCD screen? Do you not use it to judge your IQ at all? And also why do FF and crop cameras show things differently on the focussing screen?

Don't like to cite this guy, but on his website there is a very good explanation: http://www.kenrockwell​.com/nikon/50mm-f12.htm Scroll down to the Focusing Screens chapter.

eelnoraa, I'm reviewing the images on the camera LCD, not my computer. By your reasoningg, since the review images on the camera LCD are so small, both images should have the same DOF on the LCD, but they don't (LCD is approximately 834 x 1112 pixels). One clearly has label in focus, the other has label OOF, even though label was legible in the viewfinder when I took both images.

My intuition is going more along the lines of what others have said and that it has to do with the design of the focus screen and that for some reason it produces greater DOF.

tiha wrote in post #9834260
Don't like to cite this guy, but on his website there is a very good explanation: http://www.kenrockwell​.com/nikon/50mm-f12.htm Scroll down to the Focusing Screens chapter.

Read through it once. My initial thoughts are, the 7D VF has 100% image coverage, so why does it, based on Ken R., not see the other light rays from the outer edges of the lens but the sensor does? Will read through it some more and try to find other explanations. His leaves me wanting, still.

MOKK,

I'm told the older film SLR's and 1D use a course ground glass focusing screen which allows the user to judge depth of field. The semi-pro spec ff and all the croppers struggle to get the same brightness levels in the viewfinder and Canon, along with other manufacturers, have tried to solve this by using a finer ground glass. I don't understand the science behind it myself! Maybe someone else can explain it to us both.

I don't kid myself that the LCD is there for any other reason than showing the menu and the histogram. It generally is far too small and of extremely low quality (colour cast issues) AND only shows the jpeg thumbnail, to be of any real use as a tool with which to judge a shot. I wait until I'm home in front of the PC.

Having used different sorts of focusing screens for 4x5 cameras, the coarse ground glass makes it MUCH easier to judge focus and DOF. Some people prefer fresnel screens that are finer but brighter, but I never felt like it could show me the area of sharpest focus so reliably.